Nonlinear Static Seismic Response of a Building Equipped with Hybrid Cross-Laminated Timber Floor Diaphragms and Concentric X-Braced Steel Frames

Simplified seismic design procedures mostly recommend the adoption of rigid floor diaphragms when forming a building’s lateral force-resisting structural system. While rigid behavior is compatible with many reinforced concrete or composite steel-concrete floor systems, the intrinsic stiffness properties of wood and ductile timber connections of timber floor slabs typically make reaching a such comparable in-plane response difficult. Codes or standards in North America widely cover wood-frame construction, with provisions given for both rigid and flexible floor diaphragms designs. Instead, research is ongoing for emerging cross-laminated-timber (CLT) and hybrid CLT-based technologies, with seismic design codification still currently limited. This paper deals with a steel-CLT-based hybrid structure built by assembling braced steel frames with CLT-steel composite floors. Preliminary investigation on the performance of a 3-story building under seismic loads is presented, with particular attention to the influence of in-plane timber diaphragms flexibility on the force distribution and lateral deformation at each story. The building complies with the Italian Building Code damage limit state and ultimate limit state design requirements by considering a moderate seismic hazard scenario. Nonlinear static analyses are performed adopting a finite-element model calibrated based on experimental data. The CLT-steel composite floor in-plane deformability shows mitigated effects on the load distribution into the bracing systems compared to the ideal rigid behavior. On the other hand, the lateral deformation always rises at least 17% and 21% on average, independently of the story and load distribution along the building’s height.

Investigating the in-plane flexibility of steel-deck composite floors in steel structures

Purpose It is common practice in structural engineering to assume floor diaphragms infinitely stiff in their own plane. But, most of the code provisions lack clarity and unity in categorising floor diaphragms and discussing their behaviour based on the seismic response of the structures. Besides, although many of these code provisions have presented simple techniques and formulations for determining the level of flexibility in floor diaphragms, the implementation of these techniques on more complex floor systems such as the steel-deck composite floors is still under question. The paper aims to discuss these issues. Design/methodology/approach In this study, an equivalent concrete floor is employed as a representative of in-plane diaphragm action of steel-deck composite floor, using simple modelling techniques in SAP2000 and the results are validated by complex structural models developed in ABAQUS. Afterwards, the equivalent floor is inserted to 3, 5 and 7 storey steel structures with 2, 3 and 5 plan aspect ratios in two categories of structures with rigid diaphragms and analogous structures with flexible diaphragms and the responses are compared to each other. Findings The results show that the proposed technique is an effective method for evaluating the diaphragm action of steel-deck composite floors. Additionally, it is concluded that, the boundary values of plan aspect ratio equal to 3 and λ coefficient equal to 0.5 in steel-deck composite floors, mentioned in code provisions for categorising diaphragms, are not always conservative and need to be scrutinised. Originality/value The proposed methodology provides simple framework for assessing the effects of in-plane flexibility of steel-deck composite on seismic response of steel structures.