Simplified Seismic Design for Mid-Rise Buildings with Vertical Combination of Cold-Formed Steel and Concrete Framing

Presented in this paper is a practical approach for the seismic design of mid-rise buildings with vertical combination of cold-formed steel and concrete framing. In current design practice the presence of vertical irregularities on both mass and stiffness inherited in such building structures creates a challenge for the seismic design. Currently, a two stage lateral force procedure prescribed in ASCE 7 is prescribed for evaluating the seismic load if the lateral stiffness of the lower structure of the building is considerably more rigid than the upper one. In the proposed approach the requirement associated with the two stage analysis procedure on the lateral stiffness ratio between the lower and upper structures prescribed in ASCE 7 is abandoned. The seismic design can be obtained based on the required stiffness ratio determined by the proposed approach. Two examples are presented to demonstrate the efficiency of the proposed approach. The results obtained from proposed approach are justified by the verification of the dynamic analysis. Also found in this study is that in some cases over increasing the rigidity of lower structure so that the two stage analysis procedure can be applied may lead to a design that is not only uneconomical but also unsafe

An Improved Two-Stage Seismic Analysis Procedure for Mid-Rise Buildings with Vertical Combination of Cold-Formed Steel and Concrete Framing

Presented in this paper is an improved two-stage analysis procedure for evaluating the seismic load of the mid-rise buildings with vertical combination of cold-formed steel and concrete framing. By comparing the improved procedure to the one prescribed in ASCE 7, it is found the stiffness requirement of the two-stage analysis procedure stated in ASCE 7 may be over-relaxed, which may consequently result in the underestimation of the base shear of the upper structure in certain cases. Furthermore, the lateral load at the top storey of the upper structure evaluated by ASCE 7 two-stage analysis procedure may also be considerably underestimated. Therefore, an additional amount of lateral load is proposed to be applied to the top of the upper structure. The results of the improved and the existing ASCE 7 two-stage analysis procedures are compared to those of the elastic modal response spectrum analysis, respectively. Comparing to the one prescribed in ASCE 7, the proposed improved two-stage analysis procedure yields more accurate results