HOUNSLOW EAST TIMBER LAMELLA ROOF STRUCTURE
The leading edge of the roof is high above the platform and so there was no opportunity to adopt arching action as in the earlier examples of lamellae. This meant that the longer spans of the main ticket hall needed further support in the form of a central tree structure (see Figure 4).
Elsewhere, spans between wall lines were acceptable in all cases. The platform cantilever is also supported (or tied down!) using timber struts springing from the steel columns of the façade (see Figure 5).
Construction of the lamella
The pattern of lamella is shown in Figure 6 and Figure 7 each rib is two bays in length and overlapping the adjacent ribs to form a continuous grillage.
The lamella ribs were formed from Laminated Veneer Lumber (LVL) sourced from Finland, each piece of 360mm depth by 75mm width. LVL is a splendid material for this type of structure since it is supplied in large sheets (1.8m x up to 40m) from which any shape can be cut. The surface of the barrel vault was divided into nodes at every 4.3° arc and the surface thus divided into a grid with side length of 1.5m. Hence each rib is 2.5m in length. Looking along a line of ribs (see Figure 8) one can envisage how the ribs need to be progressively twisted in order to stay perpendicular to the roof deck (this requires some careful thought!).
Since each lamella is straight, the twist is achieved in two ways:
1. There is a mismatch in rib orientation across each joint of + and 4.6mm (approx. + 1.5°).
2. The top and bottom surfaces of the ribs are planed such that the decking always lies flush to the timber surface.
Cowley Structural Timberwork devised ingenious methods to achieve this fabrication to extremely tight tolerances.
The lamella ribs are all joined together by the structural decking which triangulates the system and acts as a continuous diaphragm. It is formed of tongued and grooved 450mm wide LVL sheets 27mm thick screwed to the top surface of the ribs.
The perimeter beams (see Figures 8) are complex elements curved in both plan and elevation. These were formed from thinner sheets of LVL pressure glued together in special jigs that were designed and made by Cowley Structural Timberwork.
The lamella connectionss make use of the patented Cowley Shearlock Connector in pairs to the end of each beam. These are screwed into couplers embedded in the receiving beam. This gives totally concealed jointing throughout. Connections were tested at the University of Bath, in a series of tests involving pure shear, pure bending and combined bending/shear loads in order to verify their performance and the analysis assumptions (see Figure 9 and Figure 10). Where the shear capacity of the standard connections was insufficient, an increased number of shearlock connectors/couplers were used. Three and four coupler connectionts were also tested (see Figure 11).
Two trial assemblies were carried out in the fabrication yard, to check assembly issues and to agree some details and finishes (see Figure 12).
The roof was assembled rapidly, rib-by-rib, on site. Purlin rails along grillage node lines were pre-set to the appropriate level (see Figure 3). Shearlocks were screwed in through inclined pilot holes to achieve the concealed detail.
Connection between roof and façade elements
The connection between the timber roof and the steel façade elements was generally achieved using wall plate elements as seen in Figure 13. This was the arrangement above the ticket hall curved façade, although along the platform elevation, this was not possible and brackets within the depth of the ribs were configured. Great care was taken in the configuration of strut connections. Figure 14 shows how a platform laminated oak strut (120mm diameter) interfaces with the façade steelwork externally (steelwork fabricator S. H. Structures). The strut end grain is protected by a stainless steel collar. The bar is epoxy bonded into the end of the strut and the threaded clevis attachment gives sufficient adjustment for construction assembly. Figure 15 shows a larger 225mm diameter main tree struts node. This passes through to the centre line of the rib with a simple bolted bent plate connection.
Nosing cantilever and roof build up
The nosing cantilever (1.2m long) is formed using 33mm thick LVL ribs at 450 centres, bracketed off the perimeter beam (see Figure 5). The roof build up includes waterproofing, a vapour barrier, insulation, and a further ply deck on battens. This pre-patinated copper standing seam cladding. The copper cladding is wrapped over the timber nose piece and returned to the strut support line.
Fire protection system
A fire protection system is included. This incorporates a darker stain for the decking diaphragm, and a clear stain for the ribs, which provides an interesting contrast (see Figure 4, and Figure 13).