• Steel and Composite Structures
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• 제26권5호
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• pp.635-647
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• 2018

In order to improve the efficiency of the Reinforced Concrete, RC, structures against progressive collapse, this paper proposes a procedure using alternate path and specific local resistance method to resist progressive collapse in intermediate RC frame structures. Cap truss consists of multiple trusses above a suddenly removed structural element to restrain excessive collapse and provide an alternate path. Steel strut is used as a brace to resist compressive axial forces. It is similar to knee braces in the geometry, responsible for enhancing ductility and preventing shear force localization around the column. In this paper, column removals in the critical position at the first story of two 5 and 10-story regular buildings strengthened using steel strut or cap truss are studied. Based on nonlinear dynamic analysis results, steel strut can only decrease vertical displacement due to sudden removal of the column at the first story about 23%. Cap truss can reduce the average vertical displacement and column axial force transferred to adjacent columns for the studied buildings about 56% and 61%, respectively due to sudden removal of the column. In other words, using cap truss, the axial force in the removed column transfers through an alternate path to adjacent columns to prevent local or general failure or to delay the progressive collapse occurrence.

• Steel and Composite Structures
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• 제33권3호
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• pp.389-402
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• 2019

Reinforced concrete walls and buckling restrained braces are effective structural elements that are used to resist seismic loads. In this paper, the behavior of the reinforced concrete walls coupled with buckling restrained braces is investigated. In such a system, there is not any conventional reinforced concrete coupling beam. The coupling action is provided only by buckling restrained braces that dissipate energy and also cause coupling forces in the wall piers. The studied structures are 10-, 20- and 30-story ones designed according to the ASCE, ACI-318 and AISC codes. Wall nonlinear model is then prepared using the fiber elements in PERFORM-3D software. The responses of the systems subjected to the forward directivity near-fault (NF) and ordinary far-fault (FF) ground motions at maximum considered earthquake (MCE) level are studied. The seismic responses of the structures corresponding to the inter-story drift demand, curvature ductility of wall piers, and coupling ratio of the walls are compared. On average, the results show that the inter-story drift ratio for the examined systems subjected to the far-fault events at MCE level is less than allowable value of 3%. Besides, incremental dynamic analysis is used to examine the considered systems. Results of studied systems show that, the taller the structures, the higher the probability of their collapse. Also, for a certain peak ground acceleration of 1 g, the probability of collapse under NF records is more than twice this probability under FF records.

• 한국공간구조학회논문집
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• 제22권1호
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• pp.41-49
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• 2022

In the case of columns in buildings with soft story, the concentration of stress due to the difference in stiffness can damage the columns. The irregularity of buildings including soft story requires retrofit because combined load of compression, bending, shear, and torsion acts on the structure. Concrete jacketing is advantageous in securing the strength and stiffness of existing members. However, the brittleness of concrete make it difficult to secure ductility to resist the large deformation, and the complicated construction process for integrity between the existing member and extended section reduces the constructability. In this study, two types of Steel Grid Reinforcement (SGR), which are Steel Wire Mesh (SWM) for integrity and Steel Fiber Non-Shrinkage Mortar (SFNM) for crack resistance are proposed. One reinforced concrete (RC) column with non-seismic details and two columns retrofitted with each different types of proposed method were manufactured. Seismic performance was analyzed for cyclic loading test in which a combined load of compression, bending, shear, and torsion was applied. As a result of the experiment, specimens retrofitted with proposed concrete jacketing method showed 862% of maximum load, 188% of maximum displacement and 1,324% of stiffness compared to non-retrofitted specimen.

• Computers and Concrete
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• 제31권1호
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• pp.23-32
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• 2023

Reinforced concrete flat slab (RCFS) with columns is a standard gravity floor system for tall buildings in North America. Typically, RCFS-column connections are designed to resist gravity loads, and their contribution to resisting seismic forces is ignored. However, past experimental research has shown that RCFS-column connections have some strength and ductility, which may not be ignored. Advanced numerical models have been developed in the past to determine the nonlinear cyclic behavior of RCFS-column connections. However, these models are either too complicated for nonlinear dynamic analysis of an entire building or not developed to model the behavior of modern RCFS-column connections. This paper proposes a new nonlinear model suitable for modern RCFS-column connections. The numerical model is verified using experimental data of specimens with various material and reinforcement properties. A 40-story RC shear wall building was designed and analyzed to investigate the influence of RCFS on the global response of tall concrete buildings. The seismic responses of the building with and without the RCFS were modelled and compared. The results show that the modelling of RCFS has a significant impact on the inter-story drifts and force demands on both the seismic force-resisting and gravity elements.

• Wind and Structures
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• 제31권2호
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• pp.143-152
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• 2020

The wind design of buildings is typically based on strength provisions under ultimate loads. This is unlike the ductility-based approach used in seismic design, which allows inelastic actions to take place in the structure under extreme seismic events. This research investigates the application of a similar concept in wind engineering. In seismic design, the elastic forces resulting from an extreme event of high return period are reduced by a load reduction factor chosen by the designer and accordingly a certain ductility capacity needs to be achieved by the structure. Two reasons have triggered the investigation of this ductility-based concept under wind loads. Firstly, there is a trend in the design codes to increase the return period used in wind design approaching the large return period used in seismic design. Secondly, the structure always possesses a certain level of ductility that the wind design does not benefit from. Many technical issues arise when applying a ductility-based approach under wind loads. The use of reduced design loads will lead to the design of a more flexible structure with larger natural periods. While this might be beneficial for seismic response, it is not necessarily the case for the wind response, where increasing the flexibility is expected to increase the fluctuating response. This particular issue is examined by considering a case study of a sixty-five-story high-rise building previously tested at the Boundary Layer Wind Tunnel Laboratory at the University of Western Ontario using a pressure model. A three-dimensional finite element model is developed for the building. The wind pressures from the tested rigid model are applied to the finite element model and a time history dynamic analysis is conducted. The time history variation of the straining actions on various structure elements of the building are evaluated and decomposed into mean, background and fluctuating components. A reduction factor is applied to the fluctuating components and a modified time history response of the straining actions is calculated. The building components are redesigned under this set of reduced straining actions and its fundamental period is then evaluated. A new set of loads is calculated based on the modified period and is compared to the set of loads associated with the original structure. This is followed by non-linear static pushover analysis conducted individually on each shear wall module after redesigning these walls. The ductility demand of shear walls with reduced cross sections is assessed to justify the application of the load reduction factor "R".

• 한국지진공학회논문집
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• 제6권2호
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• pp.29-37
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• 2002

본 연구는 철근콘크리트 연성 모멘트골조의 선형.비선형 정적해석을 통한 반응수정계수와 비선형 변위량을 평가하여 합리적인 내진설계의 기초자료를 제공하는 것을 목적으로 한다. 먼저 국내 내진설계 규준에 따라 각 모델을 설계한 후, 철근콘크리트 연성 모멘트골조의 반응수정계수와 비선형 변위량을 평가하였으며, 해석에 사용된 모델은 층수(10, 20, 30), 평면비(1:1, 1:2), 해석방법(2D, 3D)을 변수로 한 27개의 모델이다. 반응수정계수와 비선형 변위량의 평가는 각 모델별 선형.비선형 정적해석을 수행하여 그 결과를 비교 분석하여 산정하였다. 반응수정계수는 강도계수, 연성계수, 잉여도계수, 감쇠계수의 곱으로 산정하였고, 그 결과 해석방향의 저항골조의 수에 따라 2 스팬인 경우 3.5, 3 스팬인 경우 4.3, 4 스팬 이상인 경우에는 평면비나 층수와 상관없이 5.0에 근접한 결과를 나타내었다. 비선형 변위량은 층간변위각비(비선형 변위각/선형 변위각)에 의해 평가되었으며, 층간변위각비는 5.85에서 9.34로 나타났다.

• 한국지진공학회논문집
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• 제11권5호
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• pp.61-69
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• 2007

본 연구의 목적은 하부층에 필로티 구조, 상부층에 전단벽식구조를 가진 저층 철근콘크리트 건물의 내진설계 및 내진성능 평가를 위한 기본적인 자료를 제공하는 것으로서, 비선형 지진응답해석을 실시하여 각 층의 내력과 연성율 사이의 상관관계를 파악하여, 이것들의 비율이 건물 전체의 내진성능에 어떻게 영향을 미치는가를 검토하였다. 본 연구에서는 필로티 구조를 가진 저층 철근콘크리트 전단벽식 건물의 특성을 2질점계로 치환하였으며, 하부층인 필로티 구조는 휨파괴형으로 상부층인 전단벽식 구조는 전단파괴형 시스템으로 각각 모델링하였다. 또한 각층의 복원력 특성으로는 필로티 구조는 Degrading Trilinear Model(휨파괴형), 상부층은 Origin Oriented Model(전단파괴형)을 선정하였다. 상기 복원력 특성은 각 층의 보유내력에 의하여 변화를 하며, 지진응답해석용 입력지진파로는 8개의 피해지진의 가속도 성분을 선정하여 이들 가속도 성분의 최대가속도를 0.1g, 0.2g, 0.3g로 표준화 하였다. 각각 지진강도수준에 따라 지진 응답해석을 실시하여 하부층 필로티 구조와 상부층 전단벽식 구조의 내력비와 응답 연성율 사이의 상관관계를 파악하였다. 최종적으로 특정 연성율을 위한 필로티 구조를 가진 저층 철근콘크리트 전단벽식 건물의 요구내력을 산정하여 요구내력 스펙트럼(Required Strength Spectrum)을 제안하였다. 본 연구에서 제안한 요구내력스펙트럼은 특정 지역에서 요구하는 지진수준에 대하여 지진발생시 특정 연성율 이내로 머물게 하는 하한내력의 범위를 파악할 수 있는 등, 연구결과는 필로티 구조를 가진 철근콘크리트 전단벽식 건물의 내진성능평가 및 내진설계의 기본적인 자료로서 활용 가능하다고 사료된다.

• 한국강구조학회 논문집
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• 제18권4호
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• pp.457-468
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• 2006

얇은 웨브강판을 사용한 전단지배 강판벽의 최대 에너지소산능력 및 연성능력을 연 구하기 위한 실험 연구를 실시하였다. 이를위해서 얇은 강판을 사용한 3층 골조 강판벽 실험을 수행하였다. 주요 실험 변수는 강판의 형상비와 기둥 단면의 전단강도이며, 골조 강판벽 시스템의 상대적 비교를 위해 중심가새골조 및 모멘트저항골조와의 비교실험을 실시하였다. 골조 강판벽 실험체는 중심가새골조와 모멘트저항골조 실험체에 비하여 매우 우수한 연성도와 에너지소산능력을 나타냈다. 따라서 전단지배형 골조강판벽은 큰 강도, 강성 및 변형능력을 동시에 달성할 수 있는 이상적인 내진구조시스템으로 사용할 수 있다. 전단지배 강판벽의 주기거동을 예측하기 위하여, 본 연구의 실험결과와 기존 실험결과를 토대로 강판벽의 에너지소산능력을 예측할 수 있는 방법을 제시하였다.

• Earthquakes and Structures
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• 제9권1호
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• pp.49-71
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• 2015

The present paper aims at evaluating damage and collapse behavior of low-rise buildings with unidirectional mass irregularities in plan (torsional buildings). In previous earthquake events, such buildings have been exposed to extensive damages and even total collapse in some cases. To investigate the performance and collapse behavior of such buildings from probabilistic points of view, three-dimensional three and six-story reinforced concrete models with unidirectional mass eccentricities ranging from 0% to 30% and designed with modern seismic design code provisions specific to intermediate ductility class were subjected to nonlinear static as well as extensive nonlinear incremental dynamic analysis (IDA) under a set of far-field real ground motions containing 21 two-component records. Performance of each model was then examined by means of calculating conventional seismic design parameters including the response reduction (R), structural overstrength (${\Omega}$) and structural ductility (${\mu}$) factors, calculation of probability distribution of maximum inter-story drift responses in two orthogonal directions and calculation collapse margin ratio (CMR) as an indicator of performance. Results demonstrate that substantial differences exist between the behavior of regular and irregular buildings in terms of lateral load capacity and collapse margin ratio. Also, results indicate that current seismic design parameters could be non-conservative for buildings with high levels of plan eccentricity and such structures do not meet the target "life safety" performance level based on safety margin against collapse. The adverse effects of plan irregularity on collapse safety of structures are more pronounced as the number of stories increases.

• Steel and Composite Structures
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• 제37권2호
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• pp.175-191
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• 2020

In this article, for the first time, the seismic behavior of elliptic-braced moment resisting frame (ELBRF) is assessed through a laboratory program and numerical analyses of FEM specifically focused on the development of global- and local-type failure mechanisms. The ELBRF as a new lateral braced system, when installed in the middle bay of the frames in the facade of a building, not only causes no problem to the opening space of the facade, but also improves the structural behavior. Quantitative and qualitative investigations were pursued to find out how elliptic braces would affect the failure mechanism of ELBRF structures exposed to seismic action as a nonlinear process. To this aim, an experimental test of a ½ scale single-story single-bay ELBRF specimen under cyclic quasi-static loading was run and the results were compared with those for X-bracing, knee-bracing, K-bracing, and diamond-bracing systems in a story base model. Nonlinear FEM analyses were carried out to evaluate failure mechanism, yield order of components, distribution of plasticity, degradation of structural nonlinear stiffness, distribution of internal forces, and energy dissipation capacity. The test results indicated that the yield of elliptic braces would delay the failure mode of adjacent elliptic columns and thus, help tolerate a significant nonlinear deformation to the point of ultimate failure. Symmetrical behavior, high energy absorption, appropriate stiffness, and high ductility in comparison with the conventional systems are some of the advantages of the proposed system.