Evaluating Shear links for Use in Seismic Structural Fuses
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Advances in structural systems that resist extreme loading such as earthquake forces are important in their ability to reduce damages, improve performance, increase resilience, and improve the reliability of structures. Buckling resistant shear panels can be used to form new structural systems, which have been shown in preliminary analysis to have improved hysteretic behavior including increased stiffness and energy dissipating ability. Both of these characteristics lead to reduced drifts during earthquakes, which in turn leads to a reduction of drift related structural and nonstructural damage. Shear links are being used for seismic energy dissipation in some structures. A promising type of fuse implemented in structures for seismic energy dissipation, and seismic load resistance consists of a steel plate with cutouts leaving various shaped shear links. During a severe earthquake, inelastic deformation and damage would be concentrated in the shear links that are part of replaceable structural fuses, while the other elements of the building remain in the elastic state. In this study, by identifying the issues associated with general fuses previously used in structures, the behavior of the links is investigated and procedures to improve the behavior of the links are explained. In this study, a promising type of hysteretic damper used for seismic energy dissipation of a steel plate with cutouts leaving butterfly-shaped links subjected to shear deformations. These links have been proposed more recently to better align bending capacity with the shape of the moment diagram by using a linearly varying width between larger ends and a smaller middle section. Butterfly-shaped links have been shown in previous tests to be capable of substantial ductility and energy dissipation, but can also be prone to lateral torsional buckling. The mathematical investigations are conducted to predict, explain and analyze the butterfly-shaped shear links behavior for use in seismic structural fuses. The ductile and brittle limit states identified based on the previous studies, are mathematically explained and prediction equations are proposed accordingly. Design methodologies are subsequently conceptualized for structural shear links to address shear yielding, flexural yielding and buckling limit states for a typical link subjected to shear loading to promote ductile deformation modes. The buckling resistant design of the links is described with the aid of differential equations governing the links� buckling behavior. The differential equations solution procedures are developed for a useful range of link geometries and the statistical analysis is conducted to propose an equation for critical buckling moment. Computational studies on the fuses are conducted with finite element analysis software. The computational modeling methodology is initially verified with laboratory tests. Two parametric computational studies were completed on butterfly-shaped links to study the effect of varying geometries on the shear yielding and flexural yielding limit states as well as the buckling behavior of the different butterfly-shaped link geometries. It is shown that the proposed critical moment for brittle limit state has 98% accuracy, while the prediction equations for ductile limit states have more than 97% accuracy as well. Strategies for controlling lateral torsional buckling in butterfly links are recommended and are validated through comparison with finite element models. The backbone behavior of the seismic butterfly-shaped link is formulized and compared with computational models. In the second parametric study, the geometrical properties effects on a set of output parameters are investigated for a 112 computational models considering initial imperfection, and it is indicated that the narrower mid-width would reach to their limit states in lower displacement as compared to wider mid-width ones. The work culminates in a system-level validation of the proposed structural fuses with the design and analysis of shear link structural fuses for application in three buildings with different seismic force resisting systems. Six options for shear link geometry are designed for each building application using the design methodologies and predictive equations developed in this work and as guided by the results of the parametric studies. Subsequently, the results obtained for each group is compared to the conventional systems. The effect of implementation of the seismic links in multi-story structures is investigated by analyzing two prototype structures, with butterfly-shaped links and simple conventional beam. The results of the nonlinear response history analysis are summarized for 44 ground motions under Maximum Considered Event (MCE) and Design Basic Earthquake (DBE) ground motion hazard levels. It is shown that implementation of the butterfly-shaped links will lead to higher dissipated energy compared to conventional Eccentrically Braced Frame (EBF) systems. It is concluded that implementation of the seismic shear links significantly improves the energy dissipation capability of the systems compared to conventional systems, while the stiffness and strength are close in these two systems.
General Audience Abstract
Structural fuses are replaceable elements of a structure that are designed to yield and protect the surrounding members from damages, and then be accessible and replaceable after a major event. Several studies have indicated that steel plates with cutouts would have advantages for use in structural fuses. Having cutouts in a steel plate would make different shapes inside of the plate, which are called structural links. To have the same yielding condition all over the links, it is tried to better align the capacity of the links with the shape of the demand diagram caused by loading, which would be leading to the efficient implementation of the steel. In general, links are implemented to substantially increase the energy dissipation capacity of a structure and significantly reduce the energy dissipation demand on the framing members of a structure. For these purposes, various shapes have been proposed in this research study. The main feature of a replaceable link system is that the inelasticity is concentrated at the steel link while the beams and columns remain almost elastic. This study investigated the general behavior of the fuses, the applicability of them for space-constrained applications, the flexure, shear and buckling limit states affecting the behavior of the links. The computational analysis methodologies to model the links are explained and confirmed with the behavior of the different experiment tests as well as the proposed brittle limit state prediction equations. Subsequently, the two parametric studies are done to investigate the effect of geometrical properties on the links output results and establish prediction equations. The results from the analytical and computational studies for the seismic links are incorporated for seismic investigation of multi-story buildings. The results of seismic analysis of the two buildings are summarized for 44 ground motions.
- Doctoral Dissertations