Scalvenzi, Martina (2021) Role of material properties and retrofitting systems in structural robustness of reinforced concrete frame buildings. [Tesi di dottorato]

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Item Type:	Tesi di dottorato
Resource language:	English
Title:	Role of material properties and retrofitting systems in structural robustness of reinforced concrete frame buildings
Creators:	Creators Email Scalvenzi, Martina martina.scalvenzi@unina.it
Date:	13 December 2021
Number of Pages:	188
Institution:	Università degli Studi di Napoli Federico II
Department:	Ingegneria Chimica, dei Materiali e della Produzione Industrialea
Dottorato:	Ingegneria dei materiali e delle strutture
Ciclo di dottorato:	34
Coordinatore del Corso di dottorato:	nome email D'Anna, Andrea anddanna@unina.it
Tutor:	nome email Parisi, Fulvio UNSPECIFIED
Date:	13 December 2021
Number of Pages:	188
Keywords:	Progressive collapse, structural robustness, reinforced concrete buildings, incremental dynamic analysis, pushdown analysis, probabilistic assessment, structural retrofitting
Settori scientifico-disciplinari del MIUR:	Area 08 - Ingegneria civile e Architettura > ICAR/09 - Tecnica delle costruzioni
Date Deposited:	05 Jan 2022 06:50
Last Modified:	28 Feb 2024 11:42
URI:	http://www.fedoa.unina.it/id/eprint/14288

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Catastrophic consequences of progressive collapse of structures, particularly under extreme events, have produced a growing interest in structural robustness by different actors of construction industry (e.g., regulators, designers, construction companies, facility managers, homeland security agencies). Research programmes have been funded to simulate progressive collapse and to quantify structural robustness, both experimentally and theoretically. Significant research outcomes have thus allowed the development of advanced simulation methods for structural response analysis under abnormal loading, as well as different methods for robustness quantification and design, which are presented in several guidelines at both national and international levels. Nonetheless, a number of open issues still need to be deeply investigated, particularly regarding existing structures that were designed only to gravity loads according to past technical codes, design procedures, and practice rules. This PhD thesis deals with progressive collapse performance and structural robustness of existing reinforced concrete (RC) frame buildings, which are a significant fraction of worldwide built heritage. Special emphasis is given on the role of mechanical properties of structural materials used for construction and retrofitting of RC frame buildings. Both cast-in-place and precast buildings are considered, simulating their large-displacement nonlinear response to both single- and multi-column loss scenarios. Structural response analysis was carried out using incremental static (pushdown) analysis and incremental dynamic analysis (IDA). Among direct approaches, the alternate load path (ALP) analysis method was extensively implemented both deterministically and probabilistically, in the latter case by modelling and propagating uncertainties in material properties that are often an important uncertainty source in existing buildings. Possible catastrophic effects of improper structural retrofitting operations were also addressed, to provide a contribution to knowledge on robustness during retrofitting. The PhD thesis consists of seven chapters, starting from Chapter 1 that provides the objectives of this study and the outline of the thesis. Chapter 2 provides a state-of-the-art review to support understanding of methodologies implemented in this study. After concepts regarding extreme actions, progressive and disproportionate collapses, and structural robustness are delineated, robustness-oriented design methods and progressive collapse simulation are briefly reviewed. In the final part of Chapter 2, the role of robustness in disaster resilience of structures, infrastructures and urban systems is discussed. In Chapter 3, the role of material properties in progressive collapse resistance and robustness of cast-in-place RC frame buildings is investigated through sensitivity analysis, in order to identify the most influencing parameters. The reduction of yield strength of steel reinforcement and longitudinal reinforcement ratio of primary beams is found to have a fatal effect consisting in the progressive collapse of the framed structure. The same result comes out when the span length of primary and secondary beams is increased. The material property that least influences the progressive collapse resistance is the compressive strength of concrete. The sensitivity of the load capacity corresponding to five limit states defined for progressive collapse is also investigated in this study and all nonlinear analyses evidence a sequential occurrence of the performance limit states proposed. Tornado diagrams clearly indicate that span length of primary and secondary beams and yield strength of steel reinforcement are the capacity model properties that mostly influence the limit state load capacity. In this chapter the progressive collapse capacity of European RC framed buildings through a set of nonlinear dynamic analyses, considering multiple-column loss scenarios and alternative removal times is also investigated. This choice is due to the presence of many studies that have focused their attention on progressive collapse of building structures subjected to notional removal of single components at the ground floor. It is also noteworthy that multiple columns can be heavily damaged or totally destroyed in different time instants, for instance under events like impact of heavy objects on several parts of the structure or bomb detonation occurring at different distances from column. This study highlights that the failure removal of consecutive columns produce the lowest levels of load capacity against progressive collapse. In addition, under a scenario that involves first a corner column and afterwards a nearby column, at a time one order of magnitude higher than that of the first column, the maximum reduction in peak load is obtained. Chapter 4 deals whit the ability of existing reinforced concrete structures to prevent progressive collapse during structural retrofitting. The novelty of this study lies in the nonlinear analysis of a real existing building structure that suffered a partial progressive collapse during structural retrofitting interventions. Indeed, the majority of studies investigated the structure during its operation, assessing the progressive collapse capacity, robustness, vulnerability and risk under either abnormal load. It is worth noting that other stages of the building lifetime such as construction and retrofitting can notably undermine structural safety, frequently resulting in either the need for evacuation/demolition or even progressive collapse with huge impact on economy and people. Pushdown analysis with displacement control was performed on two different models of the structure, evidencing that the removal of concrete cover, (that is a typical retrofit measure) from an internal column results in a collapse capacity drop that is greater than that predicted for the same scenario involving a perimeter or corner column. The progressive collapse capacity of the structure reaches its maximum reduction in the case of simultaneous soil excavation at the base of three columns. Linking to structural retrofitting, chapter 5 presents a numerical study on the impact that carbon fibre reinforced polymers (CFRPs) may have on the structural robustness of low-rise RC frame buildings. The sensitivity of progressive collapse resistance to structural and material properties, is evaluated through parametric analysis. This multi-hazard assessment study outlines that robustness enhancement can be effectively driven by seismic retrofitting based on CFRP strengthening, highlighting the importance of multi-hazard approaches for design, assessment and retrofit of structures. Significant beneficial effects of local seismic strengthening on robustness (in terms of load bearing capacity and, in some cases, inelastic deformation capacity) can result from 10%–20% amplifications in shear strength at beam ends. Structural robustness of typical European precast concrete frame buildings is probabilistically assessed through a fragility analysis procedure in Chapter 6. Fragility analysis is performed to propagate the uncertainty in material properties of beam-column connections. The estimation of progressive collapse capacity is characterized by low levels of uncertainty, even degenerating into a deterministically predicted value of load capacity associated with the attainment of slight damage to the earthquake-resistant building. Seismic detailing increases the median load factor at collapse, demonstrating some effectiveness in the mitigation of progressive collapse risk. Based on the outcomes of this study, further studies in the field might be carried out to investigate the following open issues: (i) the beneficial contribution by secondary beams, floor systems and connections in 3D capacity models, particularly in case of precast RC buildings; (ii) the influence of aging and deterioration processes, as well as the extension of this kind of safety assessments in case of historical constructions that are often subjected to restoration and structural retrofitting; and (iii) the impact of other seismic retrofit methods (e.g., steel braces, RC walls, steel caging) on progressive collapse resistance.

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