Design of plywood sheathed wood floors and roofs.

摘要

Existing design methodologies for wood joist floor systems using finite element methods and T-beam models exhibit some short comings. The T-beam model is incapable of predicting deflections of floors subjected to concentrated loads or long-span floors subjected to uniformly distributed loads. On the other hand, the finite element approach is often too complicated to be adopted in practice which is one of the reasons why the T-beam model is still being used in design practice. The design issues of plywood sheathed wood joist floors and roofs are examined using orthotropic plate theory, which provides a simpler approach than finite element analysis and more comprehensive than the simple T-beam method. A simple procedure is proposed for the design of floors subjected to concentrated loads. Using the fact that plywood sheathed floors are stiffer in the direction of the joists than in the transverse direction, the determination of maximum deflections is done using a simplified procedure. The maximum deflection can be calculated from one single formula. This procedure is valid in the scope of current available sizes of joists and plywood panels. The effects of ponding on the deflection of flat roofs is also examined. Roofs are then classified as stable and unstable with respect to ponding. A criterion is derived for design purposes, which defines roofs as stable or unstable depending on the dimensions and rigidity of the roof. Limit states design of wood structures requires not only sound structural analysis procedures to determine the structural behaviour of systems, but also reliable assessment of wood members. A unified theory that combines the effects of size and loading configuration using a total damage accumulation model to assess wood member strength is proposed.