tural form which can produce a light- weight roof (at slightly greater cost than a flat roof due to fabrication cost). Where the prominence of a particular building or part of a build- ing requires a broken roofline for architectural reasons, this construction has been an answer.
Six plywood barrel vault roofs at Hull, UK. The area covered is 19,200 sq. ft.
beam action can be obtained to allow a reduction in joist size where close centre nailing is used (2in. centres along all joists) but because of the inherent slip of mechanical fastening, full T beam action is only obtained if the plywood and framing are glued. Stiffened plywood panels. This is the name given to the multiple T beam. components for use in roofs and floors formed by gluing joists to the plywood deck. The units are shallower and lighter than an independent joist with deck construction and are frequently used where fire requirements are in- volved and can be economically satisfi- ed by site fixing a plasterboard ceiling to the underside of the unit. Stressed skin panels are better structurally.
Stressed skin panels. Where plywood skins are attached to the top and bottom of longitudinal framing mem- bers an efficient spanning component is formed. Shallow and light in weight, providing preformed deck and ceiling, these components can be used either spanning between beams or as indi- vidual units. Joint and holding-down details need careful consideration because the panels are fabricated as closed units. Fire requirements some- times make the use of these com- ponents uneconomic. The longitudinal framing which resists the shear force in the panel may be replaced by a foam plastic or paper honeycomb core to achieve the same result structurally with a further reduction of weight or improved thermal properties; these are known as sandwich panels.
I beams and box beams. A widely used component and number of manufac-
facturers have their own systems. An efficient use of material produces a light beam. A development by the author provides a solid web at the place of high shear and a lattice. section in mid span giving an even lighter beam which, due to the cheap- ness and ease of fixing of the deck, has provided a roof system competitive with steel up to 80ft. span. Box beams can also be manufactured tapered or cranked. Portal frames. Cheap enough to be used for poultry buildings these same components are used on schools, li- braries and restaurants. Normally used at 10-12ft. centres, they would appear to be an economic component for 30ft. spans upwards (not competitive with steel portals for 60ft. spans and over, but may be used where the architect wants to express a bold outline of the structural frames).
Panel arches. A common structural component in N. America. These are either timber-framed ог sandwich
stressed skin curved panels, spanning as arches between structural beams. A very light roof is possible featuring multiple arches.
Triangular beams. When architectural requirements are not satisfied by the box beams profile, the same structural action can be obtained by fabricating the beams with a triangular cross section. A stable beam section is then produced which has a similarity with the folded plate structure.
Shell structures. A hyperbolic shell has been constructed using plywood in- stead of concrete or timber boarding in Scotland. A series of barrel vault shells used for a project in Hull to provide the answer to the problem of low cost space coverage on a site of bad foundation conditions are illus- trated. Plywood would seem to have more economic advantage in single curvature sandwich shells, double cur- vature multilayer shells do not need the high quality, high strength, pro- perties of plywood and may be more economic in cheaper, low quality, soft- wood boards.
Specification
The satisfactory fabrication of plywood structural components re- quires the same careful quality control as is necessary for any engineering component. BWMA (British Wood- work Manufacturers Association) Tim- ber Engineering Production Standard No. 2-Glued Assemblies other than Glulam, provides a specification pre- pared to include the accumulated ex- perience of fabricators and profes- sional engineers specialising in this work and should be used when speci- fying fabrication of components.
Where plywood is not manufactur- ed to the standard of BS Code of Practice CP 112-i.e. those described as Canadian Douglas Fir Exterior WBP, Folded plates. Another efficient struc- Finply Exterior WBP, ABPVM to
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Far East BUILDER, November 1969
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