Probability Of Collapse Research Articles

Assessing the feasibility of achieving functional recovery goals through seismic retrofit of existing reinforced concrete buildings

Damage from past earthquakes has significantly hampered post-earthquake building function, threatening community resilience, and motivating consideration of functional recovery in building design and assessment. This study examines whether it is feasible to achieve functional recovery in retrofit of existing reinforced concrete buildings, focusing on seven buildings retrofit with various motivations and strategies. The seismic response of these buildings was nonlinearly simulated, and functional recovery was probabilistically assessed. The results show that retrofits targeting life safety may or may not achieve functional recovery goals. Achieving functional recovery depends especially on the reduction of drift demands and collapse probability. However, the acceleration increase associated with many retrofits can increase function loss due to the criticality of acceleration-sensitive nonstructural components if such components are not retrofitted. We also examine other performance metrics, that is, economic losses and immediate occupancy limits of ASCE/SEI 41, showing that these provide imprecise, and in the case of the immediate occupancy conservative, proxies for functional recovery.

A Seismic Risk Assessment of Concrete-Filled Double-Skin Steel Tube (CFDST) Frames with a Beam-Only-Connection for Reinforced Concrete Shear Walls (BRWs)

The beam-only connected reinforced concrete shear wall (BRW) is used as a reinforcing component to enhance the seismic performance of concrete-filled, double-skin steel tube (CFDST) frames. The effects of the BRW on seismic risks of CFDST frames are investigated. Three performance levels of limit states are defined and described according to the typical failure of test specimens and the existing definition of current guidance. A simplified numerical model is calibrated for CFDST frame-BRW structures, and test results validate it. Nonlinear dynamic analyses on a nine-story CFDST-BRW building are performed to investigate the effects of BRW on reducing the seismic risk of CFDST buildings. The results show that the BRW reduces the probability of collapse of the CFDST frame to 2.76% in 50 years, which can effectively reduce the probability of different degrees of damage in the service cycle of the structure. The results provide information for risk-informed decision-making on the design of CFDST frame-BRW structures.

Multi-index assessment of road blockage risk due to seismic event-induced building debris

The accessibility of road systems is key to facilitating emergency responses in the aftermath of a severe disaster. However, collapsed buildings and debris may cause road blockages, impeding disaster relief efforts after a major earthquake. This study aims to present an efficient and generic assessment method for evaluating the risk of road blockage due to seismic event-induced debris. First, the probability of building collapse in city blocks and the effects of building debris after an earthquake scenario are estimated based on building types, seismic design levels, soil conditions, and local ground motion intensities of the adopted earthquake scenario. Subsequently, the seismic risk of road sections is assessed by considering the collective effects of nearby street profiles, including building materials, heights, and the shape factor of city blocks. This study considers various interpretive building environment parameters and employs three post-earthquake risk indexes to represent different aspects of seismic risk of a road system, including passability loss, affected road length ratio, and road interruption severity. To demonstrate the applicability of the proposed method, the rescue routes in Taipei City, Taiwan, are adopted as a case study. The proposed method can effectively distinguish high-risk roads as valuable information, and the results have been provided to local government disaster relief units for reference.

Collapse probability and resistance factor calibration of 2D steel frames under gravity loads

Collapse probability and resistance factor calibration of 2D steel frames under gravity loads

Seismic Performance of Multistory Chevron‐braced Steel Structures with Yielding Beams

AbstractLateral loads are resisted by braces in concentrically braced frames. Therefore, braces meet the demand for the load combination based on the seismic provisions. The rest of the system including the connections are determined according to the capacity‐based design, where the brace members yield under tension and buckle under compression. Chevron or inverted V‐braced frames are generally preferred because they allow large openings for doors and windows. However, the seismic‐design requirements in current building codes lead to deep, heavy chevron beams to resist the unbalanced load at beam members due to pre‐ and post‐buckling compressive strength deterioration and full tensile yielding of the braces. This results in uneconomical design. Recent large‐scale experiments of chevron‐braced frames have demonstrated that limited beam yielding is not detrimental to chevron‐braced frame behavior and on the contrary, it utilizes a second yielding mechanism and thereby increases the story drift capacity at which the braces fracture. Also, they propose a new design method for chevron braced frame with yielded beam. This paper expands the previous findings by investigating the seismic performance of 9‐story structures which are designed according to provisions and proposed method. Structural analysis program SAP2000 is utilized for linear design by modelling the full‐scale 9‐story building. Nonlinear finite element analysis software Perform 3D is utilized for the nonlinear time history analysis in which only a single chevron braced frame is modelled. Results show that the proposed building design has larger interstory drift ratios. However, the adjusted collapse margin ratio and the collapse probability for maximum considered earthquake satisfy the required limitations. In other words, the proposed chevron frame design offers a more economical alternative with no significant tradeoff in seismic performance.

Effects of collapse probability on cascading failure dynamics for duplex weighted networks

Effects of collapse probability on cascading failure dynamics for duplex weighted networks

Seismic performance evaluation of self-centering balloon-framed CLT building

Balloon-framed cross-laminated timber (CLT) construction has a number of advantages when compared to platform-type construction; however, many international standards only include provisions for the latter as there is little research reported on the seismic performance of the former. In this paper, a seismic fragility assessment of a 4-storey balloon-framed CLT building in Vancouver, Canada, is presented. This structure is the first in North America that adopts a self-centering friction-based technology for the hold-downs (HDs). The seismic performance of the building was assessed by performing incremental dynamic analysis (IDA) on a three-dimensional finite element model using Cascadia Subduction Zone ground motions, and compared to a second building model with conventional dowel-type HDs. The analyses at the design intensity level showed that the self-centering building had an average maximum inter-story drift ratio of 0.67%, well below the 2.5% drift limit specified in the National Building Code of Canada. Based on the IDA and taking into account uncertainties, the building had a collapse margin ratio of 2.96 and a 5.2% probability of collapse at the design level. Compared to the conventional HDs, the friction-based HDs did not improve the building’s collapse capacity, but reduced the building drift by more than 20% at lower damage states, demonstrating the effectiveness of designing balloon-framed CLT structures with a resilient self-centering HDs in high seismic zones.

Exploring risk-targeted ground motions and seismic design spectra for mainland China

Seismic design under the paradigm of uniform hazard only considers the seismic hazard of building sites, which will lead to inconsistent collapse risks of buildings in different areas. To achieve uniform collapse risk, the paradigm of conventional seismic design with uniform hazard should be shifted to the new framework of uniform risk, which accounts for both seismic hazard of sites and collapse fragility of buildings. In this study, based on the principle of risk targeting, parametric sensitivity analyses of risk-targeted ground motions for mainland China are performed considering different risk categories and site classes. Then, the risk-targeted peak ground accelerations and the uniform-risk seismic design spectra are obtained for mainland China. Seismic risk assessment is conducted for an 8-story reinforced concrete frame structure using the ground motions selected based on the uniform-risk seismic design spectra. The results indicate that the influence of logarithmic standard deviation of the target collapse fragility function on the risk-targeted maximum considered earthquake gradually increases with risk category. The impact of target collapse risk on risk-targeted maximum considered earthquake of a hard site is more remarkable than that of other sites. The effects of the logarithmic standard deviation and the conditional collapse probability under the maximum considered earthquake on the collapse risk gradually increase with the site class. In addition, the construction of uniform-risk seismic design spectra takes into account both seismic hazard and structural fragility. It is reasonable to select ground motions based on uniform-risk seismic design spectra for conducting seismic risk assessment of structures.

Fuzzy Multivariate Regression Models for Seismic Assessment of Rocking Structures

The assessment of rocking response is a challenging task due to its high nonlinearity. The present study investigates two methodologies to evaluate finite rocking rotations and overturn of three typical rocking systems. In particular, fuzzy linear regression (FLR) with triangular fuzzy numbers and a hybrid model combining logistic regression and fuzzy logic were adopted. To this end, three typical rocking structures were considered, and nonlinear time history analyses were performed to obtain their maximum response. Eighteen seismic intensity measures (IMs) extracted from recorded seismic accelerograms were considered to predict the responses. In the absence of rocking overturn, the finite rocking rotations and similarity ratios were calculated by adopting the FLR method. Moreover, extensive analysis was performed to evaluate the influence of each IM on the model’s predictions. On the other hand, rocking overturn was evaluated by logistic regression to compute the probability of collapse, followed by the FLR method to estimate the similarity between the different rocking-based structural systems. The root mean square error (RMSE) parameter and the log loss function were determined for every model to assess the predictions that emerged from the two fuzzy methods. As indicated, both methods demonstrated satisfactory results, presenting minimal deviations from the observed values. Finally, in the case of finite rocking rotation predictive models, remarkably high similarity ratios were observed among the various structures, with a median value of 0.96.

Fragility Analysis of Transmission Towers Subjected to Downburst Winds

A downburst is a typical local highly intensive wind all over the world, which is attributed to be the main cause of wind damage to transmission lines in inland areas worldwide. The collapse accidents of transmission towers under the downburst still occur every year. Therefore, it is of great significance to assess the safety of the transmission towers under downbursts. The motivation of the present study is to propose a fragility assessment method for transmission towers under the action of a downburst considering the uncertainty of wind-resistance capacity and the stochastic wind load effect. First, the downburst wind field of the transmission tower with different wind attack angles and different radial distances is simulated according to the mixed stochastic model. Then, random material characteristic samples are generated by the Latin hypercube sampling technique and applied to establish uncertain finite element models for transmission towers. Next, the static nonlinear buckling analysis is carried out by numerical methods to determine the ultimate capacity under the downburst wind load. The parameter analysis of different wind attack angles and radial distances between the downburst and the tower is conducted to determine the most unfavorable location of the maximum response. The failure mode of the transmission tower and the probabilities of the initial failure main members are summarized. Finally, the fragility curves of the transmission tower under the downburst and the atmospheric boundary layer (ABL) wind are compared. The results show that the maximum response is located at R = 1.6D. Most of the initial buckling members are located close to the first section of the tower. The fragility curves of the tower under the downburst are more dangerous than the ABL wind with the attack angle increasing from 0° to 90°. Furthermore, considering the probability model of intensity and direction of the downburst and based on the previous fragility analysis, the collapse probability of the transmission tower caused by the downburst is obtained. By probability analysis of the parameters, including layout conditions, different directions, and different wind speeds, it is found that the most favorable arrangement is 157.5°, and the most unfavorable arrangement is 112.5°.

Effect of brace failure as capacity-based design component on the EBF collapse safety

Eccentrically braced frame (EBF) is considered a suitable seismic resisting system in terms of its high ductility, and stiffness simultaneously. In this frame, nonlinear behavior and energy dissipation are limited to link beam, and other members (capacity-designed members) are designed in proportion to maximum link shear force. Despite the significance of capacity-designed members, their reliability evaluation and the effect of their failure on the overall behavior of EBF have not been studied in prior research. The aim of this paper is to evaluate the effect of brace failure on the probability of collapse by considering system overstrength, varied ratios of the capacity to demand factors (ϕ/γ) in brace design, and seismic hazard curve. To achieve this aim, Incremental Dynamic Analysis (IDA) was used to study nonlinear structural models of EBF, evaluate the force demands on the braces, and the effects of brace failure on the system collapse. The reliability analysis was performed by integrating the viewpoint of LRDF component design with the FEMA P695 system reliability methodology. The results indicate that the probability of total collapse increases significantly at low spectral acceleration due to brace failure. Additionally, the appropriate value of ϕ/γ in brace design can be calculated for different seismic areas and system overstrength to ensure an acceptable margin of collapse safety.

Evaluation of performance and storey drift ratio limits of high-rise structural systems with separated gravity and lateral load resisting systems using time history analysis and incremental dynamic analysis

To increase the efficiency of prefabricated construction of high-rise structure, a composite structural system with separated gravity and lateral load resisting systems (referred to as a ‘new separable system’) is proposed. Efficient numerical models of the composite frames, reinforced-concrete (RC) shear walls, RC coupling beams, and braces in the new separable system were developed using the finite-element program MSC.Marc. The fragility curves of these components were developed through parametric analysis and a literature review of previous research and database documents. Through time history analysis, the responses of roof displacement, storey drift, and residual drift were analysed and compared with those of the traditional composite frame–shear wall system. An incremental dynamic analysis (IDA) was performed to obtain the IDA curves of the new separable system and the traditional system, and the seismic fragility curves of the two systems were built. The seismic performance of the new separable system was compared with that of the traditional system in terms of the collapse probability, dynamic safety factor, and risk of seismic damage. The results indicated that for high-rise composite structural systems with separated gravity and lateral resisting systems under frequently occurring earthquakes, the reasonable range of the storey drift ratio limit in design is 1/800 to 1/1000, and a limit value of 1/1000 is recommended conservatively.

Capacity assessment of segment-to-segment joints in precast unbonded segmental bridge decks under vertical seismic mainshock and aftershocks

Precast segmental construction of concrete bridge decks has become increasingly popular due to short construction time and low cost compared with other construction methods. However, during earthquake excitation the segment-to-segment joints in post-tensioned bridge decks may constitute a weak part of the superstructure, particularly due to the lack of continuous rebar that is present in cast-in-place decks. This research investigates the effects of the vertical component of earthquakes on the joints of the superstructure of bridges in near-fault regions. Specifically, a sample non-continuous precast segmental bridge with unbonded cables and constructed with the span-by-span method, has been studied using a detailed two-dimensional non-linear Incremental Dynamic Analysis (IDA) and Nonlinear Pushover Analysis (NPO). The results indicate that the vertical component of the earthquake could significantly adversely affect the joints’ behavior, especially in the upward direction. This may cause failure in the critical joints at midspan earlier than other common collapse prevention (CP) limit states such as deck sliding on the seat. The joint failure occurs easier in the mainshock-damaged bridge than in the intact one. Hence, the collapse probability can increase by up to 47% for the considered cases.

Regional‐risk‐targeted seismic design: A novel approach for earthquake resistant design

AbstractCurrent seismic design codes define safety levels based on the seismic risk of individual structures, such as specifying a maximum probability of collapse in 50 years. The design remains the same whether the structure is located within a sparsely populated area or within a densely populated area provided that both areas have the same seismic hazard. A novel seismic design approach, referred to as regional‐risk‐targeted seismic design is proposed in which the design level is based on a target seismic risk at the regional scale, as opposed to at the individual structure level. The proposed approach, which is aimed at new constructions, allows local governments to define acceptable levels of regional seismic risk and modify the seismic design criteria contained in national standards to satisfy them. As the possible consequences of a seismic event increase at the regional level, the design level becomes more stringent to control the regional seismic risk to acceptable limits. A series of simplifications are considered to facilitate its application. We illustrate the application of the proposed regional‐risk‐targeted seismic design with three examples in the city of San Francisco, California. In these examples, we iteratively compute the modifications to the maximum seismic risk on individual structures required to comply with specific target levels of regional seismic risk. In the proposed approach, the maximum tolerable seismic risk in individual structures depends not only on the seismic hazard at the site, but also on the number of structures and density in the region of interest. It is concluded that the proposed seismic design approach provides a more rational tool for local government officials to set and calibrate seismic design criteria to comply with their locally‐defined acceptable levels of regional seismic risks within a probabilistic risk management framework.

Probabilistic Evaluation of Seismic Performance of Steel Buildings with Torsional Irregularities in Plan and Soft Story under Mainshock-Aftershock Sequence

Due to the tectonic plate movement, numerous aftershocks may occur following an earthquake. There are several real mainshock-aftershock ground motion records in which the peak ground acceleration of aftershock is greater than that of the mainshock. Furthermore, irregularities such as soft story and torsional irregularity may change the structural behavior under the influence of earthquakes. This study investigates the fragility curves associated with the three-, five-, and eight-story models with steel moment-resisting frames under the main earthquake and mainshock-aftershock sequence to probabilistically evaluate the aftershock impacts on the steel structures with irregularities in the plan and height. The seismic fragility curves were calculated for four damage levels by selecting the relative displacement capacity at seismic performance levels of slight, moderate, extensive, and complete damage from the US HAZUS code. The analysis was done using structural reliability relationships and incremental dynamic analysis with the OpenSees software platform. The analytical results showed that the structures with soft-story and torsional irregularity were more vulnerable with an increasing number of stories. Also, aftershocks were found to have a more destructive effect on low-rise models. The probability of structural collapse in a given peak ground acceleration for a damaged state due to the mainshock-aftershock sequence is higher than that of the mainshock sequence.

SMA-braced steel frames influenced by temperature: Practical modelling strategy and probabilistic performance assessment

This study presents a practical and efficient modelling approach for shape memory alloy (SMA)-braced steel frames under various temperatures, and examines their probabilistic performance affected by temperature. A one-dimensional constitutive model is developed in OpenSees to capture the superelasticity (SE), partial SE, shape memory effect (SME), and permanent yielding behaviors of SMA elements subjected to varying temperatures. The proposed modelling approach is partly validated against an experimental study newly conducted in the present work. The material-level investigation is followed by a comprehensive probabilistic performance assessment of a typical steel frame employing SMA-based self-centering braces (SMA-SCBs), where a practical and realistic modelling strategy for the SMA-SCB is developed to capture the degradation behavior, especially at low temperatures. Seismic risk assessment is conducted to evaluate the collapse resistance and resilience performance of the considered structures, with the influence of temperature highlighted. Among other important findings, it is shown that the decrease in environment temperature significantly increases the peak inter-story drift (PID) and residual inter-story drift (RID) responses. Under the maximum considered earthquake (MCE), the probability of collapse increases from 2.7% to 19.7% when the temperature decreases from 30 °C to −15 °C. From a self-centering capability point of view, the probabilistic of exceedance of 0.5% RID threshold soars from negligible to nearly 100%, considering the same temperature variation. The study highlights the fact that temperature plays a significant role in the annual rate of exceedance of all damage states, and warns that the idealized flag-shaped modelling approach for SMA components is extremely unsafe at low temperatures.

Probabilistic seismic performance assessment of steel moment‐resisting frames considering exposed column‐base plate connections with ductile anchor rods

AbstractThis paper presents a numerical study assessing the seismic performance of steel moment‐resisting frames (SMRFs) designed with ductile exposed column‐base plate (ECBP) connections employing yielding anchor rods. For their potential use as weak bases, the proposed ECBP detail is designed to accommodate plastic deformations in the anchors. The seismic performance of 2‐ and 4‐story archetype SMRFs with ECBPs is investigated to examine the effects of various base‐connection strengths on the frame collapse mechanisms and probabilities. To this aim, the ECBP connections of each frame are designed for a set of three strength levels, ranging from reduced seismic loads to capacity‐designed forces of the adjoining columns. These designs enable the base responses to vary from highly inelastic (i.e., weak‐base design) to elastic (i.e., strong‐base design) when subjected to earthquake‐induced ground shaking. Nonlinear time history analysis (NLTHA) is extensively performed, applying a suite of assembled ground‐motion sequences (i.e., two ground motions in series) to assess the ECBP connection response and the corresponding frame behavior. Fragility analyses accounting for only the first ground motions in each considered sequence (to derive fragility curves), and the entire ground‐motion sequences (to derive fragility surfaces) are also performed to evaluate the probabilities of frame collapse and base connection failure. Finally, key findings regarding the seismic performance of ductile ECBP connections and their effects on the frame collapse are discussed. The limitations of the study are also outlined.

Seismic performance of eccentrically braced frames with short-links: IDA approach using chilean earthquakes

In this research the seismic performance of eccentrically braced frames (EBF) with short links using Chilean earthquakes is studied. The study includes nonlinear static and dynamic analyses, calibration of numerical link models with experimental studies, and over 15,000 models analyzed. Results show that higher ductility and deformation are obtained in most archetypes, while models with 8 stories exhibit more stiffness and less ductile behavior. Furthermore, the strength capacity and ductility increase with the increment of the seismic zone and soft type of soil. Moreover, the research provides a table with the seismic design parameters for each archetype group comparing these values with Chilean seismic specifications. Finally, the study concludes that structures designed with more strength and stiffness have a lower collapse probability under extreme events. This research provides valuable information for structural design and includes fragility curves and a discussion of results that can be applied to structural design.

Seismic Performance of RC Moment Frame Buildings Considering SSI Effects: A Case Study of the New Venezuelan Seismic Code

The Soil–Structure Interaction (SSI) effect has been widely evidenced during several earthquakes around the world. In the Venezuelan context, the seismic event in Caracas in 1967 showed the significant consequences of designing buildings without considering the SSI effect. Nevertheless, limited research on the seismic performance of concrete moment frames (commonly used as structural systems in office and residential buildings in Venezuela and Latin America) considering the SSI effects has been developed, although there have been continuous updates to the Venezuelan Seismic Code. In this research, the influence of the SSI on the seismic performance of RC moment frame buildings designed according to the New Venezuelan Seismic Code was studied. An extensive numerical study of 3D buildings using concrete moment frames supported by mat foundations on sandy and clayey soils was performed. The response spectrum method, non-linear static analysis, and non-linear dynamic analysis were used to assess the seismic response of the archetypes studied. The results show that SSI effects can have a significant impact on the seismic response of RC moment frame buildings, increasing the interstory drift ratio and decreasing the shear forces. As is shown in fragility curves, the probability of collapse increases for cases with flexible bases in comparison to the cases of models with fixed bases. Additionally, in the 24-story archetype, the fixed-base model reached a maximum probability of collapse. Finally, a new proposal for the reduction of the strength-reduction factor (R) must be incorporated into the Venezuelan Seismic Code to improve the safety of the structures. Limitations in the use of RC moment frames must be incorporated for high-rise buildings since, as the present work demonstrates, for high-period structures, the normative provisions are not reached.

Sliding versus Rubber Bearings: Exploring the Difference in Collapse Probability

Sliding versus Rubber Bearings: Exploring the Difference in Collapse Probability