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Mahrenholtz C.,National Taiwan University | Lin P.-C.,National Center for Research in Earthquake Engineering | Wu A.-C.,National Center for Research in Earthquake Engineering | Tsai K.-C.,National Taiwan University | And 3 more authors.
Earthquake Engineering and Structural Dynamics

Damage to buildings observed in recent earthquakes suggests that many old reinforced concrete structures may be vulnerable to the effects of severe earthquakes. One suitable seismic retrofit solution is the installation of steel braces to increase the strength and ductility of a building. Steel bracings have some compelling advantages such as their comparatively low weight, their suitability for prefabrication, and the possibility of openings for utilities, access, and light. The braces are typically connected to steel frames that are fixed to the concrete structure using post-installed concrete anchors along the perimeter. However, these framed steel braces are not without some disadvantages such as heavier steel usage and greater difficulties during the installation. Therefore, braces without steel frames appear to be an attractive alternative. In this study, braces were connected to gussets furnished with anchor brackets, which were fixed by means of a few post-installed concrete anchors. The clear structural system and the increased utilization of the anchors allowed the anchorage to be designed precisely and economically. The use of buckling-restrained braces (BRBs) provides additional benefits in comparison with conventional braces. BRBs improve the energy dissipation efficiency and allow the limitation of the brace force to be taken up by the highly stressed anchorage. Cyclic loading tests were conducted to investigate the seismic performance of BRBs connected with post-installed anchors used to retrofit reinforced concrete frames. The tests showed that the proposed design method is feasible and increases strength as well as ductility to an adequate seismic performance level. © 2014 John Wiley & Sons, Ltd. Source

Roeder C.W.,University of Washington | Lehman D.E.,University of Washington | Clark K.,University of Washington | Powell J.,University of Washington | And 4 more authors.
Earthquake Engineering and Structural Dynamics

Braced frames are one of the most economical and efficient seismic resisting systems yet few full-scale tests exist. A recent research project, funded by the National Science Foundation (NSF), seeks to fill this gap by developing high-resolution data of improved seismic resisting braced frame systems. As part of this study, three full-scale, two-story concentrically braced frames in the multi-story X-braced configuration were tested. The experiments examined all levels of system performance, up to and including fracture of multiple braces in the frame. Although the past research suggests very limited ductility of SCBFs with HSS rectangular tubes for braces recent one-story tests with improved gusset plate designs suggest otherwise. The frame designs used AISC SCBF standards and two of these frames designs also employed new concepts developed for gusset plate connection design. Two specimens employed HSS rectangular tubes for bracing, and the third specimen had wide flange braces. Two specimens had rectangular gusset plates and the third had tapered gusset plates. The HSS tubes achieved multiple cycles at maximum story drift ratios greater than 2% before brace fracture with the improved connection design methods. Frames with wide flange braces achieved multiple cycles at maximum story drift greater than 2.5% before brace fracture. Inelastic deformation was distributed between the two stories with the multi-story X-brace configuration and top story loading. © 2010 John Wiley & Sons, Ltd. Source

Lumpkin E.J.,Thornton Tomasetti | Hsiao P.-C.,University of Washington | Roeder C.W.,University of Washington | Lehman D.E.,University of Washington | And 4 more authors.
Journal of Constructional Steel Research

Special concentrically braced frames (SCBF) are stiff, strong and economical lateral-load resisting systems, which can sustain large inelastic deformation if properly detailed. Historically, experimental research on the seismic response of braced frame research has focused on the cyclic and monotonic responses of isolated components, such as braces or gusset plate connections. However, these components do not work in isolation, and recent research shows that accurate evaluation of their seismic performance requires consideration of the complete system. A collaborative research program with investigators from National Center for Research on Earthquake Engineering (NCREE) in Taiwan, and the Universities of Washington (UW), California, and Minnesota was undertaken to investigate the full system response of SCBFs. The research results presented herein focus on two three-story SCBFs that were tested at the NCREE laboratory. The specimens evaluated a new design approach for midspan gusset plate connections. The two specimens had HSS or wide-flange braces in combination with framing members and connections typical of those used in a three-story building in regions of high seismicity. Composite, concrete slabs were placed on each story. The tests were designed using a recently proposed design method to balance the desired yield mechanisms and form yield hierarchy. The results demonstrate that multi-story SCBFs exhibit good inelastic seismic performance with proper design detailing. Together with prior test results, the test specimens advanced design recommendations for SCBFs, which result in thinner, more compact corner gusset plate connections, a rational method of dimensioning mid-span gusset plates, and a balanced-design procedure for enhanced ductility. © 2012 Elsevier Ltd. All rights reserved. Source

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