Franco Braga

Speedup of post earthquake community recovery: the case of precast industrial buildings after the Emilia 2012 earthquake

The Emilia, May–July 2012, earthquake hit a highly industrialized area, where some tens thousands industrial buldings, mainly single storey precast structures, are located. Due to the likelihood of strong after shocks and the high vulnerability of these structures, the authorities first asked for a generalized seismic retrofit after the strong shakings of May 20th. In order to accelerate community recovery, this requirement was later loosened, leaving out the buildings which had undergone a strong enough shaking without any damage; the strong enough shaking was defined with reference to the ultimate limit state design earthquake. To the authors’ knowledge, it is the first time that the information on the earthquake intensity and structural damage is used for such a large scale post earthquake simplified safety assessment. In short, the earthquake was used as large experimental test. This paper shows the details of the models and computations made to identify the industrial buildings which have been considered earthquake tested and therefore not compelled to mandatory seismic retrofit. Since earthquake indirect (e.g. due to economic halt) costs may be as large the direct ones, or even larger, it is believed that this method may considerably lower the earthquake total costs and speed up the social and economic recovery of a community.

Modified Steel Bar Model Incorporating Bond-Slip for Seismic Assessment of Concrete Structures

This paper presents a simplified model for describing the response of a longitudinal bar embedded in concrete, taking into account the bond-slip phenomenon. The model is developed by assuming a linear bond-slip field along the bar anchorage length and provides a simplified stress-strain relationship to assign to the longitudinal reinforcement. The analytical approach adopted makes the proposed model very convenient from a computational standpoint because, unlike many other refined models, it does not require a multilevel iterative process. Moreover, the assumptions made are particularly appropriate for modeling bond-slip of smooth bars generally used in older reinforced concrete buildings. The implementation strategy of the proposed bond-slip model in general-purpose nonlinear structural analysis software and comparisons with experimental results are discussed in a companion paper.

R/C Existing Structures with Smooth Reinforcing Bars: Experimental Behaviour of Beam-Column Joints Subject to Cyclic Lateral Loads

The paper illustrates the results of experimental tests performed on sub-assemblages of R/C (Reinforced Concrete) existing structures, designed only for gravity loads In order to estimate failure mechanisms and ductility properties, four internal and external R/C beam-column joints were built and tested. The specimens were built by using concrete with low strength and smooth reinforcing bars, without hoops into the panel zone. The tests were performed by increasing cyclic horizontal displacements up to the collapse. The experimental results show that seismic response of these kind of structures is mainly influenced by bond slips of longitudinal bars, and that the shear collapse regards external joints rather the internal ones. Failure mechanisms observed (column plastic hingings for internal joints, shear failure for external joints) point out the vulnerability of these structures due to the soft storey mechanism. The study is significant for better understanding of the inelastic seismic behavior of the R/C existing buildings with smooth bars, and for evaluating the effectiveness of the model classical assumptions.

Validation of a Modified Steel Bar Model Incorporating Bond-Slip for Seismic Assessment of Concrete Structures

In this paper the implementation and validation of a modified steel bar model including bond-slip of longitudinal bars that was proposed in a companion paper is discussed. The model is developed on the key assumption of linear slip field along the steel bar with different configurations at the ends of the bar. The simplified model is capable of predicting the axial slip displacement with suitable accuracy compared with a refined model but with considerably fewer computational steps. The proposed model avoids nested iterations in the context of fiber model discretization of a section that requires the representation of all actions in terms of stress and strain. The model is applied to two component tests—one with poor and another with improved reinforcing detailing. Findings from the simulations indicate that the proposed model is more suitable for use in connections with poor detailing and pronounced slip in the plastic hinge zones.

Seismic demand on steel reinforcing bars in reinforced concrete frame structures

Modern design standards for reinforced concrete (r.c.) buildings allow the achievement of ductile structures, able to globally dissipate seismic energy through the development of plastic deformations located in the dissipative regions (i.e. plastic hinges). The hysteretic capacity of r.c. structures is related to the ability of reinforcing steel bars to sustain many cycles of high plastic deformations without the exhibition significant decrease of strength and stiffness; this condition, typically due to cyclic/seismic action, shall be widely investigated in order to obtain a full and detailed knowledge of the structural behaviour of modern r.c. buildings. In the present paper, elaborated inside the European research project “Rusteel”, the evaluation of the seismic ductile demand on steel reinforcing bars due to real earthquake events was carried out. Representative r.c. case study buildings were designed following the actual European and Italian prescriptions and analyzed using the Incremental Dynamic Analysis technique for the assessment of the behaviour under real seismic events. The elaboration of a simplified mechanical model for the steel reinforcing bars, calibrated on the basis of experimental monotonic and cyclic tests, allowed the evaluation of the effective level of deformation and energy dissipation required by earthquakes and the assessment of the ability of the actual European production to satisfy the effective seismic ductile requirements.

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.

PERFORMANCE-BASED NONLINEAR RESPONSE HISTORY ANALYSIS FRAMEWORK FOR THE “PROINDUSTRY” PROJECT CASE STUDIES

Within the PROINDUSTRY project a Performance-based analysis framework is defined for seismic assessment of industrial structures, based on Nonlinear Response History Analysis (NL RHA) using in particular the Incremental Dynamic Analysis (IDA) method. This paper describes the choice of the PBEE and IDA analysis methods starting from an overview of state-of-the-art methods. The choice is analyzed in relation to: analysis goals for the selected case studies (design-based vs. risk-based), availability of databases/tools for hazard analysis and GMs selection, accuracy of criteria to scale and match GMs to a target spectrum (UHS, CMS, etc.) and treatment of record components. Three possible approaches of GMs selection are described and analyzed: (1) UHS-coherent Unscaled (Design), (2) UHScoherent GMs scaled to Sa(T1) (Risk/Loss), (3) CMS-coherent GMs scaled to Sa(T1), (Risk/Loss). Within the scopes of PROINDUSTRY, the approach (1) is proposed, as a tradeoff between simplicity of conventional PBEE design methods and probabilistic robustness for a heterogeneous portfolio of structures/facilities. M. Faggella, R. Laguardia, R. Gigliotti, F. Morelli, F. Braga, W. Salvatore.

Seismic Analysis of Concrete Gravity Dams: Nonlinear Fracture Mechanics Models and Size-Scale Effects

The phenomenon of interface crack propagation in concrete gravity dams underseismic loading is herein addressed. This problem is particularly important from the engineeringpoint of view. In fact, besides Mixed-Mode crack growth in concrete, dam failure is oftenthe result of crack propagation along the rock-concrete interface at the dam foundation. Toanalyze such a problem, the generalized interface constitutive law recently proposed by the¯rst author is used to proper modelling the phenomenon of crack closing and reopening at theinterface. A damage variable is also introduced in the cohesive zone formulation in order topredict crack propagation under repeated loadings. Special attention is given to the complexityresulting from the solution of the nonlinear dynamic problem and to the choice of the interfaceconstitutive parameters, taking into account the important size-scale e®ects observed in thesecyclopic structures. Numerical examples will show the capabilities of the proposed approachwhen applied to concrete gravity dams.

An Analytical Formulation of Stress-Block Parameters for Confined Con- crete

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 lon- gitudinal 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 pa- per concludes comparing the measured curvature ductility of the studied sections with the expected one by the codal pro- visions within the critical regions.