Large deflection behavior effect in reinforced concrete columns exposed to extreme dynamic loads

Reinforced concretes (RC) have been widely used in constructions. In construction, one of the critical elements carrying a high percentage of the weight is columns which were not used to design to absorb large dynamic load like surface bursts. This study focuses on investigating blast load parameters to design of RC columns to withstand blast detonation. The numerical model is based on finite element analysis using LS-DYNA. Numerical results are validated against blast field tests available in the literature. Couples of simulations are performed with changing blast parameters to study effects of various scaled distances on the nonlinear behavior of RC columns. According to simulation results, the scaled distance has a substantial influence on the blast response of RC columns. With lower scaled distance, higher peak pressure and larger pressure impulse are applied on the RC column. Eventually, keeping the scaled distance unchanged, increasing the charge weight or shorter standoff distance cause more damage to the RC column. Intensive studies are carried out to investigate the effects of scaled distance and charge weight on the damage degree and residual axial load carrying capacity of RC columns with various column width, longitudinal reinforcement ratio and concrete strength. Results of this research will be used to assessment the effect of an explosion on the dynamic behavior of RC columns.     

基于增量动力分析的梁贯通式支撑钢框架地震易损性研究

为了得到梁贯通式支撑钢框架节点刚度及承载力设计方法,基于增量动力分析(IDA)方法,研究了节点性能对多层梁贯通式支撑钢框架地震易损性的影响,得到了不同节点刚度和承载力设置下四种模型(刚接、全强度半刚接、半强度半刚接和铰接模型)的易损性曲线,定量评价了各模型超越各极限状态的概率和倒塌储备系数.研究结果表明:无论节点刚度和...     

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.     

Plastic design of eccentrically braced frames, II: Failure mode control

A design methodology aiming at the development of a collapse mechanism of a global type for eccentrically braced frames is presented in this paper. This result is of primary importance in earthquake resistant design, because partial and local failure modes are responsible of the worsening of the energy dissipation capacity leading to an increased risk of collapse under destructive ground motions.The proposed method is based on the assumption that horizontal member sections are known, as they are designed to resist internal actions due to vertical loads and design horizontal forces. Conversely, column and diagonal sections constitute the unknowns of the design problem. In particular, the presented approach also includes the influence of second order effects which are accounted for by means of the concept of mechanism equilibrium curve. The design requirements are derived by means of the kinematic theorem of plastic collapse. Column and diagonal sections are obtained by imposing that the mechanism equilibrium curve corresponding to the global mechanism has to lie below those corresponding to the undesired mechanisms within a displacement range compatible with the local ductility supply. Moreover, the introduction of the equivalent moment concept presented in a companion paper provides the proposed method with the ability to deal with short, intermediate and long links in the same manner.Aiming at the evaluation of the accuracy of the presented procedure, the inelastic performances of eccentrically braced frames designed by means of the proposed method are investigated, by means of static and dynamic non-linear analyses, in terms of collapse mechanism typology, available ductility and energy dissipation capacity.     

Progressive collapse analysis of seismically designed steel braced frames

The progressive collapse resistance of seismically designed steel braced frames is investigated using validated computational simulation models. Two types of braced systems are considered: namely, special concentrically braced frames and eccentrically braced frames. The study is conducted on previously designed 10-story prototype buildings by applying the alternate path method. In this methodology, critical columns and adjacent braces, if present, are instantaneously removed from an analysis model and the ability of the model to successfully absorb member loss is investigated. Member removal in this manner is intended to represent a situation where an extreme event or abnormal load destroys the member. The simulation results show that while both systems benefit from placement of the seismically designed frames on the perimeter of the building, the eccentrically braced frame is less vulnerable to progressive collapse than the special concentrically braced frame. Improvement in behavior is due to improved system and member layouts in the former compared to the latter rather than the use of more stringent seismic detailing.     

Behavior and design of reinforced concrete frames retrofitted with steel bracing against progressive collapse

Steel bracing is able to improve progressive collapse resistance of reinforced concrete (RC) frames, but the bracing design is typically based on seismic retrofitting or lateral stability. There is no approach for design of steel bracing against progressive collapse. To this end, a retrofitting approach with steel braces is proposed based on analysis of macro finite element (FE) models with fiber beam elements. The FE models were initially validated through the experimental results of a braced frame and then used to investigate the effects of pertinent parameters on the progressive collapse resistance of planar frames. The results suggest the braces should be placed at the top story. Thereafter, macro FE models are built to investigate the dynamic responses of the three‐dimensional prototype RC frames under different column removal scenarios (CRS) and show the necessity of retrofitting. Accordingly, the design approach of steel bracing is proposed with incremental dynamic analysis (IDA) and assuming independent contribution of braces and frames to resistance. Finally, the fragility analysis of the frames under a corner‐penultimate‐exterior CRS is conducted through IDA and Monte Carlo simulation, and the results confirm the validity of the proposed design approach for retrofitting RC frames.     

Application of system reliability‐based assessment for collapse fragility of braced moment‐resisting frames

This paper presents a system reliability‐based framework for collapse fragility assessment of steel braced moment‐resisting frames (BMRFs). The conditional failure of intermediate events is calculated, considering two important features in the design of BMRFs: (i) different failure scenarios (FSs) with multiple sequences of components failure formation and (ii) structural reliability analysis based on the failure propagation from components to system. The system collapse reliability‐based assessment of BMRFs is developed with an efficient algorithm using the Monte Carlo simulation procedure incorporated into a nonlinear finite element (FE) analysis program. An appropriate nonlinear FE model of such systems is demonstrated, and the probability of various predefined components' failure over the most likely FSs in the presence of both epistemic and inherent uncertainties is calculated. Then, a system‐simulated reliability index (SSRI) is computed by lower and upper bounds in the probability of BMRF system collapse. Finally, fragility curves based on the SSRI is compared with the ones from incremental dynamic analysis, and later, the outcomes from multiple FSs are compared with the codified main collapse criterion. For the BMRFs analyzed herein, it is shown that the existing allowable story drift for the collapse limit state is conservative, and a new criterion is appraised. Copyright © 2015 John Wiley & Sons, Ltd.     

Progressive collapse resisting capacity of braced frames

In this study, the progressive collapse potential of braced frames was investigated using nonlinear static and dynamic analyses. Eight different bracing types were considered and their performances were compared with those of a special moment‐resisting frame designed with the same design load. According to the pushdown analysis results, most braced frames designed per current design codes satisfied the design guidelines for progressive collapse initiated by loss of a first story interior column; however, most model structures showed brittle failure mode caused by buckling of braces and columns. Among the braced frames considered, the inverted‐V type braced frames showed superior ductile behaviour during progressive collapse. The nonlinear dynamic analysis results showed that all the braced structures remained in stable condition after sudden removal of a column, and their deflections were less than that of the moment‐resisting frame. Copyright © 2009 John Wiley & Sons, Ltd.     

Lateral cyclic behavior of zipper braced frames-considering connection details

During Northridge earthquake in USA in 1994, a variety of failures occurred in welded steel connections. Studying these structural failures has led to development of more reliable moment resisting connections and new ways of using braced frames as seismic load resisting systems. This article investigates through numerical simulations, the lateral capacity and seismic behavior of two of these newly-thought braced frames, zipper braced frames and suspended zipper braced frames. The overall seismic behavior of these frames is investigated through displacement-based pushover analyses considering the effect of connection elements such as gusset plate and shear tab. To study the efficiency of these two types of concentrically braced frames, a numerical investigation on their behaviors for low-, mid- and high-rise buildings was conducted. Three zipper braced frames and three suspended zipper braced frames with different number of stories have been modeled using OpenSEES software. For each simulation, frame maximum strength, maximum drift capacity, and weight are determined and compared with each other. It is concluded that connection modeling has significant effects on the lateral behavior of these frames. Furthermore, the suspended zipper braced frames show higher ductility when compared with the ductility of zipper braced frames. Finally, the suspended zipper braced frames are recommended to be used in high-rise buildings, however, for the lowand mid-rise buildings it is recommended to use zipper frames due to economic efficacy.     

Frame‐type load bearing system for long‐span cantilevers

Cantilevers experience high risks of vulnerability against progressive collapse and vertical ground motion effects. In addition, despite common engineering practice that regards cantilevers dominated by vertical loads, it is shown that while subjected to lateral forces, they might undergo large deflections due to the formation of plastic hinges in the supporting structural subassemblage; even if cantilevers satisfy deflection limits proposed by design codes. To overcome such vulnerabilities, successive cantilever beams can be coupled in the height of the structure using vertical elements to develop a framed cantilever system. Frame behavior in cantilevers is formulized using spring models and by employing optimization procedures, economic efficiency is compared to the conventional method. The optimization results for 6 2‐D steel moment frames showed that employing framed cantilever system has the potential to reduce required material weight up to 40%. Furthermore, nonlinear pushdown and incremental dynamic analyses were conducted to extract capacity and fragility curves. The results reveal the superior performance of framed cantilevers in both lateral and vertical loads while offering better resistance against the progressive collapse.     

Collapse analysis of regular and irregular tall steel moment frames under fire loading

This study is carried out to evaluate the progressive collapse of steel buildings under fire event. To this end, a 15‐story steel structure with moment‐resisting system and composite floors is considered. The effects of various parameters such as beam section size, gravity load ratio, vertical irregularity of resisting system, and location of fire compartments on collapse modes are investigated numerically. Different temperature‐time curves are defined across the composite floors according to the Eurocode 4. It is found that local collapse of the frames at the ground floor fire is triggered by the buckling of the interior heated columns at approximately 540°C. The redistributed loads by floors delay the global collapse at least 45 min. Increasing gravity loads accelerates the global collapse of the frames significantly. The heated columns of the middle floor buckle at higher temperature compared to the ground floor heated columns and no global collapse occur due to this scenario. In general, the potential of collapse of the regular and irregular frame due to fire in the edge bay is higher compared to the fire in the middle bay. It is also found that the local and global collapse of regular frames occur earlier than irregular frames.     

Progressive collapse analysis of concentrically braced frames through EPCA algorithm

This paper introduces a new algorithm, namely the Extended Progressive Collapse Analysis (EPCA) algorithm, whose features, such as potential and capacity of buildings for occurrence of progressive collapse, were investigated. Their failure modes were determined by using pushdown and vertical incremental dynamic analyses. Moreover, by applying this procedure, the element removal impact factor and the most critical locations of such removals were obtained. This algorithm was utilized for progressive collapse analysis of two newly designed concentrically braced frames with different numbers and locations of braced bays in order to quantitatively determine its effect on mitigating progressive collapse. Using this method, the minimum residual capacity and the most critical locations of element loss as well as element removal impact factor for the frames that were studied were determined. Results showed that the frame with two braced bays had more robustness for mitigating progressive collapse, at least to the rate of 17.21% comparing to the frame with three braced bays.     

Ductility reduction factor and collapse mechanism evaluation of a new steel knee braced frame

In this study, the suitability of a new structural system called the knee braced frames (KBFs) is investigated for seismic resistant steel structures. In these structural systems, ends of beams are connected to columns by hinges (simple connection) instead of rigid connections, and ends of knee braced elements are connected to columns and beams by hinges as well. In the present paper, in addition to a comparison between elastic behaviour and elastic fundamental natural period, the ductility reduction factor and the type of collapse mechanism in steel KBFs and steel moment resisting frames (MRFs) are compared. The study revealed that the stiffness of steel buildings can be increased considerably by applying knee braced elements and the effects of knee braced elements are highly dependent on knee braced configuration. By applying the pushover analysis, it was observed that the type of collapse mechanism of KBFs is very similar to the mechanism of MRFs. Furthermore in most cases, the ductility reduction factor, R_μ, obtained from steel KBFs is greater than the ductility reduction factor obtained for steel MRFs. Based on the similarity between type of collapse mechanism and the proximity of ductility reduction coefficients of the KBFs and MRFs systems, it can be concluded that the new steel knee braced frame systems can be categorised as steel MRFs with rigid connections.     

D型偏心支撑钢框架耗能梁段性能研究

通过对具有不同耗能梁段长度的D型偏心支撑钢框架的滞回性能及耗能梁段耗能性能的非线性有限元分析,表明耗能梁段的长度对偏心支撑钢框架的侧向刚度、延性和耗能能力有较大影响。随着耗能梁段长度的增加,D型偏心支撑钢框架的强度、刚度、延性和耗能性能均产生了不同程度的退减现象;耗能梁段越短,其塑性变形越大,进而导致耗能梁段过早塑性破坏的可能性增大。根据有限元模拟结果提出了对耗能梁段长度的设计建议。     

Extended second order approximate analysis of frames with sway-braced column interaction

Present approximate second order methods for the analysis of frames with sway are not capable of reflecting the transition from sway to partially braced, and nearly fully braced behaviour of individual columns in the frames. The main aim of the paper is to extend the approximate storey magnifier approach to account for such a transition. The key to this is in the manner local second order effects are reflected. A high order shear relationship is proposed, and general sway magnifier, critical load and effective length formulations are presented both in terms of first order lateral storey stiffness and critical, free-sway column loads. Their interrelationship, and simplifications leading to existing approaches, and adaptations in present codes and standards, are discussed. Comparisons are made with exact critical loads, sway and moment magnifiers for nearly unbraced, partially braced and nearly fully braced systems. The proposed, extended approach provides predictions that generally are in very good agreement with exact results at all axial load levels. The more simplified approaches provide good agreement for low to moderate load levels for some column end restraint combinations, and up to relatively high load levels for other combinations.     

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.     

Elastic buckling load of multi-story frames consisting of Timoshenko members

The objective of this paper is to propose a method for the evaluation of the elastic critical buckling load of columns in frames consisting of members susceptible to non-negligible shear deformations, such as built-up members in steel frames, based on Engesser's approach. To that effect, a stability matrix is proposed and three general stability equations are derived for the cases of unbraced, partially braced and braced frames. Indicative graphic interpretation of the solutions for the stability equations of the braced and unbraced cases is shown. Slope-deflection equations for shear-weak members with semi-rigid connections are also derived and used for the presentation of a complete set of rotational stiffness coefficients, which are then used for the replacement of members converging at the bottom and top ends of the column in question by equivalent springs. All possible rotational and translational boundary conditions at the far end of these members, as well as the eventual presence of axial force, are considered. Five examples are presented, dealing with braced, unbraced and partially braced frames, with rigid and semi-rigid beam to column connections, loaded with concentrated or uniformly distributed loads, in a symmetrical or non-symmetrical pattern. In all cases the proposed approach is in excellent agreement with finite element results.     

考虑预应力筋断裂失效的自复位支撑钢框架结构抗震性能与风险评估

为研究自复位支撑中预应力筋发生断裂失效对结构整体抗震性能的影响,以形状记忆合金(SMA)拉索自复位支撑结构为研究对象。通过理论分析典型单核和双核自复位支撑的失效机理,发现双核自复位支撑在提升变形能力的同时存在瞬时连锁失效风险。为此,设计了不考虑预应力筋失效的理想自复位支撑框架、考虑预应力筋失效的单核、双核自复位支撑框架以及SMA自复位支撑框架等4个原型结构模型。考虑预应力筋断裂失效的随机性分布特征,在远场和近场地震作用下对框架进行非线性时程分析;进一步采用IDA方法对框架进行倒塌及残余变形易损性分析,并结合场地特征进行风险评估。分析结果表明：考虑预应力筋的断裂失效会显著增加框架倒塌的风险,在服役期风险评估中,其倒塌概率是理想情况下的5倍左右,残余变形超越概率最高为理想情况下的6倍;SMA自复位支撑具有更大的变形能力以及额外的耗能能力,在抗倒塌和抑制残余变形方面表现更为良好。     

框架支撑类型判别中的若干问题

《钢结构设计规范》(GB 5 0 0 17-送审稿 )关于强支撑框架判别式 ,在其条文说明中没有就该判别式从理论公式到规范实际应用公式的简化、修改作出说明。为此 ,讨论强支撑框架判别式的合理性。总结了在工程设计中 ,无侧移框架判别式或强支撑框架判别式应用于仅部分层间有斜撑的框架及错层框架稳定性计算时所遇到的问题     

Evaluating response modification factors of concentrically braced steel frames

Response modification factor is one of the seismic design parameters to consider nonlinear performance of building structures during strong earthquake. Relying on this, many seismic design codes led to reduce loads. The present paper tries to evaluate the response modification factors of conventional concentric braced frames (CBFs) as well as buckling restrained braced frames (BRBFs). Since, the response modification factor depends on ductility and overstrength, the static nonlinear analysis has been performed on building models including single and double bracing bays, multi-floors and different brace configurations (chevron V, invert V and X bracing). The CBFs and BRBFs values for factors such as ductility, overstrength, force reduction due to ductility and response modification have been assessed for all the buildings. The results showed that the response modification factors for BRBFs were higher than the CBFs one. It was found that the number of bracing bays and height of buildings have had greater effect on the response modification factors.