• Structural Engineering and Mechanics
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• 제41권3호
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• pp.365-378
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• 2012

In this study, strength reduction factors are investigated for SDOF systems with period range of 0.1-3.0 s with elastoplastic behavior considering soil structure interaction for 64 different earthquake motions recorded on different site conditions such as rock, stiff soil, soft soil and very soft soil. Soil structure interacting systems are modeled and analyzed with effective period, effective damping and effective ductility values differing from fixed-base case. For inelastic time history analyses, Newmark method for step by step time integration was adapted in an in-house computer program. Results are compared with those calculated for fixed-base case. A new equation is proposed for strength reduction factor of interacting system as a function of structural period of system (T), ductility ratio (${\mu}$) and period lengthening ratio (T/T). It is concluded that soil structure interaction reduces the strength reduction factors for soft soils, therefore, using the fixed-base strength reduction factors for interacting systems lead to non-conservative design forces.

• Steel and Composite Structures
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• 제11권4호
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• pp.273-290
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• 2011

In current ductility-based earthquake-resistant design, the estimation of design forces continues to be carried out with the application of response modification factors on elastic design spectra. It is well-known that the response modification factor (R) takes into account the force reduction, strength, redundancy, and damping of structural systems. The key components of the response modification factor (R) are force reduction ($R_{\mu}$) and strength ($R_S$) factors. However, the response modification and strength factors for structural systems presented in design codes were based on professional judgment and experiences. A numerical study has been accomplished to evaluate force reduction, strength, and response modification factors for special steel moment resisting frames. A total of 72 prototype steel frames were designed based on the recommendations given in the AISC Seismic Provisions and UBC Codes. Number of stories, soil profiles, seismic zone factors, framing systems, and failure mechanisms were considered as the design parameters that influence the response. The effects of the design parameters on force reduction ($R_{\mu}$), strength ($R_S$), and response modification (R) factors were studied. Based on the analysis results, these factors for special steel moment resisting frames are evaluated.

• 콘크리트학회논문집
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• 제12권2호
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• pp.21-30
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• 2000

In the Ultimate Strength Design, the design strength of a member is determined by multiplying the strength reduction factor to the nominal strength. This concept may be a reasonable approach, however it can not consider failure modes appropriately. Moreover, column design strength diagram show an abrupt change at a low level of axial load, which does not seem to be reasonable. This research compares the design strength determined by the strength resistance factors. As the material resistance factors for flexure and compression, 0.65 and 0.90 are proposed for concrete and steel, respectively. The design strength calculation process by applying material resistance factors addresses failure modes more effectively than by applying member strength reduction factor, and provides more resnable design strength for reinforced concrete flexural and compression members.

• 한국지반공학회논문집
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• 제28권7호
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• pp.55-65
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• 2012

지오그리드의 장기허용강도를 산출할 때 사용되는 총 감소계수는 내시공성 감소계수($RF_{ID}$), 내화학성 감소계수($RF_D$), 크리프 감소계수($RF_{CR}$) 등이 적용된다. 지오그리드의 단기인장강도에 대한 감소계수를 고려한 허용인장강도 산출 모델의 경우 감소계수들 사이의 상호 작용력을 고려하지 않는 한계를 가지고 있다. 접점강도는 인장강도와 마찬가지로 시공 시 손상이나 화학적 분해에 의하여 감소하게 된다. 기존의 단일접점강도 시험 방법은 치수효과를 고려할 수 없기에 결과의 편차가 큰 시공 시 손상된 시험편의 접점강도를 측정하는데 적합하지 않다. 또한 시공 시 손상에 의한 전단강도 변화에 대한 연구도 전혀 이루어지지 않은 실정이다. 따라서 본 연구에서는 다양한 조건을 고려하여 지오그리드의 장기성능에 영향을 미치는 감소계수들을 재평가하고 감소계수 사이의 상호 작용을 고려하여 정확한 장기허용강도를 구하려고 한다. 내시공성 시험과 내화학성 시험 후 크리프 시험결과 총 감소계수는 GRI GG-4 시험값보다 작게 나타났다. 내시공성 시험과 내화학성 시험 후 접점강도의 감소계수는 인장강도 감소계수보다 더 작게 나타났다. 내시공성 시험후 전단강도 차이가 나타나지 않거나 증가함을 나타내었다.

• Earthquakes and Structures
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• 제2권1호
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• pp.65-88
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• 2011

A new method for the seismic design of plane steel moment resisting frames is developed. This method determines the design base shear of a plane steel frame through modal synthesis and spectrum analysis utilizing different values of the strength reduction (behavior) factor for the modes considered instead of a single common value of that factor for all these modes as it is the case with current seismic codes. The values of these modal strength reduction factors are derived with the aid of a) design equations that provide equivalent linear modal damping ratios for steel moment resisting frames as functions of period, allowable interstorey drift and damage levels and b) the damping reduction factor that modifies elastic acceleration spectra for high levels of damping. Thus, a new performance-based design method is established. The direct dependence of the modal strength reduction factor on desired interstorey drift and damage levels permits the control of deformations without their determination and secures that deformations will not exceed these levels. By means of certain seismic design examples presented herein, it is demonstrated that the use of different values for the strength reduction factor per mode instead of a single common value for all modes, leads to more accurate results in a more rational way than the code-based ones.

• Structural Engineering and Mechanics
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• 제62권2호
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• pp.209-224
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• 2017

Elevated water tanks are considered as important structures due to its post-earthquake requirements. Elevated water tank on reinforced concrete frame staging is widely used in India. Different response reduction factors depending on ductility of frame members are used in seismic design of frame staging. The study on appropriateness of response reduction factor for reinforced concrete tank staging is sparse in literature. In the present paper a systematic study on estimation of key components of response reduction factors is presented. By considering the various combinations of tank capacity, height of staging, seismic design level and design response reduction factors, forty-eight analytical models are developed and designed using relevant Indian codes. The minimum specified design cross section of column as per Indian code is found to be sufficient to accommodate the design steel. The strength factor and ductility factor are estimated using results of nonlinear static pushover analysis. It was observed that for seismic design category 'high' the strength factor has lesser contribution than ductility factor, whereas, opposite trend is observed for seismic design category 'low'. Further, the effects of staging height and tank capacity on strength and ductility factors for two different seismic design categories are studied. For both seismic design categories, the response reduction factors obtained from the nonlinear static analysis is higher than the code specified response reduction factors. The minimum dimension restriction of column is observed as key parameter in achieving the desired performance of the elevated water tank on frame staging.

• Structural Engineering and Mechanics
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• 제45권5호
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• pp.655-676
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• 2013

In this study, strength reduction factors and inelastic displacement ratios are investigated for SDOF systems with period range of 0.1-3.0 s considering soil structure interaction for earthquake motions recorded on soft soil. The effect of stiffness degradation on strength reduction factors and inelastic displacement ratios is investigated. The modified-Clough model is used to represent structures that exhibit significant stiffness degradation when subjected to reverse cyclic loading and the elastoplastic model is used to represent non-degrading structures. The effect of negative strain - hardening on the inelastic displacement and strength of structures is also investigated. Soil structure interacting systems are modeled and analyzed with effective period, effective damping and effective ductility values differing from fixed-base case. For inelastic time history analyses, Newmark method for step by step time integration was adapted in an in-house computer program. New equations are proposed for strength reduction factor and inelastic displacement ratio of interacting system as a function of structural period($\tilde{T}$, T) ductility (${\mu}$) and period lengthening ratio ($\tilde{T}$/T).

• 한국지진공학회논문집
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• 제14권4호
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• pp.49-60
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• 2010

구조부재와 구조시스템의 실제 비탄성 이력거동은 완만한 형태를 나타낸다. 완만한 곡선형 이력거동이 이선형 또는 분할선형 강성저하모델등에 비하여 실제 거동을 정확하게 나타낸다. 내진설계에서 강도감소계수는 탄성 거동만 허용한 경우에 요구되는 강도를 설계수준의 강도로 감소시키는데 사용된다. 강도감소계수에 대한 완만한 곡선이력거동의 영향을 근거리 지진과 원거리 지진을 받는 몇 가지 완만한 곡선이력 시스템에 대하여 평가하였다. 설계 목적을 위하여 이력거동의 완만정도와 근거리와 원거리 지진으로 대변될 수 있는 지진특성을 고려하는 간단한 강도감소계수 식을 제안하였다. 본 연구의 제안식에 의한 강도감소계수가 기존의 제안식에 의해 평가된 강도감소계수보다 비탄성 응답스펙트럼 해석으로부터 직접적으로 구한 강도감소계수에 근접한 결과를 나타낸다.

• Steel and Composite Structures
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• 제21권3호
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• pp.629-659
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• 2016

In cold-formed stainless steel lipped channel-sections, web openings are becoming increasingly popular. Such openings, however, result in the sections becoming more susceptible to web crippling, especially under concentrated loads applied near the web opening. This paper presents the results of a finite element parametric study into the effect of circular web openings on the web crippling strength of cold-formed stainless steel lipped channel-sections for the interior-one-flange (IOF) loading condition. This involves a bearing load applied to the top flange of a length of member, away from the end supports. The cases of web openings located centred beneath the bearing load (i.e. beneath the bearing plate delivering the load) and offset to the bearing plate, are considered. Three grades of stainless steel are considered: duplex EN1.4462, austenitic EN1.4404 and ferretic EN1.4003. In total, 2218 finite element models were analyzed. From the results of the parametric study, strength reduction factors for load bearing capacity are determined, where these reduction factors are applied to the bearing capacity calculated for a web without openings, to take account the influence of the web openings. The strength reduction factors are first compared to equations recently proposed for cold-formed carbon steel lipped channel-sections. It is shown that for the case of the duplex grade, the strength reduction factor equations for cold-formed carbon steel are conservative but only by 2%. However, for the cases of the austentic and ferritic grades, the cold-formed carbon steel equations are around 9% conservative. New strength reduction factor equations are proposed for all three stainless steel grades.

• Earthquakes and Structures
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• 제6권3호
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• pp.301-316
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• 2014

This paper is devoted to investigate the effects of SSI on strength reduction factor of multistory buildings. A new formula is proposed to estimate strength reduction factors for MDOF structure-soil systems. It is concluded that SSI reduces the strength reduction factor of MDOF systems. The amount of this reduction is relevant to the fundamental period of structure, soil flexibility, aspect ratio and ductility of structure, and could be significantly different from corresponding fixed-base value. Using this formula, measuring the amount of this error could be done with acceptable accuracy. For some practical cases, the error attains up to 50%.