Seismic performance of Steel MRFs retrofitted with BRBs: Influence of the design decisions for the devices

Buckling Restrained Braces (BRBs) represent an effective strategy for the seismic retrofit of existing steel Moment Resisting Frames (MRFs), as they contribute to increase the strength, stiffness and energy dissipation capacity of the frame. Nonetheless, the design choices made during the retrofit process have a significant impact on the performance of the structure. For example, the inclusion of ‘large’ BRBs (i.e., high yielding strength and stiffness) may contribute to limit the deformation demands in the MRF; nonetheless, it may also induce large forces in the beams and columns of the existing structure. On the other hand, the inclusion of ‘smaller’ BRBs (i.e., low yielding force and stiffness), while allowing reaching the required safety requirements, may not be able to protect the MRF from damage. Additionally, the sizing of the BRB elements has an influence on the seismic demand parameters affecting the global performance of structural and non-structural components (i.e., peak and residual drifts, as well as storey accelerations). The present study investigates the impact of the design choices in the seismic performance of a retrofitted three-storey case-study frame by considering three retrofit options. The case-study MRF for the bare frame and the three retrofit configurations are modelled and numerically investigated in Opensees by monitoring local damage states (e.g., damage in BRBs, beams, columns, panel zones). First, a comparison is made in terms of non-linear static analyses to identify the deficiencies of the structures. Then, a fragility analysis is carried out through Incremental Dynamic Analyses (IDAs) accounting for the influence of the recordto-record variability. Finally, a comparison is made in terms of local and global Engineering Demand Parameters, by developing fragility curves for the components, for storey drifts and accelerations

Critical Comparison of Assessment Codes for Steel Moment Resisting Frames

Many existing steel multi-storey frame buildings worldwide were designed prior to the introduction of modern seismic design provisions or based on outdated hazard maps considering low values of seismic intensity. This often resulted in buildings showing low performances with respect to earthquake loads. Assessment codes, such as the Eurocode 8 Part 3 and the ASCE 41, have been conceived to provide tools to assess the seismic performance of existing structures, to evaluate their adequacy with respect to the current safety standards and the need for seismic retrofit. However, recent research studies have revealed the necessity for a revision of these codes. In particular, for steel moment resisting frames, the current European regulation shares many similarities with older versions of the American codes, but has failed to incorporate changes based on the state-of-the-art knowledge. In addition, the undergoing update of other parts of the Eurocode motivates a full revision of the current standards. This paper compares the assessment procedures of the European and American codes. Two low-code steel Moment Resisting Frames were considered for case study purposes and the assessment was performed based on three local Engineering Demand Parameters (EDPs), i.e., column’s rotation, beam’s rotation and panel zone’s shear distortion, and the inter-story drift as global EDP. Incremental Dynamic Analyses were performed for the development of component and system fragility curves. The present work aims to identify some challenges and to provide some preliminary insights for the revision of the Eurocode 8 Part 3

Numerical modelling of masonry infill walls in existing steel frames

It is now widely recognised that masonry infill plays an essential role in the seismic behaviour of existing steel buildings; however, there is still a lack of clear guidance on the modelling of masonry infill in the current Eurocode 8-Part 3. Several methods for the numerical modelling of masonry infills have been proposed in literature over the past few decades, which either adopt a detailed approach (micro-model) or a simplified approach (macromodel). In the former case, bricks are individually modelled, taking into account the brickmortar cohesive interface, which is able to provide detailed insights of the behaviour of masonry infills and the frame-wall interaction but usually at a high computational cost. On the other hand, a simplified model can be easily built within finite element software, most of which replace the infill wall panel with one or more equivalent struts in the diagonal direction. It has been demonstrated that the strut models can simulate RC infilled structures’ global response with acceptable accuracy; however, there are still no adequate recommendations for their modelling within steel frames. Besides, these models are generally incapable of capturing the interactions between the infills and the frame members. To this end, the present paper numerically investigates an Abaqus macro-model of the infilled steel frame, which was experimentally tested as part of the recent SERA HITFRAMES project. The preliminary re-sults shows that the different detailing of steel frames could lead to different damage patterns in the infill walls when compared to RC frames. In particular, instead of a single diagonal strut, at most three struts were observed in this study. The results also suggested that the number and geometry of struts could change with increasing displacement demands, hence it might not be appropriate to use the same strut model for infill walls on different floors

Preliminary numerical analysis of the seismic response of steel frames with masonry infills retrofitted by buckling-restrained braces

Existing steel moment-resisting frames in several seismic regions worldwide are often characterised by high vulnerability to earthquakes due to insufficient local and/or global ductility. Nowadays, it is of paramount importance to assess their response under strong motions and provide cost-effective retrofitting strategies. Amongst others, the seismic behaviour of these frames is often strongly affected by the presence of masonry infills which, from one side, if adequately distributed, beneficially contribute to the seismic resistance of the structure providing stiffness and strength to the frame, from the other side often experience a brittle behaviour and are very vulnerable to seismic actions. To this end, the H2020-INFRAIA-SERA project HITFRAMES (i.e., HybrId Testing of an Existing Steel Frame with Infills under Multiple EarthquakeS) experimentally evaluated a case study building representative of non-seismically designed European steel frames with masonry infills and investigated a possible retrofit strategy. This paper takes advantage of the experimental results of the HITFRAMES project to calibrate numerical models in OpenSees of a case study building which is analysed as bare, infilled and retrofitted frame with buckling-restrained braces (BRBs). The impact of masonry infills and BRB-retrofit is investigated by comparing the response of models with different configurations. The numerical results provide some insights on the ability of BRB-retrofit option in protecting not only the steel frames from experiencing critical damage during earthquakes but also the masonry infills and on the importance of using appropriate models for the masonry infills in the assessment procedures

Dynamic response of existing steel frames with masonry infills under multiple earthquakes

Existing steel moment-resisting frames in several seismic regions worldwide are often characterised by high vulnerability to earthquakes due to insufficient local and/or global ductility. Therefore, it is of paramount importance to assess their response under strong motions and provide cost-effective retrofitting remedies. However, the current code-based assessment framework utilized in Europe for assessing existing structures is inadequate and requires improvement, especially to account for the contribution of masonry infills as they significantly influence the seismic response of steel buildings. To this end, the H2020-INFRAIA-SERA project HITFRAMES (i.e., Hybrid Testing of an Existing Steel Frame with Infills under Multiple Earthquakes) aims at experimental evaluation of a case study building representative of non-seismically designed European steel frames. This paper presents the dynamic response analyses of the case study building and serves as a theoretical prediction of the experimental results for HTTFRAMES. The case study building is analysed as a bare, an infilled and a retrofitted frame with buckling restrained braces (BRBs), respectively. It is subjected to the natural seismic sequence recorded during the 2016-2017 Central Italy earthquakes. The modal properties of the case study building are determined first, followed by the investigation of its non-linear dynamic response. The dynamic tests are performed with the earthquake records scaled to different intensity levels to simulate the structural performance under different limit states according to Eurocode 8-Part 3. The impact of masonry infills and BRB-retrofit is also investigated by comparing the response of models with different configurations. It can be concluded that appropriately-designed BRBs are effective in protecting steel frames from experiencing critical damage during earthquakes and reducing significantly the transient and residual drift

Assessment of existing steel frames: Numerical study, pseudo-dynamic testing and influence of masonry infills

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

Innovations in earthquake risk reduction for resilience: Recent advances and challenges

The Sendai Framework for Disaster Risk Reduction 2015-2030 (SFDRR) highlights the importance of scientific research, supporting the ‘availability and application of science and technology to decision making’ in disaster risk reduction (DRR). Science and technology can play a crucial role in the world’s ability to reduce casualties, physical damage, and interruption to critical infrastructure due to natural hazards and their complex interactions. The SFDRR encourages better access to technological innovations combined with increased DRR investments in developing cost-effective approaches and tackling global challenges. To this aim, it is essential to link multi- and interdisciplinary research and technological innovations with policy and engineering/DRR practice. To share knowledge and promote discussion on recent advances, challenges, and future directions on ‘Innovations in Earthquake Risk Reduction for Resilience’, a group of experts from academia and industry met in London, UK, in July 2019. The workshop focused on both cutting-edge ‘soft’ (e.g., novel modelling methods/frameworks, early warning systems, disaster financing and parametric insurance) and ‘hard’ (e.g., novel structural systems/devices for new structures and retrofitting of existing structures, sensors) risk-reduction strategies for the enhancement of structural and infrastructural earthquake safety and resilience. The workshop highlighted emerging trends and lessons from recent earthquake events and pinpointed critical issues for future research and policy interventions. This paper summarises some of the key aspects identified and discussed during the workshop to inform other researchers worldwide and extend the conversation to a broader audience, with the ultimate aim of driving change in how seismic risk is quantified and mitigated

Existing Steel Moment Resisting Frame Buildings: a Critical Comparison of Engineering Demand Parameters

Recent seismic events have continued to underline the high earthquake vulnerability of existing buildings structures and the associated direct (e.g., injuries, casualties, repair cost) and indirect (e.g., downtime) losses. Therefore, there is an urgent need for advanced code standards able to effectively assess their seismic vulnerability allowing a careful evaluation of their safety. A reliable assessment is crucial for determining whether the structure performs satisfactorily or not; if it requires of measures to mitigate the effect of the earthquake loads; or the possibility of including a retrofit solution to bring up its behavior to the required levels of performance. Therefore, modern assessment standards, such as the Eurocode 8 Part 3 in Europe, and the ASCE 41 in the United States, have been developed to provide guidelines to study the performance of existing structures. These codes propose methods to gather information from the structure, to deal with the uncertainty attained to the data collecting process and to perform linear and non-linear structural analyses. The European code provides with performance requirements and compliance criteria for different limit states in a prescriptive way. In contrast, the American regulations offer suggestions on the hazard levels related to the different limit states, and provide with acceptance criteria for each of them, allowing the stakeholders to make the final decision on the desired performance level. In particular, for steel moment resisting frames, the Eurocode has requirements for certain components, and provides detailed numerical compliance criteria for beams, columns and connections, similar to those provided by the older versions of the American code. However, in some cases these criteria ignore the interaction between simultaneous effects which may lead to the overestimation of the capacity. On the other hand, the ASCE 41-06 and ASCE 41-13 provide more detailed numerical acceptance criteria based on more recent research, but their results have been catalogued in the past as conservative for certain members. The ASCE 41-17 proposes some more advanced and less conservative criteria, however, its applicability may be limited in some cases due to the complexity of its formulations. This paper evaluates the seismic performance of existing, i.e., low-code, steel moment resisting frame buildings when assessed by using different code standards and considering different components. Two case studies are analyzed and both typologies are evaluated under the framework of the EC8-3 regulation and the three most recent versions of the ASCE 41 standard. Non-linear dynamic analyses have been carried out on a set of ground motions records perform Incremental Dynamic Analyses and hence to investigate the influence of the record-to-record variability and to derive fragility curves for the whole structure and for several local engineering demand parameters conventionally used in deterministic studies. The results confirm what observed in previous researches and show that, for the analyzed structures, the panel zones have a significant influence on their overall behavior and this outcome is common to all the considered codes. Moreover, the study allows to provide some insights on the impact of the inclusion of gravity and overturning axial loads in the definition of the rotation capacity of steel columns. This aspect is of particular interest as it is accounted for in modern code standard. In light of the revision of the Eurocode 8 Part 1, this study provides some preliminary information of the possible deficiencies of the current regulations and focuses on some of the challenges to be addressed in the new versions of the Eurocode 8 Part 3, in particular in those related to low-code SMRFs

Numerical Modelling of Masonry Infill Walls in Existing Steel Frames Against Experimental Results

The presence of masonry infills may significantly affect the seismic behaviour of existing steel moment-resisting frames, characterised by low lateral force resistance and inadequate energy dissipation capacity due to the lack of seismic detailing. Masonry infills may cause variation of internal force distribution along beams and columns, resulting in large local seismic demands at beam-column joints and consequently leading to soft-storey mechanisms. Several numerical models have been developed to account for the effects of masonry infills, among which the equivalent strut models were most widely used. However, it has been argued that despite its ability to capture the global response of structures, the single-strut model may not be adequate to correctly simulate the internal forces distributions in steel members. To this end, the present study investigates modelling strategies of infilled steel frames using both single- and three-strut models. The results from different modelling approaches are compared among them and with experimental tests, providing insights on the influence of the modelling strategies both at global and local levels

Pseudo-Dynamic Testing of Existing Steel Frames With Masonry Infills: Assessment And Retrofitting with BRBs

Many existing steel multi-storey frames in Europe were designed before the introduction of modern seismic design provisions and often exhibit low performances under earthquake loads due to their insufficient stiffness, strength and energy dissipation capacity. In this context, there is a significant need for advanced assessment procedures able to quantify the seismic performance of these structures and to evaluate the need for retrofitting. However, current procedures for the assessment of existing steel structures in Europe, included in the Eurocode 8 Part 3 (EC8-3), has demonstrated to be inadequate and should be revised. Amongst others, particular attention should be paid to the contribution from masonry infill walls as they significantly affect the modal properties and the lateral strength and stiffness of structures. To this end, the HITFRAMES (i.e., HybrId Testing of an Existing Steel Frame with Infills under Multiple EarthquakeS) SERA project, funded under the H2020-SERA Program, experimentally evaluated the seismic performance of a case study structure representative of non-seismically designed steel frames in Europe including the effects of the masonry infills. A retrofitted configuration of the structure, based on the use of buckling restrained braces, is also tested in order to provide information about the effectiveness of this retrofit strategy. This paper illustrates the analyses performed for the design and the assessment of the case study structure and the preliminary results of the tests on the infilled non-retrofitted structure. Non-linear finite element models of the frame have been developed to complement the experiment design and to forecast the outcome of the tests. The building structure is assessed as a bare and infilled frame under the EC8-3 framework by non-linear static analysis and comparisons are made between the two configurations to estimate the influence of the infills. Then, non-linear time history analyses are performed on the infilled non-retrofitted frame, which focus on the forecast of the experimental outcomes with special attention paid to the response of masonry infills. Preliminary comparison between the numerical predictions and experimental outcomes is also performed for assessing the accuracy of the finite-element model