Surrogate modeling for seismic fragility prediction of masonry infilled RC frames

Reinforced concrete (RC) frames with masonry infills have continued to remain a popular construction typology across the globe, including regions characterized by moderate to high seismic activity. Owing to the brittle nature of the masonry infills, their influence on the seismic analysis and design of framed structures has been typically neglected. However, scientific literature and field reconnaissance surveys indicate that the strength, stiffness, and distribution of masonry infills within RC frames can significantly influence their seismic performance. Typical finite element modeling of masonry infills within RC frames for nonlinear time history analysis comprises of single or multiple struts modeling approaches that require detailed information on the characteristic back-bone curve of the masonry infill. However, past studies report considerable variability associated with the material parameters of masonry infills (such as strength and elasticity) that may affect the seismic response and fragility of RC framed structures. This paper proposes a novel framework that develops parameterized seismic fragility functions for infilled RC frames conditioned on critical infill material parameters in addition to ground motion intensity. Unlike typical unidimensional fragility functions, the parameterized multidimensional fragility models offer flexibility to efficiently asses the influence of infill material parameters on seismic vulnerability. Such models are developed in the present study through a systematic approach rooted in statistical learning techniques. Initially, an experimental design is devised that considers an optimal combination of infill material parameters for computer simulations. Next, the seismic response from these simulations is obtained to develop surrogate models that predict engineering demand parameters (e.g., interstory drifts) as a function of infill material parameters and ground motion intensity. Lastly, the seismic demands obtained from the surrogate models are compared with seismic capacity estimates to generate the parameterized seismic fragility functions. The proposed methodology is applied to a case-study low-ductility RC frame with masonry infills to underline the gain in computational efficiency and accuracy for seismic response and vulnerability prediction

Seismic Performance Evaluation of Masonry Infilled RC Frame Retrofitted with BRBs

Reinforced concrete (RC) frames with unreinforced masonry infill represents a widely used construction typology across the globe, including regions characterized by moderate to high seismicity. These structures have been often designed before the introduction of modern seismic design codes, are characterized by low ductility and high seismic vulnerability and are in need for seismic retrofitting to meet the current safety standards. However, it is important to highlight that, although considered as non-structural elements, masonry infills can significantly affect the seismic response of the structure. However, their role on the seismic performance of retrofitted RC structures has been generally neglected in literature. Among the different retrofitting strategies, the use of buckling-restrained braces (BRBs) represents an effective solution to improve the seismic performance of existing RC structures. This study investigates the interaction between the BRBs and masonry infill on seismic response of a case study frame

Influence of atmospheric corrosivity on the seismic fragility of low-code steel frame structures

Low-code steel moment-resisting frames (pre-Northridge) are characterised by high seismic vulnerability due to their reduced ductility capacity. Moreover, these structures are exposed to atmospheric corrosion deterioration due to environmental corrosive agents. Corrosion deterioration leads to section mass loss, stiffness degradation, and loss of energy dissipation capacity, among others. Thus, based on the corrosive category, old steel structures could experience considerable variations in their seismic performance. The present study examines the effect of different corrosivity categories on the seismic vulnerability of steel frames. A non-seismically designed three-storey moment-resisting frame is selected for case-study purposes and exposed to increasing corrosivity categories (C3, C4, C5, and CX) as per ISO 9223: 2012. As per ISO 9224:2012, atmospheric corrosion is assessed considering a 50-year ageing time and uniform corrosion. The seismic performance of the pristine and ageing steel frames is evaluated through Incremental Dynamic Analyses (IDAs) considering a suite of 43 ground motion records to account for the record-to-record variability. The seismic performance under different exposure categories is evaluated by monitoring local and global engineering demand parameters (EDPs), allowing the development of seismic fragility functions at components- and system-levels

Impact of corrosion deterioration on the seismic performance of steel frame structures

Steel structures designed before the introduction of modern seismic design codes may be characterised by high seismic vulnerability due to their reduced ductility capacity. Additionally, these structures may be affected by significant corrosion deterioration, as one of the major atmospheric degradation phenomena when built in corrosive environments. Corrosion deterioration leads to a thickness reduction of sections, reduced bearing capacity, stiffness degradation and loss of energy dissipation capacity. Thus, old-corroded steel structures located in seismically active regions could experience a reduction of their seismic performance, significantly increasing the failure probability under earthquake events. The present study investigates the effect of atmospheric corrosion deterioration on steel frames and uses a nonseismically designed three-storey moment-resisting frame for case-study purposes. Atmospheric corrosion models based on the recommendation of ISO 9224:2012 have been adopted considering a 50-years ageing time and modelled as uniform corrosion on steel members. A probabilistic seismic performance assessment of the pristine and ageing steel frames is performed through Incremental Dynamic Analyses (IDAs). IDAs are performed for a set of 43 ground motion records accounting for the influence of the earthquake input’s uncertainty (i.e., the record-to-record variability). The corrosion effects on the seismic performance are evaluated by monitoring both global and local engineering demand parameters (EDPs), allowing the development of seismic fragility functions at components- and system-level

Influence of corrosion on failure modes and lifetime seismic vulnerability assessment of low‐ductility RC frames

Corrosion of reinforced concrete (RC) structures constitutes a critical form of environmental deterioration and may significantly increase the vulnerability of old non‐seismically designed buildings during earthquake events. This study proposes a probabilistic framework to evaluate the influence of corrosion deterioration on the lifetime seismic fragility of low‐ductility RC frame buildings. In contrast to limited past literature on this topic, the proposed framework offers novel contributions. This is one of the first study to consider potential alteration in failure modes of building components (from flexure to flexure‐shear) due to the time‐dependent aging process. Numerical models validated with past experimental test results are utilized to capture these failure modes, which are particularly relevant for low ductility RC frames designed prior to the introduction of modern seismic codes. Secondly, given the gamut of uncertainties associated with the corrosion process, this study develops condition‐dependent seismic fragility functions independent from an assumed exposure scenario, as often done in literature. These functions can be easily adopted by design engineers and stakeholders for prompt fragility assessment, and subsequent decision‐making without the need for computationally expensive finite element (FE) model runs. The proposed framework is demonstrated on a benchmark three‐story RC frame that considers time‐varying seismic demand models and damage state thresholds while accounting for the uncertain corrosion deterioration process and ground motion record‐to‐record variability

Seismic Damage Accumulation of Highway Bridges in Earthquake Prone Regions

Civil infrastructures, such as highway bridges, located in seismically active regions are often subjected to multiple earthquakes, such as multiple main shocks along their service life or main shock-aftershock sequences. Repeated seismic events result in reduced structural capacity and may lead to bridge collapse causing disruption in normal functioning of transportation networks. This study proposes a framework to predict damage accumulation in structures under multiple shock scenarios after developing damage index prediction models and accounting for the probabilistic nature of the hazard. The versatility of the proposed framework is demonstrated on a case study highway bridge located in California for two distinct hazard scenarios: a) multiple main shocks along the service life, and b) multiple aftershock earthquake occurrences following a single main shock. Results reveal that in both cases there is a significant increase in damage index exceedance probabilities due to repeated shocks within the time window of interest

Corrosion-induced failure mode alterations and implications on seismic fragility of RC frames

This study evaluates the influence of corrosion deterioration on the change in failure modes and subsequent impact on the seismic fragility of low-ductility reinforced concrete (RC) building frames. A threestory, three-bay, low-ductility RC frame is selected, and a detailed numerical model is developed by accounting for the nonlinear behavior of different components and time-dependent corrosion deterioration. The developed numerical model can capture the non-ductile failure modes of low-ductility RC frames designed without consideration of modern seismic design and detailing principles. Non-linear time-history analysis results reveal a change in the failure mode of RC columns within the frame from flexure to flexure-shear and an increase in the column peak drift due to corrosion. Time-varying probabilistic seismic demand models and damage state thresholds are used to develop seismic fragility curves. Results indicate that at the end of the design service life (50 years), corrosion deterioration significantly increases the seismic fragility by up to 38% for the Complete damage state

Seismic Fragility Updating of Highway Bridges using Field Instrumentation Data

Seismic fragility assessment of deteriorating highway bridges using analytical methods often rely on empirical, semi-empirical or numerical models to predict the rate and nature of degradation. Consequently, the predicted structural vulnerabilities of bridge components or overall bridge system during seismic shaking are only as good as the adopted deterioration models. For the sake of simplicity and ease of computational modeling, these deterioration models are often far removed from observed manifestations of time-dependent aging. One such example is the nature of corrosion in reinforced concrete bridge components under chloride attacks. While this deterioration mechanism leads to the formation of pits along the length of the rebar, past literature often adopts the simplified uniform area loss model. This study proposes a probabilistic framework that assists in improved deterioration modeling of highway bridges by explicitly modeling pit formation and also provides the opportunity of updating the analytical models with field measurement data using Bayesian techniques. The framework and case-study results presented in this study are believed to render realistic seismic fragilities for highway bridges when located in moderate to high seismic zones.This research was funded by the Science and Engineering Research Board Grant No. ECR/2016/001622. Their support is gratefully acknowledged