Experiments on Steel MRF Building with Supplemental Tendon System

Abstract

An experimental and analytical study is presented that investigates the effectiveness of a supple- mental damping system consisting of elastomeric spring dampers (ESD) and fuse-bars to mitigate the seismic response of steel structures. Steel structures generally possess significant flexibility and relatively low inherent viscous damping. Under strong earthquake ground shaking, large interstory drifts can be expected which in turn cause distress to both structural and nonstructural elements. In this study, a load-balancing tendon fuse1damper (TFD) system has been adopted, where sacrificial yielding fuse-bars are used parallel to ESD devices to initially help stiffen the structure. A total of seven different configurations were tested with various bracing as well as supplemental damping systems. It was both experimentally observed and analytically shown that the proposed supplemental tendon system significantly attenuates the overall seismic response of the structure. An improved velocity-dependent computational model for the ESD devices is presented and verified. Hence, benefits of using a load balancing-draped tendon profile are analytically investigated. INTRODUCTION Because steel-moment-frame structures did not perform par- ticularly well in the recent Northridge earthquake, it is nec- essary for the profession to investigate both direct and indirect means of mitigating the effects of inherently faulty welded steel beam-column connections designed prior to 1994. Direct methods of structural improvement principally involve strengthening or structurally modifying elements to enhance the ductility of the welded beam-column connections. Much research sponsored by SAC (a joint venture) has been directed toward this end (SAC 1997). It is the premise of the research presented here that indirect methods of mitigation should also be investigated—as an alternative or supplement to direct strengthening or ductility improvements. Whereas direct meth- ods aim to enhance the ductility capacity of components, in- direct methods embrace supplemental damping and/or bracing, with the intent to minimize the ductility demand on the struc- tural system. Another important consideration in the earthquake-resistant design and retrofit of structures is near-source ground motions and their damage potential, especially on flexible buildings. In one recent study (Hall et al. 1995) it was pointed out that the displacement pulses at or near the natural period of vibration of the structure may cause severe damage to the structure, as excessive interstory drifts are to be expected. If such pulses occur as part of the first major cycle of the response, the max- imum response is generally not a function of the damping in the structure. Furthermore, it can be shown that maximum de- formation is attained at the end of the pulse. Therefore the effect of damping in reducing the maximum response will be minimal, the dissipated energy being considerably less than that of one full cycle of response. This becomes an important issue in the design of structures with energy-dissipation sys- tems that rely merely on the added damping. For pulse-type ground motions, such as those mentioned above, damper-only systems will help to attenuate the response only after the initial