The conceptual design of an innovative seismic-resistant steel framing system capable of providing stiffness and ductility to new or existing structures is presented. The bracing system consists of concentric X-braces connected in series with rectangular sacrificial shear panels. The braces are designed to remain elastic during seismic events while the shear panels are sized and configured to dissipate ample energy through plastic deformation-induced stable hysteretic behavior. Detailed three-dimensional nonlinear finite-element analyses using ABAQUS are performed to characterize and quantify the effects of the design parameters on the local response of the bracing system and to adjust the design so that potential buckling of the elements is mitigated. The finite element predicted force-displacement curves of bracing systems that achieve the desired local behavior when subjected to a specified interstory drift are in turn translated into a SAP2000 nonlinear link element. Embedment of the link element in a two-dimensional steel frame model enables the assessment of the performance of the bracing system as applied to a seven-story steel frame subjected to different intensity levels of seismic excitation. The results demonstrate that the braced ductile shear panel framing system offers promise for decreasing the lateral displacements of structures subjected to earthquakes while minimizing damage to all structural elements other than the sacrificial panels.
Fatigue demands often control design and detailing requirements of wind turbine support structures including the tubular steel tower. The in-situ structural demands placed on a community-scale (100kW) wind turbine have been monitored for approximately one year and the fatigue demands quantified using a Palmgren-Miner approach. A rain-flow counting method is used for calculating stress range cycles during turbine response. Demand-to-capacity ratios for a few common fatigue critical steel tower details are presented and can be used for estimation of remaining fatigue life. Future investigations will provide valuable data for evaluating fatigue loading based on aero-elastic analysis and for structural design requirements of wind turbine support structures.
In this paper seismically induced overturning effects in stiffened building frames are studied by examining the response of two structures: a 20‐storey ‘core wall’ reinforced concrete frame and a 10‐storey steel braced frame. The excitations utilized in the study are the 1971 Pacoima Dam S16E Record and the 1940 El Centro N‐S Record magnified by a factor of two. Non‐linear effects of the following types are considered: plastic hinging of beams and columns, yielding and/or buckling of bracing members and transient uplift of portions of the structures from the foundation. In particular, comparisions are made between response with unlimited base overturning capacity assumed and response with dead‐weight overturning resistance only. Providing dead‐weight overturning resistance only is shown to significantly reduce seismic load levels, with relatively little or no loss in drift control. Ductility demand in these stiffened frames is shown to be limited, when transient uplift is allowed, to the link beams connecting stiffened and unstiffened portions of the structures.
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