Ductile moment connections used in steel column-tree moment-resisting frames

This paper presents analytical and experimental studies on the cyclic behavior of beam-to-column moment connections used in steel column-tree moment-resisting frames. The column-tree system is joining the column-trees and link beams in the field while the column-trees are fabricated in the shop by welding stub beams to the column. The proposed ductile column-tree connections have two distinctively improved connection details which are no weld access hole detail and a widened flange of the stub beam. Nonlinear finite element analysis demonstrated the effectiveness of the improved connection details which significantly reduce the stress concentration and plastic strain demands at the beam flange groove weld. Cyclic testing of three full-scale specimens was conducted to verify the proposed connection details. All the specimens successfully developed ductile behavior with no brittle fracture by forming the plastic hinging of the beam away from the beam-column interface. The widened flange and no weld access hole details are effective in reducing the potential of brittle fracture.     

The seismic behaviour of steel moment-resisting frames with stiffening braces

The present paper deals with the seismic behaviour of steel structures which are designed in the attempt of exploiting the dual characteristics of moment resisting frames (MRFs) and concentrically braced frames (CBFs) as lateral force resisting systems. Three prototype frames are studied within the context of Eurocode 8 (EC8) provisions; these are MRFs which are traditionally designed, against ultimate seismic actions (ULS), without checking serviceability limit state rules (SLS-interstory drift limits). To fulfill these requirements concentrical braces are inserted in the frames, which are not considered in the collapse resistance of the structural system. The non-linear behaviour of these systems under ultimate seismic environment is studied and compared to the one of the unbraced MRFs. The influence of second order effects is also investigated.     

Multi-mode pushover procedure for deformation demand estimates of steel moment-resisting frames

The main objective of the paper is the development and evaluation of a multi-mode pushover procedure for the approximate analysis of the seismic response of steel moment-resisting frames. A generalized force vector derived from modal combination simulates the instantaneous force distribution acting on the structure when the interstorey drift reaches its maximum value during dynamic response to a seismic excitation. Considering the interstorey drift for each floor, a set of generalized force vectors (each associated to maximum drift at one story) is applied separately to the structure until the corresponding target interstorey drift is attained. The maximum value of each response parameter is obtained from the envelope of results. This multi-run and multi-mode pushover procedure allows a simple implementation, reducing the computational effort compared with adaptive nonlinear static procedures and with nonlinear response history analysis. Furthermore, it does not suffer from the statistical combination of inelastic modal responses calculated separately. Both effectiveness and accuracy are verified through a comparative study involving regular steel moment resisting frames subjected to various acceleration records. The results are finally compared with those obtained from other nonlinear static procedures and with the “exact” values from nonlinear response history analysis. It is demonstrated that the proposed procedure is able to accurately predict the seismic demands of steel moment-resisting frames. In low- and middle-rise frames, the error of interstorey drift ratios of the proposed procedure is in the range 5.8-20.8% when the intensity level of the input ground motion varies in the range 0.2-0.8 g. In high-rise frames the error of interstorey drift ratios is in the range 6.38-20.9%.

     

Ductile moment-resisting frames using cold-formed steel sections: An analytical investigation

This paper presents an investigation on the potential use of cold-formed steel sections (CFS sections) in moment-resisting frames (MRFs) for seismic applications. The main limitation of CFS sections is the low out-of-plane stiffness of their thin-walled elements which leads to low ductility. The main components of MRFs are beams, beam-column connections and columns. In earthquake resistant MRFs, the beams are designed to provide considerable ductility, whereas the other elements are mainly limited to their elastic range. The performance of a new shape of CFS beam with curved flange is examined analytically and compared with that of conventional shapes. The proposed beam-column connections include through plates which potentially limit the out-of-plane action of the forces transferred through the connections. The behaviour of both individual CFS beam sections and CFS beam-column connections is studied by means of finite element analysis (FEA). The results of the analyses show that the new beam cross sections and connections exhibit a good ductile behaviour, something which cannot be achieved by conventional cold-formed frame elements.     

Dynamic increase factor for pushdown analysis of seismically designed steel moment-resisting frames

The independent threat scenario of sudden column loss under localised damage is usually considered in progressive collapse assessment. The effect of the sudden removal of a column is like the sudden application of the gravity load on the structure when significant deformations occur. This conventional approach is based on the simplifying but realistic hypothesis that the peak dynamic response can be assessed with reasonable accuracy using the nonlinear static response. In this approach, amplified gravity loads are applied to the bays that are affected by the removed column to compensate for the dynamic effects corresponding to the real load redistribution. The paper investigates the dynamic increase factor to be considered in the nonlinear pushdown analysis of seismically designed steel moment-resisting frames. The influence of the fundamental parameters involved in progressive collapse analysis was highlighted. The effect of various design variables, such as the number of stories, the number of bays, the location of the removed column and the level of seismic design load was investigated. The dynamic increase factor was estimated in a way to generate the best match of the peak dynamic responses through the nonlinear static analysis. Finally, the values obtained were expressed as a function of the vertical displacement at the location of the removed column and then compared with the GSA formulation based on the ductility factor.     

Response modification factor for steel moment-resisting frames by different pushover analysis methods

The earthquake loads imposed to the structures are generally much more than what they are designed for. This reduction of design loads by seismic codes is through the application of response modification factor (R-factor). During moderate to severe earthquakes, structures usually behave inelastically, and therefore inelastic analysis is required for design. Inelastic dynamic analysis is time consuming and interpretation of its results demands high level of expertise. Pushover analysis, recently commonly used, is however, a simple way of estimating inelastic response of structures. Despite its capabilities, conventional pushover analysis (CPA) does not account for higher mode effects and member stiffness changes. Adaptive pushover analysis (APA) method however, overcomes these drawbacks. This research deals with derivation and comparison of some seismic demand parameters such as ductility based reduction factor, R_μ, overstrength factor, Ω, and in particular, response modification factor, R, from capacity curves obtained from different methods of APA and CPA. Three steel moment-resisting frames of 3, 9 and 20 stories adopted from SAC steel project are analyzed. In pushover analyses for each frame, eight different constant as well as adaptive lateral load patterns are used. Among the main conclusions drawn is that the maximum relative difference for response modification factors was about 16% obtained by the methods of conventional and adaptive pushover analyses.     

Target roof displacement of degrading moment-resisting frames

In application of the nonlinear static procedure or static pushover analysis for estimating seismic demands of building components due to an earthquake ground motion, the peak (target) roof displacement needs to be estimated so as to quantify the global seismic demand. Most of the methods for this estimation are based on using the response of an equivalent single-degree-of-freedom (SDF) system that does not explicitly account for degradation behaviour. Hence, these methods may not be suitable for use in reinforced-concrete (RC) buildings. This study proposes that the equivalent SDF system should include the effects of degradation and its degrading properties can be determined from cyclic pushover analyses. The accuracy of the proposed method is investigated by comparing the peak roof displacements of RC moment-resisting frame buildings estimated by the proposed degrading equivalent SDF systems to the ‘reference’ value determined by nonlinear response history analysis of multi-degree-of-freedom (MDF) system building models. It was found that the use of the degrading equivalent SDF systems can predict the peak roof displacement more accurately than using non-degrading, or bilinear, equivalent SDF systems, especially in the case of non-ductile RC frames with significant degradation.     

Behaviour of mortar infilled steel frames under lateral load

An analytical method taking into account the axial deformations of the members of the frame and the slip at the interface has been summarised for linear elastic behaviour of a homogeneous and isotropic infill. The theoretical and experimental results have been compared.     

The effect of stiffener on behavior of reduced beam section connections in steel moment-resisting frames

Reduced Beam Section (RBS) connections are a new type of connection which have been used in steel moment-resisting frames since the 1994 Northridge earthquake. This study is primarily aimed at analyzing the effects of suitable web stiffeners on preventing the deterioration effect of the hysteresis curve for RBS connections. Results of more than 183 nonlinear finite element analyses on different IPE sections with radius cut, straight cut, and drilled-flange RBS connection showed that different web stiffeners considerably contribute to the enhancement of seismic performance of RBS connections. In this research, the effects of factors such as the geometry and the number of the stiffeners, the distance between the stiffener and column side, and the length and thickness of the stiffener on the seismic performance of RBS connections were also studied.     

Effect of structural uncertainty on seismic response of steel moment-resisting frames equipped with tuned mass dampers

Tuned mass dampers (TMDs) can be used to absorb the input energy of the applied loads, and reduce the response of building frames. However, inherited uncertainties in structural characteristics of building frames can significantly affect their response and counteract the effectiveness of vibration absorbing devices such as TMDs. In this study, by calculating cumulative damage indices for stories of the structure, failure probability of two steel moment-resisting frames equipped with TMDs has been studied in presence of uncertainty in characteristics of the structure. Cumulative inelastic deformation of structural elements in each story has been used to calculate the damage representative of that story, based on weighted average approach. Even though the cumulative response of the deterministic model of the structures is reduced by installing TMDs, the results of the numerical simulations on the probabilistic response of the sample structures indicate that for the records that cause excessive damage in the lower stories of the structures, the effect of TMDs on failure probability of the structure can be detrimental.     

Determination of the behaviour factor of steel moment-resisting (MR) frames by a damage accumulation approach

In most countries, seismic codes are changing, reflecting a new design philosophy based on multiple performance levels. A procedure that defines behaviour factors to reduce the elastic spectrum for different types of structures, considering all possible types of failure, is needed. The paper presents a method for the definition of the behaviour factor (q-factor) for multi-storey steel frames, accounting for cumulative damage in structural components, by means of linear elastic time history analysis, of the Palmgren-Miner rule and of S-N (constant stress or strain range versus number of cycles to failure) curves extrapolated in the low-cycle fatigue range. This technique has been verified by a non-linear procedure that is different from the previous one, as a non-linear time history analysis with damage control has been performed instead of a linear one. The proposed approach can be useful in performance based design, since the linear procedure allows the definition of the q-factor corresponding to different level of damage or collapse prevention.     

Evaluation of pre-Northridge steel moment-resisting frame joints

The 1994 Northridge earthquake caused widespread and unexpected damage to steel moment-resisting joints and connections. Shortly following the earthquake, the Federal Emergency Management Agency funded a series of full-scale tests of steel moment-resisting joints and connections to characterize the behavior of pre-earthquake connections and to evaluate the efficacy of a selected number of repair schemes. Twelve pre-earthquake connections were tested. Three of the twelve connections were tested by the authors to failure, and then repaired and re-tested. The response of the pre-earthquake connections was highly variable and uniformly poor. Premature fractures were observed in all twelve connections, and the types of fractures were similar to those observed in the field following the earthquake. The mean beam plastic rotation was 0.005 rad: one-sixth of the target value of 0.03 rad. The response of those moment-resisting connections that were repaired by replacing fractured weld and parent metal with toughness-rated weld filler metal was also poor. On the basis of the studies described in the paper, the rotation capacity of large-size moment-resisting connections built prior to the Northridge earthquake is smaller than the target values established following the earthquake; rehabilitation of earthquake-damaged moment-resisting connections by re-welding only will likely be ineffective; beam-column panel zones should be designed to remain elastic for the forces associated with plastic hinging in the beams; design equations for continuity plates should be revised; and design checks for flange compactness should be based on expected rather than nominal material properties. Copyright © 1998 John Wiley & Sons, Ltd.     

延性钢框架设计方法研究

介绍了1994年美国Northridge地震和1995年日本Kobe地震中钢框架结构断裂模式,指出利用钢框架结构梁柱新型延性节点进行延性设计是预防脆性断裂的主要途径;然后讨论了钢框架非线性分析的各种方法,以及钢框架延性设计的步骤和注意事项。     

Seismic performance of composite moment-resisting frames achieved with sustainable CFST members

The main objective of the research presented in this paper is to study the bending behaviour of Concrete Filled Steel Tube (CFST) columns made with Rubberized Concrete (RuC), and to assess the seismic performance of moment-resisting frames with these structural members. The paper describes an experimental campaign where a total of 36 specimens were tested, resorting to a novel testing setup, aimed at reducing both the preparation time and cost of the test specimens. Different geometrical and material parameters were considered, namely cross-section type, cross-section slenderness, aggregate replacement ratio, axial load level and lateral loading type. The members were tested under both monotonic and cyclic lateral loading, with different levels of applied axial loading. The test results show that the bending behaviour of CFST elements is highly dependent on the steel tube properties and that the type of infill does not have a significant influence on the flexural behaviour of the member. It is also found that Eurocode 4 is conservative in predicting the flexural capacity of the tested specimens. Additionally, it was found that the seismic design of composite momentresisting frames with CFST columns, according to Eurocode 8, not only leads to lighter design solutions but also to enhanced seismic performance in comparison to steel frames.     

装配式自复位摇摆钢框架的恢复力模型

传统的钢框架结构在强烈地震作用下梁端和柱脚通常会出现塑性铰,导致结构产生较大的塑性变形.为此,提出了一种装配式自复位摇摆钢框架结构,阐述了该结构的构造形式与工作机理,通过理论分析建立了该结构的恢复力模型.设计了单层和双层的传统钢框架结构模型和装配式自复位摇摆钢框架结构模型,通过有限元软件ABAQUS对结构模型的滞回性能...     

削弱型钢框架梁柱节点研究现状

指出加强型节点和削弱型节点是目前实现延性钢框架的主要措施,重点介绍了两种削弱型节点：狗骨式节点和腹板开圆孔型节点,分别描述了两种节点的结构形式、受力特点、削弱参数及抗震性能,以期指导实践。     

Design of unbraced multi-storey steel frames

Simple approximate methods for analysis of unbraced frames have played a part in encouraging the use of steel in construction. These methods have been supported by research, to verify and to extend their scope. This paper questions their continued use and further development. It is appropriate to do this at the present time when documentation and software are needed to introduce the Eurocodes. Approximate methods still have a role, in initial design. For unbraced frames early consideration needs to be given to limiting sway in service conditions. A very simple formulation is presented, to enable section sizes to be determined to satisfy this criterion.     

Buckling analysis design of steel frames

Steel design codes do not provide sufficient information for the efficient design of steel structures against out-of-plane failure, and what is provided is often overly conservative. The method of design by buckling analysis (DBA) specified in one code corrects this situation for beams, but the extension of this method to columns is only referred to, while there is no guidance on how to apply this method to the design of beam-columns and frames.Beam DBA uses the design code formulation for the member nominal design strengths in terms of the section moment capacities and the maximum moments at elastic buckling, accurate predictions of which may be determined by computer programs. Column DBA is similar, in that it uses the design code formulation for the column nominal design strengths in terms of the section compression capacities and accurate predictions of the elastic buckling.However, design codes do not provide formulations for the direct buckling design of beam-columns, but instead use the separate results of beam design and column design in interaction equations. The further extension to frames is not directly possible, because frames are not designed as a whole (except through the rarely used methods of advanced analysis), but as a series of individual members. This paper shows how the method of DBA can be used to design beam-columns and frames as well as beams and columns. Two example frames are designed and very significant economies are demonstrated when the DBA method is used.     

Inelastic storey-buckling factor of steel frames

This paper first briefly reviews the modified effective length method in the literature for design of steel columns considering the inter-column interaction with the limitations outlined. To overcome these limitations, a new storey-buckling approach for steel frames is presented, where each storey is treated as a whole at the sway buckling, and its resistance is evaluated using the presented storey-buckling factor, which can be obtained from the defined storey slenderness using the relationships illustrated by the column curves in current codes. In total, 30 frame examples, considering the effects of storey number, the span number and the configuration of axial forces in columns, are adopted to examine the proposed method. Comparisons between FE results show that the presented method has good performances in predicting the failure load of considered frames. In FE analysis, the geometric and material nonlinearities as well as two types of geometric imperfections and longitudinal residual stresses are considered. Predictions from the notional load approach are also compared with FE results for some cases, which shows that these predictions may underestimate or overestimate the failure load of certain frames.The interaction between the sway buckling of the whole storey and the non-sway buckling of the weakest column may reduce the failure load of steel frames. In this study, a reduction factor is presented to consider this mode interaction, and comparisons with FE results show that this method can reasonably assess the adverse effects of this mode interaction.     

Strength of flexibly connected steel frames

An analytical study of the strength of flexibly-connected steel frames is presented. A computer program which includes the effects of material and geometric nonlinearities of members and connections is developed, and used in a parametric study which indicates that in most cases, increased connection stiffness leads to an increase of frame strength, but in exceptional cases, it may result in a slight loss of strength.