Seismic Risk Analysis of Frames with Uncertain Support and PR Connection Conditions

Jungwon Huh,Achintya Haldar,Hyo-Nam Cho 대한토목학회 2001 KSCE journal of civil engineering Vol.5 No.4

Seismic response analysis of steel frames with post-Northridge connection

Ali Mehrabian,Achintya Haldar,Alfredo Reyes-Salazar 국제구조공학회 2005 Steel and Composite Structures, An International J Vol.5 No.4

The seismic behavior of two steel moment-resisting frames, which satisfy all the current seismic design requirements, are evaluated and compared in the presence of pre-Northridge connections denoted as BWWF and an improved post-Northridge connections denoted as BWWF-AD. Pre-Northridge connections are modeled first as fully restrained (FR) type. Then they are considered to be partially restrained (PR) to model their behavior more realistically. The improved post-Northridge connections are modeled as PR type, as proposed by the authors. A sophisticated nonlinear time-domain finite element program developed by the authors is used for the response evaluation of the frames in terms of the overall rotation of the connections and the maximum drift. The frames are excited by ten recorded earthquake time histories. These time histories are then scaled up to produce some relevant response characteristics. The behaviors of the frames are studied comprehensively with the help of 120 analyses. Following important observations are made. The frames produced essentially similar rotation and drift for the connections modeled as FR type and PR type represented by BWWF-AD indicating that the presence of slots in the web of beams in BWWF-AD is not detrimental to the overall response behavior. When the lateral displacements of the frames are significantly large, the responses are improved if BWWF-AD type connections are used in the frames. This study analytically confirms many desirable features of BWWF-AD connections. PR frames have longer periods of vibration in comparison to FR frames and may attract lower inertia forces. However, calculated periods of the frames of this study using FEMA 350 empirical equation is longer than those calculated using dynamic characteristics of the frames. This may result in even lower design forces and may adversely influence the design.

SFEM-based Reliability Analysis of Frames with Uncertain Support and Connection Conditions

Jungwon Huh,Achintya Haldar,Sang-Geun Seo 대한토목학회 1999 KSCE journal of civil engineering Vol.3 No.1

Seismic response and energy dissipation in partially restrained and fully restrained steel frames: An analytical study

Alfredo Reyes-Salazar,Achintya Haldar 국제구조공학회 2001 Steel and Composite Structures, An International J Vol.1 No.4

The damage suffered by steel structures during the Northridge (1994) and Kobe (1995) earthquakes indicates that the fully restrained (FR) connections in steel frames did not behave as expected. Consequently, researchers began studying other possibilities, including making the connections more flexible, to reduce the risk of damage from seismic loading. Recent experimental and analytical investigations pointed out that the seismic response of steel frames with partially restrained (PR) connections might be superior to that of similar frames with FR connections since the energy dissipation at PR connections could be significant. This beneficial effect has not yet been fully quantified analytically. Thus, the dissipation of energy at PR connections needs to be considered in analytical evaluations, in addition to the dissipation of energy due to viscous damping and at plastic hinges (if they form). An algorithm is developed and verified by the authors to estimate the nonlinear time-domain dynamic response of steel frames with PR connections. The verified algorithm is then used to quantify the major sources of energy dissipation and their effect on the overall structural response in terms of the maximum base shear and the maximum top displacement. The results indicate that the dissipation of energy at PR connections is comparable to that dissipated by viscous damping and at plastic hinges. In general, the maximum total base shear significantly increases with an increase in the connection stiffness. On the other hand, the maximum top lateral displacement Umax does not always increase as the connection stiffness decreases. Energy dissipation is considerably influenced by the stiffness of a connection, defined in terms of the T ratio, i.e., the ratio of the moment the connection would have to carry according to beam line theory (Disque 1964) and the fixed end moment of the girder. A connection with a T ratio of at least 0.9 is considered to be fully restrained. The energy dissipation behavior may be quite different for a frame with FR connections with a T ratio of 1.0 compared to when the T ratio is 0.9. Thus, for nonlinear seismic analysis, a T ratio of at least 0.9 should not be considered to be an FR connection. The study quantitatively confirms the general observations made in experimental results for frames with PR connections. Proper consideration of the PR connection stiffness and other dynamic properties are essential to predict dynamic behavior, no matter how difficult the analysis procedure becomes. Any simplified approach may need to be calibrated using this type of detailed analytical study.

FEM-based Seismic Reliability Analysis of Real Structural Systems

허정원,Huh Jung-Won,Haldar Achintya Computational Structural Engineering Institute of 2006 한국전산구조공학회논문집 Vol.19 No.2

응답면기법, 유한요소법, 일차신뢰도법 그리고 반복 선형보간기법을 합리적으로 결합한 정교한 신뢰성해석 기법이 지진하중을 포함하는 단기 동적하중을 받는 복잡한 실제 비선형 동적구조계의 신뢰성 평가를 위하여 제안되었다. 기법은 하중 및 저항과 관련된 랜덤변수의 비선형성과 불확실성의 모든 중요 원천을 명시적으로 고려한다. 본 기법의 특징은 전통적 랜덤진동방법의 대안으로서 지진하중을 시간영역에서 적용하는 것이다. 실제 강프레임의 연결부에 대한 유연성을 표현하기 위하여 4-매개변수 리차드 모델을 사용하였다. 리차드 매개변수의 불확실성에 대한 고려도 알고리즘에 포함하였다. 다음으로 횡방향으로 유연한 강프레임을 철근콘크리트 전단벽으로 보강하였다. 균열 발생 후 전단벽에서의 강도저감 또한 고려되었다. 강절 연결부를 갖는 횡방향으로 유연한 강프레임, 각기 다른 강성의 부분강절 연결부를 갖는 강프레임, 그리고 콘크리트 전단벽으로 보강된 강프레임의 세 구조물을 고려함으로써 실제 구조물의 신뢰성평가를 위한 기법의 적용성을 검증하였다. A sophisticated reliability analysis method is proposed to evaluate the reliability of real nonlinear complicated dynamic structural systems excited by short duration dynamic loadings like earthquake motions by intelligently integrating the response surface method, the finite element method, the first-order reliability method, and the iterative linear interpolation scheme. The method explicitly considers all major sources of nonlinearity and uncertainty in the load and resistance-related random variables. The unique feature of the technique is that the seismic loading is applied in the time domain, providing an alternative to the classical random vibration approach. The four-parameter Richard model is used to represent the flexibility of connections of real steel frames. Uncertainties in the Richard parameters are also incorporated in the algorithm. The laterally flexible steel frame is then reinforced with reinforced concrete shear walls. The stiffness degradation of shear walls after cracking is also considered. The applicability of the method to estimate the reliability of real structures is demonstrated by considering three examples; a laterally flexible steel frame with fully restrained connections, the same steel frame with partially restrained connections with different rigidities, and a steel frame reinforced with concrete shear walls.

Dissipation of energy in steel frames with PR connections

Reyes-Salazar, Alfredo,Haldar, Achintya Techno-Press 2000 Structural Engineering and Mechanics, An Int'l Jou Vol.9 No.3

The major sources of energy dissipation in steel frames with partially restrained (PR) connections are evaluated. Available experimental results are used to verify the mathematical model used in this study. The verified model is then used to quantify the energy dissipation in PR connections due to hysteretic behavior, due to viscous damping and at plastic hinges if they are formed. Observations are made for two load conditions: a sinusoidal load applied at the top of the frame, and a sinusoidal ground acceleration applied at the base of the frame representing a seismic loading condition. This analytical study confirms the general behavior, observed during experimental investigations, that PR connections reduce the overall stiffness of frames, but add a major source of energy dissipation. As the connections become stiffer, the contribution of PR connections in dissipating energy becomes less significant. A connection with a T ratio (representing its stiffness) of at least 0.9 should not be considered as fully restrained as is commonly assumed, since the energy dissipation characteristics are different. The flexibility of PR connections alters the fundamental frequency of the frame. Depending on the situation, it may bring the frame closer to or further from the resonance condition. If the frame approaches the resonance condition, the effect of damping is expected to be very important. However, if the frame moves away from the resonance condition, the energy dissipation at the PR connections is expected to be significant with an increase in the deformation of the frame, particularly for low damping values. For low damping values, the dissipation of energy at plastic hinges is comparable to that due to viscous damping, and increases as the frame approaches failure. For the range of parameters considered in this study, the energy dissipations at the PR connections and at the plastic hinges are of the same order of magnitude. The study quantitatively confirms the general observations made in experimental investigations for steel frames with PR connections; however, proper consideration of the stiffness of PR connections and other dynamic properties is essential in predicting the dynamic behavior.

Review of Assumptions in Simplified Multi-Component and Codified Seismic Response Evaluation Procedures

Alfredo Reyes-Salazar,Achintya Haldar,Juana L. Rivera-Salas,Edén Bojórquez 대한토목학회 2015 KSCE JOURNAL OF CIVIL ENGINEERING Vol.19 No.5

Representation of an earthquake by orthogonal normal and principal components, accuracies of several rules to combine collinear and non-collinear responses of MDOF structures, and estimation of the relative importance of the effects of individual orthogonal components are addressed in this paper. Statistically there are no differences between normal and principal components in estimating axial load for the elastic case; the principal components, however, may produce about 11% larger axial load for the inelastic case. There are no major differences in estimating base shear in all cases. To combine the two horizontal components, the 30% and SRSS combination rules underestimate the collinear axial load by about 10%. For the case of three components, the underestimation by the 30% and SRSS rules are about 5% and 10%, respectively. When responses due to axial loads and bending moments are combined, the underestimation could be between 10 and 15% for both combination rules. The assumption that the vertical response is 2/3 of that of the horizontal component appears not to be very conservative. The assumption that the response produced by the minor horizontal component is 85% of that of the major horizontal component appears to be very conservative.

Consideration of Uncertainty in the Frequency Content in the Seismic Reliability Analysis

허정원(Huh Jungwon),Haldar Achintya 대한토목학회 2007 대한토목학회 학술대회 Vol.2007 No.10

A Novel Risk Evaluation Procedure Using a Kriging-Based Surrogate Modeling for Offshore Structures

Sayyed Mohsen Vazirizade,Achintya Haldar 대한토목학회 2021 KSCE JOURNAL OF CIVIL ENGINEERING Vol.25 No.7

An innovative reliability estimation procedure for Jacket-type offshore platforms is proposed. The information on risk is extracted using multiple deterministic analyses using advanced mathematical theories resulting in compounding beneficial effects. The platforms are modeled by finite elements. Nonlinearity and major sources of uncertainty are incorporated. The wave loading model is realistically developed to satisfy the underlying physics. It is applied in three-dimension in time domain incorporating the uncertainties in the parameters. Implicit performance functions are expressed explicitly using several advanced factorial schemes and significantly improved response surface method. The efficiency and accuracy of the method are improved using a comprehensively improved Kriging-based surrogate modeling technique. The risk is evaluated for the serviceability and strength limit state functions for a jacket-type offshore platform using about two hundred nonlinear finite element analyses. The procedure was verified using simulation techniques. The concept can be considered an alternative to the conventional random vibration techniques and the standard Monte Carlo simulation procedure.

Seismic Fragility Assessment for A Newly Developed Buried Arch Bridge

Phu Van Dang,허정원,안진희,Achintya Haldar,곽기석 대한토목학회 2023 KSCE Journal of Civil Engineering Vol.27 No.11

This study aims to assess the seismic response of a newly developed buried arch bridge system. The proposed buried arch bridge system is comprised of precast concrete panels, external steel outriggers, and steel V-strips to solve the limitation encountered in the storage, transportation, and construction of traditional buried arch structures. The three-dimensional finite element analysis of this arch system is conducted under dynamic analysis emphasizing the frequency content of earthquakes. Then, the fragility curves are suggested for this arch system based on two damage states defined via incremental dynamic analysis. The results show that this proposed arch system is significantly affected under a high frequency content earthquake compared to the low and intermediate ones. Additionally, the arch system deflects the most at the mid-high position rather than at the arch crown. The deflection at the arch crown or at the mid-high position can be used for establishing the fragility curves as they both connect with the failure mechanism of the arch structure.