Glass-reinforced polyester has a high tensile strength but low modulus of elasticity. Although it can accept quite high loads, this is done only at the expense of great deformation or deflection. High deflections are undesirable as they can result in gel-coat cracking. Because of the cost of the material, thickening of the laminate overall would be too expensive. Four basic ways of stiffening panels in common use are shown in Fig. 3.l4.These are:
- shaped profiles;
- ribbed construction;
- sandwich construction with foamed core;
- sandwich construction with sheet core.
Panels can be designed for stiffness in a geometric form. Even for a nominally flat panel, a shallow profile, pyramid, dome or dish can make a considerable improvement to its stiffness. In some cases it may be possible to design the panels so that the form for stiffening also acts structurally. For example, the spandrel panels at the American Express European Headquarters building at Brighton (architects: Gollins Melvin Ward Partnership), which are 7.2 m x 1.4 m x 0.8 m Class II GRP channel sections, not only form the cladding but also act as structural elements supporting the glazing above (Fig. 3.15)
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