A simplified seismic design approach for mid-rise buildings with vertical combination of framing systems

• A simplified seismic design approach is developed.
• The approach accounts for the interaction between lower and upper structures.
• A factor αUαU is proposed to quantify the interaction.
• Empirical equations to evaluate αU are suggested.
• Two-stage analysis procedure prescribed in ASCE 7 may induce non-safe design.

Presented in this paper is a simplified seismic design approach for mid-rise buildings with vertical combination of framing systems. In current design practice, the presence of vertical irregularities on both mass and stiffness creates a challenge for the seismic design of such buildings. A two-stage analysis procedure prescribed in ASCE 7 suggests the interaction between the lower and upper structures in terms of mass and stiffness can be ignored if the lateral stiffness of the lower structure is considerably more rigid than that of the upper one. In the proposed approach the requirement associated with the two-stage analysis procedure is abandoned and the effects of the interaction between the lower and upper structures in terms of mass and stiffness on the seismic load are considered. The seismic design can be obtained based on the required relationship between the stiffness of the lower structure and that of the upper one determined by the proposed approach. An example is presented to demonstrate the efficiency of the proposed approach. The result obtained from the proposed approach is justified by the verification of the dynamic analysis. Also found in this study is that in certain cases the two-stage analysis procedure prescribed in ASCE 7 may lead to non-conservative design.