Hopetoun Alpha with John O’Hagan of Compusoft Engineering, October 2017

Concrete classicism

A lightning-fast summary: Hopetoun Alpha was built in 1875. It’s mass concrete, meaning concrete with no internal reinforcing steel. Built for the Congregationalist church, this is a Neoclassical temple in the Doric order, with a charming and luminous timber interior. It’s in Beresford Square, close to the intersection of Pitt St and K Road.

Hopetoun Alpha, exterior

If you’re reading this, you likely know that concrete is pretty strong in compression, but not great in tension. It’s hard to crush, but it doesn’t like to bend. You’ll also have a pretty good idea that 140-year-old buildings can move around a bit: slowly, as the ground settles over the course of the years; and quickly, in extreme cases—for instance, when the ground shakes. So the task that John O’Hagan of Compusoft Engineering has accepted in assessing a building like this is twofold. Firstly, to evaluate how well the building has coped with all the slow movement and the vicissitudes of time, and secondly, to consider how well it might handle an earthquake or a severe weather event. John shared some of the process of making this assessment with us, as we walked around the building.

To understand how the building will perform, you need to know exactly what’s made from, and how the pieces are put together. Over the last few weeks, John and the owners of Hopetoun Alpha have carried out a number of investigations to establish this. They’ve been underneath the building, into the roof void, and everywhere in between. John began today’s tour by explaining the structural system of the building.

Alpha’s anatomy

The building is a long rectangle, shaped somewhat like a shoebox. Its two long walls, the side walls, get narrower in two steps as they get higher. At ground level, they’re about 640mm thick. The site slopes, but the walls come up to establish a level for the floor. At floor height, they step in to 420mm, creating a ledge both inside and out. On the outside, the ledge demarcates the plinth on which the building sits, and the walls change colour to emphasise this. On the inside, the step creates a handy support for the floor joists.

Hopetoun Alpha, basement. John indicates the ledge on which the floor joists rest

It’s a very common detail for “masonry” buildings of all kinds. As we’ve learnt on our tours, brick buildings often contain a similar step (or sometimes just a socket in the course) into which floor joists can be inserted. The problem is, of course, what happens when the walls move so much that the joists fall off their ledge or out of their sockets!

The walls step in at ceiling height, too, and there the step supports the ceiling trusses, which are likely made from jarrah. John shared a drawing of the trusses.

Hopetoun Alpha, drawing of ceiling trusses. Image courtesy John O’Hagan / Compusoft Engineering

He noted that although the trusses are statically indeterminate, they were well analysed by the designer for resisting gravity loads. The steel ties that you see connecting the vertical members to the bottom chord are wrapped around and pinned, forcing the verticals into tension. The diagonals carry compression. The trusses are sound and strong, and seem to have performed well. John mentioned that nails seem to have been a scarce resource at the time of construction, as there were very few to be seen in the timberwork! Fasteners of all kinds were clearly at a premium: a single bolt connects the front truss to the gable end. (As you can imagine, this is not ideal.)

The trusses span the main hall laterally, but there is far less roof structure in the longitudinal direction. What’s there is more or less entirely cosmetic—boxwork, panels, trim, ventilators. This meant that the engineers needed to wear abseiling harnesses to get up in the ceiling for a look—there’s not much to stand on and a fair way to fall.

Hopetoun Alpha, interior from mezzanine. Note bowed ceiling panels, and mezzanine cutting across windows.

As you can perhaps divine from the photo above, the mezzanine or gallery wasn’t part of the original build. The windows on the long walls would probably look different if this were the case (c.f. the smaller auditorium at the Auckland Town Hall.) The addition of the mezzanine meant some additional supports were needed underfloor. Cast iron columns carry the load down to neat brick piers—far neater than the original footings, which seem to have been pragmatically cast inside a few handy barrels!

Hopetoun Alpha, concrete footing (and extraneous foot, far right). Note the remains of barrel hoops and staves—easy formwork!

Mass (of) concrete

The entrance to the building presents the greatest challenge for modelling and analysis. There are large volumes of concrete in the pediment and the stairwells on either side, and, as noted, this section is poorly connected to the roof truss.

Hopetoun Alpha, portico. John O’Hagan explains the structural system of the building

There are decent-sized spans between the columns—approaching two metres—and of course the concrete is being asked to cross that gap without tensile reinforcement. This part of the building may yet require more analysis.

She’ll be right

When the mezzanine was put in, Hopetoun Alpha was also extended at the rear. This allowed a stage to be built and an organ installed, and to accommodate this a large opening was made in the back wall of the existing hall. And the building didn’t fall down.

Hopetoun Alpha, model, showing the opening in the rear wall (the blue rectangle)

It didn’t fall down, but over the long years, a large crack has developed in the wall. This is probably the result of the foundations moving slightly outward, and of tension forces in the reduced thickness of wall above the stage. Whatever the cause, the crack extends from below a round window above the stage (not visible from inside) to the top of the proscenium. Then the crack starts again at the left-hand edge of the bottom of the stage aperture, and continues down to ground level.

Hopetoun Alpha, basement. John points out a deep crack in the transverse wall below the stage.

How significant the cracks are, structurally speaking, is yet to be determined. But it seems to me that some form of strengthening will be required to make up for the diminished capacity of the wall.

Solution, general form

I want to preface what follows by saying that John made it clear that he didn’t want to talk about specific solutions for Hopetoun Alpha. It’s too early for that—too early, even, to say for sure whether work is needed at all, until the NBS rating is determined. Instead, we talked about some hypothetical solutions for a building of this type. Please read the rest of this post in that spirit. Your mileage may vary.

Hopetoun Alpha, view from the mezzanine

The major concern in a building of this size and style would be the out-of-plane response of the long walls. This means, if an earthquake shook the building from side to side (as opposed to back and forth), how well would the long walls cope with being flexed?

It might be sufficient to support them by connecting the floor joists to the walls in the basement. (At the moment the joists are just resting on the ledge you saw in the picture above.) As well as that, you’d probably put a plywood diaphragm across above the ceiling panels to tie the walls together at the top. By doing this, you’d end up with the long walls far better supported by the in-plane elements of the building.

Hopetoun Alpha, longitudinal section showing position of mezzanine. Image courtesy John O’Hagan / Compusoft Engineering

But if floor and ceiling diaphragms weren’t enough, the mezzanine might present an opportunity. Site visitors may remember hearing about the truss inside the gallery at the Auckland Town Hall. There’s obviously no room, and no need, for a truss inside the mezzanine at Hopetoun Alpha. But the mezzanine itself has some inherent strength, aided by its tongue-and-groove flooring. This could be enhanced with some inserted material. If the mezzanine were then connected more securely to the wall, it would serve as a brace at about halfway up the wall height. The walls could then be modelled as rocking about the pivot of the mezzanine, improving their performance. Tying the structure together better would be completed by more securely fastening the pediment and the rest of the portico to the main building.

Thanks!

Grateful thanks to John O’Hagan for his time and enthusiasm. Thanks also to the Ashton Wylie Charitable Trust which owns the building, and to Paula King, who made it possible for us to go and see it. We’ll stay in touch with this project as it progresses.

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.