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This paper presents the fragility assessment of non-seismically designed steel moment frames with masonry infills. The assessment considered the effects of multiple earthquakes on the damage accumulation of steel frames, which is an essential part of modern performance-based earthquake engineering. Effects of aftershocks are particularly important when examining damaged buildings and making post-quake decisions, such as tagging and retrofit strategy. The procedure proposed in the present work includes two phase assessment, which is based on incremental dynamic analyses of two refined numerical models of the case-study steel frame, i.e. with and without masonry infills, and utilises mainshock-aftershock sequences of natural earthquake records. The first phase focuses on the undamaged structure subjected to single and multiple earthquakes; the effects of masonry infills on the seismic vulnerability of the steel frame were also considered. In the second phase, aftershock fragility curves were derived to investigate the seismic vulnerability of infilled steel frames with post-mainshock damage caused by mainshocks. Comparative analyses were conducted among the mainshock-damaged structures considering three post-mainshock damage levels, including no damage. The impact of aftershocks was then discussed for each mainshock-damage level in terms of the breakpoint that marks the onset of exceeding post-mainshock damage level, as well as the probability of exceeding of superior damage level due to more significant aftershocks. The evaluation of the efficiency of commonly used intensity measures of aftershocks was also carried out as part of the second phase of assessment. [ABSTRACT FROM AUTHOR]

Exposure to aggressive environments is one of the most critical problems of reinforced concrete (RC) structures, which can affect both their static and dynamic behaviour. In this paper, the linear and non-linear performance of existing corroded RC framed structures were studied through an advanced numerical model. Moreover, an extensive literature review of models and approaches used for the assessment of RC structures exposed to different levels of corrosion was presented. The numerical evaluation of an existing RC structure subjected to different exposures and degradation was considered. A new approach was presented for the evaluation of the ultimate capacity of RC elements. Such an approach has been compared and validated against a set of the experimental results from the literature. The results of comparative analyses showed that the proposed approach could predict the ultimate capacity of corroded RC components. Linear and non-linear analyses were performed using a refined Finite Element method; the seismic performance evaluated in terms of shear strength degradation, inter-storey displacements, ductility, and maximum base shear. The outcomes of the present study demonstrated that corrosion had a significant impact on the structural response of the existing building. Such an effect depended on the type of exposure. The elastic dynamic analyses of the building demonstrated that corrosion increased the fundamental periods and, changed the mass participation factor and the mode of vibration, i.e. the external exposure. Non-linear static analyses showed a significant reduction of the shear capacity and the translation ductility with the increase of the corrosion rate for all lateral loading patterns specified by the Eurocode. The results of the non-linear dynamic analyses illustrated that the damage and deterioration due to the corrosion attack increased the roof and the inter-storey drift-ratios, as well as a relevant decay of the base shear capacity and early collapse were noted for high-levels of corrosion. Comparisons between non-linear static and dynamic analyses were also provided in terms of roof drift-ratios and base shears. [ABSTRACT FROM AUTHOR]

[Display omitted] • A machine learning-based framework for deriving state-dependent fragility curves for existing steel moment frames was presented. • Feedforward neural network classification models were utilised to predict the damage state of existing steel frames with respect to prescribed limit states. • The influence of masonry infills on the seismic response of existing steel moment frame was considered. • Four Pre-Northridge steel frames covering low- and med-rise buildings designed for low and high seismicity were adopted to demonstrated the proposed framework. Seismic assessment of existing buildings is usually a building-specific task that relies on refined finite element models. Such a task may require considerable computational demand, especially when predicting the seismic fragility of existing buildings under the framework of performance-based earthquake engineering. However, the computational cost can be significantly reduced by replacing the finite element model with a well-trained machine learning-based model, for example, an artificial neural network model. This paper presents the application of feedforward neural networks to derive the state-dependent fragility curves of existing steel moment frames, taking into account the effects of masonry infills. The network models can be trained to predict explicitly whether a structure exceeds the target limit state based on representative intensity measures of ground motions, which is in nature a binary classification problem. The number of non-linear time-history analysis required to generate the training data for the network models tends to be significantly lower compared to the case of conventional incremental dynamic analysis, particularly when a great number of ground motions are adopted aiming at higher accuracy of the fragility curves. [ABSTRACT FROM AUTHOR]

[Display omitted] • A framework for developing the force–displacement model of masonry infills in existing steel moment frames is presented. • Calibration of modelling parameters using a genetic algorithm approach to find the optimal values of multiple variables. • Reliability evaluation of the optimal single-strut model in predicting the global and local response. The study described and summarised in this paper was aimed at developing a framework for the definition of force-displacement relationships for single-strut models for masonry infill walls within steel moment-resisting frames. The methodology is based on a genetic algorithm optimisation and can be used for the calibration of force–displacement curves based on databases from either experiments or numerical simulation. A case study is also tested to demonstrate the framework in detail. Due to limited available experimental data on the seismic response of existing steel frames with masonry infills, a set of comprehensive finite element micro-models developed in Abaqus are used to generate a database. The optimal values of the parameters to feed a force–displacement relationship of the single-strut model of the masonry infills are obtained for each micro-model by solving optimisation problems with a genetic algorithm. The optimisation problem involves the minimisation of the discrepancies between the global responses from the database and their corresponding single-strut models through least square minimisation. With the optimal values as the input variables, a generalised quadrilinear model of the masonry strut is obtained through regression analysis and is validated against additional micro-models of infilled steel frames. [ABSTRACT FROM AUTHOR]

The present study deals with the seismic performance of partial perimeter and spatial moment resisting frames (MRFs) for low-to-medium rise buildings. It seeks to establish perimeter configuration systems and hence the lack of redundancy can detrimentally affect the seismic response of framed buildings. The paper tackles this key issue by comparing the performance of a set of perimeter and spatial MRFs, which were "consistently designed". The starting point is the set of low- (three-storey) and medium-rise (nine-storey) perimeter frames designed within the SAC Steel Project for the Los Angeles, Seattle and Boston seismic zones. Extensive design analyses (static and multi-modal) of the perimeter frame buildings and consistent design of spatial frame systems, as an alternative to the perimeter configuration, were conducted within this analytical study. The objectives of the consistent design are two-fold, i.e. obtaining fundamental periods similar to those of the perimeter frames, i.e. same lateral stiffness under design horizontal loads, and supplying similar yield strength. The seismic behaviour of perimeter and spatial configuration structures was evaluated by means of push-over non-linear static analyses and inelastic dynamic analyses (non linear time histories). Comparisons between analysis results were developed in a well defined framework since a clear scheme to define and evaluate relevant limit states is suggested. The failure modes, either local or global, were computed and correlated to design choices, particularly those concerning the strength requirements (column overstrength factors) and stiffness (elastic stability indexes). The inelastic response exhibited by the sample MRFs under severe ground motions was assessed in a detailed fashion. Conclusions are drawn in terms of local and global performance, namely global and inter-storey drifts, beam and column plastic rotations, hysteretic energy. The finding is that the seismic response of perimeter and spatial MRFs is fairly similar. Therefore, an equivalent behaviour between the two configurations can be obtained if the design is "consistent". [ABSTRACT FROM AUTHOR]

Most of existing steel multi-storey frames in Europe have been designed before the introduction of modern seismic design provisions, hence they often exhibit low performance under earthquake loads due to their low lateral resistance and energy dissipation capacity. In addition, such structures often include rigid and brittle masonry infill walls that highly influence their lateral response and distribution of damage pattern. However, current procedures for the assessment of existing steel buildings in Europe, included in the Eurocode 8 – Part 3 (EC8–3), do not provide adequate guidance for the assessment of 'weak' steel frame with masonry infill walls. Moreover, most of available modelling approaches of masonry infills formerly developed for reinforced concrete (RC) structures do not properly represent the behaviour of infill walls in steel frames. An improved numerical has to be provided to satisfactorily mimic infill walls' behaviour in steel moment frames. To this end, an experimental and theoretical study was carried out within the framework of HITFRAMES (i.e., HybrId Testing of an Existing Steel Frame with Infills under Multiple EarthquakeS) SERA project. This paper firstly presents the limitations of current EC8–3 by conducting a code-based assessment on a case study steel moment frame using pushover analysis. Three different single strut models, widely used for simulating the presence of masonry infills in RC structures, are considered for the numerical analyses. The paper also presents the results of pseudo-dynamic (PsD) tests performed on a large-scale 3D steel frame with masonry infills. The capability of the different masonry infill models is successively evaluated by comparisons between numerical and experimental results. On the basis of the obtained results, recommendations on how to potentially improve the single strut model for masonry infills surrounded by steel frames are also provided. [Display omitted] [ABSTRACT FROM AUTHOR]

Many existing steel moment-resisting frames were constructed before the provisions of modern seismic codes; therefore, they are likely to exhibit high vulnerability to earthquakes due to their inadequate ductility design, hence low energy dissipation capacity. Public concern has been raised about the development of a reliable code-based framework for assessing existing structures, however, previous research has shown that the current European code EC8–3 may not be sufficient and requires urgent improvements. Particular attention should be paid to the contribution from infill walls in steel buildings, especially those made of masonry, and beam-column connections, as they play a significant role in the structural performance under earthquakes. To this end, an updated assessment procedure, which is an extension of the current procedure in EC8–3 but with several improvements to the non-linear analysis methods, is proposed in this paper and is applied to an existing steel building as a case study. The building is a three-storey steel moment-resisting frame located at Amatrice in Central Italy, which survived the recent 2016–2017 Central Italy earthquakes. Refined 3D models of the case study building, which account for the effects of infill walls and column panel zones, were implemented in a refined finite element platform. The effects of masonry infills on structural components, such as beams and columns, were investigated through non-linear analysis methods. The results emphasise the significance of including masonry infills in the numerical model for seismic assessment especially when an optimised retrofit solution is needed. • Assessment of existing steel moment-resisting frames with infills using non-linear static analysis. • Performance criteria for panel zones and infills introduced to the current assessment procedure in EC8–3. • Application of the proposed assessment procedure on a case study steel building. • Verification of the case study assessment results using non-linear dynamic analysis based on EC8–3. • Effects of masonry infills on the seismic performance of beams, columns and column panel zones. • Fragility analysis on the case study steel building with and without infills. [ABSTRACT FROM AUTHOR]

Nowadays, there is a lack of adequate code provisions for the seismic performance and risk assessment of steel structures to be used within European countries. At the same time, in several occasions, existing steel moment resisting frames (MRFs) have demonstrated to be very fragile with respect to seismic actions due to their inadequate ductility capacity. This combination highlights the urgent need for an update of the current Eurocode 8 – Part 3 (EC8‐3), thus promoting a reliable assessment of existing steel structures. To this aim, the present study provides a comprehensive and quantitative comparison of the EC8‐3 with the three versions of the American ASCE 41 (i.e., ASCE 41‐06, ‐13 and ‐17), which are here assumed as a reference, as they reflect the evolution of 'similar' assessment procedures during the last two decades. The comparison of the capacity values provided by the codes for different engineering demand parameters (EDPs) highlights significant differences pointing out drawbacks of the EC8‐3. In addition, the comparison is made by assessing the seismic performance of two existing steel MRFs, by performing Incremental Dynamic Analyses and deriving fragility curves in a probabilistic approach which considers local EDPs which are compliant with the codes, and that are conventionally used in deterministic studies, e.g., chord rotations in beams and columns, shear strain in panel zones. The comparison of the codes, and the probabilistic assessment of the case studies by using code-based (i.e. , local) EDPs, provide significant insights and directions for revision of the EC8‐3. [Display omitted] • Critical comparison of assessment procedures and capacity limits for steel MRFs in EC8–3 and different versions of ASCE 41. • Comprehensive comparison of code-established capacity limits, through the probabilistic assessment of two case-study MRF structures. • Focus on the urgent deficiencies to be addressed in the updated version of EC8–3 for the assessment of existing steel MRFs. [ABSTRACT FROM AUTHOR]