Seismic Design of Friction-Damped Braced Frames Based on Historical Records

This paper is concerned with the design of friction dampers designed to slip at a predetermined level and dissipate a substantial portion of the seismic energy, leaving the structure practically intact without its members having to yield or buckle. They are appropriate for use in seismic design of new buildings and in retrofitting existing structures. By choosing a practical design requirement rather than minimizing some energy criterion, a novel design procedure attains the stiffness of the individual braces and their displacements at the threshold of activation. The procedure is a two-phase process that uses in Phase 1 an equivalent single-degree-of-freedom (SDOF) system to obtain an optimal natural period of the structure by performing a full nonlinear dynamic analysis for a set of earthquake records. Phase 2 then enforces the same first mode on both the braced and unbraced frames, with the aim of ensuring simultaneous slippage. The procedure was applied to a 10-story steel frame. It yielded a rather technically attractive design of the braces since for close to mean plus standard deviation of the records, the resulting maximum roof displacements fell within the allowable design, as initially constrained to, and simultaneous slip of all braces occurred for most records. This procedure is rather simple in that the main computational effort, i.e., nonlinear analysis needed for Phase 1, is performed on an equivalent SDOF system only, whereas analysis of the multi-degree-of-freedom (MDOF) system is a linear eigenvalue analysis.