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.
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.
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.
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.
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.
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.
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.
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.
(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.
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.
Speaking of trusses
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.