CONVENTIONAL CONSTRUCTION STEEL BRACES WITH BEARING PLATE ENERGY DISSIPATION

The principle of capacity-based design is used extensively in the seismic design of steel structures. It relies on the inelastic ductility of the Seismic Force Resisting System (SFRS) to dissipate seismic energy. However, in Canada there exists the Conventional Construction (CC) category of lateral frame, for which the engineer is allowed to waive the capacity-based design principles and design a SFRS which is expected to behave principally elastically when subjected to design-level earthquakes. Type CC concentrically braced frame systems do not depend on tensile yielding of the braces to dissipate earthquake energy. Instead, the energy dissipation in tension is expected to mainly occur through localized yielding of connection components and friction within these joints. For structures located in moderate and high seismic regions, if one cannot prove that the brace connections will perform in such a ductile fashion during an earthquake, a 1.5 penalty is applied to the connection design forces as per the CSA S16 Steel Design Standard. Given that there does not exist a codified method to predict the ductility or probable resistance and failure mode of each connection type, practicing engineers most often incorporate this force penalty in design. Although Type CC concentrically braced frames are used extensively throughout Canada, there is limited research available to give insight on the ductility of these systems, particularly in the case of I-shaped braces with bolted end connections. To address this issue, a study was initiated which comprised the testing and numerical modelling of Type CC braces modified with bolted connections detailed to develop ductile bearing response. The objective of this research was to measure the response of these full-scale I-shape braces and their modified bolted connections under reversed cyclic seismic loading. A design approach for the test specimens was taken in which the probable resistances of the various failure modes of the brace connections were estimated. This information was used to design the flange bolt connections with welded steel bearing plate elements specially detailed to permit inelastic bolt bearing deformations to take place prior to all other failure modes. Two common connection configurations for I-shape braces were studied: a flange plate connection and a flange angle connection, both attached to the gusset plate. These connections were modified to include weld attached bearing plates on the flange plates and angles; the bearing plates rely on the plastic bearing deformations at the bolt holes to develop ductility and dissipate energy. The intent is to improve the predictability in the ductility and resistance of the connections such that the 1.5 force penalty can be eliminated in design. The paper includes a summary of the laboratory test program and the complementary finite element models, with commentary on the implemented design approach and on the ability of these Type CC braces modified with special bearing plates to exhibit ductile behaviour.