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  • THE EFFECTS OF GRAVITY LOADS ON THE SEISMIC BEHAVIOR OF MOMENT FRAMES WITH REDUCED WEB BEAMS

    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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  • INCREMENTAL DYNAMIC ANALYSIS FOR ASSESSING THE SEISMIC PERFORMANCE OF MOMENT RESISTING STEEL FRAMES

    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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  • PROBABILISTIC APPROACH FOR LOCAL HIERARCHY CRITERIA OF EB-FRAMES

    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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  • SEISMIC RESPONSE OF BUCKLING RESTRAINED BRACED FRAME WITH STRONG BACK SYSTEM

    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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  • INCREMENTAL DYNAMIC ANALYSIS OF COUPLED TALL REINFORCED CONCRETE WALLS SUBJECTED TO FAR-FIELD AND NEAR-FIELD EARTHQUAKES

    Near-field (NF) earthquakes, compared to far-field (FF) ones, may result in a variety of responses of the structure. In this research, the behavior of a tall building with coupled reinforced concrete (RC) core-wall, is investigated. Firstly, the structure is designed using the methods in the conventional codes and utilizing the response spectrum analysis procedure. Then, fiber elements and nonlinear shear hinge are used for preparing the nonlinear model of RC shear wall and for coupling beams, respectively. The structure is analyzed using the nonlinear time history analysis at maximum considered earthquake level, implemented for NF and FF record sets and the results are compared. Results show that the base moment demand are identical and the average base shear demand obtained from the NF record is 1.5 times the corresponding demand obtained from the FF record. Also, the inter-story drift ratio demand envelope, curvature, roof drift and stories acceleration are compared. In addition, some important earthquake intensity indices for sets of NF and FF ground motions are studied. Besides, incremental dynamic analysis is used to calculate fragility curves. This analysis demonstrates that the difference between fragility curves from NF and FF earthquakes is significant. Also, with the increase of selected performance limit state, the general difference between the two fragility curves becomes relatively smaller.

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  • A STUDY ON MITIGATION OF INTERSTORY DRIFT CONCENTRATIONS IN REINFORCED CONCRETE FRAMES UNDER EARTHQUAKE DEMANDS

    Site observations and analytical studies show that low-rise RC frames have larger interstory drift ratios at the lower stories under earthquake loading. It is obvious that due to the localization of drifts, these structures could reach their capacity limits earlier than a system with a well-distributed drift among its stories. This phenomenon is especially vital for frames with limited drift capacity and could cause them to fail below the specified design demands. The existing stock of seismically deficient RC frames in Turkey, and for that matter all over the world, are in this group. The presented study investigates the effects of a pre-organized stiffness distribution along the building height to mitigate the interstory drift concentrations. As a case study, the readily available infill walls were used for stiffness-modifying elements to obtain the organized stiffness distribution. Initially, the numerical model was calibrated and verified using the experimental results. Afterward, three planar frames were studied. These were a bare frame and two frames with infill walls. Nonlinear dynamic time-history and pushover analyses were performed to evaluate the seismic performance of the frames. The results indicate that the organized stiffness distribution could mitigate the interstory drift concentrations and provide a better distribution of interstory drift ratios along the height of the frames.

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  • INFLUENCE OF TRANSVSERSE BEAMS ON THE ULTIMATE BEHAVIOUR OF SEISMIC RESISTANT PARTIAL STRENGTH BEAM-TO-COLUMN JOINTS

    The most of existing numerical and experimental studies on seismic resistant steel beam-to-column joints were carried out on planar assemblies, thus disregarding the presence of transverse beams that typically resist gravity loads in multistorey buildings. In this article, the influence of the transverse beams and their shear connections on the ultimate response of partial strength beam-to-column joints is investigated under cyclic actions and column loss scenarios by means of finite element simulations. The results show that the presence of the transverse beams provides minor beneficial effects on the primary moment resisting joints under seismic actions. On the contrary, the presence of the transverse beams gives significant effects in the case of column loss scenario, where large plastic deformations develop following the activation of catenary effects in the connected members.

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  • 3D FINITE ELEMENT MODELLING OF NOVEL STRENGTHENING SOLUTIONS FOR RC WALL/SLAB CONNECTIONS

    The aim of this study is to create a nonlinear FE model able to predict the mechanical behavior of RC wall/slab connections strengthened by using Carbon Fiber Reinforced Polymers. The experimental mechanical behavior of 4 full scale RC wall/slab connections with a reference join and 3 strengthened by FRP are investigated from members of our researcher team. The 3D finite-element micromodel of the RC wall/slab connections is conducted utilizing the FE software ABAQUS, reproducing the actual geometry of the experiments, as well as the same boundary conditions and loading as in the experimental program. The analytical investigation is carried out through an extended comparative parametric study and is focused on the quantitative influence of certain simplified modelling assumptions and several critical modelling parameters on the response of the system. The accuracy of the 3D model was assessed against the experimental results..

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