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Precast concrete in new character commercial buildings:

Based on my Master's Thesis, much observation, discussion with other structural engineers, builders and quanitity surveyors...

Why precast concrete

One of the strongest objections to building new character commercial buildings is the cost of all the fine detail, especially the sculpted ornament. A little observation will reveal that omitting this detail results in buildings without character, and when the detail is to be carved in stone, each piece sculpted by hand, the cost today becomes more than can be justified in all but the grandest and most treasured of public buildings.

However, when looking at heritage commercial buildings it soon becomes evident that there is quite a bit of repetition in the detailing, and in most cases a building will have only two or three different window surrounds and one or two different door cases on its facade. Most of these are stucco over brick, and most of the stone ones are (or were) painted anyway. Painted, stucco and stone can be indistinguishable from one another, and can just as readily be mistaken for concrete. With an etched finish even exposed concrete can closely resemble sandstone, at least in colour and texture. Better quality garden ornaments have been doing this for some time.

Suddenly it becomes obvious: The architectural detail on pre-war commercial buildings could today be achieved in pre-cast concrete, combining the skill sets of the garden ornament industry, and the structural and architectural precast concrete industry. One each of the window and door frames, and lengths of the various cornice and plinth mouldings would be mocked up full-size in cheap timber and plasticine, latex and plaster moulds would then be taken of them, and the components could then be cast and trucked off to the site to be incorporated into the building.
A kitset of three parts and two mouldings
A kitset of three parts and two mouldings. The dormers are timber framed.

A baroque kitset of four parts
A kitset of four parts and two mouldings. The dormers again are timber framed.

Architectural precast concrete as part of the structural system

While it would be simple enough to use these components as stick-ons on tilt panel or steel framed buildings, it would probably make more sense to incorporate them into the primary structure of the building. A window element, complete with its architectural surround and concrete mullions, could be cast, perhaps the interior and exterior as two pieces to give a thermal break, and then be cast into concrete sandwich panels from which the building might be constructed. Alternatively, the window and door modules might be made up to the full storey height, and placed on the foundation or floor-edge. The formwork for in-situ concrete piers or walls would then be secured between them, stone or brick exterior cladding perhaps assembled first to act as permanent formwork to the exterior. The wall concrete would then be placed up to the underside of the structural floor. The floor structure could then be placed and its in-situ concrete topping slab and perimeter beams would then be placed, ready for the next storey.

Sandwich panel assembly
Sandwich panels are a well understood concept, but they have unexplored potential.

A base isolated gothic kitset
Assembling the kitset. This is a base isolated structure as explained later.

This construction lends itself well to very stiff and massive structures. Provided there is sufficient reinforcing, such a building is likely to perform well under seismic loadings. Under severe lateral loading a building of this kind will likely rock on its foundations, but only if its ground floor is stiff and strong enough. The problem here is shop fronts. An ideal shop front has as large an area of windows as possible facing the street, but this makes the front of the building very flimsy when shaken from side to side. Many old buildings had the facade above the shop front supported on a row of skinny brick or cast iron columns, and this has proved to be a point of considerable weakness. This has been a problem in many more modern buildings too, and is especially a problem in corner buildings. It is why the ANZ Chambers building on High Street collapsed in a heap of rubble.

Arched shop fronts

An effective means of making an open front wall very strong is to put diagonal braces across it, though this does tend to obstruct windows and doors. That said, a pair of braces could be made curved, with struts to the beam above to prevent buckling. This gives an arched opening instead of a triangle, with more room to move doors around and a less obstructed line of sight. The entire shop front panel (in a narrow building) or each of a series of bays (in a slightly larger structure) could be one precast concrete component, or instead it may be assembled from a series of concrete (or even brick or stone) voussoirs. If the front wall is pulled back slightly from the street, a faceted or curved bay window shop front could be installed in front of the arch, giving a display area rather larger than could be had with a sheet of glass right across the frontage. An added advantage in this is that the angled parts of the shop front face into the line of sight of passing pedestrians, making the shop display more visible to passers by. The entrance could also be recessed, so a person moving out of the traffic flow to look more closely at the display would find themselves right by the door to the shop. This is an old but effective idea, abandoned in the 1940s when it was thought to look ‘old-fashioned’. Presumably by then the reasons for bay shop windows had been forgotten.

Dormered Christchurch gothic
A braced gothic shop front, with traceried windows above the awning
 and projecting bay windows below

A baroque building with an arched base
Part of one bay of a baroque building with an arched shop front.

Plan and section of an arched shop front
A plan and section through an arched shop front

Base isolation

On firmer ground, or when a building is desired that can better protect its contents from shaking, it may be advisable to base-isolate the structure. This means that the building is supported on and array of flexible bearings which allow the building to slide back and forth in a controlled manner when the ground shakes. This means that the building will move much less violently than the ground beneath it. Usually this is installed in the subfloor structure, but this tends to be expensive since a double foundation is needed, and the base perimeter of the building must be detailed with a sliding joint all the way around. In a retail building this can get very complicated. Another option is to base-isolate at first floor level, as has recently been done in a few buildings in Japan and Italy. This does not protect the ground floor, but the detailing is simpler, cheaper and potentially more reliable, since it is less likely to become jammed by rubble or rubbish gathered at the base of the building.

First floor base isolation
Base isolation at first floor level. The columns do need to be rather stubby.

In this case the building would be supported on short, stubby columns cantilevered out from the foundations. At the top of these columns, perhaps 2.1 but probably not more than 2.7 metres above the ground would be the base-isolation bearings (probably either lead-rubber bearings or friction-pendulum sliders). Above these the stubby columns would continue up to tie into the structure above, which could be a quite heavy and stiff structure. The columns cantilevering out of the base of the building can of course be strongly tapered, visually becoming the spandrels of arches rather than columns, which is both visually satisfactory and very strong. At the same level as the base isolation bearings there must be a break through the entire structure. Everything in the ground floor above this point (including the lift rails) would be suspended from the floor above, and everything below would be fixed to the floor. The join would have brush seals, and perhaps flashing details to allow for some movement. If such a building had a canopy just above the bearing line or had the ground floor set back behind an arcade, the flashing details might be omitted or at least greatly simplified. In this type of building the base isolation system would be easy to inspect, easy to maintain, and fully on show to the building’s users and passers by. In any case base isolation needs space. A building may move by up to 300mm in a large earthquake, and it needs room to do so. To facilitate maintenance there really should be enough room to walk right around the building, making this approach less useful for smaller buildings on confined sites.

Precast concrete kitset base-isolated arcade
A precast concrete kitset. Note the base isolation bearings at the capitals.
James Carr - June 2012