Controllable design on failure mode for steel frames considering regional collapse-resistant mechanism

Controlling the collapse area of a structure is a direct collapse-resistant design requirement. However, existing theoretical models for collapse-resistant design establish the demand-capability relationship by considering only the lower-limit capacity of the damaged regions of a structure, and ignore the limit state of the surrounding structures. With the development of chain failure behaviors in a damaged structure, the member mechanism cannot resist accumulated unbalanced loads. Therefore, a higher level collapse-resistant mechanism should be considered in the design to ensure that the structure still has the controllable and expected failure modes. To this end, this paper proposes a collapse-resistant design concept with multilevel collapse-resistant behaviors char-acterized by a steel frame structural system with force-controlled members (FCMs). The FCM was designed based on the upper-and lower-limit internal force-controlled equations proposed in this study. Under the designed load, following the failure of a key member, unbalanced loads are redistributed throughout the remaining structure via a member mechanism. A regional mechanism is triggered (connection between FCM and sur-rounding regions is disconnected) to control the failure mode of the structure if a certain region fails. Two lines of defense at the member and regional levels are provided to prevent progressive collapse. To evaluate the efficacy of the proposed method, corresponding full-scale steel frame models were designed and tested using reliable numerical models based on the proposed method. The results indicated that the collapse-resistant behaviors of the structural system at the member and regional levels meet the design requirements.