Effects of Steel Braces on Robustness of Steel Frames against Progressive Collapse (original) (raw)
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Comparison of Conventional Brace and Buckling Restrained Brace in Steel Frame Structure
A conventional moment resisting steel frame undertakes large level of lateral deformation when subjected to strong ground motion or wind forces. If this deformation is excessive, structural and non-structural damage is evident, which damages structural integrity. To avoid such large deformations, various types of systems have been used in steel frames. Diagonal elements, called braces, have been implemented as additional structural member that increase the stiffness and energy dissipation, and control relative inter story deformation in an effective way, thus protecting the structure against damage and improving the overall behavior. The BRBs permits very high compression strength. Because there is no reduction in the available material strength due to instability, the brace can achieve great ductility..
Experimental studies have been conducted to investigate the hysteretic behavior of one story-one bay braced steel frames whose braces are made of built-up H-shapes and whose columns and beams are made of rolled H-shapes. Hysteretic behavior and transition and change of load carrying capacity of each component member of a frame, i.e., braces, columns and beams under repeated horizontal load are examined individually as well as the hysteretic behavior of a braced frame as a whole. Interaction behavior between the braces built in a frame and the components of the surrounding frame is also discussed. As a fruit of the investigation, the following is remarked. The effective slenderness ratio for buckling of the braces built in a frame could be estimated by the slope-deflection method taking the rotational rigidity by the members of the surrounding frame into account. Since hysteretic behavior of the braces built in the surrounding frame, the dimensions of whose component members are comparable with those of the brace is not much different from that of the brace reported in Part II) which was approximately rigidly fixed in the heavy surrounding frame, the effect of the deformation of the members of the surrounding frame on the hysteretic behavior of the brace would not be large. The effective slenderness ratio for the estimation of the post-buckling and hysteretic behaviors could be approximated by the assumption that the braces would be rigidly fixed at the ends. As the columns are subjected to large repeated axial load due to the deformation of the brace and the load carrying capacity of the column largely decreases when the axial load is large, the behavior of the column is largely affected by that of the brace. The load carrying capacity and the ductility of the brace are reduced and exhausted when cracks are initiated as well as reported in Part 11/ and Part 2°).
Repeated damage to frames with concentric bracings in recent earthquakes such as the 1984 Mexico City earthquake, 1989 Loma Prieta, and 1994 Northridge intensified the concerns about the ultimate ductility capacity of this class of structures. Several categories were mentioned for the poor performance of this class of braced frames. In this paper, a three-story steel structure in three dimensions is raced with buckling-resistant braces and then a dynamic analysis was applied to it. Then the results of application of buckling-resistant braces to dual system for reducing permanent deformation under impact loading have been investigated. Finally, the impact of imperfection in protective cover upon the performance of brace is studied. The results are indicated in tables and figures, hence being compared.
Cumulative Deformation Capacity of Steel Braces under Various Cyclic Loading Histories
Journal of Structural Engineering, 2015
The postbuckling behavior of seismic-resistant braces in steel frames under a cyclic axial force is often evaluated by time-history analyses; however, brace fracture is seldom considered. The authors previously proposed a physical model for predicting the moment of fracture of circular-tube braces after buckling using phenomenological hysteresis. However, the accuracy of that model was confirmed only against the test results of the gradually increasing amplitude loading protocol, and its applicability under other loading histories has not yet been verified. In this study, cyclic loading tests were carried out until fracture on circular-tube and H-section braces under various loading histories, followed by FEM analyses. The validity of the proposed formulas for evaluating the strain-concentration index under various loading histories was examined. The proposed method was used for predicting the moment of fracture and the cumulative deformation capacity until fracture, and the predictions agreed well with the test results.
A Study of the Behavior of Steel Moment Frame with Buckling Restrained Bracing (Review Article
Soil Structure Interaction Journal (SSIJ) , 2018
The use of bracing systems is one of the most suitable methods to control the lateral displacement of a structure as well as to improve the seismic rehabilitation of moment frames. The bracing systems used in the design of new systems and the rehabilitation of weak old structures are buckling restrained braces. In this study, the effect of this on the performance of steel moment frames is investigated. The use of buckling restrained braces has led to increased bracing stability and ductility under compression. Therefore, it creates a symmetry in the strength of the member under axial compressive and tensile load. For this reason, the full capacity of braces in tension and compression is used in the structural frame where braces are used double. Research results indicate that the first plastic hing in a moment frame without a restrained buckling brace is formed in base column and beam-to-column connection, which may lead to a crumbling failure.
Journal of Rehabilitation in Civil Engineering, 2019
Progressive collapse is a condition where local failure of a primary structural component leads to the collapse of neighboring members and the whole structure, consequently. In this paper, the progressive collapse potential of seismically designed steel dual systems with buckling restrained braces is investigated using the alternate path method, and their performances are compared with those of the conventional intermediate moment resisting frames. Static nonlinear Push-down and dynamic analyses under gravity loads specified in GSA guideline are conducted to capture the progressive collapse response of the structures due to column and adjacent BRBs removal, and their ability of absorbing the destructive effects of member loss is investigated. It was observed that, compared with the intermediate moment resisting frames, generally the dual systems with buckling restrained braces provided appropriate alternative path for redistributing the generated loads caused by member loss and the ...
Study of Buckling Restrained Braces in Steel Frame Building
Conventional braces have limited deformation ductility capacity, and exhibit unsymmetrical hysteretic cycles, with marked strength deterioration when loaded in compression. To overcome the above mentioned problems, a new type of brace was developed in Japan called as buckling restrained braces, designated as BRB's. These braces are designed such that buckling is inhibited to occur, exhibiting adequate behavior and symmetrical hysteretic curves under the action of both tensile and compressive cycles, produced by the action of seismic and wind forces. This paper presents experimental results concerning the lateral load carrying capacity of steel frame model by use of buckling restrained brace. This paper also includes the comparative study of lateral load carrying capacity of frame model for bare frame, frame with Conventional brace and frame with buckling restrained brace.
Ultimate behaviour of steel braces under cyclic loading
Proceedings of the ICE - Structures and Buildings, 2013
Under significant seismic loading conditions, the response of concentrically braced frames largely depends on the behaviour of the diagonal braces, which represent the key energy dissipating zones. Although the hysteretic response of steel braces under cyclic axial loading has been examined in previous studies, there is a need for further assessments which focuses on quantifying failure. This paper describes the development of detailed finite element models of hollow sections subjected to cyclic axial loading. The effects of initial imperfections and cyclic hardening are taken into consideration and the models are validated against data from 19 tests. A method to predict the fracture life of bracing members under cyclic loading is also described. Using the numerical models, parametric studies are undertaken to assess the influence of global and local slendernesses on the performance of the braces-both are found to affect the occurrence and severity of local buckling under cyclic loading, which causes high localised strains at the corner areas of sections leading to fatigue fracture. A predictive equation addressing the co-existing influence of global slenderness and local slenderness on displacement ductility is presented. The observations in the current study are compared with the conclusions from other experimental programmes and the discrepancy between the findings is discussed. Notation: A Cross-sectional area b Width of the flat area of the longer face of a hollow section c Fatigue ductility exponent Cs Empirical constant d Width of the flat area of the shorter face of a hollow section D Damage index Nip, K. H., Gardner, L. and Elghazouli, A. Y. (2013). Ultimate behaviour of steel braces under cyclic loading. Proceedings of the Institution of Civil Engineers-Structures and Buildings. 166(5), 219-234. Coefficient dependant on material yield stress p/2 Plastic strain amplitude f Fatigue ductility coefficient Dimensionless global slenderness Displacement ductility f Rotation at fracture cr Plate critical buckling stress 0 Amplitude of initial imperfection