Albert Park Keeper’s Cottage with Dave Olsen of Mitchell Vranjes and Egbert Koekoek of Cape

Your correspondent keeps a sharp eye out for old buildings under wraps. When one is spotted, this is usually followed by a spate of calls and emails requesting a site visit—what you might call (I hope!) a charm offensive. In the case of the Albert Park Keeper’s Cottage, it was almost as though the building was taunting me: for one thing, it’s right there at the University gates. And for another, it was rising into the air. Come and get me!

Albert Park Keeper’s Cottage. The Cottage has been jacked up off the ground to allow repiling work to take place. The Cottage is an 1882 timber structure, with brick piers supporting the floors and a brick chimney. One unusual feature of the building is its slate roof. The slates are an added mass high up on the structure, and have some effect on its predicted seismic behaviour.

A sign at the site explained that the building was undergoing seismic strengthening: and so, a few phone calls later, we went behind the fence to check out the project and how it was progressing. Guiding us were the project engineer Dave Olsen from Mitchell Vranjes (regular site visitors may remember him from the Melanesian Mission) and Egbert Koekoek from the construction contractors Cape.

Albert Park Keeper’s Cottage. Site visitors assemble to hear from Dave Olsen (left) about the project.

Going up

So, why was the house in the air? As with many older buildings, the basic problem is that the Cottage is not strong enough to resist horizontal loads. A structure can be fine holding up its own weight, but if it’s shoved sideways, it falls off its foundations, and that’d be that. One part of the project is to strengthen and renew the Cottage’s piles, and to brace it horizontally against loads from a future earthquake.

But buildings, especially old houses, are re-piled all the time, right? And they don’t get lifted into the air, do they? The reason for this gets at some of the differences between heritage jobs and regular engineering. In a conventional repiling, holes are cut in the floor, and those are used to dig out and place the new piles. At a heritage building, one of the first principles is to try to avoid damaging the original fabric, and to minimise any necessary damage. Rather than cut the floor to pieces, it was deemed better to lift the building—a technology more commonly associated with house removals.

Albert Park Keeper’s Cottage. Steel lifting beams support the Cottage off the ground. Note weatherboards have been removed to allow the beam to be inserted. As can clearly be seen, the beam is lifting from above the floor.

Lifting the building had other benefits. It gave enough headroom for the workers to install some larger timber piles, the deepest of which extend 900 mm below the surface. Further, because of the heritage-listed trees which surround the site, it was not permitted to use screw-piles, so workers (and the arborist!) had to be able to see where they were digging.

Albert Park Keeper’s Cottage. Lifting beams seen in the interior of the structure, photographed at the southern corner through a convenient gap. Note on the left the timber ribbon beam, which runs longitudinally through the building and is fastened to the studs.

How do you lift a house? I’d’ve imagined that this was done from the bearers, or maybe the joists. But it was plain to see that this was not the case at the Cottage. The orange steel beams running through the house are clearly above the floor level. Egbert Koekoek explained that the house lifters installed timber ribbon beams running the length of the cottage, which were attached to the studs. Weatherboards, and the internal timber lining boards (sarkings), were removed to allow the ribbon beams to attach directly to the studs. The orange steel lifting beams were inserted. Then, fourteen hydraulic jacks lifted the Cottage up into the air, a little at a time, over the course of a couple of hours. Each jack can be individually switched on and off, leading to a certain amount of racing around with a tape measure to make sure that everything’s lifting at the same rate!

Albert Park Keeper’s Cottage. Note the new timber (lighter colour) added either side of existing bearers. The line of brick piers below the bearer, still able to carry gravity loads but with no horizontal capacity, has been augmented and partly replaced by timber posts. Diagonal braces attach to new timber piles. At the right, a concrete block wall replaces bricks which have rotated outwards due to expansive soil.

With the house lifted in the air by its studs, it’s not safe to go inside, for fear that the floor might simply fall away under your feet. However, the raised house also provided the opportunity to strengthen the bearers. The need for strengthening is in part due to the new use of the building as public space, requiring a design for 3 kPa floor loads. This has been done by adding timber either side of the existing bearer—once again, unconventional practice, but in keeping with the heritage principle of retaining original material.

Albert Park Keeper’s Cottage. The original perimeter bricks are largely being retained and reintegrated into the load-bearing system.

At ground level

Geotechnical testing of the site revealed that the soil is expansive, meaning that it shrinks and swells a lot. Perhaps as a result of this, some of the original perimeter brickwork under the walls has moved around quite a lot over time. On the park side of the house, the wall had rotated about ten degrees, and had to be replaced with a concrete block wall. (The concrete will be faced with brick so that it looks much the same as the original.)

With some new perimeter walls, and with sturdy timber diagonal brace piles taking effect, the underfloor of the Cottage is now going to be fairly stiff. A site visitor asked about stiffness compatibility between the underfloor and the timber structure of the house, which can be expected to be pretty floppy by comparison to its supports. The answer to this came in several forms, if I’ve understood it correctly!

In part, the Cottage itself is getting some increased stiffness. The sarking on the internal walls is going to be renailed in a number of places, making the internal boxes of the rooms considerably firmer. The front room, in the northeastern corner, contains the chimney, about which more later. It requires extra horizontal bracing to restrain the brickwork, so a brace Gib is being added over the sarking. (The ceiling of that room gets an enhanced diaphragm, too.) But, in the main, the answer to questions of stiffness compatibility between structure and substructure is this: it doesn’t matter. The Cottage isn’t overly large. And the inherent flexibility of the timber makes it unlikely to transfer loads very far across the structure, meaning that deflections at the interface between floors and piles shouldn’t be too much for the connections to handle.

Albert Park Keeper’s Cottage. The fireplace and chimney were (naturally) not lifted with the rest of the building. The connections between chimney and structure had to be carefully broken away to allow the house to be lifted

Catching the flue

In the seismic assessment of the Cottage, the chimney was identified as the weakest link, scoring around 15% NBS. Think of the chimney as a freestanding pile of bricks. It’s supported on its foundation, and again at the ceiling level. Then there is a decent length of chimney between the ceiling and the roof, and still more again where the chimneystack protrudes into the sky. So what we have is a long brick column, with a point of restraint at the base, another at the ceiling, and a long unrestrained section above the ceiling. It’s this top section, above the ceiling, that needs extra support. In a quake, it could rock itself right off the rest of the flue, causing collapse.

Albert Park Keeper’s Cottage. A section showing the props bracing the chimney. Sturdy connections are made to the rafters. A plywood diaphragm at ceiling level increases the stiffness of the ceiling restraint. Image courtesy Dave Olsen/Mitchell Vranjes, all rights reserved.

The solution that Dave has chosen is to use timber props, creating a collar around the chimney just below the height of the roof. This creates a firm diagonal bracing for the chimney, meaning that the unrestrained section will be restrained at approximately half height. By changing the unsupported length, the period of the rocking motion expected in the chimney changes, and the resulting forces experienced by the chimney are reduced. In accordance with the NZSEE guidelines, the mortar of the chimney-bricks is assumed to have basically no tensile strength. In a quake, the chimney is expected to form cracks, breaking at predictable points into short but intact sections which will rock but not topple.

With the limited clearance beneath the Cottage’s floors, smaller workers are preferred

Local gossip

A couple more newsworthy points to share with you. Regular visitors to Albert Park will have noticed that the Band Rotunda is also under wraps. Egbert explained that, although there’s plenty to do at the Rotunda, there’s nothing structural happening: the job is mostly maintenance and repair. He also shared a few things about the work that’s been happening at Pembridge House, which is the southernmost Merchant House in the lineup along Princes St. I did make an attempt to get a site visit to Pembridge up and running, but it was too complex because the floor was taken up for a lot of the time and the site was hazardous. (Hazardous = interesting, though, doesn’t it!) A major feature of the job, structurally, was the insertion of two big two-storey steel K braces in the stairwell, which were then concealed. Nothing to see now, folks! Never mind: other opportunities will surely arise.

Thanks!

Sincere thanks to several people for helping to organise this one. We put this together against time pressure, with the Cottage due to be lowered early next week. A number of people set aside other (real) work to make this happen for us, including Richard Bland, Antony Matthews, and Stacy Vallis. Thanks to Auckland Council. We’re also most grateful to Dave Olsen and Egbert Koekoek for their time and their willingness to answer questions and discuss the project.

 

Roselle House with Peter Reed, and the Melanesian Mission with Jeremy Salmond, Andrew Clarke and Dave Olsen, April 2017

Visitors on the terrace at Roselle House...
Visitors on the terrace at Roselle House...
Visitors… on the terrace at Roselle House…
...and in the attic at the Melanesian Mission.
…and in the attic at the Melanesian Mission.

WHAT I DID ON MY HOLIDAYS

Over the Easter break, heritage enthusiasts from the U of Auckland visited building works at two 19th-century masonry buildings. The first was the 1870s mansion Roselle House, now part of St Kentigern Boys’ School. Next was the 1850s ecclesiastical training school, the Melanesian Mission, which gives its name to Mission Bay.

Melanesian Mission. Dressed-stone sill and jambs (?) around small attic window, photographed from the scaffold.
Melanesian Mission. Dressed-stone sill and jambs around small attic window, photographed from the scaffold.
Roselle House. Brick “relieving arch” built to ease strain on large internal lintel. (Note, this was hidden in the original by plasterwork, and will be covered up again.)
Roselle House. Brick “relieving arch” built to ease strain on large internal lintel. (Note, this was hidden in the original by plasterwork, and will be covered up again.)

RE-USE

One of the major topics of discussion at both sites was re-use, and how making the buildings useful for their current occupants supports their preservation. Renovating a building usually means making changes to its fabric and there are consequent losses of heritage material. To make such changes, consent is required from heritage authorities, and this has to be negotiated. Part of the negotiation comes down to demonstrating the overall benefits to the building that can be expected from the project, even if those benefits come at some cost to what is currently there.

The Roselle House tour
The Roselle House tour

At Roselle House, the school is on- trend, transforming library space into learning commons. There was a discussion of the decision-making and consenting processes that were required to allow a large opening to be cut in a wall for a new entry. Cutting the hole meant losing some heritage fabric, but the future use of the building required it. Peter Reed and his colleagues discussed how the building’s elements were classified through its conservation plan, and how the Heritage Impact Assessment (for the new aperture) was devised. (In other parts of Roselle House heritage material that has been removed has been stored for re-use when the building is made good.)

At the Melanesian Mission, a new restaurant housed in an adjacent contemporary-style building provides the financial oomph required to care for the heritage site. At the Mission, there are fewer obvious changes to the building itself than at Roselle House, but its aspect will be significantly altered by its newly-built neighbour. Jeremy Salmond termed the Mission (and other built heritage) “vertical archaeology”: a record of the past, its people, their hopes and their achievements. That’s what makes it worth the care we lavish upon these buildings, he said. Jeremy’s belief is that you best complement a good old building with a good new one, rather than attempting to replicate an older building style and thus fudging history. Yes, the Mission’s visual surroundings change, but that’s the price of maintaining the history it embodies.
Roselle House. Preparations for pouring the shear wall. Above, note existing timbers, which have been included in the design calculations. See also, for interest, the plaster oozing through the laths—this is how the plasterwork adheres to the walls.
Roselle House. Preparations for pouring the shear wall. Above, note existing timbers, which have been included in the design calculations. See also, for interest, the plaster oozing through the laths—this is how the plasterwork adheres to the walls.
Roselle House. Looking down into the cavity for the shear wall. It will sit on a slab broad enough to avoid overloading soil bearing capacity, which could lead to overturning.
Roselle House. Looking down into the cavity for the shear wall. It will sit on a slab broad enough to avoid overloading soil bearing capacity, which could lead to overturning.

HOW TOUGH IS OLD STUFF?

Both Roselle House and the Mission are being strengthened against earthquakes. There’s a good deal of new material going into each site, but, interestingly, the pre-existing fabric of the site is also having its strength recognized and used. U of A research on heritage fabric was mentioned in dispatches, and no doubt a number of you site visitors (and your professors) are working on how to assess the strength of old materials.

At Roselle, new concrete bearer beams span under the floors, and the walls, floors, and ceilings are being strapped together and connected to these beams. Plywood diaphragms at floor and ceiling are the order of the day. But the main earthquake-resisting structure will be an internal shear wall. This will be poured anew, but it incorporates pre-existing timbers, and their strength was calculated and incorporated into the shear wall’s design.

At the Mission, a good deal of new steel has gone in, to secure the gable ends and the long walls against out-of-plane loading. Jeremy Salmond and Andrew Clarke described sending their design drawings for the steelwork back and forth to each other, and they both stressed the importance of designing every detail sympathetically to the building’s original programme. For example, the 200mm beam that spans the top of the walls in the Mission Hall has been custom-welded with an angled rear flange: instead of looking like this |____| in section, it looks like this |____\ . Why? So that it fits under the slope of the roof: thus the beam will not protrude over the edge of the wall. The beam has the same dimensions on its exposed face as a now-removed timber strip that used to run around the top of the walls. When the walls are refinished, the steel beam will have the same visual effect as what has been lost.

Melanesian Mission. An internal wall is drilled at regular intervals. The Mapei grout is pumped into the holes, starting at the bottom, until it begins to flow out of adjacent holes. The process is repeated three times.
Melanesian Mission. An internal wall is drilled at regular intervals. The Mapei grout is pumped into the holes, starting at the bottom, until it begins to flow out of adjacent holes. The process is repeated three times.
Melanesian Mission, detail of another internal wall, showing the insertion tube. The hole will be re-grouted with lime mortar, so it won’t be noticeable.
Melanesian Mission, detail of another internal wall, showing the insertion tube. The hole will be re-grouted with lime mortar, so it won’t be noticeable.

But to return to the strength of the existing materials at the Mission: the engineers made an assessment of the capacity of the masonry walls, using for their calculations some results from Jason Ingham’s research. An initial plan to tie the wall together with threaded rods was abandoned in favour of a Mapei- brand lime-based grout or slurry. Regularly spaced holes were drilled in the mortar, and the sludge was pumped into the wall. (Pumped by hand, so that the pressure didn’t get high enough to pop off the other side of the wall!) The result: the void spaces between the rubble are filled, and the inner and outer skins of the wall are bonded together. And it’s invisible. So the original material, supported by some chemical wizardry, gets retained, and can now resist greater loads.

Efflorescence on the bricks, internal walls at Roselle House
Efflorescence on the bricks, internal walls at Roselle House
The highly porous volcanic stone of the Mission. The lime mortar has been renewed as part of the project, but the Mapei-grout holes are yet to be filled.
The highly porous volcanic stone of the Mission. The lime mortar has been renewed as part of the project, but the Mapei-grout holes are yet to be filled.

WATER WATER EVERYWHERE; or, THE CONSEQUENCES OF DESIGN DECISIONS

Water in the walls was a recurring theme. At Roselle House, a chain of unfortunate decisions caused considerable harm to the fabric. First, wooden verandahs were replaced with terrazzo in the 1930s, sealing off the underfloor without ventilation, and causing the timber bearers to rot. Next, sagging timber floors were replaced with concrete. Uh-oh! Now the ground water, under pressure, wicked up the rendered plaster internal walls, moving between the brick and plaster, or between the plaster and its hastily re-applied paintwork. Wherever the water went, efflorescence remained, in the form of salty stains and crystalline growths. One of the major tasks of the project is to remove the old concrete floors and to draw the moisture out of the bricks with a special clay, in a process known as poulticing. The terrazzo stays, but it will be ducted to allow proper underfloor airflow. Peter made the point that the consequences of the 1930s renovation decisions took decades to become obvious, but have also created problems for occupants for many more decades. Earlier attempts to fix the problem only made it worse. Think twice about messing with an original design!

Water has a more subtle place in the walls at the Mission. The walls are made from chunks of basalt, taken from Rangitoto, and piled up in random courses, held in place with a lime mortar. Dressed blocks of scoria form the quoins. Both scoria and basalt are highly porous, and so, in wetter months, the walls have always been permeated with damp. This, says Jeremy, is not really a problem: the walls were made to be wet—notwithstanding that water entry did ruin the original plasterwork and create efflorescence. Problems have been caused by later attempts to “solve” the dampness, in particular by repointing with Portland cement, by plastering the inner face of the walls, and by treating with an “invisible chemical raincoat”, the latter occurring in 1977. These treatments tending to combine to retain moisture within the walls—the opposite of what was intended—and deteriorate the lime mortar, so much so that Jeremy described the walls as being “two dry stone walls with sand between them.” That doesn’t sound like a structure that would resist earthquake shaking very well! In combination with the Mapei re-grouting and the steelwork, the walls have been re-limed, and will surely be much the better for it.

The Main Hall chimney at the Mission. Steel rods run down the stack to the fireplace.
The Main Hall chimney at the Mission. Steel rods run down the stack to the fireplace.
Looking up from the ground floor at Roselle House to the stub of chimney. At some stage in the building’s life, the chimney was removed from the ground floor, but the rest was left to hang on in there... somehow!
Looking up from the ground floor at Roselle House to the stub of chimney. At some stage in the building’s life, the chimney was removed from the ground floor, but the rest was left to hang on in there… somehow!

A NOOK ABOUT CHIMNEYS

Two contrasting treatments for chimneys deserve mention. At the Mission, an elegant brick chimney stands above the rock wall on the western side of the hall. The chimney has been post-tensioned with steel rods, which connect an upper plate to the floor slab, holding the stack firmly together against shaking. The solution allows space for a flue to be inserted, so that a gas fire can simulate the cozy effect of a real one.

At Roselle House, the project team discovered that in some long-forgotten she’ll- be-right renovation, a chimney which poked out of the roof had had most of its lower extent removed. This is a trick somewhat akin to climbing out on a tree branch and then sawing it off behind you—expect things to start going downhill fast! As Peter explained, there were six tons of bricks sitting up in the roof and upper storey with very little holding them in place. In this case, the majority of the remaining chimney material has been removed, and a lightweight replica will be installed to keep the roofline looking the same. A steel brace for the chimney-stump was discarded, as it would have needed to be excessively large.

Roselle House. A ceiling rose clings on to its lath, awaiting the re-finishing of the room.
Roselle House. A ceiling rose clings on to its lath, awaiting the re-finishing of the room.
Melanesian Mission. The roof sarkings, seen here from above, have been exposed by the removal of the shingles. The sarkings are being nailed off as a diaphragm, stiffening the structure of the Mission. Note the bolted connections between sarkings and purlins.
Melanesian Mission. The roof sarkings, seen here from above, have been exposed by the removal of the shingles. The sarkings are being nailed off as a diaphragm, stiffening the structure of the Mission. Note the bolted connections between sarkings and purlins.

I very much enjoyed the visits, and I’m sure that other site visitors felt the same. We’re extremely grateful to Jeremy Salmond and Peter Reed of Salmond Reed, and to Andrew Clarke and Dave Olsen of Mitchell Vranjes, as well as to the contractors and project managers who allowed us to come on site.

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