Beam–spring model for timber diaphragm and shear walls

Abstract

A roof or floor diaphragm in a wood building generally spans across multiple lateral force-resisting elements (shear walls). The diaphragm serves as a horizontal beam that distributes forces to the shear walls. For design purposes, it is common to assume the wood diaphragm is either completely flexible or completely rigid. In North American low-rise light-frame wood construction, wood diaphragms are almost always designed as flexible diaphragms and are modelled as simple beams spanning across two adjacent shear walls. Strictly speaking, neither the flexible nor the rigid assumption is accurate for modelling the behaviour of a wood diaphragm. Full-scale shake table tests of a two-storey wood-frame structure, conducted as part of the NEESWood project, have confirmed that roof and floor diaphragms are semi-rigid. This paper examines the effect of diaphragm flexibility on shear wall deflections by considering the in-plane stiffness of the diaphragm to be semi-rigid. A beam–spring analogue model is used to represent the diaphragm–shear wall system where the shear walls are modelled as springs and the diaphragm is modelled as an analogue beam that acts as a load distribution mechanism. The resulting load sharing among the shear walls is examined and possible application of the beam–spring model to seismic design of wood-frame structures is discussed.