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.

 

 

University of Auckland heritage buildings with Neil Buller and Peter Boardman, March 2017

Yesterday a large group defied the weather forecast and enjoyed a tour of several of the University’s heritage buildings.

Neil Buller, architect and project manager for the UoA, battles a sore throat and traffic noise, talking to the crowd about 10 Grafton Road.
Neil Buller, architect and project manager for the UoA, battles a sore throat and traffic noise, talking to the crowd about 10 Grafton Road.

For me, the recurring theme of the afternoon was the University’s special nature as a client. It’s unusual to have a client whose heritage properties span such a range of ages, styles, and typologies. The University’s buildings mostly have high levels of occupancy and usage, making it more critical that they be demonstrably safe and sound. And the University also has a kaitiakitanga role and a concern for preserving its history. For all these reasons and more, the University seems to take a more scrupulous position about how much retrofit it is prepared to do. The projects we saw were targeting 100% of the New Building Standard (NBS), which is a higher target than many clients choose to set.

It’s also a target that necessitates more physical intervention. Some of the conversation on the tour turned around questions of the practice of making these interventions visible, of leaving good records of work done, of reversibility, and of the suitability or otherwise of certain building technologies for heritage sites. Sometimes repairs can bring problems with them, if, for example, they bring moisture where it’s not wanted. We also heard a familiar story about working on heritage buildings–starting to fix one problem leads to finding several more!
Neil describes the proposed post-tensioning system for the Clock Tower Annexe, where work is due to start in October 2017.
Neil describes the proposed post-tensioning system for the Clock Tower Annexe, where work is due to start in October 2017.
I enjoyed the opportunity to hear and see some details about the specific problems heritage buildings experience, and how these are addressed. At Bayreuth (the 1903 Italianate building at 10 Grafton Road), tie rods have been inserted both parallel and perpendicular to the floor joists, binding the structure together. The brick and concrete Merchant Houses Belgrave, Okareta and Mona (12-16 Symonds St), dating from the 1880s, were suffering from water intrusion, and required structural improvement. The basement slab was relaid, with a ventilation system designed to improve airflow and remove moisture. Floors were taken up, the joists re-fixed to the walls, tongue-and-groove floorboards re-laid, plywood diaphragms used to stiffen the structure. Roof timbers were refreshed. All in all, a major refit, and one that necessitates tradeoffs between the future of the building as a whole and the integrity of its heritage fabric.
Crossing the road, we saw an elegant intervention at the General Library, where the services and stairwell were too stiff and inflexible in comparison to the abutting structure. In a decent shake, they’d likely knock the rest of the building to pieces. The solution proved to be “strengthening through de-strengthening”, in that vertical saw-cuts were used to weaken the walls, while the structural members of stairwell and library were tied together. There was an interesting conversation about the value of taking the time to allow designs to be re-thought. In this project, the initial design proposed masses of steel, strapping the disparate parts of the structure together. The final solution proved to be far more minimal, essentially a steel rod lashing elements together across the stairwell, forming a lattice. A conference panel on Alistair Cattanach of Dunning Thornton’s design for the library, with some photos, drawings, and some more information, can be found here.
After the tour, we moved inside to examine drawings and photographs. Above my not-so-hot picture of the design for the Library. Look up, next time you're on Alfred St!
After the tour, we moved inside to examine drawings and photographs. Above my not-so-hot picture of the design for the Library. Look up, next time you’re on Alfred St!
Lastly, we looked at the Annexe to the Clock Tower (aka the Old Arts Building), built in the 1920s. Peter Boardman, of Structure Design, who accompanied the tour, was responsible for the highly effective post-tensioning system on the Christchurch Arts Centre, which saved it from major damage. If you’re not familiar with the Christchurch project, imagine a netting of steel cables wound around the outside of a stone building and tightened! The Clock Tower Annexe is getting a more high-tech version of this treatment, where steel rods are being inserted through the major structural members of the building. The Annexe is made from concrete faced with stone, and the steel rods, once tightened, will add some very necessary tensile strength to the building.
I’ll finish by mentioning that in response to a question from the audience about the most important lessons from the Christchurch quakes, the presenters agreed that the biggest lesson was this: doing something is better than doing nothing. Buildings which had been strengthened, even those with ad hoc solutions, survived better than those which had not. Peter suggested, as an example, that shoring up verandahs at a cost of a few thousand is often a way to reduce stresses throughout the structure, and provide protection from falling decorative elements. The two presenters also reinforced the importance of communication and compromise between members of allied professions (like architecture and engineering), and I’m sure you will all concur with this sentiment. We’re very grateful to Neil Buller and Peter Boardman for their generosity, frankness, and expertise.