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WALLS AND FACADES

FOCUS ON: STRUCTURE

Enclosures require walls, so why not use them? Combining the environmental envelope and structure is, after all, intuitively efficient. Traditionally this is what we did; from the earliest cave dwellings through to brick houses and experiments in prefabrication and industrialised buildings. Then we discovered the cold bridge.

Once glazing and cladding technology became sophisticated enough to separate inside from outside by a paper-thin barrier, buildings stopped breathing and the internal load-bearing structure needed to be separated from the external weather-proofing envelope. Cladding technology continued to develop, with a trend towards lighter, glassier clip-on elevations and a tendency for buildings to be constructed as frames.

But there are many instances where the architecture demands a certain solidity, where there is intended to be an expression of mass, for example, and there are several examples where this is achieved by hanging heavy cladding panels off a heavy frame.

Confronted with similar challenges, we at Techniker have rearranged some of the old orthodoxies. Framed structures cannot and should not become extinct, but there are still many variations of load-bearing facades and wall systems to be investigated and exploited, some of which are demonstrated in the following projects.

BLUECOAT ARTS CENTRE The Bluecoat School, in Liverpool, was built in 1724 as a poor school with almshouses in a distinctive H-plan form.

Over the years, the buildings have been variously extended, damaged and repaired, but the basic massing of the site has always remained. The south-east wing was completely destroyed in World War Two and is to be replaced with a performing arts wing. The lead designer is Rotterdam-based Biq Architecten, working alongside delivery architect Austin-Smith: Lord.

The new wing is a modern interpretation of the existing brick buildings, related yet distinctly different, and is read as an extruded brick box.

Set out on a brick module, the walls are stack bonded;

the stretchers running along the flank walls terminate in header courses on the gables. There are no cut bricks in the building;

the storey heights correspond to courses and it is tempting to set the building out in numbers of bricks, but our coding system pushed tradition too hard so instead we have 0.5mm dimensions on drawings.

The brick is engineered to work for us and, as such, massive piers rise 7.5m to form the garden wall to the cloister.

There was a lack of preload from above to hold them in place so we preloaded them ourselves, with a simple device of a stainless-steel bar anchored in the foundation and a head plate. Post tensioning allows us to use the brickwork to its greatest advantage: compressive strength. Pre-stressing brickwork puts bricks into compression, increasing the flexural strength by pushing up the amount of lateral load required to put the brickwork into tension, its weakest characteristic.

Brick beams are built over the openings using bed joint reinforcement anchored into the wall on either side. This avoids the need for hidden concrete lintels, with brick glued on for decoration.

Where there is concrete, it remains exposed and forms part of the architecture.

DEVELOPER HOUSING

Set in a Georgian terrace in Belgravia, this new building is another sensitive insertion into a Classical townscape, and is a modern interpretation of terraced housing at many levels. The new 10-storey building, three storeys of which are below ground, is on the site of the former Belgravia telephone exchange, which replaced the three terraced houses which were destroyed in the war.

Firstly, the three bays are not three houses but luxurious apartments stretched across the full width of the site. The plan form is explored to give maximum flexibility and the structure is suppressed: the only internal columns are the fin walls flanking the risers.

The elevation picks up on the Portland stone facades of the terrace, both in material and modulation. Massive blocks of load-bearing reconstituted stone by Techrete are stacked on top of each other. A thermal break detail is developed to allow the slabs to be supported directly on the facade and the details are simple and robust.

In this type of development every square millimetre counts: mobilising the facade as the vertical load-bearing element and an intrinsic part of the stability of the building eliminates the need for additional structure and keeps the elements very lean. A slab works a lot harder trying to span between single points than if it is supported along its entire length.

The value-engineering exercise carried out with the contractor on the building post-tender was fascinating because none of the structure changed. On paper, we had been extravagant with our cores and flank walls and had allowed ourselves the luxury of some redundancy by using continuous reinforced concrete walls where there was the possibility of inserting a few panels of block work.

Suppressing the structure into a wall makes it as thin as you can get, and fixing simple continuous rebar is a lot quicker than fixing cages and formwork and returning later to build the walls.

The other advantage of perimeter-wall construction is that the superstructure loads are distributed evenly on the embedded retaining walls that form the three-storey basement.

SENSE OF PLACE

Another current project we are working on with David Adjaye combines the concepts discussed in the last two projects and approaches the limits of precasting technology.

As a starting point, we could have achieved the architectural intent of both the previous projects by faking it with some small compromises, although we doubt they would actually feel the same. Here, the design pushed us to mobilise the wall.

as there was nowhere to put the frame: the panels step at mid-storey height and there is no overlap. This is structure as architecture, plain and simple.

The question is asked time and time again - what is a sustainable building form?

For us, it is one with no spare parts, nothing superfluous and no redundancies.

We are working on a sports hall with specialist subcontractor KLH that is just a box; except the box is 54m long, 18m wide and 9m high and it is made from load-bearing timber panels by Massivholz GmbH KLH. Bi-axial wood panels are solid timber and they mobilise their full cross section to allow them to span long distances and carry significant loads. The inherent plane stiffness on the elements means that they act as plates and the roof plane spans horizontally between end walls to transfer wind loads, negating the need for framing or bracing: after all, the roof plate is an 18m beam spanning 54 metres.

We've used the same technology to value engineer a concrete house into a timber house. Concrete was originally selected to keep the structure thin on a very narrow site and to allow us to form the various cantilever and transfer structures without the need for complex framing.

Bi-axial timber can do exactly the same thing and the walls are less than 100mm thick, which is thinner than you can sensibly cast in concrete.

GLASS FUTURES

The ideal towards which all these experiments point is a single mediating envelope:

Reyner Banham's idea for a translucent enclosure;

a permeable, almost imperceptible, barrier.

Illustrated above is one such project for a glass penthouse, but schemes like these are becoming harder to realise with the new Part L requirements making the embedded load-bearing mullion much more difficult to use due to the cold bridge.

However, the accelerating development of structural glass boxes, including the use of coated glass, load-bearing glass panels and roof beams (all of which have become commonplace in the last decade) may well make Banham's vision a reality.

Megan Yates is a director of structural engineering company Techniker

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