Recent years have seen more architects and clients asking for tall timber buildings. In response, an ambitious timber community has been proposing challenging plans and ideas for multi-storey commercial and residential timber buildings. While engineers have been intensively looking at gravity-load-carrying elements as well as walls, frames and cores to resist lateral loads, floor diaphragms have been largely neglected.
Complex floor geometries and long span floor diaphragms create stress concentrations, high force demand and potentially large deformations. There is a lack of guidance and regulation regarding the analysis and design of timber diaphragms so structural engineers need a practical alternative to simplistic equivalent deep beam analysis or costly finite element modelling.
This paper proposes an equivalent truss method capable of solving complex geometries for both light timber framing and massive timber diaphragms. Floor panels are discretized by equivalent diagonals, having the same stiffness as the panel including its fasteners. With this method the panel unit shear forces (shear flow) and therefore fastener demand, chord forces and reaction forces can be evaluated. Because panel stiffness is accounted for, diaphragm deflection, torsional effects and transfer forces can also be assessed.
New Zealand Society for Earthquake Engineering Conference
April 10-12, 2015, Rotorua, New Zealand
This paper discusses the design of timber diaphragms, in response to the growing interest in multi-storey commercial timber structures, and the lack of guidance or regulations regarding the seismic design of timber diaphragms.
Proper performance of floor diaphragms is required to transfer all lateral loads to the vertical systems that resist them, but design for earthquake loads can be more complex than design for wind loads. This paper confirms that the seismic design of a diaphragm is intimately linked to the seismic design of the whole building. Diaphragm failure, even if restricted to a limited diaphragm portion, can compromise the behaviour of the whole building. It is therefore necessary to design and detail diaphragms for all possible load paths and to evaluate their influence on the load distribution within the rest of the structure. It is strongly recommended that timber diaphragms be designed as elastic elements, by applying dynamic amplification and overstrength factors derived from the lateral load resisting system.
This paper shows that some current design recommendations for plywood sheathing on light timber framing can be applied to massive wood diaphragms, but for more complex floor geometries an equivalent truss method is suggested. Diaphragm flexibility and displacement incompatibilities between the floor diaphragms and the lateral resisting systems also need to be accounted for.
This thesis studies the behaviour of diaphragms in multi-storey timber buildings by providing methods for the estimation of the diaphragm force demand, developing an Equivalent Truss Method for the analysis of timber diaphragms, and experimentally investigating the effects of displacement incompatibilities between the diaphragm and the lateral load resisting system and developing methods for their mitigation.
Although shortcomings in the estimation of force demand, and in the analysis and design of concrete floor diaphragms have already been partially addressed by other researchers, the behaviour of diaphragms in modern multi-storey timber buildings in general, and in low damage Pres-Lam buildings (consisting of post-tensioned timber members) in particular is still unknown.
The analysis of light timber framing and massive timber diaphragms can be successfully analysed with an Equivalent Truss Method, which is calibrated by accounting for the panel shear and fastener stiffnesses. Finally, displacement incompatibilities in frame and wall structures can be accommodated by the flexibilities of the diaphragm panels and relative connections. A design recommendations chapter summarizes all findings and allows a designer to estimate diaphragm forces, to analyse the force path in timber diaphragms and to detail the connections to allow for displacement incompatibilities in multi-storey timber buildings.