Simplified Strength Design Procedure for Plywood Web-Beams

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Nawari Omer Mohammed, Nawari
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Conventional plywood-web beams have long been designed by the traditional method of design, i.e. the "working stress method ", in which a linear stress strain (f-e) relationship has been assumed. This simplified method of design, despite its most universal acceptance and use, neglects many factors that affect strength and performance. With the improvement of knowledge about the structural resistance of wood, conversion has taken place .from the working stress design method to a method that relates the design process more clearly to strength and serviceability criteria. This method is called the "Limit states design method". In this study, expressions for the ultimate resisting moment of symmetrical and unsymmetrical plywood-web I beams have been derived, along with formulae for the panel shear stress and the flange-web shear stress in elastic-plastic bending. A computer program incorporating these expressions is reported and its use is demonstrated in design examples. Bending deflection at ultimate load has also been found using numerical techniques (linear Newton Rephson and finite difference). In plywood-web beams, the timber to plywood, flange-web joints may be rigid (glued) or semi-rigid (nailed). In this thesis only rigid joints are considered. In the rigid joint no slip is assumed (i.e no strain difference between the timber and plywood). The flange and web materials behave in compression as inelastic materials and in tension as elastic materials; whereas idealized elastic-plastic stress-strain (f-e) relationships are adopted to describe the behaviour of timber and plywood in compression, a linear one is adopted to describe their behaviour in tension. Bending failure is assumed to take place when the ultimate tensile strength either of timber or plywood is reached in the extreme fibers. The relationship of the various combinations of materils characteristics of rigid ply-wood-web beam is shown in Fig. (1). A computer program titled FFDPWB (Factors for Designing Plywood-Web Beam) has been developed. This program (FFDPWB) calculates the ultimate moment factor (Kum), the depths of yield factor (F' and F" for timber and plywood respectively), the panel shear stress factor (S p), and the top and bottom flange web shear stress factors (S’ fw and S" fw respectively), for the different cases given in Fig. (1). By using this program the design computations are simplified and combined into an easy-to-use format.
Simplified, Strength, Design Procedure, Plywood Web-Beams