Performance based earthquake assessment of an industrial silos structure and retrofit with sliding isolators

Recent seismic events pointed out the high vulnerability of existing industrial facilities, stressing on safety and high losses inherent to interruption of economic activities and release of environmentally hazardous materials. These structures often have irregular geometry and structural configuration, are subject to aging and corrosion, and are designed without specific performance-based or seismic design criteria. Due to these inherent complexities, retrofit using friction isolators can be a viable and practical solution for performance improvements. This work presents a case study of irregular industrial storage plant structure consisting of a group of six elevated silos resting on a steel frame on one side and connected to a vaulted RC structure on the other. A computational model is built incorporating nonlinearities from the components (braces, beams, columns, etc.) and from the mitigation devices. Retrofit using friction isolators is analyzed and evaluated through linear and nonlinear dynamic analyses under a set of natural ground motions. Results show the effectiveness of the mitigation strategy in terms of performance improvement

Comparative Seismic Assessment of Ancient Masonry Churches

The seismic risk assessment of the historical and architectural heritage is, nowadays, a very relevant topic due the potential human and economic losses involved in case of global or partial collapse. In order to preserve the inestimable value of such heritage, the prevention and mitigation of the seismic risk is needed and it cannot be postponed. Among the several methods available in the literature to perform vulnerability assessment on cultural heritage, this study focuses on two simplified methods proposed by the current Italian Directive containing the guide lines for assessment and reduction of cultural heritage seismic risk. Furthermore, a new simplified method is applied, capable at a territorial scale of quickly ranking the seismic behavior of ancient churches. In the paper, the considered evaluation methods are applied to the case study of the Matera Cathedral, named SS. Maria della Bruna. Then, the obtained results are compared with others of similar ancient churches, all belonging to the historical centre 'Sassi of Matera', a site protected by UNESCO having a moderate seismic hazard

Mohr circle-based graphical vibration analysis and earthquake response of asymmetric systems

The maximum seismic response of torsionally coupled plan asymmetric structures can be rationally visualized and computed through a Mohr Circle Response Spectrum Analysis (MCRSA). This is done combining the graphic modal properties of the torsional dynamic equations of motion with the structural earthquake demand in terms of a displacement spectrum as a function of the modal eigenvalues SD(ω2). A compact representation of the modal properties and of the response envelope is built and visualized in the Mohr plane. The maximum modal responses are then combined using a graphic adaptation of the SRSS and CCQ combination rules based on the elastic response spectrum. This Graphic Dynamic rule proves to be an effective response prediction tool, and is particularly suited to estimate the response of seismic base isolation systems

An analytical formulation of stress-block parameters for confined concrete

In order to evaluate the capacity of RC members, the main codes allow the use of stress-strain laws that can re- produce closely the real behaviour of concrete, as opposed to parabola-rectangular or equivalent rectangular diagrams. Both sectional strength and ductility depend on the law of concrete, therefore they are influenced by the confinement of members, as evidenced in the literature. In this paper a possible design approach is presented, based on classic section analysis methods. The method uses parameters that represent the stress-strain law of confined concrete. The studies car- ried out show that such parameters can be chosen through simple relationships depending on the strength of non-confined concrete, on the amount and geometry of longitudinal and transverse reinforcement, and on the geometry of the section. At this aim some numerical analyses have been performed using an analytical model of confined concrete, capable of tak- ing into account all the mentioned effects, even in the case of various sources of confinement, when different types of hoops and external elements (FRP wrappings, steel plates, etc.) are used. More in detail, the section interaction diagrams for the different limit states requires the definition of an appropriate upper bound for the strain of concrete. Therefore the study focuses on the possibility of using stress-blocks depending on the maximum stain assumed, or on the level of resid- ual stress accepted in concrete according to a specified limit state. Further studies will extend the parametric analysis in order to obtain design equations to be implemented in codes

Comparisons of Codal Detailing Rules for Curvature Ductility and Numerical Investigations

In moment resisting frame structures special detailing rules are applied to critical regions of primary columns and beams to ensure adequate curvature ductility. This is necessary for dissipating earthquake energy through hysteretical behavior of critical regions where inelastic flexural excursions occur. In this paper codal detailing rules for designing longitudinal and transverse reinforcement of primary elements as function of curvature ductility are assessed. Four seismic codes are considered: Italian code, New Zealand code, Eurocode 8 and American code. Non-linear monotonic moment-curvature analyses are performed on some sections of columns and beams detailed in according to the considered codal provisions. In the analyses the confinement effects within the concrete core have been taken into account as well. The paper concludes comparing the measured curvature ductility of the studied sections with the expected one by the codal pro- visions within the critical regions

Critical regions of RC primary elements detailed in according to provisions rules for curvature ductility: comparisons and numerical analyses

In moment resisting frames beams and columns are designed for flexural, axial, and shear actions due to vertical and horizontal loads. Special proportioning and detailing requirements are applied in these elements for making them capable of resisting against severe earthquakes without significant loss of strength beyond the flexural elastic limit, and avoiding brittle failure (shear mechanisms). As known, the required flexural inelastic excursions (expressed by the local ductility demand) depend on the dissipative capacity of the structure. The flexural ductility significantly increases with the transverse reinforcement amount provided to confine section core and to prevent buckling of compressed longitudinal bars. In this paper detailing provisions adopted by some seismic codes are compared. At first, the codes provisions to be applied within critical regions of RC primary frames sections are discussed and compared as a function of the curvature ductility demand. Then, non-linear monotonic moment-curvature analyses are performed on fiber sections of columns and beams, and by taking into account the confinement effects on concrete core as well. The numerical investigations are carried out for comparing the available curvature ductility with the expected one applying the provisions mentioned by the seismic codes

NONLINEAR SEISMIC ANALYSIS OF GRAVITY-DESIGNED RC STRUCTURES

This paper illustrates numerical simulations and comparisons with experimental results of existing RC beam-column sub-assemblages reinforced with smooth bars. The specimens reproduce parts of concrete structures designed only for vertical loads without any reinforcing detail rule (such as inadequate bars lap splice, absence of hoops within the joint panel) built in Italy during ‘50s-‘70s. In particular, in this paper is paid attention to the nonlinear models developed for predicting the failure mechanism experimentally observed, taking also into account the bond-slip phenomenon among the longitudinal bars and surrounding concrete. The proposed models are not time-consuming and may be easily implemented in any general-purpose finite element program for numerical simulations of concrete structures. These models represent an useful tool for seismic assessment with a good accuracy of nonductile RC existing structures

Seismic behaviour of steel modular buildings: numerical analysis and comparisons between different design solutions

Recently, there is an increasing interest to develop modular multi-storey structures that can be adapted to any use like hospitals, schools, housing, etc, characterized by versatility and speed of use in several conditions. Modular buildings may form complete building blocks with structural systems composed by several elements, such as steel frames, X-LAM timber floors and walls. These modular solutions are then completed with suspended ceilings and facilities including electrical and water systems. This work is focused on the structural behaviour of steel modular buildings with different configurations. To this scope, a case study is considered, characterized by steel elements with connections allowing a rapid on-site assemblage, without any need of skilled workmanship. A comparison among numerical results obtained with time history analyses is shown and discussed. The work presented is a first part of an on-going research addressed to propose reference solutions for modules, suitable in seismic prone areas and characterized by a low-seismic damage

La modellazione delle strutture isolate in presenza di sistemi di isolamento nonlineare e l’analisi dinamica non lineare semplificata secondo l’OPCM 3274/3431 (punto 10.7.6)

In addition to the conventional time history analysis for base isolated structures, the Italian building code OPC 3274/3431 allows a simplified nonlinear dynamic analysis, described in section 10.7.6. Although the analysis methods and criteria selection sections of the Italian code are generally very similar to EC8 and other international codes, these codes do not allow such a simplified procedure. This method appears theoretically inconsistent due to an unjustified decoupling of the nonlinear equations of motions of the superstructure and of the isolation system, since the nonlinear analysis is carried out treating the structure as a rigid SDOF system. The experimental tests performed on a full-scale building constructed at Rapolla (Potenza, Italy), pointed out some important effects of energy exchange from the first isolated mode towards the higher modes, due to the nonlinearity of the isolation system. This results in an increased contribution of higher modes to the overall structural response, as already evidenced in some of the principal studies in literature and also translated into practice by the international building codes and guidelines. Some nonlinear seismic response analyses have been carried out in order to show the inconsistency of the simplified analysis of the OPC 3274/3431. This procedure considerably underestimates the forces in the structure even for practical cases of optimal design of the isolation systems

Valutazione analitica degli effetti del confinamento sulla capacità di pilastri e travi in c.a. di strutture esistenti e di nuova progettazione

Per la valutazione della capacità delle membrature in cemento armato, le principali normative consentono l’uso di legami costituivi per il calcestruzzo ritenuti maggiormente rappresentativi del reale comportamento rispetto ai diagrammi tensione-deformazione del tipo parabola-rettangolo o rettangolare equivalente. Sia la resistenza sia la duttilità della sezione sono influenzate dal legame σ−ε del calcestruzzo e pertanto, come è noto, dal livello di confinamento. Nell’articolo si descrive un possibile approccio progettuale, basato sui metodi classici di analisi della sezione, che prevede l’uso del diagramma rettangolare equivalente per il calcestruzzo confinato. Gli studi effettuati mostrano che è possibile descrivere i parametri dello stress-block attraverso semplici relazioni, che dipendono dalla resistenza del calcestruzzo non confinato, dal quantitativo e disposizione dell’armatura trasversale e longitudinale e dalla geometria della sezione. A tale scopo, nelle analisi numeriche è stato adottato un modello analitico per il calcestruzzo confinato capace di portare in conto tutti i fattori elencati, anche nel caso di confinamento misto costituito da armature trasversali in varie configurazioni e rinforzi esterni (in acciaio, FRP, ecc.). In particolare, i domini di interazione per i diversi stati limite richiedono la definizione dei limiti appropriati per la deformazione massima del calcestruzzo. Pertanto, si indica la possibilità di definire gli stress-block in funzione della deformazione massima assunta o del livello di tensione residua del calcestruzzo ritenuto accettabile in funzione dello stato limite considerato. Occorrerà ovviamente ampliare l’analisi parametrica, allo scopo di ricavare equazioni di progetto proponibili in ambito normativo