This product, illustrated in Fig. 4.3, is invariably the product of the trussed rafter manufacturer as it uses the same engineering and manufacturing technology used to produce the now common trussed rafter roof construction assemblies. Unlike the trussed purlin illustrated in Fig. 3.13, the top and bottom timber flanges for this form of engineered joist or beam lie flat rather than vertically. This of course gives an improved bearing area for both the floor decking and the ceiling and increases the bearing area of the joist itself where it is built into the wall or set on a hanger. Posi Joist by Mitek, Eco Joist by Gang Nail, and Wolfs Easi-Joist all use similar construction to that shown in Fig. 4.3. Each, of course, have their own
design of 'V' shaped metal strut connector system, where as Alpines' Twin-I Beam uses conventional punched metal rectangular plates with vertical timber struts between the flanges as illustrated in Fig. 4.4, but Alpine revert to timbers being used vertically rather than horizontally with the systems mentioned above, although the timber is generally much thicker than one would find in roof truss construction, again to give the better support for floor and ceiling.
All of the punched metal plate connected types give copious open space for services between the joists, thus avoiding the potential problems of incorrect notching and boring for services which is so often one of the problems with the use of even conventional solid soft wood floor joists. Over notching with the installation of pipes on the upper surface, and electrical installation on the lower surface, can dramatically decrease the joist's performance. Clearly with the 'I' beam and the solid laminated beams, the question of piercing for services has to be addressed and the manufacturers' literature should be carefully adhered to as to avoid weakening the floor diaphragm being constructed.
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