Use of flexible and ductile roof diaphragms in the seismic design of single-storey steel buildings

This thesis documents an investigation of the use of the roof diaphragm flexibility in the seismic design and analysis procedure of single-storey steel buildings designed otherwise in accordance with the provisions of the 2010 NBCC and the 2009 CSA S16. The design approach considers the members of the vertical bracing system as the ductile fuse elements in the seismic force resisting system (SFRS), whereas the diaphragm remains elastic. An alternative design approach was also examined in which the steel deck roof diaphragm acts as a ductile fuse element in the SFRS; at present this procedure is not permitted by the NBCC or CSA S16. The investigation was reliant on a complementary three phase test program in which nineteen large-scale roof diaphragm specimens were dynamically excited with a sequence of increasing amplitude loading protocols. The first part of the study comprised the development of a deep horizontal plane truss numerical model using the OpenSees software platform to reproduce the dynamic characteristics as well as the elastic and inelastic response of the nineteen test specimens. The predicted fundamental period of vibration, the elastic response and the inelastic hysteretic response were compared with the test results and the models were calibrated accordingly. In the second part of the study, the detailed design and non-linear time history dynamic analyses of representative medium size and large size single-storey steel buildings were carried out. The intent was to evaluate the overall behaviour of four structural systems whose design was tailored to either rely on the flexibility of the diaphragm or to allow the roof decking / connections to deform inelastically. OpenSees building models were developed by integrating a non-linear brace model with the non-linear diaphragm model. Dynamic analyses were performed on the designed buildings using the corresponding OpenSees building model and responses were evaluated under a suite of design level earthquake signals. The study illustrated that the analytically predicted fundamental period of vibration which includes the influence of the roof deck diaphragm could be used in the design of such single-storey steel buildings. This finding leads to the recommendation to revise the expression given in 2010 NBCC for the fundamental period of vibration as well as for the period limitation. Further, compared to the different structural systems, the buildings designed with EBF structural system were found most promising in terms of the relative capacity force on the steel deck diaphragm and the building response. The study also found that the diaphragms in the EBF and CBF structural systems could be designed for the force corresponding to the seismic base shear with RdRo = 2, if it controls the design. Moreover, significant shear strength degradation and concentration of inelastic demand were observed in the diaphragm at the edge of the buildings when the steel decks were parallel to the loading direction and the diaphragm was designed as a ductile fuse element. This illustrates that the value of 2.0 that was assumed for the seismic force reduction parameter Rd may not be appropriate in the design of such buildings. Similar strength degradation and concentration of inelastic demand in the diaphragm were observed in the buildings with a Type CC structural system, which shows that the diaphragm may need to be designed corresponding to the elastic seismic force.