St Paul’s Methodist Church, Remuera, with Josiah Campbell and Richard Newman

From St Paul’s Church in Symonds St to St Paul’s Methodist Church in Remuera—22 minutes by number 75 bus. Like the Anglican St Paul’s (which we visited in March), the Methodist St Paul’s is undergoing seismic strengthening designed by EQ STRUC. The construction phase is well underway, and we were able to see some of the structural enhancements being installed. Josiah Campbell of EQ STRUC led the tour, and we were fortunate to be accompanied by Richard Newman from the contractors Aspec.

St Paul’s Methodist Church, Remuera. Point cloud model. Image courtesy EQ STRUC, all rights reserved.

St Paul’s Methodist Church is a Gothic revival brick building, designed by E. A. Pearce and completed in 1922. That puts it close to another site we’ve visited this year, the University’s ClockTower, which also dates from the ‘twenties and finds its stylistic roots in the Gothic. St Paul’s is a handsome building, with a reserved, dignified aspect from the street. It doesn’t contain the kind of flights of fancy that you’ll see at the ClockTower, but we were able to appreciate the craft and the sense of proportion that went into its design and construction.

And we were also able to see the kind of weaknesses that afflict buildings of this age and type. Reflecting on the visit, it occurs to me that the building contained examples of many of the kinds of problems—and solutions—that we’ve seen in our travels. In this post, I’ll try to give a sense of the structural solution (as I understood it) and also illustrate a few of the interesting details along the way.

St Paul’s Methodist Church. Site plan with structural additions at roof level highlighted. Note at the right the concrete-core shear wall, a later addition to the building. Image courtesy EQ STRUC, all rights reserved.

Not plane sailing

As with the ClockTower, St Paul’s Church, and other large masonry structures, a key issue at play in assessing and strengthening St Paul’s Methodist Church is the in-plane and out-of-plane capacity of the walls. (In-plane means pushing along the length of the wall, out-of-plane is pushing across the wall.) St Paul’s walls are relatively thick and sturdy—up to 410mm in places and thicker at the buttresses—and neither dizzyingly tall nor riddled with fenestration, so they are pretty good in-plane. Where the walls are less robust is out-of-plane, and a good deal of this weakness comes down to the way that connections were originally made between the walls and the roof.

Structures resist in-plane loads by transferring them to walls that lie parallel to the incoming forces. That’s a fancy way of saying that a box is stronger than four playing cards leaning against each other. At St Paul’s, the structural intervention is designed to connect the parts of the church together and to transfer loads to the parts of the building that are best placed to resist them. The new structure provides some additional strength, but its most important job is to allow the efficient  use of the existing strength of the building.

St Paul’s Methodist Church. A view of the top of the rear wall, brick veneer with concrete core. The diagonal white line running through the picture marks the position of the ceiling linings, which will be reinstated. This allows the steel PFC at the top of the picture to be installed, as it will be concealed by the ceiling. In other, visible parts of the building, a less obtrusive (and more expensive!) approach has been taken.

As it happens, there are some pieces of the church that are going to come in handy for just that reason. Most buildings that stick around for a few decades will have been modified in that time. In the case of St Paul’s Methodist Church, an extension project in the 1960s saw a concrete wall installed at the rear of the church, replacing the original wall. This newer concrete wall is hidden by brick, but there are construction drawings which give details of its dimensions and specified reinforcement. Josiah has supplemented the drawings with a GPR scan of the wall: the radar gives good indication of the reinforcement bar spacings, and from there it’s possible to drill to determine the bar diametersor in this case, the figures are on the drawings.

St Paul’s Methodist Church. A view of the original timber ceiling truss (which runs across the building) and the newly-inserted eaves truss, running longitudinally. The large metal attachment on the Reid bar is a “banana” clip, designed to allow the bar to be tensioned. The timber member running parallel to the white RHS is one of the underpurlins mentioned in the plan above.

The helpful part of all this is that the concrete wall’s capacity can be calculated, and it serves as a strong point from which to support other elements of the building. From this wall at the rear of the church, two eaves trusses are being installed along the long axis of the building. The trusses, made up of criss-crossed Reid bars and shortish lengths of RHS, connect the front and back walls together. This new metalwork also connects to the original timber trusses which run across the church, binding them into position. The front or street wall of the church is thus being given support by the other walls. The strength the eaves trusses provide to the gable end is being augmented with some steel wall braces that run across the face of the wall. In addition, the design uses the strength of some of the existing timber underpurlins.

St Paul’s Methodist Church. Connection between top of wall, timber ceiling truss, and eaves truss. The flat section of grey beneath the ceiling truss is the padstone (see below). This has been augmented by the addition of a bond beam (the concrete step at the right edge of the picture). The bond beam sits on top of the existing brick wall and is connected into the joint by a steel leg.

Looking across the building, quite a bit is being done to support the two long walls. As I mentioned above, the 1960s additions to the building have good drawings, and the engineers also found drawings made for the original construction. Anecdotally, I’m aware that the existence of original drawings is far from being a given when you’re working with older buildings. But even the designer’s drawings can only tell you so much. For example, the 1920s drawing set included a section across the building at the point where the timber roof trusses meet the top of wall. The drawing showed a solid unit—not a brick—underneath the bottom of the timber truss. Great, thought the engineers. There’s a bond beam. (A bond beam is a member that runs along the top of a masonry wall, and helps to transfer loads.)

However, when the work began in earnest, the ‘bond beam’ proved only to be a padstone, a short section of stone or concrete used to spread out the point load from the bottom of the truss onto a wider section of the brick wall. As with so many heritage projects, improvisation and redesign-on-the-fly was required, in this case taking the form of a series of bond beams cast in situ and connected to the truss system with embedded steel legs.

St Paul’s Methodist Church. Having come down from the internal scaffolding, site visitors inspect the placement of Helifix ties. The ties are being used in both horizontal directions: through the wall, to connect the brick wythes of the cavity wall together more securely, and also along the wall, to stitch cracks, especially around openings.

When describing this problem and its remedy, Josiah made an observation that I hadn’t heard before, which struck me as valuable wisdom. He wanted to avoid taking any of the old material away, he said. This was not for reasons of heritage best-practicealthough he’s certainly keenly aware of that, and earlier he had talked about how decisions have to be made about every scrap of original fabric that’s removed, from leaky roof vents to crumbling lintels. But the point that Josiah was making was that there are internal forces in play inside the existing pieces of the structure. Removing them comes at a risk. If you take things away, the internal forces will realign themselves, seeking equilibrium. If it’s not done carefully, unexpected movements or even failures could occur.

St Paul’s Methodist Church. Elevation looking from inside the church towards the road. Note the cross-bracing for the steelwork in the tower does not extend to the ground (see below). The wall braces are connected to the new internal trusses. Image courtesy EQ STRUC, all rights reserved.

You can see that it’s important to have the whole system of the building in your mind. Another example of this kind of thinking is to be found in the single new truss that spans transversally across the building. It’s being installed at the front end of the church. Notionally, says Josiah, you’d like to have it in the middle, to bridge the centres of the two long walls. But the choice to shift it to the front is part of a more holistic structural plan. Located at the front, the truss can provide connections and support for the parapet brace that’s needed in the porch, and connect to the steelwork that’s going into the tower.

St Paul’s Methodist Church. The roof is being retiled.
St Paul’s Methodist Church. A close-up view of the windows on the street side of the church, taken on the climb to the top of the tower.

Going up

At the time that we visited, we couldn’t get into the tower, but we did have the pleasure of climbing right up to the top of it from the outside. Because of the nature of towers—tall and waggly—I think it’d be fair to say that the inserted steel structure is being called on to do a bigger share of the work in the tower than it is in the main body of the church, where the design is more about using the strength of existing materials. Inside the tower, a braced frame will climb up the interior—Richard tells us it just fits. At the bottom, though, it wouldn’t be acceptable for the steelwork to block out the corner of the church, so the lowest storey of the frame—about three metres in height—has no diagonal braces. Instead, the four legs at the corners of the braced frame plunge down into a pretty hefty block of concrete, which will hold them steady.

St Paul’s Methodist Church. A concrete lintel above one of the windows was badly damaged and needed to be removed. The use of beach sand in the original concrete led to corrosion in the reinforcing steel, and the expansion caused the concrete to crack. Care needed to be taken in the removal, as it was in a delicate state. A good view here of the cavity construction of the wall. The two wythes of brick (three wythes lower down) are separated by an air gap, to keep moisture from getting inside. This can weaken the wall, but St Paul’s is not a severe case of this. Helifix ties are being used in places to connect the walls across the cavity. Image courtesy EQ STRUC, all rights reserved.
St Paul’s Methodist Church. A new lintel replaces the one above.

Climbing the tower, we got a better chance to appreciate the beauty of the church. I’d been clued in to the charm of St Paul’s Methodist by the intricacy of the timber ceiling trusses, but from the outside, we could also see the lovely slenderness of the windows with their stylish blue stripe. We saw a repaired lintel, sign of a common problem with 1920s concrete— the use of beach sand in the original mix, causing the reinforcing steel to corrode and expand.

St Paul’s Methodist Church. A site visitor inspects the top of the tower, where original lead has been removed and will be replaced. The round knobs cover fixings, as can be seen at the bottom right of the picture.

One last surprise awaited us at the top of the tower—or awaited me, I should say. I’d never seen a lead roof, at least not up close like this. It’s being re-leaded to prevent leaking, but the original round fixing-covers are going back on. They added a little hint of rococo to the Gothic, I thought. With knobs on, isn’t that the phrase?

Thanks!

It’s always a privilege to get to visit an active worksite, and we know that we are stopping real work getting done and taking up the valuable time of our guides. It makes a big difference to us—thank you for helping us to build up our experience and knowledge. Many thanks to Josiah Campbell for finding the time for this visit and for slides and for taking all our questions in his stride. Many thanks also to Richard Newman for his generous use of time for our visit. Both of these guys’ passion for the building and for the work they do was evident. Thanks also to David from the church for the background information and for permission to visit.

City Rail Link with Andrew Swan and Chris Bird

This will be a shorter post, as I am busy with study at the moment! Today we visited the City Rail Link works, looking at the cut-and-cover tunnel operation on Albert St and the ongoing work inside the Britomart Central Post Office. Our guides were Andrew Swan and Chris Bird from the City Rail Link team, who told us a little about the heritage aspects of the CRL project. We focused on the building monitoring on Albert St and the engineering work which has been done to support the CPO building while the rail tunnels are extended underneath it.

We started with a briefing from Andrew Swan, but I’m going to assume that you’re more or less familiar with the City Rail Link project, which extends the rail network up from the bottom of Queen St to Karangahape Road and then to Mt Eden. (A number of the site visits we’ve done in the past have explicitly addressed the CRL.) If you don’t know much about it, there’s a ton of info at the CRL site, and even for the well-informed, I’d highly recommend a skim through the construction blog which has loads of pictures and is updated frequently.

One facet of the heritage side that had escaped my attention until now is that the Central Post Office building is protected not just by its Category 1 status by by a Deed of Heritage Covenant, which makes it an offence to modify the building without consent. All the work that is happening at the CPO building—and there’s a lot—has been negotiated with Heritage NZ and explicitly permitted.

City Rail Link, Albert St. A monitoring total station swings around, ranging the prisms in its zone.

Hold it right there

Briefed, we headed out to site. My group began on Albert St, where a network of monitoring total stations is taking readings from a myriad of prisms every fifteen minutes.  The prisms, or reflectors, are sited along the route of the tunnel, on pavements, building facades, walls, and so on. As the work goes on, each prism is allowed to move a certain amount. If it moves any more than its pre-assigned tolerance, an alert is sounded, and the CRL team decide what needs to be done—which in extreme circumstances might include stopping the works.

City Rail Link, Albert St. A reflecting prism in place outside Link House, under the watchful eye of the total station.

I asked how the allowable deflections were set, and Chris Bird explained that this was done by consultants through geotechnical modelling and an assessment of the probable effects of the excavation on the surrounding buildings. It’s a case-by-case process, which depends upon the soil conditions at each site, building foundation type, the building’s structure, and so on. There are a range of sensitive buildings along the route, including (next to the CPO building) the Endeans Building, which still is founded on its original kauri piles.

City Rail Link, Albert St. Looking up at Link House, directly above the prism in the previous picture.

Movement monitoring continues inside the buildings themselves, which were surveyed before the excavation began. If significant pre-existing cracks were found, these were fitted with a gauge, allowing a determination of whether the works are causing any further damage. So far, deflections all along the tunnel path are well within the permitted limits.

City Rail Link, Britomart/Central Post Office. A view along one line of underpinning beams transferring load from columns. Image copyright the author. This image may not be reproduced or shared without permission from the City Rail Link.

How to lift a building by one millimetre

And so to the Central Post Office building. As you know, the major work here is to extend the train tunnels to allow trains to run both ways through Britomart Station. The problem is, the tunnels go right under a good many of the columns that hold up the building. To allow the tunnels to be dug, the loads that are coming down the columns need to be transferred out to either side of the hole, and from there down into something nice and solid.

When we visited the project last, in October 2017, the team were working on creating diaphragm walls, using a special drilling rig affectionately known as Sandrine. These walls are sturdy concrete structures, extending along the edges of the Central Post Office building, and inside the building along the edges of where the tunnels will be dug. With the diaphragm walls in place, the CRL team have put in large steel members to serve as underpinning beams. These underpinning beams span across the tops of the diaphragm walls.

The underpinning beams are there to take the weight of the columns of the CPO building. The load has to be carefully transferred from what’s supporting the columns now (the existing foundations) to the underpinning beams. This process is carried out as follows.

City Rail Link, Britomart/Central Post Office. Detail of the load transfer system. The upper red steel box is the collar. Beneath the collar are the four flat-jacks. Short members span the paired underpinning beams. Image copyright the author. This image may not be reproduced or shared without permission from the City Rail Link.

First, the concrete is chipped off from the columns, exposing the original steelwork. A collar is then clamped around the column, and the collar sits across a pair of underpinning beams. The beams aren’t taking any load yet. To transfer the load, four tiny flat-jacks are placed beneath the collar. Using hydraulic fluid pumped to 290 bar inside copper coils, the jacks lift the collars—and the columns—ever so slightly. Half a millimeter—at most a whole millimetre—but no more. They lift until they reach a given displacement or a given force, equivalent to the calculated load in the column. With the jacks in place,  lifted and shimmed, the load from the column is now being taken by the underpinning beams, and from there across and down into the diaphragm walls and into the bedrock. Then the column base, which is no longer bearing the load, can be cut away. (In this animation of the process, you can see the diaphragm walls in light grey and the underpinning beams and collars in red.)

City Rail Link, Britomart/Central Post Office. The CPO building was seismically strengthened in 2002. A recent review of the 2002 work has found that it is still adequate in a post-Christchurch era. In the blue box, one of the shear walls (?) inserted into the structure in the 2002 retrofit. Image copyright the author. This image may not be reproduced or shared without permission from the City Rail Link.

The other major part of the structures that needs support while the tunnels are being dug are the walls, in particular the East and West walls. These are the walls which lie above the tunnels.  The West wall is the Queen Street side, the grand facade of the building. To allow it to span the tunnels without cracking, the team have created two immense post-tensioned concrete beams, one inside and one outside the wall. The beams are tied together with cross members, which run underneath the wall itself. The beams have been cast and tensioned in situ, and Andrew recounted that they were complex to construct and design. On the day that we visited, some work was happening to set up the steel reinforcement for one end of the beams which will do the same work on the Eastern wall.

Parish news

A final note. Andrew mentioned the recent news stories about decision-makers looking to expand the capacity of the new stations on the CRL line, Aotea and Karangahape. I asked about the impact of this on the Pitt St Methodist Church, the Mercury Theatre, and maybe on our friends at Hopetoun Alpha. Andrew’s take was that the plans for bigger stations, if adopted, would be good for the Pitt St Church, since there would be no need for the large ventilation structure which is proposed to be placed just outside the Church. Instead, the second entrance in Beresford Square would provide ventilation for the station. Interesting to see how this all develops.

Thanks!

Warm and very sincere thanks to Andrew Swan, Chris Bird, Sonya Leahy, and Berenize Peita for their time and their willingness to share knowledge and answer questions. With special thanks also to Clare Farrant for organising the tour, and to the indefatigable Jenny Chu.

Albert Park Keeper’s Cottage with Dave Olsen of Mitchell Vranjes and Egbert Koekoek of Cape

Your correspondent keeps a sharp eye out for old buildings under wraps. When one is spotted, this is usually followed by a spate of calls and emails requesting a site visit—what you might call (I hope!) a charm offensive. In the case of the Albert Park Keeper’s Cottage, it was almost as though the building was taunting me: for one thing, it’s right there at the University gates. And for another, it was rising into the air. Come and get me!

Albert Park Keeper’s Cottage. The Cottage has been jacked up off the ground to allow repiling work to take place. The Cottage is an 1882 timber structure, with brick piers supporting the floors and a brick chimney. One unusual feature of the building is its slate roof. The slates are an added mass high up on the structure, and have some effect on its predicted seismic behaviour.

A sign at the site explained that the building was undergoing seismic strengthening: and so, a few phone calls later, we went behind the fence to check out the project and how it was progressing. Guiding us were the project engineer Dave Olsen from Mitchell Vranjes (regular site visitors may remember him from the Melanesian Mission) and Egbert Koekoek from the construction contractors Cape.

Albert Park Keeper’s Cottage. Site visitors assemble to hear from Dave Olsen (left) about the project.

Going up

So, why was the house in the air? As with many older buildings, the basic problem is that the Cottage is not strong enough to resist horizontal loads. A structure can be fine holding up its own weight, but if it’s shoved sideways, it falls off its foundations, and that’d be that. One part of the project is to strengthen and renew the Cottage’s piles, and to brace it horizontally against loads from a future earthquake.

But buildings, especially old houses, are re-piled all the time, right? And they don’t get lifted into the air, do they? The reason for this gets at some of the differences between heritage jobs and regular engineering. In a conventional repiling, holes are cut in the floor, and those are used to dig out and place the new piles. At a heritage building, one of the first principles is to try to avoid damaging the original fabric, and to minimise any necessary damage. Rather than cut the floor to pieces, it was deemed better to lift the building—a technology more commonly associated with house removals.

Albert Park Keeper’s Cottage. Steel lifting beams support the Cottage off the ground. Note weatherboards have been removed to allow the beam to be inserted. As can clearly be seen, the beam is lifting from above the floor.

Lifting the building had other benefits. It gave enough headroom for the workers to install some larger timber piles, the deepest of which extend 900 mm below the surface. Further, because of the heritage-listed trees which surround the site, it was not permitted to use screw-piles, so workers (and the arborist!) had to be able to see where they were digging.

Albert Park Keeper’s Cottage. Lifting beams seen in the interior of the structure, photographed at the southern corner through a convenient gap. Note on the left the timber ribbon beam, which runs longitudinally through the building and is fastened to the studs.

How do you lift a house? I’d’ve imagined that this was done from the bearers, or maybe the joists. But it was plain to see that this was not the case at the Cottage. The orange steel beams running through the house are clearly above the floor level. Egbert Koekoek explained that the house lifters installed timber ribbon beams running the length of the cottage, which were attached to the studs. Weatherboards, and the internal timber lining boards (sarkings), were removed to allow the ribbon beams to attach directly to the studs. The orange steel lifting beams were inserted. Then, fourteen hydraulic jacks lifted the Cottage up into the air, a little at a time, over the course of a couple of hours. Each jack can be individually switched on and off, leading to a certain amount of racing around with a tape measure to make sure that everything’s lifting at the same rate!

Albert Park Keeper’s Cottage. Note the new timber (lighter colour) added either side of existing bearers. The line of brick piers below the bearer, still able to carry gravity loads but with no horizontal capacity, has been augmented and partly replaced by timber posts. Diagonal braces attach to new timber piles. At the right, a concrete block wall replaces bricks which have rotated outwards due to expansive soil.

With the house lifted in the air by its studs, it’s not safe to go inside, for fear that the floor might simply fall away under your feet. However, the raised house also provided the opportunity to strengthen the bearers. The need for strengthening is in part due to the new use of the building as public space, requiring a design for 3 kPa floor loads. This has been done by adding timber either side of the existing bearer—once again, unconventional practice, but in keeping with the heritage principle of retaining original material.

Albert Park Keeper’s Cottage. The original perimeter bricks are largely being retained and reintegrated into the load-bearing system.

At ground level

Geotechnical testing of the site revealed that the soil is expansive, meaning that it shrinks and swells a lot. Perhaps as a result of this, some of the original perimeter brickwork under the walls has moved around quite a lot over time. On the park side of the house, the wall had rotated about ten degrees, and had to be replaced with a concrete block wall. (The concrete will be faced with brick so that it looks much the same as the original.)

With some new perimeter walls, and with sturdy timber diagonal brace piles taking effect, the underfloor of the Cottage is now going to be fairly stiff. A site visitor asked about stiffness compatibility between the underfloor and the timber structure of the house, which can be expected to be pretty floppy by comparison to its supports. The answer to this came in several forms, if I’ve understood it correctly!

In part, the Cottage itself is getting some increased stiffness. The sarking on the internal walls is going to be renailed in a number of places, making the internal boxes of the rooms considerably firmer. The front room, in the northeastern corner, contains the chimney, about which more later. It requires extra horizontal bracing to restrain the brickwork, so a brace Gib is being added over the sarking. (The ceiling of that room gets an enhanced diaphragm, too.) But, in the main, the answer to questions of stiffness compatibility between structure and substructure is this: it doesn’t matter. The Cottage isn’t overly large. And the inherent flexibility of the timber makes it unlikely to transfer loads very far across the structure, meaning that deflections at the interface between floors and piles shouldn’t be too much for the connections to handle.

Albert Park Keeper’s Cottage. The fireplace and chimney were (naturally) not lifted with the rest of the building. The connections between chimney and structure had to be carefully broken away to allow the house to be lifted

Catching the flue

In the seismic assessment of the Cottage, the chimney was identified as the weakest link, scoring around 15% NBS. Think of the chimney as a freestanding pile of bricks. It’s supported on its foundation, and again at the ceiling level. Then there is a decent length of chimney between the ceiling and the roof, and still more again where the chimneystack protrudes into the sky. So what we have is a long brick column, with a point of restraint at the base, another at the ceiling, and a long unrestrained section above the ceiling. It’s this top section, above the ceiling, that needs extra support. In a quake, it could rock itself right off the rest of the flue, causing collapse.

Albert Park Keeper’s Cottage. A section showing the props bracing the chimney. Sturdy connections are made to the rafters. A plywood diaphragm at ceiling level increases the stiffness of the ceiling restraint. Image courtesy Dave Olsen/Mitchell Vranjes, all rights reserved.

The solution that Dave has chosen is to use timber props, creating a collar around the chimney just below the height of the roof. This creates a firm diagonal bracing for the chimney, meaning that the unrestrained section will be restrained at approximately half height. By changing the unsupported length, the period of the rocking motion expected in the chimney changes, and the resulting forces experienced by the chimney are reduced. In accordance with the NZSEE guidelines, the mortar of the chimney-bricks is assumed to have basically no tensile strength. In a quake, the chimney is expected to form cracks, breaking at predictable points into short but intact sections which will rock but not topple.

With the limited clearance beneath the Cottage’s floors, smaller workers are preferred

Local gossip

A couple more newsworthy points to share with you. Regular visitors to Albert Park will have noticed that the Band Rotunda is also under wraps. Egbert explained that, although there’s plenty to do at the Rotunda, there’s nothing structural happening: the job is mostly maintenance and repair. He also shared a few things about the work that’s been happening at Pembridge House, which is the southernmost Merchant House in the lineup along Princes St. I did make an attempt to get a site visit to Pembridge up and running, but it was too complex because the floor was taken up for a lot of the time and the site was hazardous. (Hazardous = interesting, though, doesn’t it!) A major feature of the job, structurally, was the insertion of two big two-storey steel K braces in the stairwell, which were then concealed. Nothing to see now, folks! Never mind: other opportunities will surely arise.

Thanks!

Sincere thanks to several people for helping to organise this one. We put this together against time pressure, with the Cottage due to be lowered early next week. A number of people set aside other (real) work to make this happen for us, including Richard Bland, Antony Matthews, and Stacy Vallis. Thanks to Auckland Council. We’re also most grateful to Dave Olsen and Egbert Koekoek for their time and their willingness to answer questions and discuss the project.

 

Britomart, Auckland High Court, and St James Theatre: Heritage Buildings as Social Media

A brief note: apologies that this has taken so long to complete. Other deadlines compelled me more urgently!

The International Day for Monuments and Sites

What’s heritage? One facet I’m interested in is how the answer to that question changes with time. It seems inevitable (and proper) to me the contents of the basket labelled ‘heritage’ will change through the century, as New Zealand’s demographics change. I quite like the idea that heritage is a curated selection of the past, chosen by the present, on behalf of the future. And who’s curating will change.

Heritage is not interesting to everyone. But certain people, at some point in their lives, get interested in the remnants of the past that surround them. Heritage advocacy groups try to help more people to get bitten by the bug, and, with the long view in mind (always!), they want to reach out to younger generations, who’ll have to choose to take up the responsibility for looking after the old stuff.

With this aim in mind, ICOMOS (the International Council for Monuments and Sites) runs an “international-day-of-“. This year, the intention was to use social media to reach out to younger generations and foster all those warm fuzzies. Yours truly got involved in helping to organise some events to celebrate the Day, and, in discussion with the Auckland organising group, we came up with idea of going out to look at some Monuments’n’Sites and discussing the buildings themselves as pieces of social media.

You what mate? Bear with me. It’s not quite as nutty as it sounds. Public buildings don’t spring unbidden from the earth. They’re always, naturally, built with an end in mind—to communicate something about their purpose and the intentions of their builders. With that thought in mind, and with some wise guides to help us, we went to have a look at three prominent Auckland buildings. What were the messages that the buildings were made to communicate? What are they saying now, in their current context? What might happen to them in the future? When I finally finish writing this preamble, you might find out…

Jeremy Salmond and site visitors pause outside the CPO to examine the surrounding buildings: no longer “an oasis-of low-rise”?

Britomart (the CPO), with Jeremy Salmond

The Britomart story is somewhat circular, which seems fitting, given that the City Rail Link is all about completing a loop. The Britomart site was one of Auckland’s first train stations, built atop land reclaimed from the sea with the spoil from the demolition of Point Britomart. When the Central Post Office (the CPO) was built there (starting in 1909), the train tracks had to be shortened to make room. This left heavily-laden steam trains without enough flat runway to build up the speed they’d require to get up the hill to Newmarket; so, in a huff, the Railways moved to Beach Road, demolishing a couple of commemorative brick archways as they went —”out of spite,” said Jeremy.

So what does the CPO communicate? I asked. “It’s a typical Government building,” was Jeremy’s reply. Grandeur was the word he used to characterise its effect. Speeches were made in front of it, troops paraded there on their way to war, and punters meekly approached the grand elliptical counter to buy a stamp or two. The CPO was the face of government: reassuring, vigilant, stable.

Only, of course, nothing’s stable. The Post Office changed—radically—and moved on. After a period of neglect, and the threat of demolition in the 1990s, the CPO was repurposed. At last, the Railway got their station back! In the meanwhile, the warehouses of the Britomart precinct had come under threat from development, offering to turn what Jeremy called “an oasis of low-rise” into a field of tall towers. Jeremy was instrumental in developing a precinct plan, preserving some of the smaller buildings amongst their new neighbours.

The CPO’s looking a little dowdy around the edges right now, but we feel assured that it’ll get prettified when the CRL works are done. Once again, it’ll stand over an open square, projecting authority, but with far taller company looking down affectionately upon it.

Site visitors arrive at the High Court

The Auckland High Court, with Harry Allen

Up the hill, then, to the High Court. As we walked, Harry pointed out that the court’s location had been a significant choice, a signal of its prestige. It was finished in 1868, as British troops were leaving the fort at Albert Barracks. It’s vaguely military in tone. with its castellated tower, but this is clearly a fortress of justice, not of arms. We’re taking over now, was the message. The war in the Waikato had been fought. Pākehā power was here to stay. Nestled between churches, the Court asserted secular power and social order. Later the merchants of Princes St and the Northern Club came to shelter under its reassuring flanks.

The waiting room outside the main courtroom, Auckland High Court

Ecclesiastical was Harry’s term for the building. I’d be tempted to go as far as penitential. It doesn’t photograph well on a phone, but the waiting room outside the main courtroom is a clearly designed to induce a certain state of mind in witnesses or prisoners.  The Law is mighty. Do not try to fool us.

Site visitors in the waiting room outside the main courtroom, Auckland High Court

ClockTower East Wing, University of Auckland, with Neil Buller and Peter Boardman

UPDATE 15 October 2018: Tiago Almeida of Structure Design got in touch to let me know about a paper that the participants in the job had written and presented at the concrete conference. I highly recommend a read of it: it contains a good overview of the full scope of the works and has some great illustrations.

We’ve been doing these site visits for a while now. In March last year, a large group of site visitors heard Neil Buller of the U of A’s Property Services talk about planned works on the University ClockTower’s East Wing. Peter Boardman of Structure Design was along that day, too—he was there primarily to talk about the work he’d done on the Symonds St Houses. This week, we got the band back together, and went to see the progress at the ClockTower.

ClockTower, view from the site gate.

The ClockTower, the East Wing, the Annex(e), B119, B105, the Cloisters…

All the above are legitimate names for some part of the building you can see above. The East Wing was built as part of the original construction of the ClockTower, in 1923-26. The great Roy Lippincott was the architect—I’ve written about another Lippincott building, Nelson House, on this blog. The ClockTower, aka the Old Arts Building (there’s another name!) will likely need no introduction to the audience of these posts, but its East Wing is less iconic.

Originally built as student accommodation, the East Wing has served as offices, meeting rooms, and administrative space for much of the last few decades. It’s undergoing a major seismic upgrade, targeted at 67% NBS. The target is based on a 500-year return period earthquake, and the building is designated Importance Level 2. The interior has been modified considerably since the building was first constructed. At the moment it is fully stripped out, and it will be getting a contemporary refit. It’s also going back to being a teaching space.

Plans, proposed refit of ClockTower East Wing. Note the symmetrical plan of the East Wing itself. Extending at the top right are the cloisters which link the East Wing to the main ClockTower building. Image courtesy Neil Buller, drawn by Architectus, all rights reserved.
The drilling rig atop the stair tower, ClockTower East Wing. Image courtesy Neil Buller, all rights reserved.

Stronger, tougher, independent

The most arduous part of the work at the East Wing is to strengthen the walls. The building has a reinforced concrete inner shell, which is clad in an outer shell of masonry. The strengthening regime requires inserting long steel rods down the walls from top to bottom. The rods will be tensioned, squeezing the stones more tightly together.  In the horizontal direction, the masonry is being tied more firmly to the concrete. The ClockTower connects to its East Wing by a covered walkway, known as a cloister. In the cloister, rods have been inserted horizontally as well as vertically, binding the open-walled space together. A seismic joint has been cut mid-cloister, separating and de-coupling the ClockTower and the East Wing and giving each of them room to wobble about at their own rate if an earthquake strikes.

Capstones removed, top of the exterior wall, ClockTower East Wing. Picture taken in December ’17.
Capstones marked with Roman numerals to allow them to be correctly replaced. Roman numerals are used, says Neil, because they’re much easier to carve with a grinder–all straight lines!

Drilling the walls

At the roof level, the capstones have been carefully removed. At regular intervals, the drill has been worked down through the masonry parts of the wall to the foundations. As the drill is lowered, the workers add on extra length to the drill bit, carving holes down to the foundations as far as eleven metres below. If the drill jams—and sometimes it does!—in some cases a pilot hole needs to be drilled through from the inside to release the bit.

Once the hole is drilled, steel rods are inserted, then grouted into place. Grouting a wall can be tricky—unseen cavities and naturally porous materials can leave you pumping oceans of grout into a small hole. To prevent this, the hole is lined with a fabric “sock”, which deforms to fit snugly into the drilled void, but prevents the grout from branching out into the wide blue yonder.

Holes drilled down through masonry wall. Holes ~120mm diameter?
Photo taken in December ’17.

With the rods installed, a stainless steel plate connects the rod-tips together. They’re then mechanically tightened, binding the whole system into a whole. Post-tensioning works by putting the entire wall into compression. When the wall gets shoved by a quake, it wants to rock or overturn. The side that’s being shoved up gets put into tension. (To understand this, put your hands on your hips and bend sideways: you’ll feel your muscles getting stretched on the side you’re bending away from.) Stone, brick, concrete—these materials don’t like tension. They’re good at squashing, bad at stretching. By adding extra compression through the post-tensioning system, the walls get to stay in an overall compressive state, even when tensile stresses are created by rocking. The tensile stresses aren’t big enough (hopefully!) to overcome the pre-existing compression created by the post-tensioning.

Once the rods have been tightened, the capstones are drilled out to conceal the protruding rod tips and nuts. Then they’re mortared and dowelled back into place. It’s important to fix the capstones back tightly so that a shake doesn’t dislodge them. They are not something you’d want landing on your head.

Rod inserted into hole and grouted. Yellow cap is for worker safety. Note stainless steel plate connecting rods and generating compression in wall. Note shaped edge of capstone, to avoid water running into wall cavity (?) Picture taken Feb ’17. Roof of cloisters.
Stainless steel plates overlap. Tensioning rod through centre. Capstone will be hollowed out to cover bolt heads, etc. Feb ’17.

A bigger, sturdier foundation

So much for the walls, but what are the vertical rods going down into? They’re not going to help much unless they’re sturdily connected to the ground! Significant work is going into upgrading the foundations and increasing their capacity. The ground has been dug out on the outside of the building, and a new foundation strip poured against the existing one. In the interior, digging is in progress to create a second new foundation inside the existing wall. The new foundations, inner and outer, are interconnected at intervals. Soon, the base of the original wall will be sandwiched between two new foundations, with the vertical post-tensioning wall rods tied into this newer, larger foundation unit.

ClockTower, East Wing. New external foundations being prepared. Photo courtesy Neil Buller, all rights reserved.
Base of exterior wall, ClockTower East Wing. The timber is formwork for poured concrete foundation. This new concrete foundation abuts existing foundation. The dark layer of stone at the base of the wall is granite, creating a damp proof course through which water cannot travel up the walls. This was very hard to drill through! Picture Feb ’17.
Interior, ClockTower East Wing. Preparation for new internal foundation to be poured, abutting existing foundation. Note at corner in foreground, reinforcement coming through hole. This is where the new inner and new outer foundations connect.
Horizontal drilling, ClockTower cloisters. Photo courtesy Neil Buller, all rights reserved.

 Tie me up, tie me… across

In the cloisters, the drilling work has been carried out horizontally as well as vertically. Workers have drilled through the concrete vaulting of the arches, installing horizontal ties to bind the open-air structure together. The tie rods have been hidden with round pattress plates, designed to imitate the tie rod end plates that are pretty ubiquitous on older buildings. At the moment, they’re a bit shiny, but they’ll soon dull down and become essentially invisible.

Cloisters. New steel pattress plate spreads bearing load. At centre of plate, rod extends through cloister arch into wall of ClockTower. Photo courtesy Neil Buller, all rights reserved.
Cloisters. Steel bracket, used in location where drilling is not possible. Bracket styled after decorative newel post in main ClockTower building.

In one spot, drilling proved impractical, owing to the geometry of what was above. To increase the capacity of that area, a steel bracket was designed and inserted, taking up the work that the internal tie rods would have performed. In keeping with heritage principles, the bracket has been designed to be sympathetic to the character of the building, but not to pretend to be an original feature.

ClockTower, ground floor interior. Black dots on walls are the location of ResiTies, inserted to bond masonry outer wall to existing concrete inner wall.

(Not) losing face

To prevent the masonry and the concrete shell delaminating, they are being bonded together with a close-spaced grid of special ties. They’re called ResiTies, and they’re a stainless steel twist, which looks not dissimilar to a decent-sized drill bit. The system uses a resin to bond both ends of the tie, locking the masonry layer and the concrete layer together. Apparently they go in pretty easily, but it certainly seemed like a big job to install these throughout the building. The manufacturers reckon they’re good for holding together brick cavity walls, too. You can read about them here: the link goes to a commercial site but, just to be clear, I have no relationship of any kind with Helifix.

ResiTie inserted. Note epoxy blob holding stainless steel tie.
ClockTower, East Wing. First floor. Concrete floor slab, patches of drummy concrete removed. Photo courtesy Neil Buller, all rights reserved.

Augmenting the concrete

The internal floor of the building is concrete. As you will know, reader, internal floors can be pretty important when buildings are strengthened. They transfer forces between walls, and allow the structure to act as a box. Diaphragm improvement is one of the most common things we’ve seen on our tours—it’s often in the category of low-hanging fruit when it comes to improving a building’s NBS score. The East Wing is no exception.

Over the years, a certain amount of moisture has found its way into the building. This, combined with the fact that the concrete was made with unwashed beach sand, has led to some deterioration of the internal steel reinforcement. (You can tell that you’ve got unwashed sand when you find shells in your concrete, as they did at the East Wing—it’s a dead giveaway.)

On the ground floor, the undersides of some of the concrete beams have been carved away, the surface rust removed from the internal steel, and then they’ve been re-sealed. On the first floor, the team went over the floor slab with a hammer, inch by inch, whacking the concrete, listening for the ringing sound that means the concrete is drummy. That’s happened where steel has rusted and expanded, cracking the concrete, or where salts in the sand have caused adverse reactions, or both.

The drummy bits of the concrete floor slab have been raked out, leaving the floor surface more than a little Lunar. Neil pulled out a bit of the reinforcing mesh and snapped it. Not much capacity left there!The engineers have prudently decided to discount the existing reinforcement in the floor slab entirely. So, to reinforce the floor and help it do its lateral-load-transferring work, the plan is to use strips of fibre-reinforced polymer (FRP). The FRP strips will create a lattice which will resist both tension and compression. A thoughtful site visitor double-checked: FRPs? Compression? Yes, says Peter Boardman. The lattice pattern allows the FRP strips to act like a truss.

 

ClockTower, first floor. Drummy concrete removed from floor slab. Blue lines indicate proposed location of fibre reinforced polymer strips which. Lattice of strips creates truss which can resist tension and compression.

Speaking of trusses

ClockTower, East Wing. Timber ceiling battens. Timber trusses above.
Trusses, slightly better image. The trusses are mostly sound, with minor water damage in the area shown. Steel brackets will mitigate lost connection strength.

A brief note at the end, then, to say that the timber trusses that form the roof are in pretty good nick, bar a few rotten ends which are getting bypassed with steel brackets. The building’s going to be sealed and air-conditioned, and some of the plant is going up into the roof void, with the rest perching discreetly beside the cloisters. On the day we visited, the roof-level scaffold was going up, and soon the building will be wrapped to allow the concrete roof tiles to be replaced with more authentic clay ones. There’ll be the usual plywood ceiling diaphragm enhancement, too.

It’s good to be back, and thanks!

Having seen the building last year, it was great to get a chance to come back and see how the work is being done. As our ad-hoc society continues to mature, expect more “return to-” tours further down the line.

We’re sincerely and warmly grateful to Neil Buller for organising the site visit, to Peter Boardman for sharing his time and his knowledge, and to Todd and the Argon team for letting us come and get in the way of a tight timeframe. As University of Auckland students, it’s great to have the chance to use our own campus as a learning tool. We really appreciate your co-operation. Thanks also to Phillip Hartley of Salmond Reed Architects for taking me on-site at the East Wing over the summer.

 

 

St Paul’s Church with Salmond Reed Architects and EQ STRUC

Today’s visit to St Paul’s Church marked the start of the third year of activities for our ad-hoc society. Simeon Hawkins from the church’s congregation led the way. Sean Kisby of Salmond Reed Architects and Peter Liu and Dr John Jing of EQ STRUC came along to tell us about their work on the nascent project of strengthening and refreshing St Paul’s Church.

St Paul’s Church, panorama from the Symonds St entrance looking East

The church, explained

Simeon, whose Master’s thesis focusses on the church, gave us a brief overview of its history. The current St Paul’s is the third building to bear the name. It’s a Gothic Revival building, begun in 1894. The church is cross-shaped, as convention dictates. The main body of the church is stone, with brick veneers inside from shoulder height to roof and brick structure below the floor. The roof members are timber. The transepts (the arms of the cross) are also timber, and the chancel, the head of the cross, where the altar is, was made from reinforced concrete and wasn’t completed until 1936.

The building is not quite as its architect William Skinner conceived it. He’d intended for a gallery to sit above the door, housing the choir. The rough masonry you can see around the base of the church was to be covered with timber panelling. The chancel was to be faced with stone. And, most noticeably, no spire was ever built atop the northwestern stairs. Still, the building is Category I in Heritage New Zealand’s list, all the more poignantly since the HNZ listing says “St Paul’s invites comparison with Sir Gilbert Scott’s only New Zealand work, Christchurch Cathedral”.

A door to nowhere five metres in the air denotes the spot where the choir gallery was to be built.

For my part—although I don’t have favourite children!—I have a soft spot in my heart for St Paul’s. Something about the cheerful pick-and-mix of its irregularly-sized stone arches, its glorious melange of materials, and its air of patient worshipfulness makes my heathen heart glad.

Fine stonecarvings, southern entrance, St Paul’s Church

First, do no harm

Sean Kisby picked up the thread. Salmond Reed, he explained, have a dual role at the church. Firstly, there’s a fair hatful of material repairs that need to be done: there’s stone to be replaced; roof coverings to be refreshed; lead and copper flashings to replace, too. It takes expertise in heritage materials to understand what should be done, and how best to do it. Tracey Hartley (and others) from SRA are supplying this knowledge.

In parallel, there’s a real gem of a design project to be done. The building needs seismic strengthening, with the aim of reaching 67% of the New Building Standard. The internal circulation needs to be improved—at the moment, masses of people must file up and down a tiny rear staircase. The main stair, a lovely wooden spiral, has literally rotted away, the victim of a “temporary” roof over the void that was to contain the spire.

St Paul’s Church, spiral staircase seen from the crypt

With the access reconfigured, and the rooms beneath the chancel refreshed, the long-term plan is to build out into the carpark, and (best-saved-for-last here), at long last to design and build a spire for the church. Surely that’s a commission to gladden the heart of any designer.

Strengthening the church

“Bring the building up to 67% NBS” I wrote above, glibly. So, how might that be done? Peter and John from EQ STRUC have been considering that question. Over the last few months, they’ve carried out an analysis of the building, including a LiDAR  scan of the building which created a detailed 3D image of the church. The LiDAR generates a point cloud, essentially a myriad of measured dimensions, allowing a viewer to see details of colour and ornamentation as well as capturing idiosyncrasies in the plan. The engineers also need to know how the mass of the building is distributed, and the point cloud helps to identify this, making later finite element modelling in Etabs or SAP2000 more accurate.

Dr John Jing explains the proposed strengthening solution to site visitors, St Paul’s Church

The EQ STRUC blokes were too modest to say this for themselves, so I’m going to say it for them: they’re really good at assessing and then using the strength of existing materials. This is important, very important, for heritage buildings. The conservative approach would be to discount the strength of unreinforced masonry down to almost nothing, necessitating a larger and more intrusive engineering intervention. Think whacking great steel frames marching down the aisles.

For the nave and the aisles, the solution that EQ STRUC have devised uses the existing materials to their own advantage. I need to caveat this by saying that, as yet, this is just a proposed solution, but here it is: the plan is to use the mass of the walls to dissipate earthquake energy by allowing the piers to rock.

How does this work? Let’s work our way down from the roof.

Looking up at the roof truss, St Paul’s Church

Firstly, a diaphragm will be inserted above the sarking and below the roof, allowing forces to be transferred. (The nave floor’s getting a new diaphragm, too.) The existing timber trusses will be enhanced with steel, stiffening them considerably. At the junction between the trusses and the wall, a moment joint will be created, meaning that the roof and wall can’t rotate towards or away from each other.

The walls and piers of the nave. Spandrels above openings. St Paul’s Church.

Along the walls, between the openings, the spandrels will be strengthened with fibre-reinforced polymer wraps (FRPs). This will greatly increase their ability to resist tension, helping them to stay intact in a quake. [see footnote for a follow-up on this][update April ’18]

A pier, St Paul’s Church.

Lastly, as noted above, the piers will then be permitted to rock. In a design-level earthquake, the pier will form two hinges: one above the plinth (at the second moulding) and the other below the capital (the leafy bit). The pier will then wobble back and forth on the hinges, dissipating energy as it does so.

How do you stop the pier from toppling over, and bringing the roof down with it? By tuning the stiffness of the moment joints up above, the quantity of deflection at the piers can be controlled. It’s elegant—if not easy—and the great virtue of the solution is that you don’t need to ruin the spatial qualities of the interior with ugly external bracing.

The rose window and the western gable, St Paul’s Church

The real doozy of a problem might be the western gable end. How best to brace the wall, given the aesthetic and historic constraints at play? The solution may come from the planned completion of the choir gallery, absent from the church for so long. The gallery would run along the wall just below the level of the two long windows in the picture above. As at the Town Hall and Hopetoun Alpha, a gallery at the midheight of a tall wall can be a structural godsend, concealing crucial structural bracing, and reducing the risk of a potentially deadly of out-of-plane collapse

Peter Liu explains strengthening schemes to site visitors, the crypt, St Paul’s Church. Composite image.

To the Crypt!

Beneath the nave lies the crypt. Although you enter the church at ground level from Symonds St, the ground slopes away steeply to the east, and there’s a generous space beneath the floor of the nave. The space is used for a range of church activities. Here, in this unfussy rough-brick room,  it might be forgivable to insert some supplementary steel structure. EQ STRUC are proposing to do so, thus completing the rocking system described above by giving the piers a solid base to wobble on. (The slender piers of the nave are supported on the chunkier brick piers of the crypt, which you can see in the photograph above.)

Tension cracking in the transverse brick arches of the crypt, St Paul’s Church

Down in the crypt, we could see old signs of damage caused to the church  by the city growing around it. The church abuts the motorway trench of busy Wellesley Street, dug in the 1970s (? don’t quote me on this date!) As the ground settled post-trench, the foundations rotated outwards, creating tension cracking like that shown above. Don’t panic, though: a geotechnical report showed that the settlement has long subsided, and there’s no fear of further rotation and damage.

St Paul’s, south side, coming up the slope from the carpark. Timber transept meets masonry nave. Note irregular finishing of wall, perhaps intended to join to masonry veneers?
St Pauls, concrete chancel/transept and masonry nave.

 What are you made of?

As noted above, and as you can see in these images, the church is made from a variety of materials. There’s stone, brick, reinforced concrete (RC), and timber. How, I asked, would you assess the interaction between the RC and masonry elements of the building?

Here, it seems, the timber transept is invaluable. The timber’s elasticity should take up any differential movement between the RC and masonry sections. This means that the RC and the masonry were able to be assessed as two separate structures, removing a layer of complexity from the analysis.

Given the building’s Category I status, it’s not easy to make a case for destructive testing to establish the strength of materials. EQ STRUC have a database of material tests from similar structures to fall back on, for establishing likely material properties.

Signatures of the builders? Stairwell, St Paul’s Church.

Talk to me

In the crypt, I asked Peter, John, and Sean about collaboration between architects and engineers, hoping for dark and ghastly tales to befit at least the name of the surroundings. Instead, they talked about the pleasure of collaboration, of working together and modifying each others’ solutions to arrive at the best result. In all seriousness, such collaboration is a theme of our Society, since we’re composed of would-be members of each profession. It’s great to hear that the working world embraces this philosophy of co-operation.

Dr Jing made a comment that intrigued me. “You need to draw,” he said. “I always take a pen and paper to meetings and to site.” Calculations can be confusing, explanations are often ambiguous, but sketching makes for clear communication. Architects will be unsurprised by this, I think, but it’s food for thought for student engineers.

Thanks!

Sincere thanks are due to Sean Kisby from Salmond Reed Architects, and to Peter Liu and Dr John Jing of EQ STRUC, for their generous sharing of time and expertise. We’re also exceedingly grateful to Simeon Hawkins and Esther Grant from St Paul’s, and to the marvellous people who came and offered coffee and hospitality. Thank you for sharing your beautiful church with us. We look forward to seeing the project develop.

Footnote

back to article

Bricks in the spandrel area, closeup. Photo courtesy Tracey Hartley, all rights reserved.

Tracey Hartley touched base, following up on the proposal to use FRPs to reinforce the spandrel areas. Regarding the bricks in the spandrels, she notes “I believe they are a rough pale brick with large joints, originally coloured over with a red ochre finish and lined out (tuck pointed) to make the brickwork smarter looking.” The final design may involve restoring this finish, so covering with FRPs may be more difficult. Also, the breathability of FRPs needs to be determined so that moisture isn’t trapped in the brickwork.

I thought it was interesting to include this as a footnote, to illustrate how the process of design is a compromise or collaboration between the expertise and priorities of the professionals working on a project. Working together, the architects and engineers will devise an acceptable solution.

Update April ’18

back to article

I ran into Dr Jing today and we talked about the FRPs for the spandrels and piers in the nave. Dr Jing explained that the FRPs in the spandrels were never going to be added onto the surface of the bricks, as I had thought. Instead, they would be mounted into little slots cut into the bricks. The slots would be ~3-4mm wide and span across several brick units, being anchored at top and bottom into bolts. This near-surface mounting would make the FRP intervention much less visually intrusive than if it were covering the surface (as per Tracey Hartley’s note above). To reiterate, it was never the plan to mount the FRPs on the surface: the EQ STRUC team were always intending them to be near-surface mounted.

One final point. I hadn’t picked up that the piers were also to be FRP wrapped, not just the spandrels. The FRP wrapping would naturally add great confinement strength to the pier. However, by varying the length and position of the wrap, the EQ STRUC engineers can force the plastic hinge to form in the places they want it to: at the top and bottom, as per the pictures above.

Britomart CPO and the City Rail Link with Jenny Chu, October 2017

Regular readers of this irregular journal may remember that sometimes your correspondent organises tours but can’t attend ’em. Unfortunately, I missed the Britomart visit in favour of a crook two-year-old—although luckily she was well enough to come down to town with me and hand over the hard hats to the site visitors.

Thanks to the great kindness of Matt Goodall, I have a few pictures from the tour to share with you all. My apologies for the delay in getting these on the web.

This brief post will (hopefully) serve as something of a placeholder. It has been suggested that we might be in with a shot of going back in 2018—which will no doubt be welcome news to the sixty-odd people who signed up but didn’t get a spot.

Those of you who are interested in following the project may also enjoy seeing an animation of the proposed work and the well-illustrated work-in-progress blog.

Many thanks to Jenny Chu and John Fellows of the City Rail Link for their kind hosting and for giving generously of their time.

CPO, general view of the interior. Sandrine le Drilling Machine lurking in the background. Note wrapped columns and protected ceiling. Image courtesy of Matt Goodall, all rights reserved.
Detail of column base. Image courtesy of Matt Goodall, all rights reserved.
Drilling operations. Image courtesy of Matt Goodall, all rights reserved.
Backfilling trench. Image courtesy of Matt Goodall, all rights reserved.

Hopetoun Alpha with John O’Hagan of Compusoft Engineering, October 2017

Concrete classicism

A lightning-fast summary: Hopetoun Alpha was built in 1875. It’s mass concrete, meaning concrete with no internal reinforcing steel. Built for the Congregationalist church, this is a Neoclassical temple in the Doric order, with a charming and luminous timber interior. It’s in Beresford Square, close to the intersection of Pitt St and K Road.

Hopetoun Alpha, exterior

If you’re reading this, you likely know that concrete is pretty strong in compression, but not great in tension. It’s hard to crush, but it doesn’t like to bend. You’ll also have a pretty good idea that 140-year-old buildings can move around a bit: slowly, as the ground settles over the course of the years; and quickly, in extreme cases—for instance, when the ground shakes. So the task that John O’Hagan of Compusoft Engineering has accepted in assessing a building like this is twofold. Firstly, to evaluate how well the building has coped with all the slow movement and the vicissitudes of time, and secondly, to consider how well it might handle an earthquake or a severe weather event. John shared some of the process of making this assessment with us, as we walked around the building.

To understand how the building will perform, you need to know exactly what’s made from, and how the pieces are put together. Over the last few weeks, John and the owners of Hopetoun Alpha have carried out a number of investigations to establish this. They’ve been underneath the building, into the roof void, and everywhere in between. John began today’s tour by explaining the structural system of the building.

Alpha’s anatomy

The building is a long rectangle, shaped somewhat like a shoebox. Its two long walls, the side walls, get narrower in two steps as they get higher. At ground level, they’re about 640mm thick. The site slopes, but the walls come up to establish a level for the floor. At floor height, they step in to 420mm, creating a ledge both inside and out. On the outside, the ledge demarcates the plinth on which the building sits, and the walls change colour to emphasise this. On the inside, the step creates a handy support for the floor joists.

Hopetoun Alpha, basement. John indicates the ledge on which the floor joists rest

It’s a very common detail for “masonry” buildings of all kinds. As we’ve learnt on our tours, brick buildings often contain a similar step (or sometimes just a socket in the course) into which floor joists can be inserted. The problem is, of course, what happens when the walls move so much that the joists fall off their ledge or out of their sockets!

The walls step in at ceiling height, too, and there the step supports the ceiling trusses, which are likely made from jarrah. John shared a drawing of the trusses.

Hopetoun Alpha, drawing of ceiling trusses. Image courtesy John O’Hagan / Compusoft Engineering

He noted that although the trusses are statically indeterminate, they were well analysed by the designer for resisting gravity loads. The steel ties that you see connecting the vertical members to the bottom chord are wrapped around and pinned, forcing the verticals into tension. The diagonals carry compression. The trusses are sound and strong, and seem to have performed well. John mentioned that nails seem to have been a scarce resource at the time of construction, as there were very few to be seen in the timberwork! Fasteners of all kinds were clearly at a premium: a single bolt connects the front truss to the gable end. (As you can imagine, this is not ideal.)

The trusses span the main hall laterally, but there is far less roof structure in the longitudinal direction. What’s there is more or less entirely cosmetic—boxwork, panels, trim, ventilators. This meant that the engineers needed to wear abseiling harnesses to get up in the ceiling for a look—there’s not much to stand on and a fair way to fall.

Hopetoun Alpha, interior from mezzanine. Note bowed ceiling panels, and mezzanine cutting across windows.

As you can perhaps divine from the photo above, the mezzanine or gallery wasn’t part of the original build. The windows on the long walls would probably look different if this were the case (c.f. the smaller auditorium at the Auckland Town Hall.) The addition of the mezzanine meant some additional supports were needed underfloor. Cast iron columns carry the load down to neat brick piers—far neater than the original footings, which seem to have been pragmatically cast inside a few handy barrels!

Hopetoun Alpha, concrete footing (and extraneous foot, far right). Note the remains of barrel hoops and staves—easy formwork!

Mass (of) concrete

The entrance to the building presents the greatest challenge for modelling and analysis. There are large volumes of concrete in the pediment and the stairwells on either side, and, as noted, this section is poorly connected to the roof truss.

Hopetoun Alpha, portico. John O’Hagan explains the structural system of the building

There are decent-sized spans between the columns—approaching two metres—and of course the concrete is being asked to cross that gap without tensile reinforcement. This part of the building may yet require more analysis.

She’ll be right

When the mezzanine was put in, Hopetoun Alpha was also extended at the rear. This allowed a stage to be built and an organ installed, and to accommodate this a large opening was made in the back wall of the existing hall. And the building didn’t fall down.

Hopetoun Alpha, model, showing the opening in the rear wall (the blue rectangle)

It didn’t fall down, but over the long years, a large crack has developed in the wall. This is probably the result of the foundations moving slightly outward, and of tension forces in the reduced thickness of wall above the stage. Whatever the cause, the crack extends from below a round window above the stage (not visible from inside) to the top of the proscenium. Then the crack starts again at the left-hand edge of the bottom of the stage aperture, and continues down to ground level.

Hopetoun Alpha, basement. John points out a deep crack in the transverse wall below the stage.

How significant the cracks are, structurally speaking, is yet to be determined. But it seems to me that some form of strengthening will be required to make up for the diminished capacity of the wall.

Solution, general form

I want to preface what follows by saying that John made it clear that he didn’t want to talk about specific solutions for Hopetoun Alpha. It’s too early for that—too early, even, to say for sure whether work is needed at all, until the NBS rating is determined. Instead, we talked about some hypothetical solutions for a building of this type. Please read the rest of this post in that spirit. Your mileage may vary.

Hopetoun Alpha, view from the mezzanine

The major concern in a building of this size and style would be the out-of-plane response of the long walls. This means, if an earthquake shook the building from side to side (as opposed to back and forth), how well would the long walls cope with being flexed?

It might be sufficient to support them by connecting the floor joists to the walls in the basement. (At the moment the joists are just resting on the ledge you saw in the picture above.) As well as that, you’d probably put a plywood diaphragm across above the ceiling panels to tie the walls together at the top. By doing this, you’d end up with the long walls far better supported by the in-plane elements of the building.

Hopetoun Alpha, longitudinal section showing position of mezzanine. Image courtesy John O’Hagan / Compusoft Engineering

But if floor and ceiling diaphragms weren’t enough, the mezzanine might present an opportunity. Site visitors may remember hearing about the truss inside the gallery at the Auckland Town Hall. There’s obviously no room, and no need, for a truss inside the mezzanine at Hopetoun Alpha. But the mezzanine itself has some inherent strength, aided by its tongue-and-groove flooring. This could be enhanced with some inserted material. If the mezzanine were then connected more securely to the wall, it would serve as a brace at about halfway up the wall height. The walls could then be modelled as rocking about the pivot of the mezzanine, improving their performance. Tying the structure together better would be completed by more securely fastening the pediment and the rest of the portico to the main building.

Thanks!

Grateful thanks to John O’Hagan for his time and enthusiasm. Thanks also to the Ashton Wylie Charitable Trust which owns the building, and to Paula King, who made it possible for us to go and see it. We’ll stay in touch with this project as it progresses.

The City Rail Link, Auckland Baptist Tabernacle, Mercury Theatre, Hopetoun Alpha, and the Pitt St Methodist Church with Edward Bennett and John Fellows, August 2017

Terry Gilliam’s 1985 film Brazil, designed by Norman Garwood, tells its story partly by creating contrasting spaces. There are cramped domestic interiors; imposing civic buildings; sparse and frightening chambers of horrors. The look of the film, Gilliam said, came from “looking at beautiful Regency houses, Nash terrace houses, where, smashing through the cornices, is the wastepipe from the loo… …all these times exist right now and people don’t notice them. They’re all there.*”

On Friday, as site visitors toured around four significant buildings in the Karangahape Road precinct, Brazil was on my mind. Mostly, this was because I knew we were going to go past the ghost of what used to be my favourite cafe in Auckland, named and themed after the film. But as we toured, it seemed to me that the film’s aesthetic echoed something about the sites we were looking at. All of them had hidden beauty; odd spaces; unexpected textures and histories to reveal. The face they show the street doesn’t always match what’s inside. And all of them exist in the anachronistic mish-mash that is K Road, a space that’s being opened up and re-invented by the imminent arrival of the City Rail Link tunnel.

In company with Edward Bennett, K Road historian, and joined along the way by John Fellows of the City Rail Link, we learned a little more about the tunneling, discovered a couple of the loveliest interiors in Auckland, and even climbed through a trapdoor on a folding ladder—seemingly a recurring theme of these site visits. Follow me and I’ll show you some of what we saw.

Auckland Baptist Tabernacle

Auckland Baptist Tabernacle, Queen St.

The Auckland Baptist Tabernacle is a study in hierarchies. Front on, its Classical rigour is imposing—its design was based on the Pantheon in Rome. But from any other angle than dead centre, the building reveals its more prosaic brickwork—to me, generous and well crafted, but to Victorian tastes, horribly patchy and common. The walls were intended to be stucco’d to a shiny white, but this never happened.

Well-proportioned windows at the Auckland Baptist Tabernacle. Note the brick lintel and the irregular colour of the bricks.
Auckland Baptist Tabernacle, looking down into the main hall from the gallery.

Inside, the Tabernacle shifts gears again. The spaces are large—indeed, this was the largest room in Auckland when built in 1885—but not imposing. It’s perhaps not the authoritarian space that the portico might suggest. The authority, Edward explained, came from the moral rigour that the congregation practiced, and was intended to set clergy and flock on a more level footing.

Structurally, the room is noteworthy for the curved rear wall, intended to bounce sound back into the room. There are slender cast iron pillars supporting the gallery. But, most of all, this is a large span. And the span had to be crossed without the aid of structural steel. Luckily for the church’s builders, then, that they lived in a country where 2000-year old kauri grew strong and straight! Thirteen good-sized ‘uns were ordered up from the North, and were duly sawed to size. We climbed through a hatch in the ceiling to have a look. Here’s where it got a little Brazil.

In the ceiling, Auckland Baptist Tabernacle

As you can see above and at the top of this post, there’s a large-ish ceiling space above the main hall. Truthfully, I was a little preoccupied with my fear that a site visitor would put a foot wrong and crash sixty feet to their doom (“how can Santa Claus get in if we don’t have a chimney?”), but nevertheless I managed to cast an eye over the structure. There are large kauri rafters, long straight members which make up the top and bottom chords of the truss. The hall doesn’t run the length of the building—there are sizeable rooms behind for other kinds of functions, and so the building is divided about half way by a brick shear wall, which goes up through the whole building almost to the underside of the roof.

As the tour continued, I butted in to a conversation that Professor Jason Ingham was having about the Tabernacle. For those of you who don’t know him, Jason is responsible—among a number of other things!—for developing methods to analyse the strength of unreinforced masonry buildings. Jason explained that this kind of building is a classic example of a structure that isn’t explained well by conventional structural dynamics. Instead, said Jason, the ceiling has to be thought of as a flexible diaphragm (not a rigid one), and assessment and strengthening should be designed on that basis. That doesn’t, of course, solve the problem that (like most churches) you are dealing with a big empty box with long not-so-strong sides. Still, there may be more strength in the building than conventional analysis would suggest.

Mercury Theatre

Mercury Theatre, the stalls and the corner of the gallery. Showing the “restored” but perhaps over-garish colour scheme

Next stop was the Mercury Theatre, opened in 1910 and Auckland’s oldest surviving theatre. It has been through a number of reinventions in its time: as a picture palace; a 1970s black-box theatre; a church; a language school; and so on. Like the Tabernacle, it’s a brick building, but in the intervening 25 years between the Tab’ and the Mercury, structural steel was introduced: so the Mercury’s large roof is held up by I-beams, not kauri. [Edit (25 Aug 17): Thanks to Mike Skinner on the K Road Heritage Facebook page who pointed out that the Mercury’s roof beams are timber and provided a picture.]

The theatre is ornate, having kept most of its plasterwork intact even through the austerities of a 1970s all-black paintjob. When it was last restored, the paint was scraped back revealing the bright blues and reds you see in the photo. These colours were duly reinstated—but Edward’s opinion is that the bright colours would’ve been more muted in the original, overlaid with paint effects: in fact, he thinks the bright blue was probably an undercoat.

Pressed-metal ceiling, Mercury Theatre foyer

There’s a large expanse of lovely pressed-metal ceiling still to be seen in the entrance foyer, and Edward explained that at the time of construction, this was believed to be a fireproof material. Sadly, fires in other buildings with pressed-metal ceilings disproved this notion, and these ceilings were mostly torn out, becoming quite rare.

Complex forms beneath the gallery, Mercury Theatre

For my part, I enjoyed the profusion and contradiction of the forms and decorations of the theatre. It’s hard, on first sight, to get a sense of the exact extent of the space and its orientation, and this slightly warren-like quality is exacerbated by the theatre’s position, tucked down the lane, its façade declaiming bravely and boldly at an audience who are not there to watch.

John Fellows now took the stage at the Mercury. This was the perfect place for him to speak, as, come 2019, ground will be broken next door for the new Karangahape Station, part of the City Rail Link. The project involves digging a large pit at the south edge of the theatre, a pit which descends some ten stories. The station’s platforms will extend underneath the Mercury, underneath K Road, and underneath some of Pitt St on the other side.

It’s an audacious project, but of course one with plenty of precedents in all the major cities of the world. John explained that careful consideration has been given to minimising the impact that the CRL will have on the surrounding  buildings, both during construction and in operation. For example, the tunnels that will take passengers down from the Mercury Lane entrance to the station will veer out under the roadway, rather than passing under the theatre. This is to avoid noise and vibration passing up into the structures above.

John also explained that the results of subsurface core sampling have been encouraging. The soil, at the depth where the work has to be done, is East Coast Bays sandstone—common throughout Auckland. This soil can vary widely in its strength, but the good news is that the stuff underneath the Mercury is stronger than expected. This will make shoring up the pit next to the Mercury easier, and makes settlement less likely.

Decorative profusion, Mercury Theatre

As a sidenote, John described the system that is protecting the heritage buildings of Albert St, where the cut-and-cover tunnel work for the lower end of the line is currently proceeding. A network of over 1300 laser sensors is trained on the buildings’ exteriors, measuring in real time any deflections that might occur. If the movements of the buildings were to exceed the design parameters—hold the phone! The work stops immediately until the problem is resolved.

John had  plenty more to say about the plans for the station, about its design programme, mana whenua, use of local materials, bicycle integration, green design, and other topics. He said, just as Jeremy Salmond said at the Melanesian Mission, that he doesn’t see the purpose of trying to make a new building look like an old one just to “blend in” with its surroundings. Instead, John says, why not try to design a building that in 50-100 years will become a historic building in its own right? There was more to say and more to ask about all this, and the good news is that there will be an opportunity to hear more from John when he speaks at an ACE event in September. Keep an eye on their Facebook page for details.

We site visitors moved on to one of the loveliest hidden treasures in the city: the palm court in the disused K Road entranceway to the Mercury. To increase foot traffic to the Mercury, shortly after it was opened the owners purchased a narrow sliver of land and built a barrel-vaulted entranceway that took punters down into the theatre. As I mentioned, some will remember it as Brazil cafe. Now it’s a fast food joint. With brick-pattern wallpaper.

The Palm Court, Mercury Theatre

Tucked away, though, in between the Mercury and the now-disconnected entranceway, is the palm court. This was intended as a scene of Hollywood glamour to pass through on the way to the movies. Designed by Daniel Patterson, topped with a stunning leadlight dome, the room has retained its glamour and charm through decades of disuse. Fashionistas, artists, clairvoyants: what a studio space! Get in there, you muggs! (The author confesses to having once practiced one of the three professions listed above.)

Hopetoun Alpha

Hopetoun Alpha

Hopetoun Alpha is a delight. I felt the same sense of joy and astonishment as when I first entered St-Matthew-in-the-City, last year. It’s a light, delicate, finely-proportioned space—a Leipzig shoebox, just like Auckland Town Hall. Before you even get inside, the portico is unusual enough to warrant a better look. It’s painted a bold red with a pale blue soffit, creating a sense of interiority in comparison to the pale sides.

Red portico, Hopetoun Alpha
Blue ceiling, portico, Hopetoun Alpha. Note the marked curvature of the wall.
The “oak” door, Hopetoun Alpha, in fact made of kauri. Note the “ashlar” lines on the wall, which is in fact made of concrete.

From the pictures above, you can see that the front wall is curved, once again to produce sound reflection and natural amplification inside. The wall looks a bit like ashlar, doesn’t it? But in fact it is mass concrete, unreinforced. Timber trusses span the walls, just like at the Tabernacle. Speaking of timber and things that look like other things, the main door to Hopetoun Alpha appears to be oak—but scratches on its surface show that the oak is a paint effect, and the door is kauri. Fashions have changed, and now real fake oak is rarely seen.

It’s inside that Hopetoun Alpha truly shines. We were all delighted with its lightness and grace.

Interior, Hopetoun Alpha
Interior, Hopetoun Alpha. Detail of decorative elements. Slender cast iron columns.

Like many other buildings of its age and general type, Hopetoun Alpha and its owners are now having to give consideration to earthquake strengthening. There’s some hope that the gallery or mezzanine could act as a diaphragm, strengthening the outer walls.  [Edit: Edward Bennett kindly corrected me: the gallery was inserted into the 1875 building in 1885, “which is why it rather awkwardly passes in front of the windows”. The point I was trying (and failing) to make is that perhaps a retrofit can strengthen the gallery or be concealed inside it, to brace the long walls. HT]

Visitors to the Auckland Town Hall will remember that its gallery conceals a large truss designed to brace the long walls. Subsequent to our visit, I spoke with John O’Hagan of Compusoft Engineering, a firm known to site visitors from the St James Theatre visit last year. John’s supervising some investigations into the materials, foundations, and structural members of the building. We may yet have the chance to return and learn more.

Pitt St Methodist Church

Pitt St Methodist Church, with the 1962 porch

Last but not least we arrived at the Pitt St Methodist Church, nipping in through the Wesley Bicentennial Hall, for which there’s sadly no more space in this post. The Pitt St Methodist is determinedly Neo-Gothic, echoing the style of an English parish church, and deliberately eschewing the Classical. It’s a brick building, spanned with timber arches, and incorporating wrought-iron tie rods to muscularly and pointedly restrain the springings of the arches. Edward explained that this style reflected the Neo-Gothic designers’ conception of the power of the Gothic—Gothic church-builders would have done this too if they’d had wrought iron.

Pitt St Methodist Church, interior

Earlier, I wrote about John Fellows’ contention that to design for a great historic building, you make a great contemporary design. Here at Pitt St, there are two shades of this theory in evidence. The first is the organ, which was rebuilt and rehoused in the 1960s into a large “tabernacle”, looking something like an enormous jukebox. Secondly, there is the porch, added on at the same time. The porch is concertina-folded, with windows and doors shaped as stylised versions of the Gothic ogive. It’s very likely inspired by the Coventry Cathedral of a similar date, says Edward. Both the organ and the porch inspired mixed feelings from visitors, some feeling that they added a new dimension, others that they detracted from the original form of the building.

Pitt St Methodist Church, the organ in its 1962 “Tabernacle” rehousing.

Heritage buildings are living things, truth to tell, and there’s no one point at which you can freeze them and say, that’s it. For me, the comment that resonated was Paula King’s—she works for the Trust that owns Hopetoun Alpha. Paula said that using Hopetoun Alpha for good things “keeps its battery charged”; and keeping it charged gives it the power to last longer and speak louder, perhaps loud enough that future generations will still be able to hear it.

Thanks!

Our thanks to Edward Bennett and to John Fellows. You can read more about K Road’s buildings and their history at the kroad.com site, written by Edward. You can also read about the plans for Karangahape Station on the City Rail Link’s site.

* The quotation at the start of this post is from Bob McCabe’s book Dark Knights and Holy Fools: The Art and Films of Terry Gilliam: From Before Python to Beyond Fear and Loathing 1999 p.141.

Disclaimer: all ideas, information, insight are Edward’s and John’s. Errors of fact or interpretation are all my own work. HT

Ōtāhuhu Station with Sara Zwart, Tessa Harris, Joshua Hyland, Tony Berben, August 2017

Site visitors at Otahuhu Station

Sadly, your correspondent was crook and did not attend the site visit. Many thanks to the professionals for their time and effort.

I can highly recommend having a look at the Ōtāhuhu Station case study published by the Auckland Design Manual.

Update: I’m delighted to be able to share with you some pictures and thoughts by Yi (Sophie) Huang, a civil engineering student who attended the visit. Thanks Sophie!

The Maunga Moana facade by Graham Tipene. Image courtesy Sophie Huang, all rights reserved.
The most interesting part for me was the stormwater collection system and the water treatment pond.

At the station, there are two small rain gardens. The one we saw is approximately 5-7 square metres. There are plants, grass and flowers above the ground surface. Beneath the soil, the bed is deeply filled with sand and other materials which have a very good permeability, to absorb the stormwater. The storm water is collected underneath the rain garden then discharges through a stormwater channel to the water treatment pond. After several processes, the water is good enough to release into the wetland. We did not have a chance to see the water treatment pond, because the traffic was busy and it was too dangerous to walk there.

Site plan. Wetland on the right. Image courtesy Sophie Huang, all rights reserved.

The stormwater system was built to reduce the possibility of rainwater being contaminated by oil underneath Ōtāhuhu Station. Furthermore, one of the iwi consultants was very concerned about water quality: she strongly suggested that it would be a good idea to include an on-site water treatment pond. Sarah Zwart, of Jasmax, said that if the iwi had not persisted in the idea, the on-site water treatment pond might not be there.

Site visitors watch an informational video about the artworks. Image courtesy Sophie Huang, all rights reserved.

There were several difficulties involved in the construction process of this project. Firstly, constructability: several things did not go as planned. There was a problem with the piles, and this had to be solved. The schedule and money at that time were very tight. Tony Berben, of Aurecon, said “most of the time we think on our feet. There are always problems we did not expect; we had to find solutions for each of them.”

Tessa Harris artwork on window and other features. Image courtesy Sophie Huang, all rights reserved.

Secondly, time and money were constrained. Joshua Hyland, of AT, said it was very important to make sure the right materials were delivered on time.  Because of the tight schedule and budget, they didn’t have time and money to reorder and wait for the right material to be delivered.

Purapura whetū mahau by Tessa Harris under awning. Image courtesy Sophie Huang, all rights reserved.

We saw all the art works from the case study. They look simple, amazing with deep meanings and were not expensive. For example, at the station entrance, there is an x-shaped pattern, the purapura whetū mahau [by Tessa Harris of Ngā Tai Ki Tāmaki]. Joshua Hyland said it represents the past and future, and ancestors.

Professionals offer advice on the train. Image courtesy Sophie Huang, all rights reserved.

I learnt so much and was amazed by the two hour site visit. On the train back to the CBD, the professionals gave us tips on finding jobs, networking with people and how to be better engineers. They are super-friendly people and professional in their field. Moreover, I made amazing friends through the trip as well.