Why Requiring Frame Capable of Resisting at Least 25% of Prescribed Seismic Forces in Dual Systems

Static behavior of resisting lateral-force of steel frame, compressive and tensile braces is reviewed, emphasis is put on the deteriorating of compressive capacity of the brace as the lateral drift increases. Two series of dual structural systems are analyzed to reveal their lateral force-drift behavior, the lateral capacity ratio of frame and braces is 1:1 and 1:3 respectively. It is found that, although the contribution of the compressive brace decreases continually, the curve of the series with 1:1 ratio does not deteriorate as the lateral drift increases, due to coming into play of the frame's capacity; while the series with 1:3 ratio has a 20% decrease in the total lateral capacity due to weaker compensating capacity of the frame. As the 20% decrease is almost the maximum amount that is acceptable by a system with good aseismic behavior, it implies that if the frame cannot resist at least 25% of the seismic force, the aseismic behavior will be short of expectations.