Displacement-based seismic retrofit design for non-ductile RC frame structures using local retrofit interventions at beam-column joints

Paper 192In the past design codes, infill panels/walls within frame buildings have been considered as non-structural elements and thus have been typically neglected in the design process. However, the observations made after major earthquakes even in recent times (e.g. Duzce 1999, L’Aquila 2009, Darfield 2010) have shown that although infill walls are considered as non-structural elements, they can interact with the structural system during seismic actions and modify the behaviour of the structure significantly. More recent code design provisions (i.e.,NZS4230, Eurocode 8, Fema 273) do now recognize the complexity of such interactions and require either a) consider these effects of frame-infill interaction during the design and modelling phase or b) assure no or low interaction of the two systems with proper detailing and arrangements in the construction phase. However, considering the interaction in the design stage may not be a practical approach due to the complexity itself and in most cases does not solve the actual problem of brittle behaviour and thus damage to the infills. Therefore, the purpose of this particular research is to develop technological solutions and design guidelines for the control or prevention of damage to infill walls for either newly designed or existing buildings. For that purpose, an extensive experimental and numerical research programme has been planned. The concept, background on infill practice in New Zealand and the research programme will be briefly described in this paper

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

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

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

New Zealand contributions to the global earthquake model’s earthquake consequences database (GEMECD)

The Global Earthquake Model’s (GEM) Earthquake Consequences Database (GEMECD) aims to develop, for the first time, a standardised framework for collecting and collating geocoded consequence data induced by primary and secondary seismic hazards to different types of buildings, critical facilities, infrastructure and population, and relate this data to estimated ground motion intensity via the USGS ShakeMap Atlas. New Zealand is a partner of the GEMECD consortium and to-date has contributed with 7 events to the database, of which 4 are localised in the South Pacific area (Newcastle 1989; Luzon 1990; South of Java 2006 and Samoa Islands 2009) and 3 are NZ-specific events (Edgecumbe 1987; Darfield 2010 and Christchurch 2011). This contribution to GEMECD represented a unique opportunity for collating, comparing and reviewing existing damage datasets and harmonising them into a common, openly accessible and standardised database, from where the seismic performance of New Zealand buildings can be comparatively assessed. This paper firstly provides an overview of the GEMECD database structure, including taxonomies and guidelines to collect and report on earthquake-induced consequence data. Secondly, the paper presents a summary of the studies implemented for the 7 events, with particular focus on the Darfield (2010) and Christchurch (2011) earthquakes. Finally, examples of specific outcomes and potentials for NZ from using and processing GEMECD are presented, including: 1) the rationale for adopting the GEM taxonomy in NZ and any need for introducing NZ-specific attributes; 2) a complete overview of the building typological distribution in the Christchurch CBD prior to the Canterbury earthquakes and 3) some initial correlations between the level and extent of earthquake-induced physical damage to buildings, building safety/accessibility issues and the induced human casualtie

Damage Mitigation Strategies of ‘Non-Structural’ Infill Walls: Concept and Numerical-Experimental Validation Program

In the past design codes, infill panels/walls within frame buildings have been considered as non-structural elements and thus have been typically neglected in the design process. However, the observations made after major earthquakes even in recent times (e.g. Duzce 1999, L’Aquila 2009, Darfield 2010) have shown that although infill walls are considered as non-structural elements, they can interact with the structural system during seismic actions and modify the behaviour of the structure significantly. More recent code design provisions (i.e.,NZS4230, Eurocode 8, Fema 273) do now recognize the complexity of such interactions and require either a) consider these effects of frame-infill interaction during the design and modelling phase or b) assure no or lowinteraction of the two systems with proper detailing and arrangements in the construction phase. However, considering the interaction in the design stage may not be a practical approach due to the complexity itself and in most cases does not solve the actual problem of brittle behaviour and thus damage to the infills. Therefore, the purpose of this particular research is to develop technological solutions and design guidelines for the control or prevention of damage to infill walls for either newly designed or existing buildings. For that purpose, an extensive experimental and numerical research programme has been planned. The concept, background on infill practice in New Zealand and the research programme will be briefly described in this paper

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

Material Property Uncertainties versus Joint Structural Detailing: Relative Effect on the Seismic Fragility of Reinforced Concrete Frames

This paper investigates the relative effect of material properties and structural details in the joint panels on the seismic fragility of existing reinforced concrete (RC) frames. Five building classes with different structural details (particularly in the joint panels) and material characteristics are defined according to different past design codes, for a three-story and a six-story archetype geometry. Based on nonlinear static or nonlinear dynamic analysis procedures, results from the study show that the effect of structural details on seismic fragility of the considered structures is negligible for damage states involving an essentially elastic behavior. Conversely, it is much higher for life-safety and near-collapse damage states, and it is considerably higher than the effect due to materials. Therefore, in the diagnosis phase, higher emphasis should be given to on-site investigations of actual reinforcement content/layout rather than to invasive material testing. The uncertainty related to the structural details described here is practically related to exterior, rather than interior, joint panels. Cover removal for one of those joints may potentially eliminate this specific uncertainty. As a practical action, in situ testing of RC frames should involve the cover removal of at least one exterior joint panel regardless of the required target “level of knowledge” of the existing structure

A computational framework for selecting the optimal combination of seismic retrofit and insurance coverage

Economic earthquake losses can be mitigated through either building retrofit strategies and/or, to some extent, risk-transfer to the (re)insurance market. This paper proposes a computational framework to select the optimal combination of seismic retrofit and insurance policy parameters for buildings. First, a designer selects a suitable retrofit strategy. This is implemented incrementally to define interventions with increasing retrofit performance levels. The cost of each intervention is calculated, along with the cost of property rental while the retrofit is implemented. Alternative insurance options are considered. For each retrofit-insurance combination, the insured and uninsured economic losses within a given time horizon are estimated. The optimal retrofit and insurance combination minimizes the tail value at risk of the life cycle cost. The selected confidence level for this metric depends on the homeowner's risk aversion. The proposed framework is illustrated for a case-study archetype Italian reinforced concrete frame building retrofitted with concrete jacketing, also considering the Italian retrofit tax incentives/rebates called “Sismabonus.