Numerical investigation of the displacement incompatibility between masonry infill walls and surrounding reinforced concrete frames

In the European building practice, masonry infill panels have been widely adopted as facade elements in Reinforced Concrete (RC) frames in order to provide architectural needs such as thermal and acoustic insulation. During seismic shakings, infill wall panels and the surrounding RC frame have a strong interaction, potentially leading to local brittle failures of both structural and non-structural elements or even to global collapse mechanisms (e.g., soft-story mechanism). In the past years, a significant research effort has been dedicated at the international level to better understand the seismic performance of infilled RC frame structures as well as to develop suitable and practical design/retrofit techniques to reduce the negative effects of infill-frame interaction. However, past numerical and experimental investigations mainly focused on the diagonal compression strut mechanism and associated stress path. On the other hand, a procedure to assess the local infill-frame displacement incompatibility (i.e., detachment due to the relative deformation mechanism) in terms of shape and values is still missing in the literature. Therefore, this paper investigates and discusses the seismic displacement incompatibility between infill walls and the RC frame structure as well as the key parameters affecting the infill-frame detachment. Specifically, the concept of shape functions is introduced and proposed to assess the seismic infill-frame displacement incompatibility, in line with and extending the state-of-the-art investigations on the relative deformation mechanism between seismic-resisting frames and precast flooring units. The proposed methodology can support a displacement-compatible design check of specific connection solutions, in the form of either shear keys and/or steel dowels, as part of either strengthening or decoupling seismic retrofit strategies, as well as of energy rehabilitation solutions, such as external thermal insulation systems, in order to protect these components during earthquakes

Displacement incompatibility shape functions between masonry infill wall panels and reinforced concrete frames

During an earthquake, the detachment and local interaction between infill wall panels and surrounding frame can occur, potentially leading to significant local damage to both structural and non-structural elements, if not global collapse. Yet, a procedure to assess the relative deformation mechanism in terms of detachment shape and values, rather than, and in addition to, the diagonal compression strut mechanism and associated internal panel strain and stress path, is still missing in the literature. Therefore, in this paper the concept of shape functions is proposed and adopted to assess the seismic displacement incompatibility between infill walls and the surrounding frame structure. A parametric study on different typologies of infilled frames is developed to investigate the key parameters affecting the infill-frame detachment. The proposed concept of shape functions can support the design/retrofit of improved construction details, such as shear keys and/or steel dowels, in view of either decoupling or strengthening retrofit/repair strategies. Moreover, as infill-frame detachment can lead to damage to energy enhancement rehabilitation solutions, such as external thermal insulation systems, which are becoming more common nowadays in view of the international target towards a significant reduction of energy consumption and CO2 emission, it is suggested to implement the proposed displacement-compatible design check to assess and detail for adequate displacement capacity

Comparative analysis of code-compliant seismic assessment methods through nonlinear static analyses and demand spectrum: N2 Method vs. Capacity Spectrum Method

This paper investigates the main differences in evaluating the seismic performance of buildings through nonlinear static procedures according to different code-compliant approaches, with a specific focus on the two alternative methods reported in the Italian Building Code, namely “Method A” and “Method B”, referring to the N2 Method and the Capacity Spectrum Method, respectively. An extensive parametric analysis is carried out by performing several nonlinear static analyses on Multi-Degree-of-Freedom (MDoF) models of different Reinforced Concrete (RC) frame structures. Seismic assessment is then performed by applying the two spectrum-based methods, and results are compared in terms of safety evaluation and loss assessment. Results of the comparison highlight that the ductility capacity of the structure strongly affects the seismic assessment, leading to larger differences when more ductile structures are considered. This work could be considered as a preliminary step toward the development of specific guidelines including provisions on the recommended simplified approach to be adopted for seismic assessment of buildings (also based on the observed/expected seismic behavior) in practical applications

Adaptive knowledge-based seismic risk assessment of existing reinforced concrete buildings using the SLaMA method

This paper presents and discusses the ongoing developments towards the definition of a multi-knowledge level seismic assessment procedure for large-scale seismic risk applications. The procedure involves the analytical-mechanical SLaMA (Simple Lateral Mechanism Analysis) method and allows for an adaptive and updatable assessment of the seismic performance of buildings accounting for different data acquisition (knowledge) levels. By coupling this approach with vulnerability assessment survey forms, a range/domain of expected capacity curves of a structure can be obtained and used to evaluate the seismic safety and the expected economic losses according to the state-of-the-art procedures in literature. Moreover, the results of the analytical assessment method can be used to develop fragility curves through simplified spectrum-based procedures. Combining the results of the fragility analysis with the hazard analysis, the seismic risk of a structure can be assessed in terms of Mean Annual Frequency (MAF) of collapse, as well as in terms of Expected Annual Losses (EAL). The proposed SLaMA-based approach is illustrated for an existing reinforced concrete building. Results confirm the effectiveness of the methodology for seismic-risk assessment studies at large scale, thus overcoming the issue related to limited building information, yet allowing for a continuous update of the “digital twin” model as further data/information becomes available

SIMPLIFIED ANALYTICAL/MECHANICAL PROCEDURE FOR THE RESIDUAL CAPACITY ASSESSMENT OF EARTHQUAKE-DAMAGED REINFORCED CONCRETE FRAMES

The series of recent catastrophic earthquakes worldwide have further emphasized the evident complexity and difficulty related to the evaluation of the post-earthquake seismic residual capacity of buildings. In the aftermath of a major seismic event, a fast, yet effective, safety evaluation procedure for earthquake-damaged buildings is critical to speed up and support the definition of emergency planning strategies, as well as to provide useful intel to the stakeholders and aid the decision-making process to enhance community resilience. Therefore, this paper aims to investigate the possible implementations of a procedure based on SLaMA (Simple Lateral Mechanism Analysis) methodology for the seismic assessment of damaged Reinforced Concrete (RC) frame buildings. The proposed procedure is based on the use of reduction coefficients for damaged structural members, in line with the FEMA 306 approach, and an update of the “hierarchy of strength” at the subassembly level by accounting for the earthquake-related damage. Results are compared against a numerical model in terms of a Capacity vs. Demand Safety Index” (IS-V or %New Building Standard, %NBS) and Expected Annual Losses (EAL). Moreover, the simplified procedure can be used to assess the feasibility and effects of a repair/retrofit solution. Results show that the proposed analytical procedure is able to estimate with reasonable accuracy, considering its simplified nature, and the performance of the building when compared to numerical analyses. Finally, the effect of the use of low-damage exoskeletons based on the PRESSS low-damage technology has been evaluated via the application of the Displacement-Based Retrofit procedure

Inelastic response spectra for an integrated displacement and energy-based seismic design (DEBD) of structures

The severe socio-economic impact of recent earthquakes has further highlighted the crucial need for a paradigm shift in performance-based design criteria and objectives towards a low-damage design philosophy, in order to reduce losses in terms of human lives, repair/reconstruction costs, and recovery time (deaths, dollars and downtime). Currently, displacement-based parameters are typically adopted to design/assess the seismic performance of the structures, by limiting the maximum displacement or the maximum interstorey drift ratio (IDR) reached by the structure under different earthquake intensities. However and arguably, displacement-based quantities are characterized by inherent weaknesses, since, for instance, they are not cumulated parameters, thus not able to capture directly the effects of multiple cycles, deterioration and damage cumulation. Therefore, in the last decades, energy-based approaches were investigated and developed in order to establish alternative engineering demand parameters for the assessment of post-event damage through a dynamic energy balance. Towards the main goal of developing an integrated Displacement and Energy-Based Design/assessment procedure (DEBD) for actual use in practice, this research work proposes an innovative approach based on the use of inelastic spectra correlating the energy components with the corresponding maximum displacement response parameters of the structure. In practical terms, the proposal is to further integrate and develop the well-known Direct Displacement-Based Design, by directly adopting the hysteretic energy as an additional design parameter. The energy inelastic spectra are developed through an extensive parametric analysis of Single-Degree-of-Freedom (SDoF) systems, with different nonlinear hysteretic models. In such an approach, the maximum seismic energy demand imparted to a structure can be directly predicted and controlled, whilst distinguishing the various components of the energy balance, including the hysteretic one. The effects of near-field and far-field earthquakes are also investigated. Results show that in the first case the seismic demand is concentrated in the peak of a few large cycles that absorb the demand energy induced by the high component in peak ground velocity in the second case the higher equivalent number of plastic cycles tends to become critical for structures with inadequate structural details and prone to suffer by cumulative cycles and overall plastic fatigue mechanisms

Post-earthquake seismic residual capacity and economic loss assessment of reinforced concrete buildings

After an earthquake, a detailed assessment of the residual capacity of damaged buildings is critical to support decision-making related to both re-occupancy and repair vs demolition. However, the evident complexity and lack of knowledge/guidelines both in terms of evaluation of the capacity of damaged buildings to sustain subsequent aftershocks and selection of suitable repair techniques have often led to extensive demolition or to, possibly, inadequate repair interventions. Both options have resulted in additional indirect economic losses and restoring time or, in the case of repair options, in underestimation of the post-repair safety level and associated economic losses. Therefore, this paper investigates the residual capacity of earthquake-damaged buildings, aiming to support their safety evaluation and loss assessment in practical application. Specifically, the influence of residual capacity on the economic losses of Reinforced Concrete (RC) frame structures is investigated. A case-study RC building is selected, and pre- and post-earthquake loss assessments are carried out considering different damage levels. A pushover-based methodology is adopted to assess the seismic performance of the damaged build-ing, based on the use of capacity reduction factors for damaged structural members. Results highlight that higher economic losses can be expected when cumulative damage is considered. This output could support decisions on the post-earthquake repair/retrofit/demolition

Multi-knowledge level seismic assessment procedure for reinforced concrete existing buildings

Recent catastrophic earthquakes have further highlighted the crucial need to develop and implement a medium-long-term national plan of seismic risk reduction in most of the seismic-prone countries worldwide. However, the constraint of economic resources and the lack of a prioritization plan are often deemed as primary obstacles to the practical implementation of such project. Moreover, when dealing with existing buildings, the technical complexity is further increased by building knowledge, often limited thus leading to higher uncertainty. To overcome this issue, improved and adaptive assessment procedures and tools should be developed. Following this goal, this paper presents a multi-knowledge level seismic assessment procedure based on the analytical-mechanical SLaMA (Simple Lateral Mechanism Analysis) method. An effective and supporting tool is developed to rapidly estimate the seismic safety and the socio-economic consequences/impact of buildings based on the data collected through assessment forms. An application of the SLaMA-based procedure is presented for a case-study building. Alternative scenarios are involved by assuming different data acquisition (knowledge) levels, from limited to complete. The range/domain of expected capacity curves, safety indexes and risk classes for all scenarios are identified. Results confirm the effectiveness of the proposed methodology and its possible implementation for seismic risk assessment studies at national scale

Alternative retrofit strategies for seismic risk-reduction: studying the attractiveness of low-damage external exoskeletons.

Recent earthquakes that occurred worldwide have further highlighted the high vulnerability of existing Reinforced Concrete (RC) buildings designed prior to the enforcement of modern seismic codes. These types of structures are expected to be affected by critical structural weaknesses, mainly related to the absence of the “hierarchy of strength” principles, potentially leading to a “no-ductile” global behaviour. As a result, in the last decades, a significant research effort has been undertaken to develop and implement retrofit strategies able to enhance seismic safety and community resilience. Several retrofit strategies and techniques are nowadays available for improving the seismic performance of existing RC buildings, involving both local (e.g., Fibre Reinforced Polymers FRP, metallic haunches, selective weakening, concrete or steel jacketing) and global interventions (e.g., elastic or dissipative braces, external exoskeletons consisting of walls or frames). Among the others, exoskeletons are deemed a promising solution, since they can be implemented entirely from outside, significantly reducing the invasiveness of the intervention (i.e., owners’ disruption), yet providing the possibility of a holistic refurbishment of the building system based on the concept of a high-performance “double-skin”. Further advantages of exoskeletons can be achieved by implementing low-damage technologies, enhancing the seismic performance of the retrofitted structure. Therefore, this paper aims to investigate the use of external exoskeletons based on the low-damage PREcast Seismic Structural System (PRESSS) technology [1]. Specifically, this advanced seismic-resistant system is based on “jointed ductile” connections, replacing the traditional “plastic hinge” in monolithic systems with a rocking and dissipative mechanism at the interface of structural members. To demonstrate the benefits of adopting low-damage exoskeletons rather than traditional retrofit techniques, an illustrative application is herein presented. Specifically, a pre-1970 existing RC building is considered, and alternative retrofit strategies are implemented targeting different performance levels. The seismic behaviour of the as-built and the alternative retrofitted structures is evaluated in terms of probability of collapse through non-linear dynamic analyses. This allows to provide a correlation between the “Safety Index” (i.e., the ratio between the capacity of the structure to the demand of a newly designed building on the same site) and the expected annual probability of collapse, thus highlighting the advantages of adopting high-preforming low-damage exoskeletons. The seismic residual capacity of the structure in its as-built and retrofitted configuration is also evaluated through a scenario-based framework. Specifically, the variation of the Safety Index and Expected Annual Losses (EAL) index is assessed considering ground-motion sequences. The concepts shown in this research work, if supported by experimental data and ad-hoc design/implementation guidelines, can represent a significant step toward the seismic risk reduction together with the improvement of the community resilience at the national level