Collapse Capacity Research Articles

Dynamic loading history and collapse analysis of the pipe during deepwater S-lay operation

In the deepwater S-lay operations depending on the required stinger radius and rollers configuration, relatively large plastic deformation is induced when the pipe passes over the stinger, under the combined loadings of bending, axial tension, roller reaction force and the pipelay vessel motion. The resulting plastic deformation does not vanish after the pipe leaves the stinger. It accumulates until the pipe reaches the seabed. The inherited residual deformation might reduce the collapse capacity of the pipe under the external pressure loading. The present paper investigates the dynamic loading history of the pipe during the S-lay operation based on a test-verified finite element model, and then calculates the residual plastic deformation of the pipe cross-section after the pipe reaches the seabed. Finally, the nonlinear collapse analysis is implemented based on the modified RIKS method to evaluate the capacity of the installation-induced deformed pipe. The results confirm that the deepwater S-lay operation will lead to obvious plastic deformation of the pipe, which decreases the pipe collapse capacity to some extent.

Aftershock seismic assessment taking into account postmainshock residual drifts

SummaryThis paper introduces and evaluates a methodology for the aftershock seismic assessment of buildings taking explicitly into account residual drift demands after the mainshock (i.e., postmainshock residual interstory drifts,RIDRo). The methodology is applied to a testbed four‐story steel moment‐resisting building designed with modern seismic design provisions when subjected to a set of near‐fault mainshock–aftershock seismic sequences that induce five levels ofRIDRo. Once the postmainshock residual drift is induced to the building model, a postmainshock incremental dynamic analysis is performed under each aftershock to obtain its collapse capacity and its capacity associated to demolition (i.e., the capacity to reach or exceed a 2% residual drift). The effect of additional sources of stiffness and strength (i.e., interior gravity frames and slab contribution) and the polarity of the aftershocks are examined in this study. Results of this investigation show that the collapse potential under aftershocks strongly depends on the modeling approach (i.e., the aftershock collapse potential is modified when additional sources of lateral stiffness and strength are included in the analytical model). Furthermore, it is demonstrated that the aftershock capacity associated to demolition (i.e., the aftershock collapse capacity associated to a residual interstory drift that leads to an imminent demolition) is lower than that of the aftershock collapse capacity, which mean that this parameter should be a better measure of the building residual capacity against aftershocks. Copyright © 2014 John Wiley & Sons, Ltd.

Effect of gravity framing on the overstrength and collapse capacity of steel frame buildings with perimeter special moment frames

SummaryThis paper investigates the effect of the gravity framing system on the overstrength and collapse risk of steel frame buildings with perimeter special moment frames (SMFs) designed in North America. A nonlinear analytical model that simulates the pinched hysteretic response of typical shear tab connections is calibrated with past experimental data. The proposed modeling approach is implemented into nonlinear analytical models of archetype steel buildings with different heights. It is found that when the gravity framing is considered as part of the analytical model, the overall base shear strength of steel frame buildings with perimeter SMFs could be 50% larger than that of the bare SMFs. This is attributed to the gravity framing as well as the composite action provided by the concrete slab. The same analytical models (i) achieve a static overstrength factor,Ωslarger than 3.0 and (ii) pass the collapse risk evaluation criteria by FEMA P695 regardless of the assigned total system uncertainty. However, when more precise collapse metrics are considered for collapse risk assessment of steel frame buildings with perimeter SMFs, a tolerable probability of collapse is only achieved in a return period of 50 years when the perimeter SMFs of mid‐rise steel buildings are designed with a strong‐column/weak‐beam ratio larger than 1.5. The concept of the dynamic overstrength,Ωdis introduced that captures the inelastic force redistribution due to dynamic loading. Steel frame buildings with perimeter SMFs achieve aΩd > 3 regardless if the gravity framing is considered as part of the nonlinear analytical model representation. Copyright © 2014 John Wiley & Sons, Ltd.

Aftershock collapse vulnerability assessment of reinforced concrete frame structures

SummaryIn a seismically active region, structures may be subjected to multiple earthquakes, due to mainshock–aftershock phenomena or other sequences, leaving no time for repair or retrofit between the events. This study quantifies the aftershock vulnerability of four modern ductile reinforced concrete (RC) framed buildings in California by conducting incremental dynamic analysis of nonlinear MDOF analytical models. Based on the nonlinear dynamic analysis results, collapse and damage fragility curves are generated for intact and damaged buildings. If the building is not severely damaged in the mainshock, its collapse capacity is unaffected in the aftershock. However, if the building is extensively damaged in the mainshock, there is a significant reduction in its collapse capacity in the aftershock. For example, if an RC frame experiences 4% or more interstory drift in the mainshock, the median capacity to resist aftershock shaking is reduced by about 40%. The study also evaluates the effectiveness of different measures of physical damage observed in the mainshock‐damaged buildings for predicting the reduction in collapse capacity of the damaged building in subsequent aftershocks. These physical damage indicators for the building are chosen such that they quantify the qualitative red tagging (unsafe for occupation) criteria employed in post‐earthquake evaluation of RC frames. The results indicated that damage indicators related to the drift experienced by the damaged building best predicted the reduced aftershock collapse capacities for these ductile structures. Copyright © 2014 John Wiley & Sons, Ltd.

Impact of earthquake ground motion characteristics on collapse risk of post-mainshock buildings considering aftershocks

This paper investigates the influence of duration and frequency content of earthquakes on the collapse risk of post-mainshock buildings accounting for four damage states. The 5–95% significant duration Ds and the mean period Tm are selected as the index parameters to represent the duration and frequency content of ground motions, respectively. The modified Ibarra–Krawinkler hysteretic model is used in the structural models to capture strength and stiffness degradation associated with structural damages. The ground motion intensity is measured by inelastic spectral displacement (Sdi) to implicitly capture the spectral shape effect. Structural collapse capacities are determined using a suite of 62 records with a broad range of earthquake ground motion characteristics. The results demonstrate that both the duration and frequency content of ground motion play a significant role in structural collapse capacity. The degree of influence of aftershock characteristics on post-mainshock building collapse capacities becomes more significant as the structural damage level from the mainshock increases. As a result, post-mainshock structures with more serious damage states may be more fragile when subjected to the aftershocks with longer duration and lower frequency. Aftershocks are usually characterized by shorter duration and higher frequency than those of the corresponding mainshocks, thus the collapse risk of post-mainshock buildings may not be properly estimated using seeding scaled mainshock record as an aftershock compared to those using as-recorded mainshock–aftershock sequences.

Collapse capacity of soil‐structure systems under pulse‐like earthquakes

SummaryIn this paper, a comprehensive study is carried out to examine the possibility of dynamic instability produced in soil‐structure systems using an ensemble of 50 pulse‐like records. A number of structural models with various vibration periods varying from 0.1 to 2 s are used in this study. The superstructure is simulated as a non‐linear SDOF oscillator with a two‐segment backbone curve having negative post‐yield stiffness. The soil is idealized based on the cone model concept widely used for practical purposes. The results of this investigation demonstrate that as the pulse period increases, the collapse relative lateral strength ratio decreases and probability of dynamic instability enhances. Moreover, soil flexibility makes the system dynamically more unstable, and as the non‐dimensional frequency increases, the collapse relative lateral strength ratio highly reduces. Additionally, the aspect ratio has insignificant effects on the collapse relative lateral strength ratio. Furthermore, comparison of the collapse relative lateral strength ratios resulting from pulse‐like motions with those obtained from studies under non‐pulse‐like motions (Miranda and Akkar; FEMA 440) for fixed‐base conditions shows that high‐velocity pulses exacerbate the dynamic instability problem and decrease the collapse relative lateral strength ratio. Copyright © 2014 John Wiley & Sons, Ltd.

Effect of P-delta uncertainty on the seismic collapse capacity and its variability of single-degree-of freedom systems

This study assesses the effect of parameter uncertainty of non-deteriorating P-delta vulnerable single-degree-of-freedom systems on the median and dispersion of the collapse capacity. The post-yielding negative slope is a necessary condition of P-delta induced collapse that dominates the failure mode, and thus, it is the primary system parameter to be considered as a random variable. The parameter uncertainty on the collapse capacity is quantified with the first-order-second-moment method, and verified with the Latin hypercube sampling (LHS) technique. The total variability of the collapse capacity is estimated by combining the parameter uncertainty with record-to-record variability according to the square-root-of-sum-of-squares rule. Alternatively, the total variability of the collapse capacity is obtained from LHS realizations that simultaneously account for uncertainty of the post-yielding negative stiffness ratio and the earthquake excitation. The importance of uncertain post-yielding negative slope on the collapse capacity is underlined, and the main observations of the parameter uncertainty and total uncertainty of the collapse capacity are discussed.

Development of Seismic Collapse Capacity Spectra and Parametric Study

Single-Degree-of-Freedom (SDOF) systems are widely used to investigate the collapse resistance of building structures. A SDOF model capable of representing the key properties associated with earthquake-induced collapse is outlined in this study. The seismic collapse capacity of SDOF systems is evaluated via an incremental dynamic analysis, based on which the collapse capacity spectrum is developed. The computational procedure and engineering significance of the collapse capacity spectrum are elaborated in some detail. The influence of various structural properties on the collapse resistance of SDOF structural systems is also comprehensively discussed, which include fundamental period, ductility ratio, post-capping stiffness, hysteretic pinching, cyclic deterioration and P-Δ effect. Furthermore, the influence of ground motion features on the collapse resistance of SDOF systems is examined by using the proposed collapse capacity spectrum.

Seismic Collapse Capacity–Based Evaluation and Design of Frame Buildings with Viscous Dampers Using Pushover Analysis

A simplified procedure is developed for estimating the seismic sidesway collapse capacity of frame building structures incorporating linear and nonlinear viscous dampers. The proposed procedure is based on a robust database of seismic peak displacement responses of viscously damped nonlinear single-degree-of-freedom systems for various seismic intensities and uses nonlinear static (pushover) analysis without the need for nonlinear time history dynamic analysis. The proposed procedure is assessed by comparing its collapse capacity predictions on 1,190 different building models with those obtained from incremental nonlinear dynamic analyses. A straightforward collapse capacity-based design procedure aimed at achieving a predetermined probability of collapse under maximum considered earthquake event is also introduced for viscously damped structures without extreme soft story irregularities.

Validation of Ground-Motion Simulations through Simple Proxies for the Response of Engineered Systems

We propose a list of simple parameters that act as proxies for the response of more complicated engineered systems and therefore can be studied to validate new methods of ground-motion simulation for engineering applications. The primary list of parameters includes correlation of spectral acceleration across periods, ratio of maximum-to-median spectral acceleration across all horizontal orientations, and the ratio of inelastic-to-elastic displacement, all of which have reliable empirical mod- els against which simulations can be compared. We also describe several secondary parameters, such as directivity pulse period and structural collapse capacity, that do not have robust empirical models but are important for engineering analysis. We then demonstrate the application of these parameters to exemplify simulations computed using a variety of methods, including stochastic finite fault, Graves-Pitarka hybrid broadband, and a composite source model. In general, each simulation method matches empirical models for some parameters and not others, indicating that all rel- evant parameters need to be carefully validated. Online Material: Tables of ground-motion records and simulations selected to have comparable response spectra, and MATLAB code to compute simple proxies for the response of engineering systems.

Assessment of seismic collapse uncertainties of steel moment-resisting frames by means of single-degree-of-freedom systems

The collapse evaluations presented to date are of various approximations, such as significant degrees of uncertainty in ground motion characterization and structural collapse modeling. Accordingly, determining the variance of collapse capacity has an important role in evaluation of the seismic risk of structures. However, its process includes time-consuming steps, for example, incremental dynamic analysis and nonlinear dynamic collapse analyses of the archetype models. In order to save time and preserve accuracy, this study attempts to assess the seismic collapse uncertainty of single-degree-of-freedom systems for evaluating the same uncertainties in steel moment frames. A combination of extended incremental dynamic analysis and Latin hypercube sampling method was utilized to study the epistemic and aleatory effects on multi-story steel moment frames (3-, 5-, 8- and 15-story frames) as well as single-degree-of-freedom models. The results indicated that due to aleatory and epistemic uncertainties, the variance of collapse capacity in three-, five- and eight-story frames was close to the results of single-degree-of-freedom systems in which P-Δ effects were assumed as small, whereas the variance of collapse capacity of a 15-story frame ( T1>2 s) was compatible with the results of single-degree-of-freedom systems when P-Δ effects were considered large.

Computational Approach for Collapse Assessment of Concentrically Braced Frames in Seismic Regions

This paper proposes a computational approach for the collapse assessment of concentrically braced frames (CBFs) subjected to earthquakes. Empirical formulations for modeling the postbuckling behavior and fracture of three main steel brace shapes that are commonly used in CBFs are developed. These formulations are based on extensive calibrations of a fiber-based steel brace model with available information from a recently developed steel brace database. As part of the same computational approach, the representation of strength and stiffness deterioration associated with plastic hinging in steel columns and gusset-plate beam-to-column connections is considered. Through a case study of a 12-story Special Concentrically Braced Frame (SCBF), the influence of classical damping on the collapse capacity of CBFs is investigated. It is demonstrated that when SCBFs attain a negative stiffness during an earthquake, their collapse capacity can be significantly overestimated, if viscous damping is based on a commonly employed Rayleigh assumption with initial stiffness approximation. It is shown that sidesway collapse of CBFs should be traced based on a combination of criteria that associate large story drift ratios and the story shear resistance of a CBF at the corresponding story drift ratios. © 2014 American Society of Civil Engineers.

철골중간모멘트골조의 붕괴성능 II

연속하는 두 논문중 첫 번쩨 논문에서 도출한 철골중간모멘트골조의 붕괴성능에 가장 큰 영향을 미치는 요인중 하나가 기둥의 소성변형능력이라는 점에 주목하여 최대급지진에서 붕괴방지성능을 만족시키는 기둥의 소성변형능력을 규명하고자 한다. 이를 위해서 첫 번쩨 논문에서 소개한 세 개의 표본건물을 이용하여 40개 지진파에 대한 증분동적해석을 수행하였고 FEMA355F의 신뢰도계수법과 FEMA-P695의 방법론을 이용하여 붕괴성능을 평가하였다. The first paper of continuing two papers observed that the one of most important factor of determining collapse capacity is plastic deformation capacity of column. This study investigated the plastic deformation capacity satisfying collapse prevention level in maximum considered earthquake. For this purpose, incremental dynamic analysis was performed with the prototype structure introduced in former paper using 40 ground motion records. The Collapse capacity was evaluated with confidence factor method of FEMA355F and methodology of FEMA-P695.

철골중간모멘트골조의 붕괴성능 I

국내 구조기준인 KBC2009는 성능기반설계의 설계지진에서의 인명안전수준을 만족시키기 위한 기준은 명시적으로 제시하고 있지만 최대급지진에서의 붕괴방지수준을 만족하기위한 기준은 암시적으로 제시하고 있다. 따라서 본 연구에서는 층수별로 구분된 세 개의 철골중간모멘트골조의 표본건물을 설계 및 모델링하였다. 비선형 시간이력해석을 통하여 설계지진에서의 인명안전수준의 만족여부와 최대급지진에서 붕괴방지수준의 만족여부를 조사하였다. 그리고 이러한 내진성능목표들을 달성하기 위한 붕괴성능에 가장 큰 영향을 미치는 변수인 저층부 기둥의 소성변형능력을 변화시켜 표본건물의 전체적, 국부적 동적응답을 관찰하였다. Domestic structural design code, KBC2009 specify the criteria for satisfying life safety level in design earthquake. But criteria for satisfying collapse prevention level in maximum considered earthquake are stated allusively. This study designed and modeled three prototype structure of steel intermediate moment resisting frame classified with the number of story. Through nonlinear time history analysis, adequacy of life safety level in design earthquake and collapse prevention level in maximum considered earthquake are evaluated in this study. global and local dynamic response of prototype structures were observed changing the plastic deformation capacity which is one of the most important factor in collapse capacity to achieve two seismic capacity goals; life safety level in design earthquake and collapse prevention level in maximum considered earthquake.

Modeling of the composite action in fully restrained beam‐to‐column connections: implications in the seismic design and collapse capacity of steel special moment frames

SUMMARYThis paper investigates the effect of the composite action on the seismic performance of steel special moment frames (SMFs) through collapse. A rational approach is first proposed to model the hysteretic behavior of fully restrained composite beam‐to‐column connections, with reduced beam sections. Using the proposed modeling recommendations, a system‐level analytical study is performed on archetype steel buildings that utilize perimeter steel SMFs, with different heights, designed in the West‐Coast of the USA. It is shown that in average, the composite action may enhance the seismic performance of steel SMFs. However, bottom story collapse mechanisms may be triggered leading to rapid deterioration of the global strength of steel SMFs. Because of composite action, excessive panel zone shear distortion is also observed in interior joints of steel SMFs designed with strong‐column/weak‐beam ratios larger than 1.0. It is demonstrated that when steel SMFs are designed with strong‐column/weak‐beam ratios larger than 1.5, (i) bottom story collapse mechanisms are typically avoided; (ii) a tolerable probability of collapse is achieved in a return period of 50 years; and (iii) controlled panel zone yielding is achieved while reducing the required number of welded doubler plates in interior beam‐to‐column joints. Copyright © 2014 John Wiley & Sons, Ltd.

비선형 증분동적해석을 통한 철골 중간모멘트 골조의 붕괴성능 평가

철골 중간모멘트골조는 강한 지반운동에 대하여 적합한 저항능력을 확보하기 위한 지진력저항시스템으로서 일반적으로 사용되고 있다. 하지만 국내의 대다수 중 저층 철골건축물은 내진설계가 도입되기 이전에 건설되었거나 현재의 내진설계기준의 요구조건을 준수하지 않은 것들로, 이러한 건물들이 가지는 내진성능에는 의문점이 존재한다. 이와 같은 문제점의 인식에 기반하여 본 연구에서는 국내 철골 중간 모멘트골조의 내진성능에 대한 정량적 제시를 목표로 우선 층수 종류, 지진에 대한 보유내력, 부재 연성도, 제진장치의 유무를 변수로 하여 표본 건물을 설계하였다. 표본 건물의 내진 성능과 붕괴 매커니즘은 비선형 정적해석과 증분동적해석으로부터 획득한 붕괴여유비와 붕괴확률을 이용하여 분석하였다. 해석결과를 통하여 현행 국내기준에 따라 내진설계된 신축건물은 설계지진에 대해 충분한 내진성능을 가졌으며, 이에 반해 구조부재의 연성저감이 발생하거나 낮은 설계 밑면전단력에 대한 저항력을 가진 기존건물의 경우에는 높은 붕괴확률을 가지며 목표로 한 내진성능을 만족시키지 못하는 것으로 나타났다. 이와 같은 내진성능을 충족시키지 못하는 내진설계 도입 이전의 건물에 대해서 에너지 소산장치를 통해 보강하게 되면 장치의 에너지 소산능력뿐만 아니라 소성힌지의 재분배를 통해 붕괴확률 및 내진성능이 신축건물 수준으로 향상되었다. Steel intermediate moment frames (IMFs) have been generally used as seismic load resisting systems (SLRSs) of a building to provide resistances against strong ground shaking. However, most of low and mid-rise steel buildings in Korea were constructed during pre-seismic code era or before the introduction of well-organized current seismic codes. It has been recognized that the seismic performance of these steel IMFs is still questionable. In order to respond to such a question, this study quantitatively investigates the seismic capacities of steel IMFs. Prototype models are built according to the number of stories, the levels of elastic seismic design base shear and the ductilities of structural components. Also, the other prototype models employing hysteretic energy dissipating devices (HEDDs) are considered. The collapse mechanism and the seismic performance of the prototype models are then described based on the results obtained from nonlinear-static and incremental-dynamic analyses. The seismic performance of the prototype models is assessed from collapse margin ratio (CMR) and collapse probability. From the assessment, the prototype model representing new steel IMFs has enough seismic capacities while, the prototype models representing existing steel IMFs provide higher collapse probabilities. From the analytic results of the prototype models retrofitted with HEDDs, the HEDDs enhance the seismic performance and collapse capacity of the existing steel IMFs. This is due to the energy dissipating capacity of the HEDDs and the redistribution of plastic hinges.

Effects of Different Record Selection Methods on the Transverse Seismic Response of a Bridge in South Western British Columbia

The seismic response of a continuous 4-span bridge designed according to the current Canadian seismic provisions is investigated using Incremental Dynamic Analysis (IDA). Different earthquake types, including shallow crustal events, interface Cascadia subduction, and deep inslab subduction are considered. The median collapse capacities calculated using different record selection methods including Conditional Mean Spectrum (CMS)-based, Uniform Hazard Spectrum (UHS)-based, and epsilon-based methods are compared. The use of the epsilon-based method generally resulted in the highest collapse capacity predictions, but the CMS-based method was less sensitive to the number of records considered in the IDA.

Simplified Seismic Sidesway Collapse Capacity-Based Evaluation and Design of Frame Buildings with Linear Viscous Dampers

This article describes a simplified procedure for estimating the seismic sidesway collapse capacity of frame building structures incorporating linear viscous dampers. The proposed procedure is based on a robust database of seismic peak displacement responses of viscously damped nonlinear single-degree-of-freedom systems for various seismic intensities and uses nonlinear static (pushover) analysis without the need for nonlinear time history dynamic analysis. The proposed procedure is assessed by comparing its collapse capacity predictions on 272 different building models with those obtained from incremental dynamic analyses. A straightforward collapse capacity-based design procedure is also introduced for structures without extreme soft story irregularities.

A new proxy for ground motion selection in seismic collapse assessment of tall buildings

SUMMARYTall buildings are long‐period structures that are sensitive to the long‐period content of ground motions. Selection of appropriate ground motions is an important step in seismic collapse assessment of tall buildings using nonlinear dynamic analyses. Epsilon (εSa) and eta (η) are two spectral shape indicators, which have been recently proposed for ground motion selection in the technical literature. In this study, a new parameter gamma (γ) is proposed, which has considerable correlation with the collapse capacity of long‐period structures having a fundamental period greater than 1 s. This parameter is a linear combination of εSa and the displacement spectrum intensity epsilon (εDSI). The parameter γ is obtained and optimized by applying the particle swarm optimization algorithm. Since γ has significant correlation with the collapse capacity of long‐period structures, it can be used as an efficient proxy for ground motion selection in seismic collapse assessment of tall buildings. The results of this study show that ground motion selection considering the new proxy γ causes reduction in the dispersion of structural response and also decrease in the mean annual frequency of collapse, when compared with ground motion selection based on εSa and η. Copyright © 2013 John Wiley & Sons, Ltd.

Importance of seismic design accidental torsion requirements for building collapse capacity

SUMMARYSeismic ground motions induce torsional responses in buildings that can be difficult to predict. To compensate for this, most modern building codes require the consideration of accidental torsion when computing design earthquake forces. This study evaluates the influence of ASCE/SEI 7 accidental torsion seismic design requirements on the performance of 230 archetypical buildings that are designed with and without accidental torsion design provisions, taking building collapse capacity as the performance metric. The test case archetypes include a broad range of heights, gravity load levels, and plan configurations. Results show that the ASCE/SEI 7 accidental torsion provisions lead to significant changes in collapse capacity for buildings that are very torsionally flexible or asymmetric. However, only inconsequential changes in collapse capacity are observed in the buildings that are both torsionally stiff and regular in plan. Therefore, the study concludes that accidental torsion provisions are not necessary for seismic design of buildings without excessive torsional flexibility or asymmetry. Copyright © 2013 John Wiley & Sons, Ltd.