Month: December 2020

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There are two point of views regarding the presence of beam web openings. The first attitude is treating the opening as non-intentional weakness imposed by non-structural necessities. The second and newly proposed attitude is to use these weakening parts as intentionally placed fuse elements to improve the seismic behaviour of the entire frame. In both cases, the existence of gravity loads on the beam causes an asymmetry in distribution of the internal forces of the beam when the frame is subjected to the lateral loads. This will cause, the different behaviour of the openings at two sides of the beam. This study evaluates the effects of gravity loads on the seismic behaviour of steel moment resisting frames with reduced web beams. For this purpose, two series of numerical models at beam and frame scales are developed and analysed under different levels of gravity loads. Based on the results, the unbalancing gravitational bending moment at the centre of openings causes the asymmetric yielding of the openings making one of the openings more critical.

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The seismic performance of steel frames can be efficiently evaluated via the Incremental Dynamic Analysis (IDA) approach. Generally, reference is made only to the transient interstory drift as a damage intensity measure (DM) but, as herein demonstrated, additional DMs could be suitably accounted for. The present paper is focused on the application of the IDA approach to Moment-Resisting (MR) steel frames by considering in total four DMs: the transient and the residual interstory drift, the residual load carrying capacity and the low-cycle fatigue of joints and members. The discussed numerical applications are related to four planar frames differing for beam-to-column connections and member cross-sections. Research outcomes underline how the additional information obtained from the proposed DMs contributes to provide a more accurate prediction of the effective post-earthquake performance than the one based on the sole transient interstory drift.

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The objective of this work is the study and the evaluation of the overstrength of short links in Eccentrically Braced Frames (EBFs) to be considered in the application of local hierarchy criteria. Starting from the results of experimental tests devoted to short links, linear multivariate regression is provided to evaluate the overstrength of links. The obtained mathematical relation accounts for the following geometrical and mechanical properties: the web slenderness of the link, the slenderness of the stiffening plates, the non-dimensional distance between the stiffening plates, the steel hardening, the non-dimensional length of the link and its ultimate rotation.  Moreover, to account for the uncertainty of the regression model and the material variability, the first-order reliability method (FORM) is applied to the limit state function, which is used to describe the local hierarchy criterion for short links based on the rigid-plastic analysis. So, the authors defined an overstrength factor accounting for the geometrical and mechanical properties of links and including a safety factor to account for uncertainties.

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In this research, Concentrically Braced Frames coupled with an elastic truss as a strong back system are analysed. The elastic system has the scope of reducing the permanent residual lateral displacement of steel frames subjected to strong ground motion. Responses of this system are compared with those of the buckling restrained braced frame without elastic truss. All members of the vertical truss, including the diagonal members, should remain in the elastic range during the earthquake, while other braces, that are buckling restrained braces (BRB), come into the plastic zone in severe earthquakes and provide the system with ductility and energy dissipation. Frames are initially designed according to current design codes and then the results are studied by nonlinear time history analysis subjected to the maximum considered earthquake level records and static nonlinear analysis. For the structures under study, the amount of residual inter-story drift ratio decreased on average from 1.33 percent to 0.58 percent, due to the use of an elastic truss in the BRB.

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