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.

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

Eight things I learned by reading Section C8

I know, I know: a listicle. How 2011! But this is a site about heritage, after all… Here’s the pitch. I’ve been reading Section C8 Unreinforced Masonry Buildings from The Seismic Assessment of Existing Buildings, written by a team of researchers, academics and professionals and published at eq-assess.org.nz. (I mentioned it in the post on bed joint sliding shear.)

You might think it sounds tedious, but it’s not. It’s well-written, well-illustrated, and provides some really useful and clear categorisation of structures and the ways they can fail. I needed to read it: this is the work I want to do. Some of you might be in the same position. But even if you’re not interested in doing seismic retrofit, if you’re involved in any way with building works on heritage buildings made from URM, you should probably leaf through the first few sections.

It would be presumptuous and preposterous for me to write a “review” of Section C8. I don’t understand it well enough to do so. But I thought I’d share my own highlights, as a way of enticing others to have a look. We’ll see how it goes.


Christchurch

On a more serious note, this article includes images of damaged and collapsed buildings from Christchurch. The following is presented with sincere respect to the 185 people who died in the 2011 earthquake and to their families. Let’s work to try to stop something like that happening again. Kua hinga te tōtara i Te Waonui-a-Tāne. HT


 

Section through a cavity wall. Note also the change in thickness at the first floor, creating a ledge for the joist to sit on. From Section C8 Unreinforced Masonry Buildings, in The Seismic Assessment of Existing Buildings, at eq-assess.org.nz. Attributed to Holmes Consulting Group.

1.  Spot cavities with algebra

It was common practice to have a vertical cavity in brick walls. The cavity provided a barrier to moisture, and the outer wall could be made from higher-quality bricks that gave a finer appearance to the building. (The cavity is the black line running down the wall in the picture above.)

There are various complexities pertaining to the ties that were used to hold the layers of wall together across the cavity, how those ties have lasted, and how to assess the relative motion of the outer wythe (the veneer of good bricks) and the inner wythe(s). I won’t go into those here beyond noting that they exist. The question is, how do you know whether there’s a cavity, if you haven’t made a hole in the wall?

Algebra to the rescue! Well, multiplication, anyway. Brick are standard sizes. They’re usually 110mm thick (taking “thickness” as the dimension going into the wall). With some mortar in between wythes, that means that two wythes (layers) ≈ 230mm, three wythes ≈ 350mm, and four wythes ≈ 450mm. So if your wall thickness isn’t pretty close to one of those numbers—check for a cavity.


Insufficient connections between (floor and ceiling) diaphragms and walls leading to out-of-plane collapse. Section C8, as above, eq-assess-org.nz

2. Loose diaphragms fall out (sorry)

We’ve heard on site about floor joists sitting on ledges and maybe falling off when the ground shakes. We’ve also heard, on almost every site we’ve visited, about how tying the diaphragm into the walls at floor height, ceiling or both, can improve the structural performance of the building. What Section C8 makes clear is that the diaphragm can act to redistribute loads from out-of-plane to in-plane walls. If the diaphragm deforms too much, though, it won’t be able to support the walls effectively. In fact, too much diaphragm deformation can actually shove a face-loaded wall right off the edge of the building.


In-plane sliding on a damp-proof course. Section C8, as above, eq-assess.org.nz

3. Slip-‘n’-slide on a damp-proof course

For walls loaded in-plane (along their long axis), one of the possible failure modes is in-plane sliding. I wrote about one version of this in my post on bed joint shear testing. Section C8 points out that damp-proof courses can also be vulnerable to sliding.

To stop moisture wicking upwards through porous masonry, a layer of damp-proof non-porous material was commonly included in brick walls. Bitumen, slate, lead, or similar waterproofing was laid down in a continuous layer, usually not far above the foundations. (At a building I saw this summer, the DPC is granite, but that’s exceptional.)

Turns out, those damp-proof materials are softer than the masonry, or perhaps bond less well to the surrounding materials. Or maybe the change of material just provides a stress concentration. I don’t know! Still, I’d never seen or heard about DPCs as a site for sliding until I read C8.


Parapets tied back to a roof with raking braces — but are they sufficiently restrained in the vertical direction? Section C8, as above, eq-assess.org.nz. Attributed to Dmytro Dizhur.

4. Vertical tie-down for parapets

It’s a common sight around the traps to look up and see a steel brace holding back a parapet. Job done, right? Maybe not. Section C8 points out that such braces may not have enough capacity to deal with vertical displacement, especially when shaking of the roof plane is amplified by the brace and transmitted to the parapet. Quoth C8: “The danger of non-robust strengthening is that the parapet still fails, but collapses in larger, more dangerous pieces.” Not good. The parapet may need to be drilled and post-tensioned onto the top of the wall below.


As axial load increases, masonry walls gain strength from confinement. From Section C8, as above, eq-assess.org.nz. Attributed to Dunning Thornton.

5. Look out above

I suspect if you’d asked me whether masonry buildings suffered more damage at the top or the bottom in earthquakes, I’d’ve said the bottom. Makes sense, right? The walls crush and they turn over? Wrong. Generally speaking, the more axial load that is on the masonry walls, the stronger they are and the better they resist disintegration. Of course, this depends on the building form, the shaking, and other things, but the relationship between axial load and strength is useful to know.


Failure modes. Left, out-of-plane failure: instability of wall insufficiently tied in. Right, in-plane failure: spandrel failure, diagonal tension cracking, toe-crushing of piers. [Right, Section C8, as above, eq-assess.org. Attributed to Sharpe. Left, CCC Heritage, licensed under a Creative Commons Attribution-Noncommercial-Share Alike 3.0 New Zealand License]

6. We don’t talk about failure here

Section C8 enumerates the failure modes of URM buildings. I won’t write a list or a summary, but I will say that I found it really clarified my thinking about how buildings work to consider a finite list of failure modes. When I look at a structure now, I feel much better informed about how to break it into chunks and think about how each chunk might move and how it might fail. From that perspective, reading C8 was like taking your American mate to the cricket and telling him where to look.


Hierarchy of vulnerability, Section C8, as above, from eq-assess.org.nz

6b. And what to look for

An afterthought to the above. In assessing the structure, the C8 guidelines suggest working from left to right on this hierarchy chain, with the idea that the most vulnerable components of the building, those which are likely to fail first, are at the left. No good wasting time and money diagnosing a complex in-plane failure mode if the parapet’s not secured.


Ten steps. The assessment procedure, Section C8, as above, from eq-assess.org.nz

7. Ten simple steps…

I suppose I haven’t much to add to the image above. These are the guidelines which C8 provides to engineers as advice on how they should approach assessing a building. As with #6 above, for me this helped to understand how engineers divide the building into a set of observations and parameters which allow a model to be created—and don’t bother creating one that exceeds the complexity of the structure! I will be trying to hold these ideas in my head on our next site visit (St Paul’s Church on the 13th of March) and to think about how I’d approach the task of assessing the building. Thankfully we’ll have Peter Liu from EQ STRUC with us to show us how it’s really done!


A video from the Uminho Research Group on Historical and Masonry Structures. This is apparently a strengthened model. Still, watch the upper sections of the wall crack at the floor line and rock.

8. Walls rock

Walls under face load can be modelled as rocking. This means if the load is perpendicular to the wall, the wall can be “assumed to form hinge lines at the points where effective horizontal restraint is assumed to be applied… At mid height between these pivots… a third pivot point is assumed to form.”

When I read those words I recalled John O’Hagan talking about this at Hopetoun Alpha, but I think I understand it a little better now. To me, it feels different to think about a masonry wall in an earthquake as two rigid panels teetering one atop the other, as opposed to thinking about n bricks shuffling about independently, or as one rigid surface.

And this is the note on which I’ll leave this post. C8 offers quite a lot of guidance about how to make simplifying assumptions that allow analyses to be made, the rocking walls being one. It also offers suggestions for how to calculate important parameters if you can’t or haven’t tested them—things like tensile strength of the masonry. The impression I had, on reading these guidelines, was that the task of doing this kind of work myself someday seemed not impossible. Surely that’s the most sincere praise I can offer.

Testing mortar on-site with EQ STRUC, February 2018

What’s this then?

I’ve mentioned here before that I’m working for Salmond Reed Architects this summer. Recently, I had the opportunity to observe an on-site test of the material properties of some mortar at a heritage building in Auckland, and I thought I’d share some of the details here. The site is a late-nineteenth century two-storey brick building. (To clarify some jargon used henceforth:  bricks = unreinforced masonry = URM). I’ve been given permission to share the test procedure, but I haven’t sought permission to disclose exactly where the testing was being carried out, so that is a deliberate omission from the post. Still, I thought the process was novel and interesting enough that some of you might enjoy hearing more about it.

Borderline punching shear failure, wall anchor. From Section C8 Unreinforced Masonry Buildings, in The Seismic Assessment of Existing Buildings at eq-assess.org.nz. Attributed to Dymtro Dizhur, et. al.

So you think you’re pretty tough

If you’re going to assess an existing building, you have to decide how you think the building might fail. For URM buildings, there is a hierarchy of failure modes from most likely to least likely, and an engineer needs to work down through  the hierarchy, examining all of the possible modes. At a certain point, the assessment will find a mode that causes something unacceptable to happen under the predicted load. That doesn’t mean that the assessment stops there: but certainly, something needs to be done about the potential failures and their resulting risk to life.

So far, so tidy. But there’s a problem. Heritage materials are far from homogenous. The 1920s concrete at the St James Theatre  is soft and drummy, whereas on another site I recently saw 1920s concrete that was described to me as “rock-hard”. Bricks can be low-fired and soft, or fired at high temperature and hardened. Stone’s anisotropic. And mortar’s really idiosyncratic. How much lime went in? Were there shells in the sand? How wet or dry has it been throughout its life? It’s hard to predict the strength of hand-mixed materials from a time before the standardisation of products. The true in-situ strength of the material, the actual number, will make a difference to which failure modes come out of the analysis as critical weaknesses, and how much work you have to do to the building. Hence also the dollars involved. So how do you find out how strong the materials at your specific building truly are, and hence, how they will fail? You test them.

Sliding shear along a defined plane. From Section C8 Unreinforced Masonry Buildings, in The Seismic Assessment of Existing Buildings at eq-assess.org.nz. Attributed to Dunning Thornton.

In due course

Mortar’s not brick glue. Its primary purpose is not to stick bricks together. Instead, it provides a slightly compressible joint between the bricks as they sit in their stacks, allowing them to expand and contract without cracking each other to shards. Generally, mortar is softer than the bricks, especially lime mortar, and this is a good thing.

Notwithstanding the above, the mortar is the thing that stops the bricks sliding across each other if the wall gets shoved along its length, for example by an earthquake. The test that I observed, a bed joint shear test, examines how well the mortar prevents the bricks sliding across each other. It seeks to establish a cohesion value for the mortar. Key takeaway: it’s a mortar test, not a brick test.

Diagonal tension cracking, piers. From Section C8 Unreinforced Masonry Buildings, in The Seismic Assessment of Existing Buildings at eq-assess.org.nz. Attributed to Dmytro Dizhur.

The cohesion value influences several important failure modes: embedded anchor pullout; punching shear for plate anchors; (diagonal tensile strength leading to) diagonal tensile cracking and spandrel shear due to flexure; and bed-joint sliding. In this post, you’re seeing  pictures of several of these modes. For the building that was being tested, the  EQ STRUC engineers told me that the cohesion value was going to be used for calculating bed joint sliding shear.

Bed joint sliding, stair-step crack sliding in low axial load walls. From Section C8 Unreinforced Masonry Buildings, in The Seismic Assessment of Existing Buildings at eq-assess.org.nz. Attributed to Bothara.

The bed joint shear test is carried out by taking a brick out of the wall and using a hydraulic jack to push on the bricks either side of the hole. When the brick that’s being pushed starts to move, that means that the mortar has failed. Simple. In a moment, I’ll show some images of the steps. But before that… this.

Bed joint sliding shear

Bed joint sliding shear is calculated thusly:

Bed joint sliding shear, equation C8.33, from Section C8 (Unreinforced Masonry Buildings) of The Seismic Assessment of Existing Buildings, from eq-assess.org.nz

I hope it won’t totally destroy my credibility if I tell you that my first reaction to seeing the above was yuck. But on closer inspection, the equation is pretty straightforward, and considering its terms really helps to understand what is being measured.

Firstly, the µf(P + Pw) bit. That’s just saying that the load pressing on the bricks makes them rub on each other, which makes it harder for them to slide across each other. More load on top: harder to slide the bricks.

Secondly, the tnomLwc bit. tnom and Lare the thickness and the length, so really that’s an area: the area of the bed joint. And the cohesion value is the stickness. How sticky is the mortar, per area? That’s what this part is describing.

In the real-world bed joint shear test, the engineers try to make the equation even simpler, because the friction bit mightn’t be completely obvious: exactly how much load IS on the wall at the time you test? You can measure the height of the wall, but what about superimposed loads? So, to avoid worrying about this, the EQ STRUC team carried out the testing on bricks just underneath windows, where there wouldn’t be much imposed load. That means the cohesion can be found by knowing how much area of mortar you were testing (the bed joint size) and how hard you shoved the brick.

Opening up and finding a stretcher course

The test, in detail

First, catch your brick. I’ve noted above that the best spot to test is under a window. Another limitation is that the test should only be carried out with stretcher bricks in a stretcher course. If you’re not sure what that means: stretchers are laid along the wall, headers are laid across the wall. Like so:

A brick wall, anatomised.

The tests I saw were all carried out on the internal wythe of the brick wall. (A wythe is a layer of thickness–ie, a wall that’s three bricks thick is laid in three wythes, usually interlocked. There’s more complexity to this but that’ll do for now.) So, to carry out the test, you need to open up the linings and identify the right course to test.

Using a mortar saw to remove a brick; and a closeup of the mortar saw

Secondly, a single brick is removed. This means using a mortar saw to take out all the mortar above, below, and to the side of the bricks. On the picture two up you can see that these are the two bed joints and the two head joints. The brick gets extracted whole and can be replaced later.

A brick is removed. The mark reads “W Hunt, Auckland”

Mortar is removed from the extracted brick, and set aside for future testing—think of it as something like a concrete cylinder test. This testing isn’t part of determining cohesion.

The head joint is removed from the far side of a brick adjacent to the hole

Thirdly, the head joint is removed from the far side of one of the bricks adjacent to the hole. This is to give the brick that will be shoved some room to move. If the mortar wasn’t removed, the test results would be affected by the compressibility of the mortar in the head joint.

A jack exerts force on the brick, shearing the mortar in the bed joints above and below

Finally, the jack is inserted. It’s pumped by hand, exerting a force on the brick. The mortar in the bed joints above and below the brick resists the force. Since we’re testing under a window, there’s basically not much friction from the weight above. So the thing that’s stopping the brick moving is the two bed joints.

When the shove gets strong enough to move the brick—that’s deemed to be the point of failure, and the value is recorded. This is the peak value, meaning how much force would have to be exerted on the bricks by an earthquake to get them to start moving. Once they’re moving, they still need some force to keep them moving. So the engineers reset the jack, and pump it up once more until the brick begins sliding again—at which point, the residual value has been found.

The brick on the other side of the hole can now be tested. The engineers wedge something into the vacant head joint on the first brick—to prevent further sliding—and then cut out the head joint on the second brick, before testing again as described above. With a set of values from paired tests carried out around the building, an average value can be determined. Remember, we know the area of the bed joints, we know how much force we used, we’re ignoring friction, so there’s only one unknown: the cohesion value.

Engineers read the gauge on the jack during bed joint shear testing

Mouthfeel

There’s a somewhat subjective element to the tests. As for most things, it takes experience to determine exactly when sliding has begun. The tester gets a certain amount of physical feedback from the unloading of the jack as the brick slides, but even then, the exact moment of failure and the associated peak value are not precisely defined.

And this is only appropriate. After all, the mortar isn’t going to be homogenous throughout the whole building. Nor will other conditions be exactly similar. What’s required is something more like a geotechnical value—which is to say, a good rigorous estimate of the cohesion, into which some safety margins can be built. It’d be possible, with more elaborate equipment, to measure the load and the deflection more precisely; but a more precise number wouldn’t really be more meaningful.

Thanks!

Thanks are very much due to Romain Knowles and Antti Wallenius from EQ STRUC. Thanks also to Phillip Hartley from Salmond Reed Architects for getting me in the door at the site.

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

Auckland Town Hall with George Farrant, May 2017

The clock mechanism, Auckland Town Hall.

To potential clock room taggers and graffiti artists: All tags, names and graffiti will be promptly removed and you will be forever haunted by the ghost of the clock tower.

 Thus read the notice meeting the eyes of Auckland Town Hall site visitors on Monday and Tuesday, as they put their heads through the trapdoor at the top of the ladder to the clock room. As Ray Parker, Jr. said, I ain’t afraid of no ghost: but it seemed to me what we saw on the tour, the fruits of the restoration work carried out from 1994-97, was certainly the resurrected spirit of the original design of the Town Hall. No effort was spared to return the building to something approaching its original state, and, at the same time, to make it safe and strong for the future. True to form for ghosts, the structural upgrades are in many cases invisible—or at least, hidden from the eye where members of the public can ordinarily go. On this visit, we went behind the scenes.

Bluestone on the lower floor, Oamaru stone above, but right at the top the true building material of the Town Hall: bricks.
Town Hall exterior, northeastern corner. Bluestone on the lower floor, Oamaru stone above, but right at the top, the true building material of the Town Hall: bricks.

BIG EMPTY BOXES

The Auckland Town Hall is essentially a brick building. It is faced with Oamaru stone and with Melbourne bluestone, the latter brought over by the Australian
architects of the Town Hall—a prime example of coals to Newcastle in this basalt-bottomed town. There’s reinforced concrete in the foundations, in the form of piers and floor beams. Structurally, however, the Town Hall suffered from some of the usual flaws of unreinforced masonry buildings: vulnerability to face loading of external walls, and insufficient shear strength.

 

In out-of-the-way areas, remedies for these problems could be visible. On lateral cross walls, shear strength was improved by adding a 100mm-thick concrete skin to the bricks. That doesn’t stop the building rocking itself off its foundations, so the basement-level concrete was tied into new piles, hand-dug for lack of headroom. Longitudinal walls at the upper level had fibreglass glued to them, and this was covered with plaster. For various reasons, some of the internal brick walls had new openings cut into them. To retain the shear strength of the wall and to leave a record of the intervention, these openings were finished with a visible internal frame of structural concrete. This frame-within-a-frame motif, used to signify a modern alteration, was lifted from parts of the original design.

The original (non-structural) frame-within-a-frame design.
The original (non-structural) frame-within-a-frame design.
In the light well, new openings cut into the brick shear wall were denoted by the frame-in-frame treatment.
In the light well, new openings cut into the brick shear wall were denoted by the frame-in-frame treatment.

Strengthening the main performance spaces was trickier. Of necessity, these rooms are high-ceilinged and large, and, designed for natural light, their walls have many openings, separated by slender columns. Out-of-plane loading would wreck them. But the walls are beautiful inside and out, and the spaces are well-beloved and well-known in their current form. Structural enhancements had to be invisible.

The wall of the Great Hall. The curtains cover large windows--note the slender piers between openings.
The wall of the Great Hall. The curtains cover large windows–note the slender piers between openings.
George explains the design solution underneath the gallery which conceals the truss.
George explains the design solution underneath the gallery which conceals the truss.

In the Great Hall, the solution was obvious—once someone had thought of it. A gallery runs around three sides of the room, providing extra seating. It was easily large enough to conceal a gigantic U-shaped horizontal truss, which provides stiffness to resist lateral movement of the weak outer walls. A plywood diaphragm hidden in the ceiling cavity tied the tops of the walls together. In the somewhat smaller Concert Chamber, the gallery is small too, and it doesn’t extend around three sides of the room. With no opportunity to conceal a truss, the strengthening in the Concert Chamber took the form of reinforced concrete columns inserted into 400×400 slots cut into the wall—or, in one case, cut right through the wall and out into the weather (Oamaru stone is pretty soft!). As part of the refit, air conditioning was inserted into the walls, and the vents are partially hidden by the decorative plasterwork.

Air conditioning vents between plaster corbels, Concert Chamber. The steelwork is inserted into the columns between the windows.
Air conditioning vents between plaster corbels, Concert Chamber. The steelwork is inserted into the columns between the windows.

 OUT OF THE FRYING PAN, INTO THE FOYER

 For a number of years, prior to the restoration project, the floor tiles in the foyer often exploded. This alarming phenomenon was at first put down to excessive compaction caused by floor buffing machines, but the installation of a sprinkler system into the concrete slab on which the tiles sat revealed the true problem. The reinforcing bars in the slab were in the wrong place—the lower bar sitting far too close to the top surface. What was causing the tiles to explode was the floor slabs deflecting under the weight of concertgoers: alarming indeed! Thankfully, none of the floors failed, but many of the tiles, under strong compression, did.

The problem for the design team was how to support the floors without changing the proportions of the spaces, since there is nowhere to hide any supplementary structure. The deflection was reduced by adding carbon fibre strips to the underside of the floors—likely the first time that this material had been used for structural repair in NZ. With the floors strengthened, a repair job had to be done on the tiles.

The tiled floors of the foyer. Floors extend over two levels. In the centre, the round tile is an encaustic tile, stained orange by the acid bath. The square brown tiles are original--I think!
The tiled floors of the foyer. Floors extend over two levels. In the centre, the round tile is an encaustic tile, stained orange by the acid bath. The square brown tiles in this photo are original–I think!

The fanciest tiles, made with a light-coloured slip poured into a relief-moulded dark-coloured base (encaustic tiles), came through OK, barring some orange stains caused by an overzealous acid bleaching. But the plain, square, brown tiles which cover the greatest part of the floor were seemingly impossible to source: they couldn’t be bought, and, scour the world though they might, the team could not find a manufacturer capable of exactly matching the original colour, given an understandable reluctance on the part of modern potters to use lead oxide in their glazing. All seemed lost—until one day, a project manager from the Town Hall team had lunch at a well-known franchise restaurant specialising in Scottish food. To his utter astonishment, the kitchen tiles at McD’s appeared to be an exact match, and he nearly earned himself a cell next to the Hamburglar by bursting unannounced into the restaurant kitchen with his tape measure get the exact size of them. To cut a long story short—the tiles matched matched perfectly, and McD’s eventually agreed to give the Town Hall enough of their custom-made tiles to repair the floors.

In a similar spirit of desire for perfection, George mentioned several other examples of the lengths to which the project team went to get as close to the original design as possible, including scraping the walls painstakingly to find the original wall colour (not to be mistaken for the colour of the primer or the basecoat). They trawled through archival pictures to find the patterns of the original leadlight windows. Of course, the pictures are in black-and-white, but the glass colours were revealed by the discovery of one large window, which had literally been rolled up and stashed away. Picture the restorers hunting in a dark basement for scraps of coloured glass. That’s dedication.

 A TICKING TIME BOMB?

The clock tower rises above the administrative offices of the Town Hall.
The clock tower rises above the administrative offices of the Town Hall.

On Monday’s tour, George willingly expressed his “diffidence” over the threat that earthquakes pose to Auckland’s buildings. He qualified his position to the extent of saying that the risk is non-zero—and with a non-zero risk in mind, the clock tower on the southern end of the Town Hall presented a serious engineering challenge. It’s extremely heavy, and being taller than the adjoining structure, it would have a different period under earthquake acceleration.

The exposed steel frame in an upper storey of the clock tower. Note the steel rods running across the window instead of solid beams.
The exposed steel frame in an upper storey of the clock tower. Note the steel rods running across the window instead of solid beams.
In the storey below, the exposed steel frame (white) joins up with steel inserted into the walls (grey). In lower (public) floors, the steel strips are hidden in the walls.
In the storey below, the exposed steel frame (white) joins up with steel inserted into the walls (grey). In lower (public) floors, the steel strips are hidden in the walls.

The initial design solution, a steel framework inside the tower designed to hold the tower up, was rejected—by George. It would have dramatically altered the staircase below it, which winds up to the council offices. George’s name was mud among the engineering team for some weeks, until an alternative solution occurred: why not hold the tower down, instead of up? This developed into a solid steel frame, in the upper tower; connected to cross bracing cut into the walls, in the storey below the steel frame; connected to thin steel strips inserted into the walls of the stairwell, and anchored into the foundations. These steel strips are 200×19 galvanised steel flats, sitting in 270×120 slots packed with a Denso felt, and tensioned. This holds the tower together, using the tension in the steel against the crush strength of the masonry, but doesn’t eliminate the possibility of swaying. In addition to the post-tensioning, then, a transfer truss connects the tower to the top of the longitudinal walls of the Town Hall, holding it fast. The truss is hidden under a sloping roof. One final touch—in the clock tower, where the steel frame sits, instead of steel cross-members going over the windows, the bracing consists of four 40mm steel bars, painted in a dark colour. You’ll see it (at night) now that you know it’s there, but it’s far less noticeable than steel beams would be—seen out of the corner of your eye, you might pass it off as a mere apparition.

 FURTHER READING

There’s a really lovely post on the Timespanner blog with some great archival images of the construction of the Town Hall.

I also sent site visitors a link to Downer Senior Engineer Mark Hedley’s 2014 paper on the strengthening of five major civic buildings in Auckland.

 THANKS

 With sincere thanks to George Farrant, redoubled since he generously agreed to host a second tour in the face of extremely high demand. For he’s a jolly good fellow, and so say all of us.

Roselle House with Peter Reed, and the Melanesian Mission with Jeremy Salmond, Andrew Clarke and Dave Olsen, April 2017

Visitors on the terrace at Roselle House...
Visitors on the terrace at Roselle House...
Visitors… on the terrace at Roselle House…
...and in the attic at the Melanesian Mission.
…and in the attic at the Melanesian Mission.

WHAT I DID ON MY HOLIDAYS

Over the Easter break, heritage enthusiasts from the U of Auckland visited building works at two 19th-century masonry buildings. The first was the 1870s mansion Roselle House, now part of St Kentigern Boys’ School. Next was the 1850s ecclesiastical training school, the Melanesian Mission, which gives its name to Mission Bay.

Melanesian Mission. Dressed-stone sill and jambs (?) around small attic window, photographed from the scaffold.
Melanesian Mission. Dressed-stone sill and jambs around small attic window, photographed from the scaffold.
Roselle House. Brick “relieving arch” built to ease strain on large internal lintel. (Note, this was hidden in the original by plasterwork, and will be covered up again.)
Roselle House. Brick “relieving arch” built to ease strain on large internal lintel. (Note, this was hidden in the original by plasterwork, and will be covered up again.)

RE-USE

One of the major topics of discussion at both sites was re-use, and how making the buildings useful for their current occupants supports their preservation. Renovating a building usually means making changes to its fabric and there are consequent losses of heritage material. To make such changes, consent is required from heritage authorities, and this has to be negotiated. Part of the negotiation comes down to demonstrating the overall benefits to the building that can be expected from the project, even if those benefits come at some cost to what is currently there.

The Roselle House tour
The Roselle House tour

At Roselle House, the school is on- trend, transforming library space into learning commons. There was a discussion of the decision-making and consenting processes that were required to allow a large opening to be cut in a wall for a new entry. Cutting the hole meant losing some heritage fabric, but the future use of the building required it. Peter Reed and his colleagues discussed how the building’s elements were classified through its conservation plan, and how the Heritage Impact Assessment (for the new aperture) was devised. (In other parts of Roselle House heritage material that has been removed has been stored for re-use when the building is made good.)

At the Melanesian Mission, a new restaurant housed in an adjacent contemporary-style building provides the financial oomph required to care for the heritage site. At the Mission, there are fewer obvious changes to the building itself than at Roselle House, but its aspect will be significantly altered by its newly-built neighbour. Jeremy Salmond termed the Mission (and other built heritage) “vertical archaeology”: a record of the past, its people, their hopes and their achievements. That’s what makes it worth the care we lavish upon these buildings, he said. Jeremy’s belief is that you best complement a good old building with a good new one, rather than attempting to replicate an older building style and thus fudging history. Yes, the Mission’s visual surroundings change, but that’s the price of maintaining the history it embodies.
Roselle House. Preparations for pouring the shear wall. Above, note existing timbers, which have been included in the design calculations. See also, for interest, the plaster oozing through the laths—this is how the plasterwork adheres to the walls.
Roselle House. Preparations for pouring the shear wall. Above, note existing timbers, which have been included in the design calculations. See also, for interest, the plaster oozing through the laths—this is how the plasterwork adheres to the walls.
Roselle House. Looking down into the cavity for the shear wall. It will sit on a slab broad enough to avoid overloading soil bearing capacity, which could lead to overturning.
Roselle House. Looking down into the cavity for the shear wall. It will sit on a slab broad enough to avoid overloading soil bearing capacity, which could lead to overturning.

HOW TOUGH IS OLD STUFF?

Both Roselle House and the Mission are being strengthened against earthquakes. There’s a good deal of new material going into each site, but, interestingly, the pre-existing fabric of the site is also having its strength recognized and used. U of A research on heritage fabric was mentioned in dispatches, and no doubt a number of you site visitors (and your professors) are working on how to assess the strength of old materials.

At Roselle, new concrete bearer beams span under the floors, and the walls, floors, and ceilings are being strapped together and connected to these beams. Plywood diaphragms at floor and ceiling are the order of the day. But the main earthquake-resisting structure will be an internal shear wall. This will be poured anew, but it incorporates pre-existing timbers, and their strength was calculated and incorporated into the shear wall’s design.

At the Mission, a good deal of new steel has gone in, to secure the gable ends and the long walls against out-of-plane loading. Jeremy Salmond and Andrew Clarke described sending their design drawings for the steelwork back and forth to each other, and they both stressed the importance of designing every detail sympathetically to the building’s original programme. For example, the 200mm beam that spans the top of the walls in the Mission Hall has been custom-welded with an angled rear flange: instead of looking like this |____| in section, it looks like this |____\ . Why? So that it fits under the slope of the roof: thus the beam will not protrude over the edge of the wall. The beam has the same dimensions on its exposed face as a now-removed timber strip that used to run around the top of the walls. When the walls are refinished, the steel beam will have the same visual effect as what has been lost.

Melanesian Mission. An internal wall is drilled at regular intervals. The Mapei grout is pumped into the holes, starting at the bottom, until it begins to flow out of adjacent holes. The process is repeated three times.
Melanesian Mission. An internal wall is drilled at regular intervals. The Mapei grout is pumped into the holes, starting at the bottom, until it begins to flow out of adjacent holes. The process is repeated three times.
Melanesian Mission, detail of another internal wall, showing the insertion tube. The hole will be re-grouted with lime mortar, so it won’t be noticeable.
Melanesian Mission, detail of another internal wall, showing the insertion tube. The hole will be re-grouted with lime mortar, so it won’t be noticeable.

But to return to the strength of the existing materials at the Mission: the engineers made an assessment of the capacity of the masonry walls, using for their calculations some results from Jason Ingham’s research. An initial plan to tie the wall together with threaded rods was abandoned in favour of a Mapei- brand lime-based grout or slurry. Regularly spaced holes were drilled in the mortar, and the sludge was pumped into the wall. (Pumped by hand, so that the pressure didn’t get high enough to pop off the other side of the wall!) The result: the void spaces between the rubble are filled, and the inner and outer skins of the wall are bonded together. And it’s invisible. So the original material, supported by some chemical wizardry, gets retained, and can now resist greater loads.

Efflorescence on the bricks, internal walls at Roselle House
Efflorescence on the bricks, internal walls at Roselle House
The highly porous volcanic stone of the Mission. The lime mortar has been renewed as part of the project, but the Mapei-grout holes are yet to be filled.
The highly porous volcanic stone of the Mission. The lime mortar has been renewed as part of the project, but the Mapei-grout holes are yet to be filled.

WATER WATER EVERYWHERE; or, THE CONSEQUENCES OF DESIGN DECISIONS

Water in the walls was a recurring theme. At Roselle House, a chain of unfortunate decisions caused considerable harm to the fabric. First, wooden verandahs were replaced with terrazzo in the 1930s, sealing off the underfloor without ventilation, and causing the timber bearers to rot. Next, sagging timber floors were replaced with concrete. Uh-oh! Now the ground water, under pressure, wicked up the rendered plaster internal walls, moving between the brick and plaster, or between the plaster and its hastily re-applied paintwork. Wherever the water went, efflorescence remained, in the form of salty stains and crystalline growths. One of the major tasks of the project is to remove the old concrete floors and to draw the moisture out of the bricks with a special clay, in a process known as poulticing. The terrazzo stays, but it will be ducted to allow proper underfloor airflow. Peter made the point that the consequences of the 1930s renovation decisions took decades to become obvious, but have also created problems for occupants for many more decades. Earlier attempts to fix the problem only made it worse. Think twice about messing with an original design!

Water has a more subtle place in the walls at the Mission. The walls are made from chunks of basalt, taken from Rangitoto, and piled up in random courses, held in place with a lime mortar. Dressed blocks of scoria form the quoins. Both scoria and basalt are highly porous, and so, in wetter months, the walls have always been permeated with damp. This, says Jeremy, is not really a problem: the walls were made to be wet—notwithstanding that water entry did ruin the original plasterwork and create efflorescence. Problems have been caused by later attempts to “solve” the dampness, in particular by repointing with Portland cement, by plastering the inner face of the walls, and by treating with an “invisible chemical raincoat”, the latter occurring in 1977. These treatments tending to combine to retain moisture within the walls—the opposite of what was intended—and deteriorate the lime mortar, so much so that Jeremy described the walls as being “two dry stone walls with sand between them.” That doesn’t sound like a structure that would resist earthquake shaking very well! In combination with the Mapei re-grouting and the steelwork, the walls have been re-limed, and will surely be much the better for it.

The Main Hall chimney at the Mission. Steel rods run down the stack to the fireplace.
The Main Hall chimney at the Mission. Steel rods run down the stack to the fireplace.
Looking up from the ground floor at Roselle House to the stub of chimney. At some stage in the building’s life, the chimney was removed from the ground floor, but the rest was left to hang on in there... somehow!
Looking up from the ground floor at Roselle House to the stub of chimney. At some stage in the building’s life, the chimney was removed from the ground floor, but the rest was left to hang on in there… somehow!

A NOOK ABOUT CHIMNEYS

Two contrasting treatments for chimneys deserve mention. At the Mission, an elegant brick chimney stands above the rock wall on the western side of the hall. The chimney has been post-tensioned with steel rods, which connect an upper plate to the floor slab, holding the stack firmly together against shaking. The solution allows space for a flue to be inserted, so that a gas fire can simulate the cozy effect of a real one.

At Roselle House, the project team discovered that in some long-forgotten she’ll- be-right renovation, a chimney which poked out of the roof had had most of its lower extent removed. This is a trick somewhat akin to climbing out on a tree branch and then sawing it off behind you—expect things to start going downhill fast! As Peter explained, there were six tons of bricks sitting up in the roof and upper storey with very little holding them in place. In this case, the majority of the remaining chimney material has been removed, and a lightweight replica will be installed to keep the roofline looking the same. A steel brace for the chimney-stump was discarded, as it would have needed to be excessively large.

Roselle House. A ceiling rose clings on to its lath, awaiting the re-finishing of the room.
Roselle House. A ceiling rose clings on to its lath, awaiting the re-finishing of the room.
Melanesian Mission. The roof sarkings, seen here from above, have been exposed by the removal of the shingles. The sarkings are being nailed off as a diaphragm, stiffening the structure of the Mission. Note the bolted connections between sarkings and purlins.
Melanesian Mission. The roof sarkings, seen here from above, have been exposed by the removal of the shingles. The sarkings are being nailed off as a diaphragm, stiffening the structure of the Mission. Note the bolted connections between sarkings and purlins.

I very much enjoyed the visits, and I’m sure that other site visitors felt the same. We’re extremely grateful to Jeremy Salmond and Peter Reed of Salmond Reed, and to Andrew Clarke and Dave Olsen of Mitchell Vranjes, as well as to the contractors and project managers who allowed us to come on site.

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