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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 diameters—or 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-practice—although 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.

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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.

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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.

 

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Christchurch Arts Centre, with Owain Scullion of Holmes Consulting

In memoriam

As with earlier posts about Christchurch, the post following is written with  respect and sympathy for the victims of the quake sequence and for their families. As students who want to work with heritage buildings, we can learn from Christchurch and be part of doing better. Ka mate te kāinga tahi, ka ora te kāinga rua. HT

Christchurch Arts Centre, tower restored by stonemasons.

What I did on my holidays

With exams out of the way, I was delighted to be offered the opportunity to go down to Christchurch and have a look at the reconstruction and strengthening work taking place at the Christchurch Arts Centre. The Arts Centre is a cluster of historic buildings, the majority of which made up the original campus of the University of Canterbury. It’s a closely-spaced collection of good-sized masonry structures, dating from the second half of the 19th century and the early years of the 20th: more or less the exact kind of buildings that copped a fair hiding in the Canterbury earthquake sequence. A ~$300 million, multi-year programme is restoring and strengthening the buildings in the complex.

Christchurch Arts Centre, Girls’ High entrance. A Corinthian column is cased for protection during the building works.

I had the great pleasure of going through the site with Owain Scullion of Holmes Consulting. Owain patiently explained the strengthening programme to me, and showed me the current state of the works. What I thought I’d do, for the audience of this blog (who are mostly clustered up here on Pig Island) is talk about a few of the technical details that I was able to see, and try to give an impression of how the work is proceeding. I can hardly claim to be comprehensive, but I’d like to give a snapshot of what I saw. There’s a site map here to see where the buildings are as I refer to them; scroll down a bit on the linked page to find it.

Christchurch Arts Centre, Chemistry building. Horizontal and vertical stainless steel cables are connected to the structure through junction boxes, enhancing the compression in the wall caused by gravity loads to ensure that the wall stays in overall compression during shaking.

A stitch in time

Buildings at the Arts Centre had been strengthened before the quakes began. (Holmes’ involvement with the Arts Centre stretches back well before the 2010 quake.[update]) Regular attendees of Heritage Now events may remember Peter Boardman talking about his involvement with applying post-tensioning to the Chemistry building at the Arts Centre, which happened in the 1980s. As Peter told us, there was controversy over the appearance of the post-tensioning, which takes the form of a visible network of cables wrapped around the building. But what is undeniable is that the system largely preserved the Chemistry block during the quakes, despite the collapse of the building’s (unstrengthened) tower. As a measure of the post-tensioning system’s success, it has been renewed and reinstated, this time using stainless steel cables. (For more about how the post-tensioning cable system performed, you might like to read Dmytro Dizhur (et al)’s paper from 2013, which also talks about the post-tensioning of College Hall at the Arts Centre. TL;DR: it worked bloody well, and had some fringe benefits like increased out-of-plane capacity that weren’t predicted by the retrofit designers.)

Christchurch Arts Centre, Chemistry building. The post-tensioning terminates at steel plates. Note the cable entering the structure on the right to travel along the inside face of the wall. The stone buttresses to which the plate is attached were the original design mechanism to resist out-of-place forces in the gable end walls, but unfortunately their capacity to do so is insufficient.
Christchurch Arts Centre, Girls’ High, pre-quake tension brace retrofit, attaching to gable end. Note valley beam at springing of arches.

Not all of the pre-quake work was suitable for unmodified reinstatement. However, I did see examples of earlier retrofits being integrated into the new strengthening systems. For example, in the Girls’ High building, a system of tension braces had been installed to restrain the gable end and to transfer loads towards the core of the building. In the 2010-11 quakes, the loads were transferred pretty successfully, but in doing so, the valley beam at the base of the tension braces pushed on an internal wall, moving it by 50mm. This internal wall is now being beefed up and stiffened to drag loads out of several surrounding rooms.

Christchurch Arts Centre, Girls’ High. A brick internal wall is being strengthened to collate loads from adjoining rooms.

As an aside, Owain mentioned that a fringe benefit of working on these unique structures is that you get to see examples of fine nineteenth-century crafting. In the Girls’ High building, tusk tenons connect the floor joists to the beams, a detail that points to the relative cost of fasteners vs. workmen’s time in the early days of the NZ construction industry!

Christchurch Arts Centre, Girls’ High. A profusion of materials require strengths to be assumed or determined: timber sarking; bricks and mortar; timber beams; steel tension braces (upper left), plus masonry walls (not shown).

The right ANSR?

Owain explained that it has been important for Holmes to test the properties of the materials at the site—particularly for critical structural elements. For example, the internal wall mentioned above that’s aggregating loads has been subjected to pull-out testing to determine the cohesion value of its mortar, in a process analogous to the bed joint shear tests I’ve written about on this blog. The testing allows the engineers to close the loop on their modelling.  Having made assumptions about the material properties of the elements, the engineers model the structures in a software package called ANSR. Using data recorded in the last few decades of earthquakes around the globe, the engineers subject the model to a succession of simulated earthquakes. This modelling identifies structural hot-spots that need to be addressed. Materials testing allows the team to to be satisfied that their underlying assumptions are valid and that they can therefore trust the results of the modelling.

Christchurch Arts Centre, Girls’ High. First floor interior, showing lower edge of timber ceiling trusses with hammerbeam-style half-arches. Hidden structural steelwork supports the gable end.

Variations on a theme

For an all-too-brief moment, I got to put my steelies on and go up the scaffolding, even if I did have to wear the hard-hat-of-shame marked Visitor. At the top of the walls of block DA, aka Girls’ High, below a steeply-pitched roof, some supplementary structural steel has been inserted. The frames support the gable end, and diagonal braces lead down to a PFC that runs along the top of the wall. (Jargon alert! PFC = “parallel flange channel”, eg a C-shaped section.)

Christchurch Arts Centre, Girls’ High. Taken from the scaffold outside the room in previous picture. Note the PFC running along the top of the wall, and at left the diagonal brace heading towards the gable end. The thicker timber beams at the left are the upper chord of the trusses shown in the previous picture.

At the time I visited, the workers were setting up to drill some three-metre holes horizontally down through the wall. My first thought was that what I was looking at was similar to what’s being done at the University of Auckland’s ClockTower Annexe, where a series of drilled holes accommodate rods that are putting the entire wall into compression. I was wrong. In fact, here the drilling was to allow for Macalloy bars to be inserted, which will hold down the PFC securely and hence restrain the gable end against excessive rocking. The three metre length of the drill holes won’t get them to ground level—it’s just to allow the Macalloy bars enough space to develop good friction. Plenty of length is needed, because the drillholes tend to be smooth, meaning that the grout or epoxy that holds the bars in place doesn’t get a very strong bond to the walls of the hole.

Christchurch Arts Centre, Girls’ High. In a ground-floor room, a concrete core has been created and attached to the existing masonry. This necessitates building up and moulding window reveals. Dowels secure the concrete core to the masonry.

In another part of the building, however, there was a remediation technique being used that’s closer to what’s happening at the ClockTower. (Incidentally, although the ClockTower is about 50 years younger than the Girls’ High building, the coursing of the masonry is similar.) A ground floor space is being converted into a movie theatre, and shotcrete has been used to create a concrete core inside the masonry. This concrete core is connected to the exterior material by a closely-space system of dowels, similar to the Helifix screws that are being used at the ClockTower. (At the ClockTower the concrete core was part of the original design.)

Christchurch Arts Centre, Great Hall

The great Great Hall

One of the best things about the site visits we’ve done is getting to see some of the unique and remarkable interiors of heritage buildings. (St Matthew-in-the-City is a standout example for me.) While I’m sure that College Hall aka the Great Hall at the Arts Centre is well known to many people, it was a complete (and stunning) surprise to me. It’s an incredible interior: ornate, stately, and imposing. My first reaction was that I’d never seen anything like it in New Zealand; but then, as I looked more closely, I realised that it reminded me a little of St Paul’s Church on Symonds St which we visited earlier this year.

Christchurch Arts Centre, Great Hall. Successive layers of materials. The piers contain concrete cantilevers and post-tensioning systems.

There’s a roundabout connection to be made between the two buildings, in that Benjamin Mountfort, who designed the Great Hall, also worked on  ChristChurch Cathedral, to which St Paul’s Church has been compared by Heritage NZ. That part I’ll confess I looked up: what spoke to me immediately, though, was the palette of materials and the hierarchy of how they were deployed. At the Great Hall, moving horizontally upwards, timber panelling gives way to fine red bricks, then to Oamaru stone, then timber hammer beams and a a patterned timber ceiling. St Paul’s follows a similar pattern of material proportions and transitions. Gothic Revival, innit? One day St Paul’s will look as stunning as the Great Hall does.

At the Great Hall, the strengthening has involved inserting concrete cantilever columns inside the piers of the long side walls, enhancing the in-plane and out-of-plane capacity of the walls. Post-tensioning was also re-installed and enhanced. I suspect that the much larger ground accelerations expected in Christchurch compared to Auckland meant that a rocking-pier model that EQ STRUC proposed for St Paul’s wouldn’t have worked for the Great Hall—and, of course, the site topography’s different.

(Here’s an interesting article on Newsroom about the work at the Great Hall and the UNESCO prize it won, including a few images.)

Christchurch Arts Centre, Engineering building. Note missing gable end, steel wire strapping, vertical cracks through masonry.

Next up

Once the Girls’ High work is complete, the works will move on to the eastern end of the Engineering building, where extensive damage is still visible. A gable end is missing; there are deep vertical cracks in the masonry; and, in the building’s tower, the Oamaru stone quoins and the basalt wall have parted company. Steel ropes lash the structure together for temporary support. When works begin, tall steel buttresses will be attached to the walls, allowing the connections between walls and diaphragms to be cut and reinstated without triggering a collapse. Stonemasons are working to recreate the lost finials and many other decorative elements of the building, using archival photographs as visual references.

Christchurch Arts Centre, Engineering building. Steel buttress frames await re-use in supporting the structure. Cut off at bottom centre is the poster for the Teece exhibition.

A colossal amount of work has been done to restore the Arts Centre complex. While planning for the Engineering Building restoration is doubtless far advanced, I’m sure that lessons learned from working on the other buildings will be carried into the final part of the project, and will go on to inform strengthening and repair of similar buildings.

ChristChurch Cathedral

Christchurch

Walking around the city, it was sad and sobering to see the two hulks of the great churches standing unrestored, the Cathedral and the Basilica. The first sadness is for the human tragedy, certainly, but then you feel the loss of the buildings and the stories they contained.

Inside the Chemistry building at the Arts Centre, I spent time at a small exhibition in the Teece Museum which contained artefacts from Greek and Roman burial sites. One way of reading the objects in this collection was suggested by the inclusion of a Piranesi engraving, emphasising the splendour of decay and Romanticism of ruin.

The Cathedral of the Blessed Sacrament, aka the Christchurch Basilica. Shipping containers as temporary supports are a not uncommon sight in the city.

Without a doubt the Cathedral and Basilica would fit the bill for the Piranesi treatment, and I do love Piranesi. But for me, the most affecting object in the Teece show was the tiny child’s sarcophagus, and the memorial plaques beside it. Death and loss seem overwhelming, all-consuming, and hope is lost. But what can survivors do in response except make beautiful objects, and offer dedications to the memory of those who (to quote the plaques) “well deserved” to be remembered? As hopeful humans, we keep piling up stones, making new, making good, even though we know that nothing we make can truly last forever.

I’ve just been reading Joseph Conrad’s Nostromo, and my thought above is expressed far better by him. “…for life to be large and full, it must contain the care of the past and of the future in every passing moment of the present. Our daily work must be done to the glory of the dead, and for the good of those who come after.” From afar, I’ve been unconvinced that it’s worth reconstructing the Cathedrals. But, faced with the sight of them, and seeing them on the same day as the Arts Centre, I’ve changed my mind. It’s worth trying to restore those churches, just as the Arts Centre has been restored. What else are poor hopeful humans to do? We’re fortunate, then, to have engineers, architects, historians, stonemasons—and many others—to shoulder the task.

Thanks!

With sincere thanks to John Hare, Owain Scullion, and Alistair Boyes of Holmes Consulting, for inviting me to visit the Arts Centre and for their generosity with their valuable time.

Update

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John Hare checked in to provide some more background on Holmes’ involvement with the Arts Centre project.

“The strengthening being done now is now part of a much more comprehensive programme that was always intended, but is now being amalgamated into the repairs. That commences in 1978 or thereabouts, when the Arts Centre came into existence. The general approach was:

  • Stage 1 – secure the buildings to make them a good as they could be, ie tie in the floors and roofs and add a few elements to take out the worst of the structural weaknesses, and address the most critical life safety falling hazards – chimneys for example.
  • Stage 2 – protect as much as possible of the fragile heritage secondary structural and non-structural elements
  • Stage 3 – full seismic upgrading.

“In practice, money was always short so Stage 1 was done bit-by-bit with parts of stage 2 added in where practical. A big push in the late 90s saw most of the Stage 1 worked completed with outside help, but most of the work to that time was done by Jim Loper, the on-site Master of everything! And later in the piece, with Chris Whitty, now the Site and Restoration Manager.

“We did a few big studies, starting as far back as 1996, as I remember it, for the Stage 3, but other matters were judged more important, such as fire protection (which I supported at the time – fire would be hugely destructive there).

“As a consequence, only the Old Registry (HA) and the Old Girls High (DA) were strengthened to anything like what we now regard as a reasonable level (I think both were to 67% at the time, therefore around 50% against current code).

“The original post-tensioning (College Hall and Chemistry) was quite adventurous at the time! Done to a relatively low level and as such, prevented the worst of the damage, but needed significant upgrade for the current work.”