Ductility Estimation for a Novel Timber–Steel Hybrid System

In current force-based seismic design procedures, structures are allowed to behave inelastically during significant seismic events. For this reason, the applied design base shear of a structure is calculated by reducing the elastic strength demand by a ductility factor that represents the ability of a structure or structural system to deform inelastically beyond yielding. Whereas North American building codes provide ductility factors for traditional structural systems, including timber-based or steel-based systems, there are currently no design provisions available for novel hybrid structural systems. Thus research is required to define the force reduction factors, and specifically the ductility factor, $R_d$, to safely and efficiently design novel systems within the existing seismic design provisions. One such novel system is the so-called finding the forest through the trees (FFTT), proposed in 2012, where mass-timber panels act as shear walls and are connected to each other and to perimeter frames through steel beams. The research reported in this paper presents nonlinear dynamic analyses to evaluate the suitability of $R_d$ factors ranging from 1.5–6.0 to design Option 1 of the FFTT system, which is proposed for buildings up to 12 stories tall. Potential $R_d$ factors are evaluated by limiting interstory drift as the main performance criterion in nonlinear time history analyses to an acceptable limit of 2.5% drift with 90% probability of nonexceedance. Based on the analyses presented in this paper, a ductility factor of 5.0 is suggested for the considered layout of the novel timber–steel hybrid FFTT system.