Technical Frame
that’s when the structure attracts wind loading, so at that point all the necessary bracing needs to be in place.” Ridley-Thompson explains that unlike
Left: the Scape Technology system is designed ideally as a homogenised frame of similar beam spans. Where columns are missed, or loads increased, the penalty is thicker beams and increased cost
Left: Perimeter columns are hidden in the cladding zone. With no cross-bracing, the perimeter could potentially be completely glazed
the cross-bracing of the past (see box), in today’s Scape system, “it’s the roof that gives the skeleton true rigidity. The stressed 7.4m long D60 metal deck roof and ply layer acting as a diaphragm, stitching it all together.” Vertical bracing is evident on all four sides of the main hall, but on the single-storey classroom blocks coming off it, there’s none at all. Instead, Ridley-Thompson points out
a detail where columns meet the edge beam, where there’s a double-bolt connection that stiffens up the whole structure. “Just this detail gives enough stiffness that could free up the whole perimeter elevation for glazing,” he says. Attention to detail is therefore everything — Pinder says that “the most difficult thing for the erector is the positioning of the column bolts ‘floated’ into the wet raft concrete, you have to ensure that they don’t move to make sure they keep the structure plumb”.
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are necessary, such as in the hall, the design defaults to perforated, deeper sections; any roof penetrations that break the steel rafter centres are achieved using parallel flange channels. The thinner steelwork sections are due to the fact that these are low-rise structures. Great Denham’s main hall is a double-
height space, but Scape design director Tom Ridley-Thompson says that the system would never run above four storeys. It’s all about keeping the physical weight of the structure down and, being single-storey, here it doesn’t even require any fire-proofing. And it shows. The most obvious thing about the system is its slenderness, something Scape virgin Pinder attests to. “I came from a retail background, and when I saw the structure first I thought it looked very thin, but I’m a convert,” he says. But this slenderness is an aspect that comes with provisos. “It’s important
32 | APRIL 2012 | CONSTRUCTION MANAGER
to educate the site team to the nature of this structure and its constraints, as stability is key. Erection, for instance, has to be done starting at the right location.” In the case of Great Denham, the frame is being erected like a race track. The two- storey hall section, the structure’s most rigid element, had to be built first, with the fingers of single-storey classrooms coming off it. Every return then gives any “run” of structure greater stability. The figure-of-eight form is structurally efficient as each side is strengthened by its adjacency with another. So is this super-optimised structure actually an exercise in engineering brinkmanship? Ridley-Thompson thinks not. “In its skeletal state, the structure really is quite stable, as every column has a four-bolt base connection, giving it column rigidity,” he says. But there’s a qualification coming. “Where care needs to be taken is when the cladding goes on, as
“There’re only nine ground workers on site currently, four fabricators, four labourers and a crane driver. ”
Mark Pinder, Willmott Dixon
Structurally independent The main idea with the Scape system is that the steel is structurally independent of anything else in the build. Ridley- Thompson explains that even on their taller buildings, where there might be blockwork lift shafts, the structural design would still isolate the steel frame from other elements. “It was key that the system doesn’t rely on wet trades for its integrity, as typically coming later in the project, these would have programme implications.” The lightness of the structure also simplifies site logistics. Les Carter, Willmott Dixon local account manager, says that crane access for lifting them in only needed to be from one side with a single 80-tonner. Carter says also helping is the fact that with the structure concentrated on the outside and with shallow excavations, “the pour is only constrained by the rate the concrete comes out of the pump. Here, the raft is 125mm thick, with a 450mm downstand — there’s only mesh reinforcing too; no rebar and no chairs or bending of steel”. All these facts might contribute to the fact that the site seems oddly devoid of operatives “There’re only nine ground workers on site currently, four fabricators,
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