Fire after earthquake analysis of steel moment resisting frames

Fire following earthquake can cause substantially loss of life and property, added to the destruction already caused by the earthquake, and represents an important threat in seismic regions. On the other hand, even when no fire develops immediately after an earthquake, the possibility of later fires affecting the structure must be adequately taken into account, since the earthquake induced damages make the structure more vulnerable to fire effects than the undamaged one. The paper presents the evaluation of the fire resistance time for some unprotected steel moment resisting frames, in the hypothesis that they are already damaged by the earthquake, using advanced methods for earthquake and subsequent fire analysis, and using both standard and natural fire scenarios. Moderate and severe seismic actions are used for designing the steel structures. The influence of the damage level induced by the earthquake on the fire resistance is emphasized.     

Seismic performance evaluation of moment resisting frames using high performance steel

A new high quality steel material (SN) was developed by reducing the distribution at yield point. This steel material has an advantage of having accurate yield strength compared to regular steel materials (SS and SM). Therefore, by using SN steel, the collapse mechanism can be controlled as intended for design. SS, SM, and SN steel materials were tested, and variations of yield strengths were investigated. The effect of having dispersed yield strength was investigated by conducting experiments on 8 test specimens. Column to beam yield strength ratio was changed from intended column to beam yield strength ratio because of the difference between the specified minimum yield stress and the actual yield strength of the steel materials, and it affected the collapse mechanism and the overall behavior of the specimen. It was verified through static cyclic test and static pushover analysis that the seismic performance of buildings designed using SS and SM steel could be decreased by as much as 20% compared to using SN steel. It is observed that the provision of upper bound limit on yield point in SN steel is effective in securing seismic performance of steel buildings.     

Seismic performance evaluation of steel moment resisting frames through incremental dynamic analysis

Earthquake hazards effect significant damage to structures and cause widespread failure throughout buildings. Moment resisting frames are widely used as lateral resisting systems when sufficient ductility is to be met. Generally three types of moment resisting frames are designed in practice namely Special, Intermediate and Ordinary Moment Frames, each of which has certain level of ductility. Comparative studies on the seismic performance of these three different types of structure are performed in this study. Analytical models of connections are employed including panel zone and beam to column joint model. Incremental dynamic analysis is then utilized to assess the structural dynamic behavior of the frames and to generate required data for performance based evaluations. Maximum annual probability of exceeding different limit states may reveal the superiority of a ductile structure in which a greater behavior factor is employed. Special moment resisting frames are expected to perform better once a certain level of ductility is to be met but the amount of superiority may be the subject of investigation especially from a performance based design standpoint.     

Aseismic behaviors of steel moment resisting frames with opening in beam web

Opening in beam web short away clear of beam-to-column connection is an effective method to improve the aseismic behaviors of steel moment resisting frames (MRFs). The pseudo-dynamic (PSD) test and the quasi-static test on the aseismic behaviors of full-size steel MRFs with opening in beam web are carried out. The PSD test shows that the tested frame can satisfy the design requirement and its stiffness isn’t weakened by the web opening. It can be judged from the strain distribution around the beam-to-column connection that the seismic energy is dissipated by local deformation in the weakened area of the beam due to the opening in the case of severe earthquake action, and the expected failure mode of a ductile frame (‘strong column but weak beam’ and ‘strong connection but weak component’) is reached. In the quasi-static test, the failure mode of the tested frame is in conformity with the judgement, i.e., Vierendeel mechanism is formed in weakened areas due to web opening and brittle weld fracture is avoided, which results in an improvement of the aseismic behaviors of steel MRF. Based on numerical analysis, the non-linear analysis model of steel MRF with opening in beam web is provided. Some experimental tests are numerically re-analyzed by applying the proposed model and the numerical results are in conformity with the test results, which verify the validity of the model. A 17-story steel MRF building, damaged during the Northridge earthquake and measured in detail after the earthquake, is selected as the studied case. Push-over analysis shows that the ultimate displacement of the modified building with web openings increases a lot due to the opening and the building’s ductility is improved greatly. Plastic hinge distribution in time-history analysis indicates that brittle weld fracture can be avoided in the frame including connection with opening and the maximum plastic zone moves to the weakened areas. It can be concluded that the aseismic behaviors of steel MRF are improved due to the opening in beam web.     

A modified moment resisting connection for ductile steel frames (Numerical and experimental investigation)

The seismic behavior of steel moment resisting building systems are directly affected by the local behavior of beam to column connections. In welded connections, severe stress concentrations at the connection edge coupled with imperfections of the weld roots, may contribute to significant reduction of the seismic performance. In this study, a reduced plate section connection is proposed to shift the stress concentration from the connection face. With this aim, the authors propose drilled holes at cover plates to create an intentional weak point. Applying this method, the stress concentration at the weld roots is significantly diminished and the failure mechanism is altered from premature weld fracture to ductile plate yielding around the holes. This technique eliminates unfavorable local beam failures present in some similar methods and also forces the cover plate to behave as the replaceable fuse of the connection system. As it is revealed from the finite element and experimental investigations, the strength and ductility capacity of the considered connection type is significantly improved and the behavior of the connection seems to be far less dependant of the weld root quality. This might improve the structural reliability of the connection due to the guided failure mechanism.     

Effect of inverted-V bracing on retrofitting against progressive collapse of steel moment resisting frames

The effect of inverted-V bracing on enhancing progressive collapse resistance of steel Moment Resisting Frames (MRF) were investigated in this study. A series of nonlinear static and dynamic analyses were performed to determine the resistance of four generic MRFs retrofitted by ten inverted-V bracing element. These structures were subjected to an exterior column loss and had a different number of stories and span lengths in order to study the effect of these variations on the structural response. Both force-controlled and deformation-controlled actions were implemented to determine if the column loss would lead to a failure progression. Results showed that structural configuration affects the structural resistance against failure progression and hence the appropriate brace element to retrofit it. Also, it was shown that for the studied 4-story frames, by increasing the span length by 20%, the structural resistance decreases by 42% on average. Finally, it was observed that by decreasing the span length, the Dynamic Increase Factor (DIF) suggested by the UFC, will lead to underestimating the required cross-sectional area of the brace for strengthening the unbraced structures.     

Performance evaluation of different types of steel moment resisting frames subjected to strong ground motion through incremental dynamic analysis

Comparative studies on seismic performance for various types of steel moment resisting frames subjected to near field and far field earthquakes are performed through Incremental dynamic analysis (IDA) method in this study. Near field earthquake has a pulse like effects on the structures. It imports immediate force in very short duration to buildings. Therefore, destructive effects of surge energy are not negligible. Four intensity indices are used, namely, peak acceleration (PGA), spectral acceleration at the structure??s first-mode period (S_a(T₁, 5%)), spectral acceleration at the structure??s nth effective-mode period (S_a(T_n, 5%)) and the Spectral velocity at the structure??s first-mode period (S_v(T₁, 5%)). Numerical results illustrate that the intensity measure parameters related to ground velocity and the higher mode-related parameters present better correlation with the seismic responses of near source ground motion for given systems. The higher mode-related parameters are more suitable for tall systems subjected to near field earthquakes. Moreover, the chosen parameters S_a(T_n, 5%) and S_v(T₁, 5%) of near-fault impulsive ground motions enhance the performance of intensity measure of corresponding conventional parameters, i.e. S_a(T₁, 5%). A comparison for the special and intermediate steel moment resisting frames is made as regard to performance using IDA method. A more efficient performance is observed for the special moment resisting frames compare to intermediate ones.     

Displacement-restraint bracing for seismic retrofit of steel moment frames

This paper presents a seismic retrofit method using wire rope (cable) bracing for steel moment-resisting frames. The retrofitted frame using the proposed bracing system exhibits ductile behavior and maintains seismic energy dissipation capacity to the same extent as the original bare frame. The bracing member does not act for small and medium vibration amplitudes. For large vibration amplitudes, the bracing member acts and restrains unacceptably large story drift. This retrofit method prevents an increase in the column compression force resulting from the brace action. Cyclic loading test results of the portal frames reveal fundamental characteristics of the proposed bracing system. Seismic response analyses are also conducted for the three-story moment-resisting frames. The effectiveness of the retrofit method is discussed in light of these test and analysis results.     

Cyclic response characteristics of retrofitted moment resisting connections

Experimental studies were conducted in order to investigate the behavior of the weld in steel moment resisting connections. A total of forty seven reduced-scale subassembly model specimens were tested that represent welded moment resisting connections of the beam to box column. These specimens were used to study the following aspects in fabrication and retrofitting of moment resisting connections: Complete joint penetration (CJP) groove weld behavior, transversely loaded fillet weld behavior, grinding and rewelding influence on weld behavior, reinforcing of connection with T-stiffener and rib plates, strain rate effect on weld and material characteristics and electrode toughness effect on weld behavior. Following test result interpretations, five full-scale moment resisting connections of beam to box column were fabricated and tested. These models included one specimen fabricated with details of an outdated connection, two specimens with an improved CJP groove weld detail, and two specimens retrofitted by T-stiffeners. Each specimen was subjected to a standard quasi-static cyclic load pattern. Overall, the improved and retrofitted specimens performed well, achieving total (elastic plus plastic) story drift ratios of at least 4% radians in magnitude before experiencing 20% strength degradation.     

Development of floor slab for steel post-tensioned self-centering moment frames

A floor slab constructed in conventional moment-resisting frames (MRFs) limits opening of gaps at the beam-to-column interfaces, causing losses of self-centering capability and preventing its use in steel post-tensioned (PT) moment frames. This work presents two novel slab schemes to reduce the restraints on the seismic responses of a PT frame. The first scheme uses a slab in typical MRFs with a modification so that near the connection, the metal deck is discontinuous and the longitudinal bar in the deck is debonded from concrete to minimize slab restraints. The second scheme uses truss elements to connect the slab and beams in only one bay (rigid bay) of the PT frame for transferring inertial forces from the floor to the frame. The sliding device is provided at one end of floor beams transverse to the PT frame, where sliding of the slab is expected. The cyclic behaviors of the bare connection and the connection with the first slab scheme are also evaluated by testing four connections. Additionally, how the rigid bay affects the cyclic behavior of the frame, distribution of column shear, beam axial force variation, and gap opening response near the beam-to-column interface is evaluated by conducting frame subassembly tests. The test results demonstrate that (1) the PT frame with any of two slab schemes develops large deformation capacities with small residual deformations, and (2) the column shear and beam axial force can be estimated based on the proposed method.     

钢框架结构基于位移的抗震设计方法

与传统基于力的抗震设计方法相比,基于位移的抗震设计方法更易于实现基于性能的抗震设计.在若干文献的基础上,提出了一种钢框架结构基于位移的抗震设计方法.该方法建立在钢框架结构屈服位移可由几何尺寸确定的基础上,然后根据结构的性能水平确定其目标位移(即极限位移),计算相应的延性系数,采用相应的折减弹性谱,据此计算出结构的设计基底剪力,然后对钢框架结构进行刚度设计和承载力设计.算例分析表明,钢框架结构基于位移的抗震设计方法安全可靠,便于操作,而且还能够实现钢框架结构在不同性能水平下的抗震设计.     

Assessment of progressive collapse-resisting capacity of steel moment frames

In this study the progressive collapse-resisting capacity of steel moment resisting frames was investigated using alternate path methods recommended in the GSA and DoD guidelines. The linear static and nonlinear dynamic analysis procedures were carried out for comparison. It was observed that, compared with the linear analysis results, the nonlinear dynamic analysis provided larger structural responses and the results varied more significantly depending on the variables such as applied load, location of column removal, or number of building story. However the linear procedure provided more conservative decision for progressive collapse potential of model structures. As the nonlinear dynamic analysis for progressive collapse analysis does not require modeling of complicated hysteretic behavior, it may be used as more precise and practical tool for evaluation of progressive collapse potential of building structures.     

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.     

抗弯钢框架结构自振周期经验公式研究

为完善钢结构自振周期计算经验公式,改善该类结构抗震设计。针对36座抗弯钢框架结构建筑物详细资料及其结构强震记录,采用在线多模型遗忘算子算法探测每个结构的反应记录是否为非线性时变反应时程,并利用离线的自回归系统辨识模型,识别所有钢结构损伤前后的自振周期。在此基础上,对结构完好状态时的自振周期进行回归分析,给出基于结构强震记录的抗弯钢框架结构自振周期计算公式,并将公式与钢筋混凝土框架剪力墙结构的计算公式进行对比。所得公式可以计算抗弯钢框架结构的自振周期,可供该类结构的抗震设计或相关研究参考。     

Simplified nonlinear progressive collapse analysis of welded steel moment frames

In this study, two nonlinear analysis methods are proposed that can be used for a simplified but accurate evaluation of progressive collapse potential in welded steel moment frames. To this end, the load-resisting mechanism of the column-removed double-span beams in welded steel moment frames was first investigated based on material and geometric nonlinear parametric finite element analysis. A simplified tri-linear model for the vertical resistance versus chord rotation relationship of the double-span beams was developed. The application of the developed model to energy-based nonlinear static progressive collapse analysis was then proposed. The relationship between the gravity loading and the maximum dynamic chord rotation or the concept of collapse spectrum was also established for a quick assessment of the maximum deformation demands.     

Collapse analysis of steel moment frames with various seismic connections

Seismic connections with high ductile capacity are generally considered to be effective for resisting seismic loads. However, additional studies are still needed to evaluate the performance of seismic connections during progressive collapse. In this study the progressive collapse resisting capacity of the Reduced Beam Section (RBS), Welded Cover Plated Flange (WCPF), and Welded Unreinforced Flange-Welded Web (WUF-W) connections, which are seismic connections recommended by the FEMA/SAC project, was investigated. For progressive collapse analysis, two types of steel moment frame buildings were considered; one designed for high-seismic load and the other designed for moderate-seismic load. The vertical displacement at the point of column removal and the plastic hinge rotation at beam ends were checked by using an alternative load path method proposed in the guidelines. The analysis results showed that the performance of the Cover Plate connection turned out to be the most effective in resisting progressive collapse, especially in structures located in moderate-seismic regions.     

Effect of tension bracing on the collapse mechanism of steel moment frames

The nonlinear response of Tension Braced Moment Resisting Frames (TBMRF) under lateral loading with regards to their sequential failure mechanisms and ductility is studied. Both distributed and lumped plasticity finite element approaches are considered and parametric studies are carried out to evaluate the sequence of element failures in various single and multi-storey TBMRFs. Three possible failure mechanisms representing different specific characteristics of TBMRF systems are presented and their differences are outlined. Also, several multi-storey dual-system TBMRFs are analyzed and the results are utilized to set up an analytical model which can appropriately differentiate and evaluate the load carrying contribution of brace and frame elements. Finally, an empirical formula and design curves to calculate frame-brace contribution ratios in different performance levels are proposed.     

Design approach of cold-formed steel portal frames

A structural arrangement of a portal frame that is easy to fabricate and assemble is developed. The rafter and column members are formed from single cold-formed channel sections, which are bolted, back-to-back at the eaves and apex joints, and connected to the foundation through angle irons. Variables in the frames include the span, width of the channel flanges and the strength of the channels. The design of the frame members follows the traditional approach of applying the effective width method to control local buckling, followed by an assessment of cross-sectional and buckling requirements. However, the yield and overall buckling requirements are modified by a factor of 0.8 to account for stress concentrations, shear lag and bearing deformations in the connections. All deflections of the frames considered are found to be within the limits provided in the literature and by the two codes of practice. It is proposed that the frames of the structure should be provided in a kit form and erected in “meccano” fashion     

钢框架结构鲁棒性评估方法

结构鲁棒性体现了结构对局部破坏的不敏感性。为了定量评估钢框架结构的抗连续倒塌能力,提出一种适用于钢框架结构的鲁棒性定量评估方法。在综合考虑构件在结构能量流分布中的贡献和构件失效影响面积的基础上,给出构件重要性系数计算方法,为确定结构的最不利初始损伤模型奠定基础。探讨钢结构构件的承载能力冗余度,并结合构件应变能灵敏度,提出一个基于损伤模型的钢框架结构冗余度评估指标,该指标与构件的承载能力冗余度成正比,而与构件的应变能灵敏度成反比。从定义上对结构冗余度与结构鲁棒性之间的差异进行讨论,提出以结构最小冗余度作为评价结构鲁棒性能的观点。在此基础上,给出一种适用于钢框架结构的鲁棒性评估方法,同时对该方法的适用范围进行分析。最后,通过一个8层空间钢框架结构算例验证所提出的钢框架结构鲁棒性评估方法的可行性。