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.
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.
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.)
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.
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!