Auckland Town Hall with George Farrant, May 2017

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.