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.

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

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

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

ClockTower, view from the site gate.

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

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

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

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

Stronger, tougher, independent

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

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

Drilling the walls

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

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

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

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

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

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

A bigger, sturdier foundation

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

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

 Tie me up, tie me… across

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

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

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

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

(Not) losing face

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

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

Augmenting the concrete

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

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

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

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

 

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

Speaking of trusses

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

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

It’s good to be back, and thanks!

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

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

 

 

St James Theatre with Anthony McBride, August 2016

On Friday a few of us visited the St James Theatre in central Auckland, where a major refit is taking place. Anthony McBride of Compusoft Engineering took the group around the site.

Anthony McBride describes the structure of the theatre.

The major theme of the talk was how to deal with a large, crumbly, but precious building. The theatre is an inherently tricky shape: a large, empty box, with high slender walls, and a big span between them. It’s also inherently high-risk — if the building fails, a lot of people could be inside. (Anthony noted that the live load of the circles (galleries) is five times the dead load.) And, as it happens, the St James Theatre is a weak structure. Its concrete is drummy and crumbling–more on that in a moment. However, as is likely to be the case with heritage buildings, the fabric is beautiful, unique, and carries its own value. Trying to brace this big crumbly box with steel would mean obliterating a good deal of that fabric.

​View from backstage through the proscenium to the upper and lower circle.
​View from backstage through the proscenium to the upper and lower circle.

The solution that the engineers have decided upon is base isolation. If it’s impractical to strengthen the walls to resist strong shaking, the logical step is therefore to reduce the loads they experience by dissipating the quake energy. Anthony described the state-of-the-art triple pendulum bearing system which is being installed at the St James, which will allow the building to move up to 250mm in any direction. (Or perhaps it might be better to say, the ground moves and the building doesn’t move with it–its period is increased considerably.)

Looking down into the excavated floor from the upper gallery--the view from the gods.
Looking down into the excavated floor from the upper gallery–the view from the gods.

To illustrate the parlous state of the building, and also its charm, we had a thorough walk through the site. Starting in the lower circle, we filed down to the ground upon which the building stands. To get at the foundations in order to install the isolation, the floor has been removed. As I noted in the invitation, what was uncovered beneath the floor was a large section of nineteenth-century cobbled street, and what appeared to be the brick foundations of the butcher’s shop that stood on the site long before the theatre was built. A number of artefacts have been removed from the site, including bottles and china, and we were told that the floor slab of the restored theatre will include glass windows allowing visitors to inspect the cobblestones. It was eerie to stand right next to the paved street, while 25 metres above your head is an ornate ceiling complete with dome — the shabby-grand remains of a vaudeville house —  and to think of the different lives that have been lived inside this bubble of space. It’s true of any space in any city; but the enclosure and the contrasts are what make the thought hit home.

​​​The cobblestones, seen from the lower circle.
​​​The cobblestones, seen from the lower circle.
​​Looking up from the floor to the dome.
​​Looking up from the floor to the dome.

Looking up from the floor we saw the precarious south wall and the bulging brickwork of the proscenium arch. Looking down, Anthony showed us the foundations. The original design drawings of the theatre specified 6 metre deep solid foundations; but what was really built was more like 2.5 metres, tapering irregularly in a hand-dug caisson, and filled with building rubble. A real “oh shit” moment for the refit team. Even digging beneath these foundations looks like a no-go, as it might disrupt the skin friction between pile and soil, and you wouldn’t want to be there when that happens!

​The foundations as revealed by excavation.
​The foundations as revealed by excavation.
Passing through a basement that pre-dates the theatre, we climbed the scaffolding on the north wall, and paused to inspect the steel reinforcement that has been exposed by exploratory drilling. In a number of places, it was corroded, poorly interconnected, or simply inadequate, and considerable repairs will be required. We emerged onto the roof, where we could see the demolition crew working on the adjoining apartment tower, the profit from which is making the refit possible. I’ll spare you the detail of this, but we did get to hear (from the developer Steve Bielby) a little about the way that the development deal funds the heritage project, which was most interesting. From the roof we entered the upper circle, from where we could better see the ornate detailing of the decorative plasterwork and the dome. One day, but not soon, it will all be finished, and it will be glorious.
​Inspecting steel on the north wall.
​Inspecting steel on the north wall.