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Structure

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aj building study

The structure to the new visitor centre consists of simple, load-bearing blockwork external walls supported on mass concrete strip foundations with a mesh-reinforced concrete ground-bearing slab.The first floor construction consists of 254 x 254 UC beams at approximately 2.3m centres acting compositely with a 120mm-thick lightweight concrete Holorib slab.

The lightweight canopy roof over the first floor restaurant and cafe area is supported off two Y-shaped stainless steel columns.The roof plate is constructed as a plywood stressed-skin panel with a carcass made from 6mm-thick intersecting plywood ribs at 400mm centres in both directions.This plywood egg-crate carcass is shaped to form the tapered profile section of the roof, having a maximum depth of 750mm at mid span reducing to 30mm at the edge.The top and bottom skins consist of 18mm and 12mm exterior-grade plywood respectively, connected to the internal core structure using 50mm softwood blocks glued and screwed into position at each intersection. This stressed-skin plywood construction has two-way spanning characteristics and is capable of cantilevering up to 4.5m from the Y-shaped columns.Steel-plate reinforcement is provided locally where the steel columns support the roof structure in order to deal with the high local shear stresses at the connection points.

A finite element analysis was carried out on the plywood roof construction to predict anticipated live load deflections and sway movements.The depth and stiffness of the roof plate result in low stresses in the principal elements, allowing relatively thin plywood to be used.

The restaurant is surrounded on three sides by glass, to form a facade without any vertical structural members. Full-height frameless glass along one long side and the return end is supported off the first floor slab construction, with lateral stability being provided by a concealed channel within the plywood roof construction at its head.Along the back wall of the restaurant, the glazing is supported at sill height from a blockwork wall.This reduced height of glazing allows it to be used structurally to provide stability to the roof for out-of-balance load condition.The 10mmthick glass panels are glued into a channel section, which in turn is fixed to both the timber roof and the supporting blockwork wall.These glass panels act as a tension or compression member to provide a continuous line of vertical support along one long edge of the roof.

Triangulation between the head of each Y-shaped column and the roof plate provides fixity in the lateral direction.Consequently, the combination of the Y-shaped column with a pin base, the plywood roof plate and the glass vertical side support creates a three-pin portal frame arrangement to provide overall lateral stability.

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