Design and seismic response of tall chevron panel buckling‐restrained braced steel frames

The numerical analysis of the seismic performance for tall chevron panel buckling‐restrained braced steel frames (PBRBFs) under small and strong earthquake excitations has been carried out to investigate a capacity design procedure for chevron PBRBFs and to examine the effects of axial strength distribution of braces along the height of buildings, vertical supports of braces for the braced beams and the overstrength of braces on the seismic response of PBRBFs. It revealed that the chevron braces that remained elastic can actually provide the vertical supports for the braced beams. Under severe earthquake excitations, the vertical supports deteriorated greatly after braces yielding. The PBRBFs designed by omitting vertical supports of braces for the braced beams and considering the overstrength of braces exhibited superior performance with smaller plastic deformations for braced beams and reduction in ductility demands for panel buckling‐restrained braces (PBRBs) as compared with the others. The distribution of yielding for PBRBs in 10‐story buildings verified that the participation from the higher modes is not very remarkable and that the capacity design based on the first‐mode response can be considered for multistory PBRBFs. Moreover, on the basis of the analysis results of the 30‐story PBRBF, the participation of the higher modes should be taken into account for high‐rise PBRBFs. Copyright © 2011 John Wiley & Sons, Ltd.     

偏心支撑钢框架的设计

简述了偏心支撑钢框架结构的工作原理及特点,介绍了偏心支撑钢框架的设计计算方法,其中重点介绍了各杆件的内力计算：耗能梁段设计、非耗能梁段设计、支撑设计和框架柱设计,为工程设计人员提供了指导。     

Seismic testing and numerical analysis of Y‐shaped eccentrically braced frame made of high‐strength steel

In eccentrically braced frame made of high‐strength steel (HSS‐EBF), link and brace are made from conventional steel whereas other structural members use high‐strength steel. Using HSS for beams and columns in EBF can reduce steel consumption and increase economic efficiency. In this paper, one shake table test of a 1:2 scaled three‐story Y‐shaped HSS‐EBF (Y‐HSS‐EBF) specimen was carried out to study its seismic behavior underground motions with different peak ground accelerations. The dynamic properties, base shear force, displacement, and strain responses of the specimen were obtained from this test. In addition, the finite element models of two 10‐story Y‐HSS‐EBF buildings and one 10‐story conventional Y‐EBF building were evaluated for seismic effects. Nonlinear pushover and dynamic analyses were conducted to compare their seismic performance and economy. The results indicated that the specimen exhibited sufficient lateral stiffness and safety but suffered some localized damages. During the high seismic intensity earthquakes, the links of the test specimen were in inelastic to dissipate the earthquake energy, whereas other structural members remained in the elastic state. Under the same design conditions, Y‐HSS‐EBF used less steel than that of conventional Y‐EBF, which could reduce the amount of steel used in Y‐HSS‐EBF. The Y‐HSS‐EBF is a safe, dual system with reliable seismic performance.     

高强钢组合K形偏心支撑框架抗震性能对比分析

为研究不同强度组合的高强钢组合K形偏心支撑框架结构的抗震性能,设计了一组不同强度(Q345、Q460、Q690钢材)组合的5层K形偏心支撑框架结构算例Q345-5、Q460-5和算例Q690-5,选取10条地震动记录对其进行动力时程分析,得到各算例在不同水准地震作用下的耗能梁段转角和层间位移角。研究表明：8度罕遇地震作用下,高强钢组合K形偏心支撑框架的层间位移角比传统K形偏心支撑钢框架大,各算例耗能梁段全部进入塑性变形阶段;塑性层间位移角到达规范限值时,算例Q460-5框架梁开始进入塑性变形阶段,算例Q690-5框架柱、框架梁和支撑均处于弹性变形阶段,还可以承受更大的地震作用;达到定义的极限状态时,与传统偏心支撑钢框架相比,算例Q460-5能够承受的地震作用和耗能梁段转角更小;算例Q690-5承受的地震作用和耗能梁段转角更大。     

Recent research on eccentrically braced frames

Recent research developments on eccentrically braced steel frames for seismic design are reviewed. The emphasis is placed on the design of links, which are short sections of beams between columns and braces, and similar elements at eccentric joints. The review includes some highlights of the latest experiments with one-third scale models employing different eccentric bracing schemes, an updated classification of links, and special design requirements for different types of links. Some results are given on recent cyclic tests of full-size links.     

Seismic response of stainless steel braced frames

The present paper investigates the feasibility of the application of stainless steel (SS) in the seismic design of braced frames, either concentrically (CBFs) or eccentrically (EBFs) braced. A sample of regular multi-storey CBFs and EBFs was designed in compliance with modern seismic standards based on capacity-design rules. The results of pushover and inelastic response history analyses demonstrate that systems employing SSs exhibit enhanced plastic deformations and excellent energy absorbing capacity with respect to mild steel braced frames. The augmented strain hardening of SS, which is nearly twice that of carbon steels, is beneficial to prevent local buckling in steel members, especially those subjected to high axial compression. The performed analyses also demonstrate that in CBFs with SS braces and columns the increase in overstrength is about 40% with respect to the configuration in mild steel. For EBFs, the use of SS in the diagonals or in braces and links increases the global overstrength of the lateral resisting system by 20%. When the EBFs employ braces and columns in SS the increase can be as high as 50%.     

某偏心支撑框架结构设计

偏心支撑框架在弹性阶段的刚度接近中心支撑框架,弹塑性阶段的延性和耗能能力接近延性框架,是一种新型的高层钢结构抗震体系。以咸阳丽彩.天玺广场商住楼工程为例,详细介绍偏心支撑框架的耗能梁段、非耗能梁段、支撑和柱的设计以及构造措施,有助于工程设计人员更好地了解该结构体系设计过程,可为类似工程设计提供参考。     

偏心支撑框架的设计计算方法

我国《建筑抗震设计规范》(GB5 0 0 11- 2 0 0 1)规定 ,8度以上宜采用偏心支撑 ,但我国至今没有一个自行设计的工程用过这种支撑。参考美国计算例题、相关规范 ,并依据我国规范 ,对偏心支撑框架的实际计算方法进行讨论 ,总结出适合我国规范和实际情况的偏心支撑框架设计计算方法。其中 ,重点介绍了各杆件的内力计算、消能梁段的设计、非消能梁段的设计、支撑的设计、柱的设计等 ,给工程设计提供理论基础和实用方法 ,有助于设计人员对该体系设计过程的了解和此种支撑的推广应用     

空间受力巨型钢柱滞回性能的影响因素分析

采用双重非线性有限元程序ANSYS5.7,分析了不同加载方式、轴向荷载、支撑长细比、支撑轴向刚度和横梁刚度等因素对人字形和X形支撑的巨型钢柱滞回性能的影响规律。为巨型钢框架结构中巨型钢柱的设计提供了理论依据和合理建议。     

Dual‐steel frame consisting of moment‐resisting frame and shape memory alloy braces subjected to near‐field earthquakes

In this paper, concentric braced frames are combined with moment‐resisting frame (MRF) as a dual system subjected to near‐field (NF) pulse‐like and far‐field ground motions. The braced frame in this system configuration consists of steel buckling‐restrained braces (BRB model), braces with shape memory alloy (SMA model), or combination of BRB and SMA braces (COMBINED model). Some prototype structures of the proposed systems are designed according to the code recommendations. Then, the nonlinear models of the considered structures are developed in SeismoStruct software, and nonlinear time history analysis (NLTHA) is implemented. NLTHA is performed subjected to earthquake record sets at maximum considered earthquake (MCE) and design base earthquake (DBE) levels, and responses of the systems are investigated and compared with each other. Among the examined models, the SMA and COMBINED models exceed the CP level subjected to NF‐MCE record set. Therefore, more investigation is needed for using short‐segment SMA braces in the dual‐steel frames in NF area.     

On the plastic analysis of concentrically braced frames with shear panel,obtaining predetermined collapse mechanism

Conventional design methods do not ensure that the desired collapse mechanism is developed at target displacement. In this paper, a case study is presented to analyze concentrically braced frames with steel shear panel (CBFSP). Also, extensive investigation in the failure modes are made, to have the global yielding mode at the final state. For this purpose, each of one‐story, three‐story, six‐story and nine‐story CBFSP models were decomposed into three parts where the members' closed‐form equations of internal forces were identified and superimposed. On the basis of the kinematic theorem of plastic collapse, the possible mechanisms and the related energy equations were defined to estimate the lateral load multiplier. First, the shear panels, columns, vertical and horizontal boundary elements were designed using the values of internal forces and seismic loads. Next, sections of the beams and braces were selected by constraining, where the mechanism equilibrium curve of the desired mechanism had to be placed below the others within the admissible roof displacement. Finally, for assessment of the precision of the method, results of the pushover analysis of the finite element models were compared with the theoretical ones. The findings show that, despite more effort for design, the investigated method is reliable and satisfactory. Copyright © 2014 John Wiley & Sons, Ltd.     

Effect of gravity columns on mitigation of drift concentration for braced frames

In strong earthquake events, multi-story concentrically braced steel frames (CBFs) are prone to form a story-collapse mechanism after buckling and yielding of the braces in a story. With the addition of continuous gravity columns and the increased lateral stiffness and strength they provide, however, it becomes possible to reduce the concentration of story drift and prevent the story collapse. Nonlinear dynamic analysis is implemented for a three-story CBF, and it is demonstrated that a drift concentration is inevitable, particularly in the first story, when the CBF is subjected to a set of large ground motions with a probability of exceedance of 2% in 50 years. A simplified theoretical formulation is presented to characterize the effect of gravity columns. Two cases are considered: gravity columns perfectly fixed, and gravity columns pinned at the base. It is noted that significant mitigation of drift concentration can be achieved, particularly when gravity columns are fixed at the base. Nonlinear time-history analysis is further performed to validate the theoretical observations and quantify the stiffness/strength demands of gravity columns to avoid drift concentration. The analysis results support the trends and observations obtained from the theoretical formulation. Finally, numerical simulations are implemented with the seismic force reduction factor Ds, slenderness ratio of braces λ, and number of stories, as major analysis variables. The results demonstrate that a few (e.g. four) fixed-base gravity columns can lead to collapse prevention for reasonably designed low-rise CFBs under the set of large ground motions.     

Test description of steel X‐bracing system using low‐grade steel and internal oblique transverse stiffeners

A sample of steel X‐braced frame was studied using laboratory tests by which system behaviour and its failure modes were driven. The braces were designed based on the idea of using low‐grade steel in lateral load resisting system. Therefore, stocky braces were used. A rigid central core at intersection point of braces was considered. Oblique transverse stiffeners were provided inside the braces' cross‐section for more lateral stiffness and reduction of system yield displacement. Intervals between stiffeners were determined to create guided local buckling before global buckling of braces and thus dissipate energy by accordion behaviour. The hysteresis and energy absorption diagrams were presented. The system ductility has also been calculated. The obtained results represent uniform and stable energy absorption and reasonable system ductility by using low‐grade steel, rigid central core and oblique transverse stiffeners. Copyright © 2010 John Wiley & Sons, Ltd.     

Drift-based and dual-parameter fragility curves for concentrically braced frames in seismic regions

This paper discusses the development of drift-based and dual-parameter fragility curves for steel braces as part of concentrically braced frames designed in seismic regions. The experimental results from 24 different research programs are compiled into a database for this effort. Drift-based fragility curves are developed for three damage states of steel braces subjected to cyclic loading associated with brace flexural buckling, local buckling and brace strength loss due to fracture. The effects of material variability, brace cross sectional shape and loading protocol on the drift-based fragility curves are investigated. The effect of global and local slenderness ratios on the fracture ductility of various shapes of steel braces is examined through dual-parameter fragility curves that relate these geometric ratios with the expected story drift ratios that each of the three pre-described damage states occur. The proposed fragility curves can be employed for rapid assessment of the seismic vulnerability of concentrically braced frames.     

高层钢结构支撑方案抗震性能对比分析

高层钢结构中框架—支撑结构是一种常用的结构形式,文中介绍了支撑结构体系的发展情况.以某实际工程为例,说明了普通钢支撑和屈曲约束支撑两种不同支撑对结构弹性性能的影响,通过静力弹塑性推覆分析对比研究了两种方案的弹塑性性能,并对屈曲约束支撑采用偏心布置形式对框架梁的影响进行了探讨.     

Prevention of story drift amplification in a 20‐story steel frame structure by tension‐rod displacement–restraint bracing

This paper proposes an application of tension‐rod displacement–restraint bracing to prevent story drift amplification in tall steel moment frames. Seismic response analyses of a 20‐story bare steel frame are performed first, revealing that story drift amplification occurs in the upper and lower stories at different times. Characteristics observed for the seismic response of the bare frame suggest the efficacy of the delay action of bracing. Subsequently, seismic response analyses of the 20‐story braced frame with tension‐rod displacement–restraint bracings reveals that the increment of the column axial force by addition of bracing is reduced dramatically by the delay action of bracing. The story rotation angles within partial stories where the story drift amplification occurs in the bare frame are also reduced efficiently by the displacement–restraint bracing. The delay action of bracing influences the floor response acceleration and the residual displacement. Finally, parametric analysis results indicate an appropriate value of the story rotation angle at which the brace action starts.     

Experimental and Analytical Study of Eccentrically Braced Frames Combined with High-Strength Steel

In the structure of high-strength steel composite eccentrically braced steel frames (HSS–EBFs), the links and braces are made of Q345 steel, while the non-energy-dissipation segments (columns and beams) are made of high-strength steel (HSS). HSS reduces the cross-section of the members and increases the economic efficiency. Here, four groups of K-HSS–EBFs are designed by performance-based plastic design method in this paper, which includes 5-storey, 10-storey, 15-storey and 20-storey, and each group contain four different link length (900, 1000, 1100 and 1200 mm). The cyclic test loading was applied to 1:2 scale three-storey K-type HSS–EBFs (K-HSS–EBFs) with shear links to investigate their seismic performance. The results indicate that the as-prepared K-HSS–EBF structure exhibits excellent bearing capacity, ductility, and energy dissipation. We also find that the fracture of the link web in the second storey led to the degradation of the load-carrying capacity. The non-designated yield members remained in the elastic stage, whereas the links ultimately experience inelastic rotations, and thus dissipate the energy in the K-HSS–EBFs. Moreover, nonlinear pushover analyses and nonlinear dynamic analyses are conducted, and the loading capacity, link rotations, ductility, interstory drifts and failure mode under rare earthquake of all models are compared. The results indicate that K-HSS–EBFs with different link length have similar deformation characteristic and failure mode under pushover analysis or rare earthquakes, and the interstory drifts, link rotations and ductility of HSS–EBFs are increased with rising the link length.     

A design procedure for dual eccentrically braced systems: Numerical investigation

The paper investigates the seismic response of dual braced structures constituted by eccentrically braced frames and moment resisting frames. The response of dual braced structures designed according to the procedure described in the companion paper is first examined with the aim of suggesting behaviour factors varying with the mechanical length of the links. A larger set of structures is then designed according to the proposed values of the behaviour factor to validate the expression adopted. The seismic response of these structures is obtained by incremental dynamic analyses with reference to deterministic and random values of strength of steel. To highlight the qualities of the response of these structures the results of the dynamic analyses are compared to those obtained from structures designed according to procedures suggested by codes. A comparison is also carried out with conventional eccentrically braced frames in which the degree of interaction between the eccentrically braced frames and the other parts of the structure is purposely held very low.     

Seismic performance of steel mega braced frames equipped with shape‐memory alloy braces under near‐fault earthquakes

This paper has two main objectives. The first objective is to compare the dynamic behavior of mega shape‐memory alloy (SMA) braced frames subjected to far‐fault and near‐fault ground motions. Therefore, four mega SMA braced frames with various stories located in the vicinity of an active fault were considered. Fourteen near‐fault records with two well‐known characteristics of these records, i.e. forward directivity and fling step, were selected to test near‐fault earthquake characteristics. Furthermore, other seven far‐field records were selected for comparison. Through the nonlinear dynamic analyses, the results showed that for high‐rise frames, the near‐fault earthquakes resulted in more demands than the far‐field, but for low‐rise frames, far‐fault records imposed more demands. It was also found that mega configuration and SMA stiffening at large strains played key roles in seismic vibration control of frames. The second objective of this paper is to study the superior performance of SMA braces over the buckling restrained braces by exploiting the super‐elastic characteristic of the SMA. Identical buildings equipped with buckling restrained braces were also studied for comparison purposes. The results revealed the excellent performance of SMA braces under near‐fault records by reducing both interstory drift and residual displacement of the top floor. Copyright © 2015 John Wiley & Sons, Ltd.     

Topology optimization and seismic collapse assessment of shape memory alloy (SMA)-braced frames: Effectiveness of Fe-based SMAs

This paper presents a seismic topology optimization study of steel braced frames with shape memory alloy (SMA) braces. Optimal SMA-braced frames (SMA-BFs) with either Fe-based SMA or NiTi braces are determined in a performance-based seismic design context. The topology optimization is performed on 5- and 10-story SMA-BFs considering the placement, length, and cross-sectional area of SMA bracing members. Geometric, strength, and performance-based design constraints are considered in the optimization. The seismic response and collapse safety of topologically optimal SMA-BFs are assessed according to the FEMA P695 methodology. A comparative study on the optimal SMA-BFs is also presented in terms of total relative cost, collapse capacity, and peak and residual story drift. The results demonstrate that Fe-based SMA-BFs exhibit higher collapse capacity and more uniform distribution of lateral displacement over the frame height while being more cost-effective than NiTi braced frames. In addition to a lower unit price compared to NiTi, Fe-based SMAs reduce SMA material usage. In frames with Fe-based SMA braces, the SMA usage is reduced by up to 80%. The results highlight the need for using SMAs with larger recoverable strains.