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 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.
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the walls are less robust is out-of-plane, and a good deal of this weakness comes down to the way that connections were originally made between the walls and the roof.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.
As it happens, there are some pieces of the church that are going to come in handy for just that reason. Most buildings that stick around for a few decades will have been modified in that time. In the case of St Paul’s Methodist Church, an extension project in the 1960s saw a concrete wall installed at the rear of the church, replacing the original wall. This newer concrete wall is hidden by brick, but there are construction drawings which give details of its dimensions and specified reinforcement. Josiah has supplemented the drawings with a GPR scan of the wall: the radar gives good indication of the reinforcement bar spacings, and from there it’s possible to drill to determine the bar diameters—or in this case, the figures are on the drawings.
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.
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.
When describing this problem and its remedy, Josiah made an observation that I hadn’t heard before, which struck me as valuable wisdom. He wanted to avoid taking any of the old material away, he said. This was not for reasons of heritage best-practice—although he’s certainly keenly aware of that, and earlier he had talked about how decisions have to be made about every scrap of original fabric that’s removed, from leaky roof vents to crumbling lintels. But the point that Josiah was making was that there are internal forces in play inside the existing pieces of the structure. Removing them comes at a risk. If you take things away, the internal forces will realign themselves, seeking equilibrium. If it’s not done carefully, unexpected movements or even failures could occur.
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.
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.
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.
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?
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.