1,721,023 research outputs found
Extending the concepts of response spectrum analysis to nonlinear static analysis: Does it make sense?
No full textThe study presented in this paper aims to assess the recent novelties proposed by new generation building codes about the extension of some concepts at the base of linear analyses to the procedures of nonlinear static analysis. The nonlinear static analysis (pushover) is an extensively adopted approach by engineers and practitioners for purpose of assessing the seismic performance of new and existing buildings, although several drawbacks characterize this methodology, as evidenced by the existing scientific literature. In order to overcome the well-known limitations characterizing nonlinear static analysis, and to offer an alternative to nonlinear dynamic analysis, the recent upgrades of some building codes, such as the Italian one, provide different rules to employ in nonlinear static procedures. Among these new provisions, one regards the possibility to use a horizontal load profile proportional to the storey forces derived from a response spectrum analysis in all cases (i.e., also for irregular buildings). The main goal of this study is to demonstrate the reliability of this new load profile when strong irregularities arise. To this scope, the seismic behaviour of a sample of archetype buildings characterized by increasing in-height irregularity was investigated. The sample of buildings was subdivided in two subsets characterized by different design levels, one conceived by considering the prescriptions of the new Italian building code and one designed according to the oldest Italian building code (i.e., without considering anti-seismic details). The sample of buildings, simulating new and existing cases, was firstly modelled and after investigated through nonlinear static analyses by employing both traditional (i.e., inverse triangular- and uniform-like) and new (i.e., derived from response spectrum analysis) load profiles. After, nonlinear dynamic analyses were run, in order to assess the capacity of both traditional and new load profiles to capture the dynamic behaviour of the investigated archetype buildings. The performed analysis campaign demonstrated the efficiency of the new load profile proposed by the code, and the output of the work provided an answer to the main question posed by the authors: does it make sense to extend the concepts of response spectrum analysis to nonlinear static analysis
The influence of torsion on acceleration demands in low-rise RC buildings
No full textThis paper presents a study of acceleration demands in low-rise reinforced concrete (RC) buildings with torsion, evaluated by quantifying peak floor accelerations (PFAs) and floor response (acceleration) spectra (FRS). The study was performed with the aim to provide simple empirical formulas to quantify the amplification effects due to torsion, which can occur in most of the existing and new RC buildings. With this goal in mind, a set of eight archetype buildings was selected, characterized by an increasing floor eccentricity obtained by moving the centre of rigidity (CR) away from the centre of mass (CM). Numerical models of the proposed set of archetype RC buildings were considered in both linear elastic and nonlinear configurations. For the latter, the properties of models were widely varied, by systematically modifying parameters of plastic hinges, in order to obtain a sample of 1000 models. Non-structural components (NSCs) were considered linear elastic in all cases. To investigate acceleration demands, a set of forty Eurocode 8 spectrum-compatible ground motion records were used as input. For linear elastic building models, it was observed that the change of demands depends on the position of the NSC (in-plan and in-height), and on the distance between CR and CM. On the other hand, for nonlinear models, additional parameters must be considered, such as the building ductility (μ) and yielding force (Vy). New regression models were proposed for quantifying the observed differences in PFAs and FRS when torsion occurs. The efficiency of the proposed models was assessed by testing the new formulas on an existing case study building, as well as on the well-known SPEAR building
Acceleration demands in single-storey RC buildings with flexible diaphragms
No full textThis paper presents a study on acceleration demands in single-storey reinforced concrete (RC) buildings with flexible diaphragms, expressed through peak floor accelerations (PFAs) and floor response (acceleration) spectra (FRS). Firstly, an extensive parametric study was performed on idealized single-storey buildings for which geometrical parameters were varied for inducing different degrees of diaphragm flexibility. The influence of diaphragm flexibility was taken into account through the commonly used in-plane displacement ratio. To investigate acceleration demands, three sets of Eurocode 8 spectrum-compatible ground motion records were used as input, selected according to three different soil categories. RC buildings were considered as both linear elastic and nonlinear, whereas non-structural components (NSCs) were considered only as linear elastic. In all cases, it was observed that diaphragm flexibility significantly influences PFAs and FRS, which can both increase and decrease compared to buildings with rigid diaphragms. In the case of FRS, a shifting of peaks was observed as well. Despite the very complicated dependence between the diaphragm flexibility and PFAs and FRS, based on the obtained results, simple empirical formulas for the estimation of the ratios between PFAs and FRS in RC buildings with rigid and flexible diaphragms were proposed. Their accuracy was assessed through a case study example performed on an existing single-storey RC building
Advanced strategies for the seismic assessment of existing RC moment-frame buildings: appraisal of modelling assumptions and development of parsimonious PBEE-based methods of analysis
No full textIl problema della verifica di vulnerabilità sismica di edifici esistenti in calcestruzzo
armato è stato oggetto negli ultimi anni di studi approfonditi, che hanno favorito
lo sviluppo di un quadro di riferimento internazionale sul tema molto ampio, sia dal
punto di vista della ricerca scientifica che da quello delle normative tecniche vigenti.
Tuttavia, sono ancora molte le questioni irrisolte a riguardo di temi come la modellazione
numerica e i metodi di analisi sismica, fasi fortemente influenzate da continue
fonti di incertezza (conoscenza dei dettagli geometrici e strutturali, proprietà dei materiali,
input sismico, accuratezza e affidabilità di modelli di capacità e strategie di discretizzazione).
Ai fini di una valutazione affidabile delle prestazioni sismiche, tali problematiche
richiedono lo sviluppo di strategie di modellazione e analisi innovative ed efficaci,
soprattutto da un punto di vista di una accurata valutazione probabilistica e con uno
sguardo attento alla pratica progettuale, dove la facilità di implementazione e i tempi di
calcolo assumono un’importanza prioritaria.
Dopo un’estesa ricerca bibliografica degli approcci proposti e utilizzati per effettuare
verifiche di vulnerabilità sismica di edifici esistenti in calcestruzzo armato, proposti
dalla letteratura scientifica e dalle normative tecniche vigenti, nella tesi sono stati
discussi inizialmente alcuni aspetti critici di modellazione, relativi alle consuete ipotesi
semplificative adottate. Nella fattispecie, l’influenza dell’ipotesi di piano rigido, con riferimento
agli elementi strutturali secondari come il solaio, è stata analizzata, con l’obiettivo
di proporre un’idonea strategia efficiente di modellazione per una pratica applicazione,
rivolta a ricercatori e professionisti. Ciò stante, un’analisi iniziale di sensibilità è
stata condotta, investigando quali parametri influenzano significativamente la risposta
sismica globale della tipologia di edifici in oggetto. Sulla base dei risultati ottenuti, una
nuova procedura numerica di modellazione dell’impalcato è stata proposta, atta a definire
una piastra ortotropa equivalente capace di simulare la reale rigidezza nel piano,
per azioni orizzontali. La metodologia adottata, nonostante incrementi lo sforzo computazionale
dell’analisi, ha il vantaggio di evitare le assunzioni aprioristiche sulla rigidezza
dell’impalcato. Al fine di validare quanto proposto, il metodo è stato applicato ad
un edificio esistente in calcestruzzo armato, valutando i risultati e comparandoli con
altre metodologie proposte dalla letteratura scientifica per considerare il comportamento
nel piano dell’impalcato, come quella a puntoni equivalenti. Infine, è stata valutata
la possibilità di applicare la procedura nei casi in cui si considera l’influenza delle
tamponature esterne e successivamente, in una prospettiva di miglioramento o adeguamento
sismico dell’edificio. In quest’ultimi casi, la prestazione dell’edificio alle
azioni orizzontali è stata migliorata, mediante l’uso di tamponature rinforzate e mediante
l’inserimento di pareti in calcestruzzo armato sul perimetro dell’edificio.
Per quanto riguarda la fase di analisi sismica, stabilire quale sia la metodologia
più efficace per identificare la risposta strutturale in campo elastico e inelastico assume
una grande importanza, considerando soprattutto la vasta casistica di procedure proposte
dalla letteratura scientifica e dalle normative tecniche vigenti. A valle di un’estesa
valutazione di quest’ultime, con particolare attenzione ai metodi di analisi non lineari,
sia statici che dinamici, la dissertazione presenta alcune applicazioni di analisi statiche
non lineari, metodo che rappresenta la prima scelta da parte dei professionisti. Inizialmente,
un’applicazione di analisi statica non lineare convenzionale è stata condotta su
un campione di edifici esistenti ideali in calcestruzzo armato, con l’obiettivo di verificare
il ruolo del nodo di controllo. Tuttavia, come già evidenziato dalle normative tecniche
vigenti (Normativa Tecnica Italiana e Eurocodice 8), le procedure di analisi statica non
lineare non possono essere sempre applicate, a causa di alcune limitazioni dovute alle
caratteristiche dell’edificio analizzato, come le irregolarità e la forte influenza dei modi
superiori. Con l’obiettivo di proporre una strategia che possa colmare i limiti sopraelencati,
una possibile soluzione è rappresentata dai metodi non convenzionali come le
analisi statiche non lineari multimodali o adattive. A questo proposito, una procedura
semplificata di analisi statica non lineare multimodale è stata proposta. La peculiarità
di tale metodologia è dovuta ad un algoritmo capace di fornire un singolo profilo di
carico, facilmente implementabile nelle stesse modalità di un’analisi convenzionale. Al
fine di verificare l’efficienza del metodo, quest’ultimo è stato applicato ad un edificio
esistente in calcestruzzo armato, caratterizzato da irregolarità dinamiche e da elevata
inomogeneità dei materiali in situ.
Nella parte finale della tesi, è stata analizzata la possibilità di implementare i
concetti alla base del Performance Based Earthquake Engineering (PBEE), metodo di
elevata rilevanza scientifica, per applicazioni pratiche nella verifica di vulnerabilità sismica
di edifici in calcestruzzo armato. Generalmente, l’applicazione del PBEE richiede
conoscenze specifiche circa le teorie della probabilità e competenze specialistiche nel
campo della modellazione e analisi non lineare, qualità non sempre comuni tra i professionisti.
Con l’obiettivo di ridurre i sopramenzionati ostacoli, una metodologia di analisi
dinamica non lineare è stata proposta, consistente in un’applicazione del metodo
“multi stripe analysis” su modelli numerici redatti con programmi di calcolo commerciali.
Nella fattispecie, la nuova procedura, chiamata “Few Stripe Analysis” (FSA) è stata
applicata e testata su un campione di 15 edifici scolastici esistenti in calcestruzzo armato
(nella provincia di Foggia, Sud Italia) e i risultati ottenuti, in termini di stato di
danno e curve di fragilità, sono stati confrontati con quelli ottenuti utilizzando il programma
di calcolo SPO2FRAG. Quest’ultimo consente di calcolare curve di fragilità,
partendo da curve di capacità ottenute da analisi statiche non lineari.
Infine, una nuova procedura di modellazione per valutare la risposta sismica
globale di edifici in calcestruzzo armato è stata proposta. In particolare, la metodologia
consente di produrre modelli 3D ad ordine ridotto (caratterizzati da pochi gradi di libertà),
partendo dalle caratteristiche geometriche e meccaniche di un edificio esistente.
Il vantaggio principale del presente approccio è quello di cogliere molti degli effetti predicibili
con un MDoF, ma con bassi tempi di calcolo e analisi e elevata capacità di
convergenza, caratteristiche tipiche dei modelli SDoF. L’efficienza di questi modelli
semplificati è stata testata sul campione di edifici esistenti sopramenzionato e i risultati,
in termini di risposta strutturale, stato di danno e livello di confidenza, sono stati confrontati
con quelli ottenuti precedentemente dall’applicazione della metodologia FSA. La
rilevanza e l’impatto futuro del lavoro di ricerca presentato può essere valutato in una
prospettiva più ampia e relativa ad un’analisi di vulnerabilità del patrimonio costruito a
scala territoriale, che risulta essere attualmente un aspetto critico sia per la comunità
scientifica che per le autorità governative. Infatti quest’ultime hanno il difficile compito
di proporre strategie di mitigazione del rischio sismico per un ampio e disomogeneo
patrimonio strutturale, ma con risorse economiche spesso molto limitate. Pertanto, lo
sviluppo di metodologie per la stima della vulnerabilità basata su dati limitati è un tema
soggetto ad intense attività di ricerca. Le proposte presentate nella tesi possono fornire
un potenziale strumento di analisi di grande utilità, in quanto potrebbero consentire,
attraverso l’uso dei modelli 3D ad ordine ridotto combinati con la metodologia FSA, di
superare le ben note limitazioni mostrate dagli approcci empirici, a favore di metodi
meccanici, utilizzati in un quadro completo di analisi probabilistica.The issue of seismic assessment of existing RC buildings has been extensively
studied in the last few years and the international reference framework, both with regard
to the scientific research and the development of technical codes, is very wide. Nevertheless,
there are still a lot of challenging questions about the definition of reliable numerical
models and methods of analysis, which are strongly affected by many uncertainty
sources (knowledge of structural details, material properties, seismic input; accuracy
and reliability of capacity models and discretization strategies). The management
of these issues, especially in view of practice-oriented applications, requires the
availability of effective strategies, so to allow a probabilistic assessment approach that
can be relatively accessible in terms of implementation hurdle the computational time.
After an extensive background about the approaches to vulnerability assessment
proposed by recent scientific literature and technical codes, the dissertation discusses
the critical aspects related to some assumptions commonly adopted in the
seismic modelling of existing RC buildings, with the aim of proposing proper sanitization
strategies, which can be particularly useful in view of practical applications. As a
first issue, the influence on the global response of alternative modelling assumptions
for secondary structural elements such as slabs is investigated. The usual hypothesis
of rigid floor is assessed by performing a sensitivity analysis based on several parameters,
which are particularly significant for the structural response evaluation. Then,
based on the results of the analyses, a numerical procedure for modelling the floor
system is proposed, defining an orthotropic equivalent shell element capable to simulate
the in-plan stiffness of the floor. The methodology actually increases the computational
efforts, but has the significant advantage of avoiding aprioristic assumptions
about the floor stiffness. An application of the method to the numerical modelling of
existing RC buildings is then proposed, by appraising the variation of results in comparison
with alternative models for considering in-plan stiffness (namely, equivalent
strut models). Lastly, the application possibilities of the proposed procedure are appraised,
by presenting a number of examples. As an additional effect, the presence of
infill panels is considered, in the perspective of retrofit solutions. More specifically, the
possibility of increasing the capacity to horizontal actions by reinforcing the infilled
frames or by introducing additional RC shear walls on the building perimeter is appraised.
The second issue addressed in the dissertation is the definition of the most
effective methodology to be used for identifying the structural response both in the
elastic and inelastic field. After a review of the nonlinear methods of analysis provided
by the scientific literature, both static and dynamic, the dissertation presents some applications
of the pushover method, which is by far the most popular choice of practitioners.
Firstly, an application of conventional pushover analysis is performed on a set
of ideal buildings, with the aim of appraising the role of the control node position. Anyway,
as highlighted by current technical laws (Italian building code and Eurocode 8),
nonlinear static procedure cannot be always applied in its conventional formulation. In
particular, some limitations arise in the presence of structural irregularities or in the
cases where higher modes have a strong influence. With the aim to bridge these gaps,
a solution can be represented by non-conventional methods as multimodal or adaptive
pushover analysis. With regard to this question, a simplified multimodal pushover procedure
is proposed in the dissertation. The main advantage of the proposal is represented
by the easiness of application, thanks to the adoption of a single load profile in
the computation, which is moreover an approach very familiar to practitioners. For assessing
the reliability of the procedure, it is tested on a real case study characterized
by relevant dynamic irregularity and a consistent inhomogeneity of in-situ materials.
The final part of the dissertation is devoted to the possibility of extensively bringing
the concepts at the base of Performance Based Earthquake Engineering (PBEE) to
a wider audience of users, considering that this method has a high scientific relevance
for the assessment of existing RC buildings. Generally, the application of PBEE needs
a specialist knowledge about probability theories and about nonlinear modelling and
analysis, which are skills not always common among practitioners. With the aim of
reducing these obstacles, a methodology of nonlinear dynamic analysis is proposed,
which consists in an application of the multi-stripe analysis on numerical models implemented
through a commercial software. In particular, the new procedure, called Few
Stripe Analysis (FSA), is applied on a sample of 15 existing RC school buildings (located
in the province of Foggia, Southern Italy) and the results, in terms of damage
states, are compared with the ones obtained from SPO2FRAG software, an userfriendly
tool able to compute the fragility curves starting from pushover curves.
Finally, a new simplified modelling procedure for estimating the global response
of existing RC buildings is presented. It is able to produce 3D reduced-order models
(characterized by very few degrees of freedom) starting from the geometrical and mechanical
features of the case study. The main advantage of the present approach is to
account for the effects predictable with MDoF models, but with low analysis time and
computational efforts, with elevate convergence capacity, typical of the SDoF models.
The performance of this simplified numerical modelling procedure has been tested by
the application on the previously mentioned sample of school buildings and comparing
the results, in terms of structural response, damage states and confidence levels, with
the ones previously obtained from the application of FSA. The relevance and perspective
impact of the research work here presented should be seen in the wider field of the
vulnerability analysis of the building stock at the regional scale, which is a crucial issue
for the scientific community and for the civil society. Governments and administrations
are invested with the difficult task of providing mitigation strategies for the seismic risk
for a very wide and inhomogeneous portfolio of buildings and the economic resources
are often very limited. Therefore, the development of methods for estimating the vulnerability
with limited data has been a subject of intense research activity. The framework
that is depicted in the dissertation can provide a tool potentially very impactful,
since it could allow, by the exploitation of the 3D Reduced Order Models combined
with FSA, to overcome the well-known limitations of empirical vulnerability approaches
in favor of mechanical based methods managed in a full probabilistic framework
Jerk in Earthquake Engineering: State-of-the-Art
No full textThe time derivative of acceleration, termed jerk, represents a physical property reflected through a sudden change of acceleration, and is expressed in m/s3. Since jerk is felt by humans, it has been widely used as a common (dis)comfort parameter. In earthquake engineering, due to the inevitable need for further progress in understanding ground motions and soil, structural and non-structural responses, new frontiers need to be examined. Therefore, lately, there has been an increasing interest in jerk, and various research efforts have been made towards its applications. Since a proper overview of the jerk-related literature applicable to earthquake engineering is missing, the main purpose of this paper is to fill the gap and provide a starting point for future studies
Accounting for the Spatial Variability of Seismic Motion in the Pushover Analysis of Regular and Irregular RC Buildings in the New Italian Building Code
Full text linkPushover analysis is the main methodology adopted in practice-oriented applications for investigating the non-linear response of reinforced concrete (RC) buildings; it is applicable for both new and existing buildings. It is well-known that several limitations characterize this methodology and the scientific literature proposes several non-conventional approaches to provide results comparable to those of the more efficient nonlinear dynamic analysis. In most recent seismic guidelines, some improvements have been introduced, in order to overcome the main drawbacks of conventional pushover methods, in view of practice-oriented applications. In particular, new prescriptions are related to the load profiles and the choice of control nodes, aspects that lead to different results in terms of capacity curves and in the safety assessment. Another relevant point is represented by the spatial combination of effects, which suggests the opportunity of executing simultaneous bi-directional pushover analyses. The aim of this paper is to investigate the effects of the new trends followed by some guidelines about pushover analysis, such as the recent 2018 release of the Italian Building Code. In particular, after a general test of the new conventional procedure for the case of RC buildings, a set of case studies has been generated, consisting of three-dimensional RC-archetypes specifically designed and investigated in order to cover the more significant scenarios. The results in terms of global and local performances are processed and critically analyzed, with the aim of appraising the main differences between the traditional and new approaches and identifying the effectiveness and of the actual improvements achieved
Seismic fragility and risk assessment of isolated bridges subjected to pre-existing ground displacements
No full textThis study focuses on the seismic fragility of isolated continuous-superstructure bridges affected by pre-seismic differential ground displacements that affect the deformation capacity of the isolators. The research methodology is designed based on observations of a case-study bridge in Southern Italy that was subjected to a slow-moving landslide leading to shear strains of the isolators. In this study, fragility analyses are carried out for ten bridge realisations, with varying number of spans and isolator types subjected to ground displacements at the base of the substructure components. The study presents a methodology to compute the probability of exceedance of a damage state conditioned to a given seismic intensity and pre-seismic isolation strains. In addition, vertical ground displacements (e.g. subsidence, settlements) on the substructure components lead to a negligible effect on seismic fragility. Findings reveal that localised horizontal ground displacements involve significant pre-seismic isolator strains, especially in shorter bridges with isolators having low rubber heights. Conversely, vertical displacements and variation of axial loads have a reduced effect on seismic fragility. In presence of pre-seismic isolator strains, the mean annual frequency of suffering slight damage increases significantly, while the mean annual frequency of reaching a severe damage is less affected. In most of the analysed cases, the effect of the presence of ground displacements has minimal influence on the expected annual loss, provided that pre-seismic shear strains are lower than 0.40
Assessing the dredging vibrational effects on surrounding structures: The case of port nourishment in Bari
No full textThe paper presents a study on the dredging vibrational effects, for nourishment purpose, on the existing structures surrounding the worksite. Nourishment is a common operation when beach (or coasts, or ports) protection is required, allowing to reduce far-field impacts of coastal structures and improve navigability. Nourishment is then performed to reshape underwater land, and it is usually practiced by locating in the zones in which is required, soil coming from nearby areas. This latter is often obtained by a dredging process, in which the phases of excavation, transportation and soil placement are carried out. From the structural point of view, of interest is the excavation phase, which is usually performed in the water environment by a ship equipped with a dredge that mines the seabed, generating a new source of vibrations for the existing structures facing the working area. The aim of this paper is to assess the effects of vibrations induced by dredging operations, by taking as reference the recently performed nourishment in the port of Bari, Southern Italy. To this scope, an existing structure was selected and identified as sentry building, considering its extreme proximity to the worksite. Hence, a structural monitoring was performed, by investigating the behaviour of the structure before, during and after the dredging. Three main controls were carried out within the monitoring campaign: (a) check of the vibration levels and comparison with thresholds provided by the current Italian prescriptions for human comfort and structural damages; (b) operational modal analysis to assess the possible variations of the structural behaviour during dredging; (c) calibration of a numerical model to simulate the structural behaviour of the sentry building and to derive unknown geometrical and mechanical parameters. A full description of the reference building (characterized by a certain irregularity degree) and all the monitoring phases are reported throughout the manuscript. The results show that, over the monitoring period, the dredging vibration levels never exceeded the thresholds provided by code provisions, and subsequently, the sentry building did not report structural damages, as confirmed by the continuous control of dynamic parameters from experimental and numerical models. In addition, the contents of the paper show the paramount importance of the structural health monitoring, and the experience herein reported can inspire the management of buildings under particular actions like the ones herein investigated
A Multisource Methodology for the Regional Seismic Fragility Assessment of Existing Masonry Buildings in Historical Centres
No full textThis article presents a methodology tailored for estimating the seismic fragility at regional scale of existing masonry buildings in historical centres, using multisource data about typological, morphological, constructive features of urban fabric and building stock of the considered geographical area. In detail, the proposed procedure exploits these data for defining a specific taxonomy, aimed at identifying specific macroclasses. After, by exploiting architectural, geometrical, and structural information, an abacus of architectural types is proposed. Associating the macroclasses and the abacus of regional architectural types through data derived by surveys carried out on specific municipalities of the region, the most representative regional building typologies can be identified, which can be subsequently investigated through modelling and analysis for deriving seismic fragility curves. The proposed procedure was released by specifying the minimum requirements for the application and was tested on the case of Puglia Region, Southern Italy. As shown throughout the manuscript, several advantages can be highlighted: (a) definition of a new taxonomy for historical centers within a region; (b) possibility to associate fully characterized architectural types to different historical centers presenting the same macroclass by exploiting few data; (c) reduction of uncertainties in the characterization of existing building stock, typical of regional-scale studies
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