Britomart, Auckland High Court, and St James Theatre: Heritage Buildings as Social Media

A brief note: apologies that this has taken so long to complete. Other deadlines compelled me more urgently!

The International Day for Monuments and Sites

What’s heritage? One facet I’m interested in is how the answer to that question changes with time. It seems inevitable (and proper) to me the contents of the basket labelled ‘heritage’ will change through the century, as New Zealand’s demographics change. I quite like the idea that heritage is a curated selection of the past, chosen by the present, on behalf of the future. And who’s curating will change.

Heritage is not interesting to everyone. But certain people, at some point in their lives, get interested in the remnants of the past that surround them. Heritage advocacy groups try to help more people to get bitten by the bug, and, with the long view in mind (always!), they want to reach out to younger generations, who’ll have to choose to take up the responsibility for looking after the old stuff.

With this aim in mind, ICOMOS (the International Council for Monuments and Sites) runs an “international-day-of-“. This year, the intention was to use social media to reach out to younger generations and foster all those warm fuzzies. Yours truly got involved in helping to organise some events to celebrate the Day, and, in discussion with the Auckland organising group, we came up with idea of going out to look at some Monuments’n’Sites and discussing the buildings themselves as pieces of social media.

You what mate? Bear with me. It’s not quite as nutty as it sounds. Public buildings don’t spring unbidden from the earth. They’re always, naturally, built with an end in mind—to communicate something about their purpose and the intentions of their builders. With that thought in mind, and with some wise guides to help us, we went to have a look at three prominent Auckland buildings. What were the messages that the buildings were made to communicate? What are they saying now, in their current context? What might happen to them in the future? When I finally finish writing this preamble, you might find out…

Jeremy Salmond and site visitors pause outside the CPO to examine the surrounding buildings: no longer “an oasis-of low-rise”?

Britomart (the CPO), with Jeremy Salmond

The Britomart story is somewhat circular, which seems fitting, given that the City Rail Link is all about completing a loop. The Britomart site was one of Auckland’s first train stations, built atop land reclaimed from the sea with the spoil from the demolition of Point Britomart. When the Central Post Office (the CPO) was built there (starting in 1909), the train tracks had to be shortened to make room. This left heavily-laden steam trains without enough flat runway to build up the speed they’d require to get up the hill to Newmarket; so, in a huff, the Railways moved to Beach Road, demolishing a couple of commemorative brick archways as they went —”out of spite,” said Jeremy.

So what does the CPO communicate? I asked. “It’s a typical Government building,” was Jeremy’s reply. Grandeur was the word he used to characterise its effect. Speeches were made in front of it, troops paraded there on their way to war, and punters meekly approached the grand elliptical counter to buy a stamp or two. The CPO was the face of government: reassuring, vigilant, stable.

Only, of course, nothing’s stable. The Post Office changed—radically—and moved on. After a period of neglect, and the threat of demolition in the 1990s, the CPO was repurposed. At last, the Railway got their station back! In the meanwhile, the warehouses of the Britomart precinct had come under threat from development, offering to turn what Jeremy called “an oasis of low-rise” into a field of tall towers. Jeremy was instrumental in developing a precinct plan, preserving some of the smaller buildings amongst their new neighbours.

The CPO’s looking a little dowdy around the edges right now, but we feel assured that it’ll get prettified when the CRL works are done. Once again, it’ll stand over an open square, projecting authority, but with far taller company looking down affectionately upon it.

Site visitors arrive at the High Court

The Auckland High Court, with Harry Allen

Up the hill, then, to the High Court. As we walked, Harry pointed out that the court’s location had been a significant choice, a signal of its prestige. It was finished in 1868, as British troops were leaving the fort at Albert Barracks. It’s vaguely military in tone. with its castellated tower, but this is clearly a fortress of justice, not of arms. We’re taking over now, was the message. The war in the Waikato had been fought. Pākehā power was here to stay. Nestled between churches, the Court asserted secular power and social order. Later the merchants of Princes St and the Northern Club came to shelter under its reassuring flanks.

The waiting room outside the main courtroom, Auckland High Court

Ecclesiastical was Harry’s term for the building. I’d be tempted to go as far as penitential. It doesn’t photograph well on a phone, but the waiting room outside the main courtroom is a clearly designed to induce a certain state of mind in witnesses or prisoners.  The Law is mighty. Do not try to fool us.

Site visitors in the waiting room outside the main courtroom, Auckland High Court

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.

 

 

St Paul’s Church with Salmond Reed Architects and EQ STRUC

Today’s visit to St Paul’s Church marked the start of the third year of activities for our ad-hoc society. Simeon Hawkins from the church’s congregation led the way. Sean Kisby of Salmond Reed Architects and Peter Liu and Dr John Jing of EQ STRUC came along to tell us about their work on the nascent project of strengthening and refreshing St Paul’s Church.

St Paul’s Church, panorama from the Symonds St entrance looking East

The church, explained

Simeon, whose Master’s thesis focusses on the church, gave us a brief overview of its history. The current St Paul’s is the third building to bear the name. It’s a Gothic Revival building, begun in 1894. The church is cross-shaped, as convention dictates. The main body of the church is stone, with brick veneers inside from shoulder height to roof and brick structure below the floor. The roof members are timber. The transepts (the arms of the cross) are also timber, and the chancel, the head of the cross, where the altar is, was made from reinforced concrete and wasn’t completed until 1936.

The building is not quite as its architect William Skinner conceived it. He’d intended for a gallery to sit above the door, housing the choir. The rough masonry you can see around the base of the church was to be covered with timber panelling. The chancel was to be faced with stone. And, most noticeably, no spire was ever built atop the northwestern stairs. Still, the building is Category I in Heritage New Zealand’s list, all the more poignantly since the HNZ listing says “St Paul’s invites comparison with Sir Gilbert Scott’s only New Zealand work, Christchurch Cathedral”.

A door to nowhere five metres in the air denotes the spot where the choir gallery was to be built.

For my part—although I don’t have favourite children!—I have a soft spot in my heart for St Paul’s. Something about the cheerful pick-and-mix of its irregularly-sized stone arches, its glorious melange of materials, and its air of patient worshipfulness makes my heathen heart glad.

Fine stonecarvings, southern entrance, St Paul’s Church

First, do no harm

Sean Kisby picked up the thread. Salmond Reed, he explained, have a dual role at the church. Firstly, there’s a fair hatful of material repairs that need to be done: there’s stone to be replaced; roof coverings to be refreshed; lead and copper flashings to replace, too. It takes expertise in heritage materials to understand what should be done, and how best to do it. Tracey Hartley (and others) from SRA are supplying this knowledge.

In parallel, there’s a real gem of a design project to be done. The building needs seismic strengthening, with the aim of reaching 67% of the New Building Standard. The internal circulation needs to be improved—at the moment, masses of people must file up and down a tiny rear staircase. The main stair, a lovely wooden spiral, has literally rotted away, the victim of a “temporary” roof over the void that was to contain the spire.

St Paul’s Church, spiral staircase seen from the crypt

With the access reconfigured, and the rooms beneath the chancel refreshed, the long-term plan is to build out into the carpark, and (best-saved-for-last here), at long last to design and build a spire for the church. Surely that’s a commission to gladden the heart of any designer.

Strengthening the church

“Bring the building up to 67% NBS” I wrote above, glibly. So, how might that be done? Peter and John from EQ STRUC have been considering that question. Over the last few months, they’ve carried out an analysis of the building, including a LiDAR  scan of the building which created a detailed 3D image of the church. The LiDAR generates a point cloud, essentially a myriad of measured dimensions, allowing a viewer to see details of colour and ornamentation as well as capturing idiosyncrasies in the plan. The engineers also need to know how the mass of the building is distributed, and the point cloud helps to identify this, making later finite element modelling in Etabs or SAP2000 more accurate.

Dr John Jing explains the proposed strengthening solution to site visitors, St Paul’s Church

The EQ STRUC blokes were too modest to say this for themselves, so I’m going to say it for them: they’re really good at assessing and then using the strength of existing materials. This is important, very important, for heritage buildings. The conservative approach would be to discount the strength of unreinforced masonry down to almost nothing, necessitating a larger and more intrusive engineering intervention. Think whacking great steel frames marching down the aisles.

For the nave and the aisles, the solution that EQ STRUC have devised uses the existing materials to their own advantage. I need to caveat this by saying that, as yet, this is just a proposed solution, but here it is: the plan is to use the mass of the walls to dissipate earthquake energy by allowing the piers to rock.

How does this work? Let’s work our way down from the roof.

Looking up at the roof truss, St Paul’s Church

Firstly, a diaphragm will be inserted above the sarking and below the roof, allowing forces to be transferred. (The nave floor’s getting a new diaphragm, too.) The existing timber trusses will be enhanced with steel, stiffening them considerably. At the junction between the trusses and the wall, a moment joint will be created, meaning that the roof and wall can’t rotate towards or away from each other.

The walls and piers of the nave. Spandrels above openings. St Paul’s Church.

Along the walls, between the openings, the spandrels will be strengthened with fibre-reinforced polymer wraps (FRPs). This will greatly increase their ability to resist tension, helping them to stay intact in a quake. [see footnote for a follow-up on this][update April ’18]

A pier, St Paul’s Church.

Lastly, as noted above, the piers will then be permitted to rock. In a design-level earthquake, the pier will form two hinges: one above the plinth (at the second moulding) and the other below the capital (the leafy bit). The pier will then wobble back and forth on the hinges, dissipating energy as it does so.

How do you stop the pier from toppling over, and bringing the roof down with it? By tuning the stiffness of the moment joints up above, the quantity of deflection at the piers can be controlled. It’s elegant—if not easy—and the great virtue of the solution is that you don’t need to ruin the spatial qualities of the interior with ugly external bracing.

The rose window and the western gable, St Paul’s Church

The real doozy of a problem might be the western gable end. How best to brace the wall, given the aesthetic and historic constraints at play? The solution may come from the planned completion of the choir gallery, absent from the church for so long. The gallery would run along the wall just below the level of the two long windows in the picture above. As at the Town Hall and Hopetoun Alpha, a gallery at the midheight of a tall wall can be a structural godsend, concealing crucial structural bracing, and reducing the risk of a potentially deadly of out-of-plane collapse

Peter Liu explains strengthening schemes to site visitors, the crypt, St Paul’s Church. Composite image.

To the Crypt!

Beneath the nave lies the crypt. Although you enter the church at ground level from Symonds St, the ground slopes away steeply to the east, and there’s a generous space beneath the floor of the nave. The space is used for a range of church activities. Here, in this unfussy rough-brick room,  it might be forgivable to insert some supplementary steel structure. EQ STRUC are proposing to do so, thus completing the rocking system described above by giving the piers a solid base to wobble on. (The slender piers of the nave are supported on the chunkier brick piers of the crypt, which you can see in the photograph above.)

Tension cracking in the transverse brick arches of the crypt, St Paul’s Church

Down in the crypt, we could see old signs of damage caused to the church  by the city growing around it. The church abuts the motorway trench of busy Wellesley Street, dug in the 1970s (? don’t quote me on this date!) As the ground settled post-trench, the foundations rotated outwards, creating tension cracking like that shown above. Don’t panic, though: a geotechnical report showed that the settlement has long subsided, and there’s no fear of further rotation and damage.

St Paul’s, south side, coming up the slope from the carpark. Timber transept meets masonry nave. Note irregular finishing of wall, perhaps intended to join to masonry veneers?
St Pauls, concrete chancel/transept and masonry nave.

 What are you made of?

As noted above, and as you can see in these images, the church is made from a variety of materials. There’s stone, brick, reinforced concrete (RC), and timber. How, I asked, would you assess the interaction between the RC and masonry elements of the building?

Here, it seems, the timber transept is invaluable. The timber’s elasticity should take up any differential movement between the RC and masonry sections. This means that the RC and the masonry were able to be assessed as two separate structures, removing a layer of complexity from the analysis.

Given the building’s Category I status, it’s not easy to make a case for destructive testing to establish the strength of materials. EQ STRUC have a database of material tests from similar structures to fall back on, for establishing likely material properties.

Signatures of the builders? Stairwell, St Paul’s Church.

Talk to me

In the crypt, I asked Peter, John, and Sean about collaboration between architects and engineers, hoping for dark and ghastly tales to befit at least the name of the surroundings. Instead, they talked about the pleasure of collaboration, of working together and modifying each others’ solutions to arrive at the best result. In all seriousness, such collaboration is a theme of our Society, since we’re composed of would-be members of each profession. It’s great to hear that the working world embraces this philosophy of co-operation.

Dr Jing made a comment that intrigued me. “You need to draw,” he said. “I always take a pen and paper to meetings and to site.” Calculations can be confusing, explanations are often ambiguous, but sketching makes for clear communication. Architects will be unsurprised by this, I think, but it’s food for thought for student engineers.

Thanks!

Sincere thanks are due to Sean Kisby from Salmond Reed Architects, and to Peter Liu and Dr John Jing of EQ STRUC, for their generous sharing of time and expertise. We’re also exceedingly grateful to Simeon Hawkins and Esther Grant from St Paul’s, and to the marvellous people who came and offered coffee and hospitality. Thank you for sharing your beautiful church with us. We look forward to seeing the project develop.

Footnote

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Bricks in the spandrel area, closeup. Photo courtesy Tracey Hartley, all rights reserved.

Tracey Hartley touched base, following up on the proposal to use FRPs to reinforce the spandrel areas. Regarding the bricks in the spandrels, she notes “I believe they are a rough pale brick with large joints, originally coloured over with a red ochre finish and lined out (tuck pointed) to make the brickwork smarter looking.” The final design may involve restoring this finish, so covering with FRPs may be more difficult. Also, the breathability of FRPs needs to be determined so that moisture isn’t trapped in the brickwork.

I thought it was interesting to include this as a footnote, to illustrate how the process of design is a compromise or collaboration between the expertise and priorities of the professionals working on a project. Working together, the architects and engineers will devise an acceptable solution.

Update April ’18

back to article

I ran into Dr Jing today and we talked about the FRPs for the spandrels and piers in the nave. Dr Jing explained that the FRPs in the spandrels were never going to be added onto the surface of the bricks, as I had thought. Instead, they would be mounted into little slots cut into the bricks. The slots would be ~3-4mm wide and span across several brick units, being anchored at top and bottom into bolts. This near-surface mounting would make the FRP intervention much less visually intrusive than if it were covering the surface (as per Tracey Hartley’s note above). To reiterate, it was never the plan to mount the FRPs on the surface: the EQ STRUC team were always intending them to be near-surface mounted.

One final point. I hadn’t picked up that the piers were also to be FRP wrapped, not just the spandrels. The FRP wrapping would naturally add great confinement strength to the pier. However, by varying the length and position of the wrap, the EQ STRUC engineers can force the plastic hinge to form in the places they want it to: at the top and bottom, as per the pictures above.

Auckland Town Hall with George Farrant, May 2017

The clock mechanism, Auckland Town Hall.

To potential clock room taggers and graffiti artists: All tags, names and graffiti will be promptly removed and you will be forever haunted by the ghost of the clock tower.

 Thus read the notice meeting the eyes of Auckland Town Hall site visitors on Monday and Tuesday, as they put their heads through the trapdoor at the top of the ladder to the clock room. As Ray Parker, Jr. said, I ain’t afraid of no ghost: but it seemed to me what we saw on the tour, the fruits of the restoration work carried out from 1994-97, was certainly the resurrected spirit of the original design of the Town Hall. No effort was spared to return the building to something approaching its original state, and, at the same time, to make it safe and strong for the future. True to form for ghosts, the structural upgrades are in many cases invisible—or at least, hidden from the eye where members of the public can ordinarily go. On this visit, we went behind the scenes.

Bluestone on the lower floor, Oamaru stone above, but right at the top the true building material of the Town Hall: bricks.
Town Hall exterior, northeastern corner. Bluestone on the lower floor, Oamaru stone above, but right at the top, the true building material of the Town Hall: bricks.

BIG EMPTY BOXES

The Auckland Town Hall is essentially a brick building. It is faced with Oamaru stone and with Melbourne bluestone, the latter brought over by the Australian
architects of the Town Hall—a prime example of coals to Newcastle in this basalt-bottomed town. There’s reinforced concrete in the foundations, in the form of piers and floor beams. Structurally, however, the Town Hall suffered from some of the usual flaws of unreinforced masonry buildings: vulnerability to face loading of external walls, and insufficient shear strength.

 

In out-of-the-way areas, remedies for these problems could be visible. On lateral cross walls, shear strength was improved by adding a 100mm-thick concrete skin to the bricks. That doesn’t stop the building rocking itself off its foundations, so the basement-level concrete was tied into new piles, hand-dug for lack of headroom. Longitudinal walls at the upper level had fibreglass glued to them, and this was covered with plaster. For various reasons, some of the internal brick walls had new openings cut into them. To retain the shear strength of the wall and to leave a record of the intervention, these openings were finished with a visible internal frame of structural concrete. This frame-within-a-frame motif, used to signify a modern alteration, was lifted from parts of the original design.

The original (non-structural) frame-within-a-frame design.
The original (non-structural) frame-within-a-frame design.
In the light well, new openings cut into the brick shear wall were denoted by the frame-in-frame treatment.
In the light well, new openings cut into the brick shear wall were denoted by the frame-in-frame treatment.

Strengthening the main performance spaces was trickier. Of necessity, these rooms are high-ceilinged and large, and, designed for natural light, their walls have many openings, separated by slender columns. Out-of-plane loading would wreck them. But the walls are beautiful inside and out, and the spaces are well-beloved and well-known in their current form. Structural enhancements had to be invisible.

The wall of the Great Hall. The curtains cover large windows--note the slender piers between openings.
The wall of the Great Hall. The curtains cover large windows–note the slender piers between openings.
George explains the design solution underneath the gallery which conceals the truss.
George explains the design solution underneath the gallery which conceals the truss.

In the Great Hall, the solution was obvious—once someone had thought of it. A gallery runs around three sides of the room, providing extra seating. It was easily large enough to conceal a gigantic U-shaped horizontal truss, which provides stiffness to resist lateral movement of the weak outer walls. A plywood diaphragm hidden in the ceiling cavity tied the tops of the walls together. In the somewhat smaller Concert Chamber, the gallery is small too, and it doesn’t extend around three sides of the room. With no opportunity to conceal a truss, the strengthening in the Concert Chamber took the form of reinforced concrete columns inserted into 400×400 slots cut into the wall—or, in one case, cut right through the wall and out into the weather (Oamaru stone is pretty soft!). As part of the refit, air conditioning was inserted into the walls, and the vents are partially hidden by the decorative plasterwork.

Air conditioning vents between plaster corbels, Concert Chamber. The steelwork is inserted into the columns between the windows.
Air conditioning vents between plaster corbels, Concert Chamber. The steelwork is inserted into the columns between the windows.

 OUT OF THE FRYING PAN, INTO THE FOYER

 For a number of years, prior to the restoration project, the floor tiles in the foyer often exploded. This alarming phenomenon was at first put down to excessive compaction caused by floor buffing machines, but the installation of a sprinkler system into the concrete slab on which the tiles sat revealed the true problem. The reinforcing bars in the slab were in the wrong place—the lower bar sitting far too close to the top surface. What was causing the tiles to explode was the floor slabs deflecting under the weight of concertgoers: alarming indeed! Thankfully, none of the floors failed, but many of the tiles, under strong compression, did.

The problem for the design team was how to support the floors without changing the proportions of the spaces, since there is nowhere to hide any supplementary structure. The deflection was reduced by adding carbon fibre strips to the underside of the floors—likely the first time that this material had been used for structural repair in NZ. With the floors strengthened, a repair job had to be done on the tiles.

The tiled floors of the foyer. Floors extend over two levels. In the centre, the round tile is an encaustic tile, stained orange by the acid bath. The square brown tiles are original--I think!
The tiled floors of the foyer. Floors extend over two levels. In the centre, the round tile is an encaustic tile, stained orange by the acid bath. The square brown tiles in this photo are original–I think!

The fanciest tiles, made with a light-coloured slip poured into a relief-moulded dark-coloured base (encaustic tiles), came through OK, barring some orange stains caused by an overzealous acid bleaching. But the plain, square, brown tiles which cover the greatest part of the floor were seemingly impossible to source: they couldn’t be bought, and, scour the world though they might, the team could not find a manufacturer capable of exactly matching the original colour, given an understandable reluctance on the part of modern potters to use lead oxide in their glazing. All seemed lost—until one day, a project manager from the Town Hall team had lunch at a well-known franchise restaurant specialising in Scottish food. To his utter astonishment, the kitchen tiles at McD’s appeared to be an exact match, and he nearly earned himself a cell next to the Hamburglar by bursting unannounced into the restaurant kitchen with his tape measure get the exact size of them. To cut a long story short—the tiles matched matched perfectly, and McD’s eventually agreed to give the Town Hall enough of their custom-made tiles to repair the floors.

In a similar spirit of desire for perfection, George mentioned several other examples of the lengths to which the project team went to get as close to the original design as possible, including scraping the walls painstakingly to find the original wall colour (not to be mistaken for the colour of the primer or the basecoat). They trawled through archival pictures to find the patterns of the original leadlight windows. Of course, the pictures are in black-and-white, but the glass colours were revealed by the discovery of one large window, which had literally been rolled up and stashed away. Picture the restorers hunting in a dark basement for scraps of coloured glass. That’s dedication.

 A TICKING TIME BOMB?

The clock tower rises above the administrative offices of the Town Hall.
The clock tower rises above the administrative offices of the Town Hall.

On Monday’s tour, George willingly expressed his “diffidence” over the threat that earthquakes pose to Auckland’s buildings. He qualified his position to the extent of saying that the risk is non-zero—and with a non-zero risk in mind, the clock tower on the southern end of the Town Hall presented a serious engineering challenge. It’s extremely heavy, and being taller than the adjoining structure, it would have a different period under earthquake acceleration.

The exposed steel frame in an upper storey of the clock tower. Note the steel rods running across the window instead of solid beams.
The exposed steel frame in an upper storey of the clock tower. Note the steel rods running across the window instead of solid beams.
In the storey below, the exposed steel frame (white) joins up with steel inserted into the walls (grey). In lower (public) floors, the steel strips are hidden in the walls.
In the storey below, the exposed steel frame (white) joins up with steel inserted into the walls (grey). In lower (public) floors, the steel strips are hidden in the walls.

The initial design solution, a steel framework inside the tower designed to hold the tower up, was rejected—by George. It would have dramatically altered the staircase below it, which winds up to the council offices. George’s name was mud among the engineering team for some weeks, until an alternative solution occurred: why not hold the tower down, instead of up? This developed into a solid steel frame, in the upper tower; connected to cross bracing cut into the walls, in the storey below the steel frame; connected to thin steel strips inserted into the walls of the stairwell, and anchored into the foundations. These steel strips are 200×19 galvanised steel flats, sitting in 270×120 slots packed with a Denso felt, and tensioned. This holds the tower together, using the tension in the steel against the crush strength of the masonry, but doesn’t eliminate the possibility of swaying. In addition to the post-tensioning, then, a transfer truss connects the tower to the top of the longitudinal walls of the Town Hall, holding it fast. The truss is hidden under a sloping roof. One final touch—in the clock tower, where the steel frame sits, instead of steel cross-members going over the windows, the bracing consists of four 40mm steel bars, painted in a dark colour. You’ll see it (at night) now that you know it’s there, but it’s far less noticeable than steel beams would be—seen out of the corner of your eye, you might pass it off as a mere apparition.

 FURTHER READING

There’s a really lovely post on the Timespanner blog with some great archival images of the construction of the Town Hall.

I also sent site visitors a link to Downer Senior Engineer Mark Hedley’s 2014 paper on the strengthening of five major civic buildings in Auckland.

 THANKS

 With sincere thanks to George Farrant, redoubled since he generously agreed to host a second tour in the face of extremely high demand. For he’s a jolly good fellow, and so say all of us.

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.