Seismic Response Of Steel Frames Research Articles

Evaluation of the effect of local rotation and vertical displacements modes in the nonlinear seismic response of steel frames

Seismic analysis of buildings is normally based on a damping matrix derived from the Rayleigh Model (CR). However, it is accepted that the damping matrix (CS) derived from the superposition of modal damping matrices (SMDM) gives more accurate results. Another issue related to seismic analysis of structures consists in the overlooking of the contribution of the modes associated to local rotations (LR) and vertical displacements (VD). The main purposes of this paper are to illustrate the inconvenience of using the CR matrix and to evaluate the contribution of the LR and VD modes. To this aim, the nonlinear seismic responses of three steel building models idealized as complex 2D MDOF systems under de action of several seismic records are calculated. The major finding of the paper are: (a) The underestimation of axial loads and bending moments can be larger than 40% and 30%, respectively, if the matrix CR is used; (b) if only lateral displacements (LD) modes are used to generate CS, some damping should be given in LR and VD modes to avoid amplification of the response; (c) the combined contribution of the LR and VD modes to axial loads, bending moment can be, on an average basis, larger than 50 % and 17 %, respectively, while for interstory shears and drifts it can be larger than 20 %. In light of the findings of this study, it is strongly recommended to use the SMDM procedure to form the damping matrix and to consider the contributions of the LR and VD modes when calculating the seismic response. The presented study was limited to plane frames that are regular in-plan and elevation. Other aspects like irregularity and 3D models need to be considered to get more general conclusions.

Effect of Friction Dampers on Seismic Response of Steel Frames

Abstract This paper deals with effect of friction dampers on seismic response of 2D steel frames in comparison with structures stiffened by large sections and centrally braced. For the present study three structures with six storeys are subjected to a time history analysis. To study the effect of dampers in structures in comparison with structures stiffened by large sections and centrally braced, are analysed the relative level displacements, maximum displacement on the top, variation of lateral acceleration at the last level and variation of the maximum seismic base shear force depending on the lateral rigidity. The results indicate that placement of friction dampers improve the response of structures in all analyzed terms.

Effect of the Improved Pall Friction Damper on the Seismic Response of Steel Frames

Energy-absorbing dampers are used to reinforce structures which are vulnerable to earthquakes. This study evaluates the performance of Improved Pall Frictional Dampers (IPFD) which is a type of Pall Frictional Damper (PFD). For this purpose, this study compares the performances of steel frames with concentric steel bracing reinforced by IPFD and steel frames with concentric steel bracing with no damper. Frames with different stories and pans were modelled in sap2000 and exposed to accelerograms of earthquakes for non-linear time history analysis. Results of analysis were studied; parameters such as story displacement, base shear and absorbed energy were compared in steel frames with damper and without damper.

Effect of the Improved Pall Friction Damper on the Seismic Response of Steel Frames

Effect of the Improved Pall Friction Damper on the Seismic Response of Steel Frames

Comparative Response Assessment of Steel Frames With Different Bracing Systems Under Seismic Effect

The field of earthquake engineering and seismology is of a great importance to structural engineers around the world. Choosing an appropriate lateral force resisting system has a significant effect on performance of the steel structure. The paper presents a comparison of the seismic response of steel frames by using different types of bracing systems. The bracing systems are X-braced frames, V braced frames, inverted V braced frames, Knee braced frames and zipper braced frames. The steel frames are modeled and analyzed in four different height levels. Nonlinear static and dynamic analyses were performed. The frames consist of three bays and steel braces were inserted in the middle bay of each frame. The structural responses of frames are studied in terms of capacity curve, drift ratio, global damage index, base shear, storey displacements, roof displacement time history and plastification. The results showed a good improvement in the seismic resistance of frames with the incorporation of bracing. The results revealed that the bracing elements were very effective in diminishing drifts since the reduction of inter-storey drifts with respect to unbraced frames were on the average 58%. Also steel braces considerably reduced the global damage index.

Response of Steel Moment and Braced Frames Subjected to Near-Source Pulse-Like Ground Motions by Including Soil-Structure Interaction Effects

Most seismic regulations are usually associated with fixed-base structures, assuming that elimination of this phenomenon leads to conservative results and engineers are not obliged to use near-fault earthquakes. This study investigates the effect of soil–structure interaction on the inelastic response of MDOF steel structures by using well known Cone method. In order to achieve this, three dimensional multi-storey steel structures with moment and braced frame are analysed using non-linear time history method under the action of 40 near-fault records. Seismic response parameters, such as base shear, performance of structures, ductility demand and displacement demand ratios of structures subjected to different frequency-contents of near-fault records including pulse type and high-frequency components are investigated. The results elucidate that the flexibility of soil strongly affects the seismic response of steel frames. Soil–structure interaction can increase seismic demands of structures. Also, soil has approximately increasing and mitigating effects on structural responses subjected to the pulse type and high frequency components. A threshold period exists below which can highly change the ductility demand for short period structures subjected to near-fault records.

Investigating effects of various base restraints on the nonlinear inelastic static and seismic responses of steel frames

This paper deals with effects of various base restraints on the nonlinear inelastic static and seismic response of plane and space steel frames. The inelastic behavior is captured by a plastic fiber beam-column method, in which the beam-column member is monitored by integration points along the member length, and the cross-section is meshed into several sub-sections. The second-order effects are considered through the use of stability functions and the geometric stiffness matrix. The effect of shear deformation is also taken into account. The column-base restraint is simulated by using a multi-spring connection element developed by authors. The independent hardening model is employed for performing hysteretic loops of rotational springs under seismic loadings, whereas mathematical models are adopted for representing moment-rotation curves of those springs under static loadings. The accuracy and efficiency of the proposed program are compared with an experimental test, numerical examples of previous studies or SAP2000 commercial software. The results show significant differences of steel frames with various base restraints. Nonlinear semi-rigid base restraints perform strongly damping ability due to energy dissipation through moment-rotation hysteretic loops.

Innovative multi-level control with concentric pipes along brace to reduce seismic response of steel frames

The Painful experiences of past earthquakes and their related financial and physical damages have shown poor seismic performance of some structures; thus using new tools and equipment are inevitable. Application of two-level control systems is one of the cases that recently attracted the attention of researchers. The main idea of these systems is to combine two separate control devices with different strength and stiffness resulting in dual seismic behaviors due to their different energy dissipation levels. In this study, at first a multi-level pipe in pipe passive control system is presented and its cyclic behavior is evaluated with nonlinear static and dynamic analyses using finite element method by ABAQUS software. Then, hysteresis curves are studied representing a highly ductile behavior for the proposed damper. In addition, obtained hysteresis curves show that the multi-level system as expected can reliably dissipate energy in different energy levels leading to ductility ratios about 15 to 37 and equivalent viscous damping ratios about 36 to 50%. Finally for the 5, 10 and 15-story steel frames with the proposed dampers, maximum displacements decreased up to 79%, 63% and 27%, respectively, compared to bare frames.

Seismic response of steel frames considering the hysteretic behaviour of the semi-rigid supports

The influence of the steel column base plate semi-rigid behaviour on the seismic behaviour of steel frames was examined. In order to evaluate this influence, three typical frames with different types of semi-rigid column base connections were modeled. The investigation of the seismic behaviour of these frames was performed with the implementation of nonlinear dynamic analyses using a recorded ground motion of the fault normal component of the 1995 Aigion earthquake in Greece. The analyses were performed using a sequence of hysteretic curves corresponding to the axial load levels during the process evaluation, including the base plate flexibility related phenomena of joint materials’ yielding (base plate, anchor bolts and concrete). The direct comparison of the response analyses results, in terms of variation of the fundamental periods, the peak displacements, the column base bending moments and shear forces and the plastic hinges sequence, indicated that the column base flexibility strongly affects the seismic response of the frames and therefore should be always considered.

SSI in steel frames subjected to near-fault earthquakes

Modern seismic codes are usually associated with far-field earthquakes and generally neglect soil–structure interaction, assuming that omission of this phenomenon leads to conservative results. This study investigates the effect of soil–structure interaction on the inelastic response of two-dimensional steel frames subjected to near-fault earthquakes, which have been recorded near to seismic faults with reverse and strike–slip mechanisms. In order to achieve this, thirty-six planar moment resisting steel frames which have been designed for seismic and vertical loads according to European codes are examined under the action of sixty near-fault pulse-like records. Simplified soil–structure interaction is carried out using springs and dashpots to simulate the flexibility of soil at the soil–foundation interface and adopting the effective properties of soil. Seismic response parameters, such as, interstorey drift ratios, maximum floor accelerations and inelastic displacement ratios are numerically determined by dynamic inelastic analyses. After a comprehensive statistical analysis, empirical expressions for these parameters in terms of the number of storeys of structures, the type of fault mechanism as well as the consideration or not of soil–structure interaction are obtained. Critical examination of the results indicates that the flexibility of soil strongly affects the seismic response of steel frames. Furthermore, it is concluded that the structural response parameters under consideration are noticeably influenced by the type of fault mechanism.

Prediction of the Seismic Response of Steel Frames with Concentric Diagonal Bracings

Concentrically braced frames and eccentrically braced frames are efficient seismic resistant systems but they are generally prone to develop storey collapse mechanisms. As underlined in some previous papers with regard to eccen- trically braced frames, this tendency depends on the overstrength factors and damage distribution capacity factors result- ing from the use of common design procedures. In this paper a previously proposed procedure which predicts the height- wise distribution of the damage at collapse of eccentrically braced frames is applied to concentrically braced frames. To apply this procedure, proper definitions of the overstrength factors and damage distribution capacity factors are derived. The effectiveness of the proposed procedure is tested on frames with concentric diagonal bracings characterised by differ- ent storey numbers and designed by common design procedures. The target response is provided by nonlinear dynamic analyses. The seismic input is constituted by ten artificial accelerograms. The paper proves that the proposed procedure is able to predict accurately the nonlinear dynamic response of systems in which the damage is mainly restricted to a few storeys. In the other cases, some not negligible scattering between actual and expected values of damage can be found at some storeys.

Sensitivity-based study of the influence of brace over-strength distributions on the seismic response of steel frames with BRBs

Steel frames with buckling-restrained braces (BRBs) have low lateral post-elastic stiffness, especially in the case of non-moment resisting beam-to-column connections, due to the brace limited post-yield stiffness. Thus, a tendency to soft storey formation, fostered by nonhomogeneous distributions of brace over-strength, might be observed during seismic events. The objective of this paper is to study the effects of brace over-strength distributions on the expected maximum reduction of seismic performance, as measured by local and global engineering demand parameters (EDPs). This goal is pursued using response sensitivity analysis and solving a constrained extreme problem. The general framework of the proposed sensitivity-based approach is presented and its application to steel frames with BRBs is described in detail. Realistic case studies are considered, taking as reference solution the design based on the modal response spectrum method. Interstorey drifts are used as global EDPs related to the damage of nonstructural elements, while maximum plastic strains and cumulative plastic strains of the braces are considered as local EDPs to monitor the damage of braces. The brace over-strength patterns that are the most unfavourable for each EDP are identified and the relevant maximum increment of each EDP with respect to the reference design solution is determined, obtaining a relation between the maximum seismic performance reduction and the brace over-strength used in the design. In this way, the proposed approach can be used as a tool for safer and more effective seismic designs of steel frames with BRBs.

Analytical Prediction of Seismic Response of Steel Frames with Superelastic Shape Memory Alloy

Analytical Prediction of Seismic Response of Steel Frames with Superelastic Shape Memory Alloy

SEISMIC RESPONSE ANALYSIS CONSIDERING HYSTERESIS CHARACTERISTIC OF HIGH STRENGTH BOLT CONNECTION OBTAINED FROM LOADING TEST

Seismic response of the passive controlled structure using buckling-restrained braces (BRB) can be ensured when the BRB connection have sufficient strength. This paper investigates demand of connection strength considering hysteresis characteristics of high-strength bolt connections. Cyclic loading test are conducted to clarify hysteresis characteristics of connection including slip behavior under excessive bearing displacement, and the analytical model of connection is proposed from test results. By using the proposed analytical model, seismic response of steel frames with BRB dumpers are numerically analyzed and the strength of BRB bolt connections necessary to secure sufficient energy dissipation and dumping effect is suggested as connection factor.

Compatibility of the endurance time method with codified seismic analysis approaches on three‐dimensional analysis of steel frames

SUMMARYThe endurance time (ET) method is a time history‐based dynamic pushover analysis procedure that utilizes special intensifying acceleration functions for estimating the seismic performance of structures at different excitation levels. One of the potential applications of the ET method is in the three‐dimensional analysis of buildings considering multidirectional excitation. In this paper, a procedure for multi‐component analysis by the ET method has been developed. Several steel moment frames with heights from 9.6 to 48 m were designed according to the Iranian National Building Code. By applying the proposed algorithm for three‐dimensional analysis of structures by the ET method, these frames were analyzed, and results have been compared with static, response spectrum and response history methods considering typical code recommendations. Results show that for all studied models, even when irregularity and height of structure increase, the ET method estimates the seismic response of steel frames with reasonable accuracy compared with other investigated approaches. Copyright © 2010 John Wiley & Sons, Ltd.

Seismic Response of Steel Frames Including Brittle Fractures at Hysteretic Dampers

Seismic Response of Steel Frames Including Brittle Fractures at Hysteretic Dampers

Seismic response of steel frames under repeated earthquake ground motions

Seismic sequences characterized by the repetition of medium–strong earthquake ground motions occurring at short time one after the other were observed in many parts of the world. The paper investigates the effects produced by these events on steel structures. Single-degree-of-freedom (SDOF) systems with elastic–plastic laws, moment resistant frames with rigid and semi-rigid joints, and a concentrically braced steel frame have been analysed under seismic sequences. Both the SDOF systems and steel frames are characterized by a significant damage accumulation with respect to only one event. According to the approach where, for damage control limit state verifications, the seismic analysis of a non-linear system is performed through an elastic analysis by using the q-factor, a reduction in the q-factor is hence proposed. This reduction in the behaviour factor should be considered in earthquake-prone regions, where the repetition of seismic events may have a high probability of occurrence.

Effects of column axial force – bending moment interaction on inelastic seismic response of steel frames

AbstractIt is well known that axial force – bending moment interaction (N–M interaction) affects to a large extent the cyclic inelastic behaviour of structural elements, especially columns in framed structures, with reduction in bending capacity and loss of available ductility. A few studies have also shown that significant inelastic axial shortening affects the response of column elements subjected to medium–high levels of axial loads and cyclic bending.This paper is primarily aimed at evaluating the effects of column N–M interaction on the inelastic seismic response of steel frames. By considering the contemporaneous action of vertical loads, due to gravity, and of horizontal seismic excitation, it is shown that the progressive axial shortening of adjacent columns may differ substantially, thus inducing significant relative settlements at the ends of the connecting beams and, then, remarkable amplifications in beam plastic rotations. An evaluation of additional beam plastic rotations induced by column N–M interaction is carried out for real structures by investigating the inelastic response of steel frames designed according to European standards under horizontal and vertical earthquake excitations. Copyright © 2003 John Wiley & Sons, Ltd.

Modal analysis and seismic response of steel frames with connection dampers

To reduce dynamic response of a steel frame of bolted connections, energy dissipation materials may be placed at a connection between the end plate and column flange or between the angle and member flange. By idealizing bolted connections and energy dissipation materials as rotational spring and damper respectively, this paper derives the mass matrix, stiffness matrix and damping matrix for the frame using a combination of the finite element method and the direct stiffness method. The complex modal analysis is then carried out to determine dynamic characteristics of the frame and to investigate the effects of connection stiffness and rotational damper on natural frequency and modal-damping ratio. By further introducing a generalized pseudo-excitation method, the vibration analysis of the frame subject to earthquake excitation is performed in the frequency domain to see how the connection stiffness and rotational damper affect the seismic performance of the frame. The parametric studies on the example frame with and without the connection dampers show that there is an optimal damper damping coefficient by which the modal-damping ratio of the frame can be considerably increased and the seismic responses, including both lateral displacements and internal forces, can be significantly reduced.

Modelling of lightweight sandwich shear diaphragms for dynamic analyses

A procedure interpreting the behaviour of screwed lightweight sandwich shear walls is presented in this paper. The study is part of a general research project aimed at the development of a suitable methodology of analysis, which should allow the effect of cladding panels on the structural response of steel frames to be properly accounted for. Firstly, the monotonic behaviour of panels in shear is predicted by means of an adequate finite element model, whose reliability is checked on the basis of available experimental results. Then, the cyclic response of the system is established through a purposely-developed analytical hysteretic model. Appropriate strength degradation functions are introduced, aimed at simulating the cyclic response of the system as experimentally stated. On the whole, the proposed methodology is profitable for predicting the seismic response of steel frames, accounting for the contributing effect due to infill sandwich panels.