Extension of the effective modal seismic design method to generally irregular RC wall structures

Abstract

Structural irregularity in new buildings is sometimes desired for aesthetic reasons. Often it is unavoidable due to different uses in adjacent spaces within the building. The seismic behaviour of irregular structures is harder to predict than that of regular buildings. More comprehensive analysis techniques are often required to achieve adequate accuracy. Designing irregular structures poses additional challenges as the structural characteristics are unknown. There is a lack of practical design methods that reliably produce economic and seismically robust design solutions for highly irregular RC structures. This paper presents an extension of the Effective Modal Design (EMD) method from asymmetric-plan RC wall buildings to vertically setback asymmetric-plan RC wall buildings. EMD is a generalization of the Direct Displacement-Based Design method for highly irregular ductile uncoupled RC wall structures. EMD reverse engineers a multi-degree of freedom Equivalent Linear System to produce the most economic design solution that achieves the target performance levels. The utility of EMD is verified for a wide range of setback asymmetric-plan reinforced concrete wall structures using nonlinear time history analysis of reasonably realistic 3D structural models. Advantages of EMD include explicit consideration of nonlinear, torsional and ‘higher mode’ effects. The method produces capacity-designed design actions for all reinforced concrete walls in the seismic structural system. EMD only requires three response spectrum type analyses. It does not require time history analysis or pushover analysis. EMD is a practical seismic design method for generally irregular RC wall buildings that uses analysis techniques that most engineering practitioners are familiar and confident with. It was found that for over 95% of the structures considered, EMD achieved critical mean peak responses between − 20 and + 15% of the target response values, with a median of − 5%. This significant improvement in design accuracy and reliability (compared to traditional force based design) was achieved at the relatively small additional computational effort of two Response Spectrum Analyses. This demonstrates the value that the proposed Effective Modal Design method adds to the current spectrum of seismic design methods for irregular ductile RC wall structures.