Gerardo M. Verderame

A simulated design procedure for the assessment of seismic capacity of existing reinforced concrete buildings

This paper presents the general criteria and implementation of an automatic procedure to evaluate the seismic capacity of existing reinforced concrete (RC) regular buildings. The method represents a useful tool in the framework of mechanical based vulnerability assessment methods. In particular, the seismic capacity is retrieved via pushover analyses on a lumped plasticity model for the building. Unlike recent approaches that rely on a single representative structural model for an entire building population, the proposed method allows virtually all the buildings of the population to be analysed in an automatic loop. With the aim of expediting and automatizing the analysis process, a dedicated software was implemented, whose main functions are: identifying possible structural systems compatible with regular building shapes of assigned dimensions and designing its elements in terms of cross-section and reinforcement; constructing the related nonlinear model and performing pushover analyses in order to determine synthetic capacity parameters useful for preliminary vulnerability assessment at a large scale. The software application and potentialities are shown in an example for the generic building of a class.

A MULTILEVEL APPROACH TO THE CAPACITY ASSESSMENT OF EXISTING RC BUILDINGS

In the present paper a rational mechanical based approach for the seismic capacity assessment of classes of buildings is presented and capacity curves in terms of ultimate strength and deformation capacities are derived. The proposed procedure allows the main parameters (morphologic and geometric configuration, mechanical properties etc.) to be chosen and their relative influence on the capacity of RC buildings to be evaluated. This evaluation is helpful, since it permits, in the framework of vulnerability assessment at a regional or sub-regional scale, to have a measure of the error that is related to the amount of the information available. Consequently, depending on the relative influence of parameters on response, and also considering the different type of parameters and related availability (by public databases for low order parameters, or by field survey for high order ones), a multilevel building class specification in function of input variables is proposed.

Influence of infill distribution and design typology on seismic performance of low- and mid-rise RC buildings

A growing attention has been addressed to the influence of infills on the seismic behavior of Reinforced Concrete buildings, also supported by the observation of damage to infilled RC buildings after severe earthquakes (e.g. L’Aquila 2009, Lorca 2011). In this paper, a numerical investigation on the influence of infills on the seismic behavior of four different case study buildings is carried out: four- and eight-storey buildings, designed for seismic loads according to the current Italian technical code or for gravity loads only according to an obsolete technical code, are considered. Seismic capacity at two Limit States (Damage Limitation and Near Collapse) is assessed through static push-over analyses, within the N2 spectral assessment framework. Different infill configurations are considered (Bare, Uniformly Infilled, Pilotis), and a sensitivity analysis is carried out, thus evaluating the influence of main material and capacity parameters on seismic response, depending on the number of storeys and the design typology. Fragility curves are obtained, through the application of a Response Surface Method. Seismic performance is also expressed in terms of failure probability, given a reference time period.

Vulnerability Analysis for Gravity Load Designed RC Buildings in Naples – Italy

Damage scenario and risk analyses are helpful tools for the local administrators for mitigation of potential earthquake losses at the urban-regional level. One of the main issues in developing such scenarios is the choice of proper capacity functions expressing the effective seismic supply for the existing building classes and the convolution with demand in the so-called fragility analysis. This article, as a further implementation of a recently developed mechanical based procedure for class scale quantitative risk evaluation, presents the derivation of class representative capacity curves and the relative fragility curves for slight, moderate, extensive, and complete damage states as defined by the well-known HAZUS methodology. Starting from an extensive building survey of Arenella district in Naples, southern Italy, statistics on main model input parameters are obtained for selected building classes of existing and/or pre-code RC buildings. Accordingly, a number of building models is simulated designed and analyzed in order to determine building class capacity. Fragility curves are computed simulating the fraction of “failures” within a capacity spectrum method framework. These capacity and fragility curves have been used in a companion paper by Lang et al. [2008] for the computation of damage scenarios in Arenella.

2012 Emilia earthquake, Italy: reinforced concrete buildings response

Data of the Italian National Institute of Statistics are collected aimed at characterizing Reinforced Concrete (RC) building stock of the area struck by the 2012 Emilia earthquake (number of storeys, age of construction, structural typology). Damage observations, collected right after the event in reconnaissance reports, are shown and analyzed emphasizing typical weaknesses of RC buildings in the area. The evolution of seismic classification for Emilia region and RC buildings’ main characteristics represent the input data for the assessment of non-structural damage of infilled RC buildings, through a simplified approach (FAST method), based on EMS-98 damage scale. Peak Ground Acceleration (PGA) capacities for the first three damage states of EMS-98 are compared with registered PGA in the epicentral area. Observed damage and damage states evaluated for the PGA of the event, in the epicentral area, are finally compared. The comparison led to a fair agreement between observed and numerical data.

SEISMIC ASSESSMENT OF GRAVITY LOAD DESIGNED R.C. FRAMES: CRITICAL ISSUES IN STRUCTURAL MODELLING

Reinforced concrete frames designed according to early seismic provisions or, sometimes, without any seismic provision, have usually low strength and, in most cases, show limited ductility. Very often details are poor and, consequently, the critical zones do not behave in a ductile way, showing brittle mechanisms of failure. Because of these problems, the assessment of existing reinforced concrete (RC) structures requires a refined procedure. A summary of models that permit the analysis of the non-linear behaviour of RC structures is discussed. An innovative numerical model is presented wich takes into account the most important mechanical phenomena affecting the non-linear behaviour of the RC frames. In conclusion, the influence of different strength and deformation sources on the global behaviour of existing buildings is studied and the needed capabilities of the numerical models are underlined.

Experimental Behavior of Nonconforming RC Columns with Plain Bars under Constant Axial Load and Biaxial Bending

AbstractAvailable capacity models for the assessment of seismic capacity of RC columns are mainly based on experimental tests under axial load and uniaxial bending. Furthermore, experimental tests have been mainly performed on columns representative of new constructions. To analyze the effects of biaxial bending on the seismic performances of existing RC columns with design characteristics nonconforming to present day seismic codes and practices, this paper presents the results of a first stage of tests on RC columns under both axial load and biaxial bending, which effectively simulate the seismic actions. In particular, this paper presents results of four tests on full-scale square columns reinforced with plain bars subjected to constant axial load and biaxial cyclic actions. Two different cyclic displacement paths were investigated: horizontal displacements with an inclination angle of 45 or 30°, with respect to the cross-section principal axes. The influence of biaxial bending actions on the global beh...

Cyclic Behavior of Nonconforming Full-Scale RC Columns

Capacity models to assess structural safety are primarily based on experimental outcomes typical of new constructions. In order to analyze deformation and strength mechanisms of members that do not comply to current seismic codes and practices, an experimental investigation was undertaken on full-scale columns reinforced with deformed or plain rebars. The experiments involved eight tests under monotonic or cyclic actions on square or rectangular reinforced concrete columns designed according to provisions, construction practice, and material properties enforced between the 1940s and 1970s. Experimental outcomes allowed pointing out global and local behavior of RC columns typically found in existing constructions, in which both plain and deformed bars can be found. Theoretical plastic rotation capacities provided by the proposed supplement to the current ASCE standard are discussed in this paper and compared to those experimentally obtained. The results indicate that predicted plastic rotations corresponding to significant loss of lateral-force capacity are very conservative if compared with experimental outcomes. This seems especially clear in the case of columns reinforced with plain rebars. For these columns, the contribution of the base rotation on the global deformation mechanism is considerable.

Empirical fragility curves from damage data on RC buildings after the 2009 L’Aquila earthquake

The study analyses the data related to a database of 7597 private Reinforced Concrete buildings located in the city and the province of L’Aquila surveyed after the 2009 earthquake. Survey data were collected by the Italian Department of Civil Protection during post-earthquake usability inspections including information on building characteristics, level and extent of damage to structural and non-structural components. For each building, the Peak Ground Acceleration demand has been determined according to data available from the ShakeMap of the event and the georeferenced building location. The analysis of data highlights the key role played by the damage to non-structural components—namely, infills and partitions. Damage Grades according to the European Macroseismic Scale EMS-98 have been derived from damage data to single building components. Two building classes have been defined in the study in order to investigate the influence of number of storeys of buildings on the observed damage. Damage Probability Matrices have been derived for the assumed building classes and bins of Peak Ground Acceleration, and observed trends are analyzed. Different methodologies for estimating fragility functions from data on Damage Grades and Peak Ground Acceleration demand are illustrated, discussed and applied to the database, leading to the derivation of EMS-98-based fragility curves for the defined building classes. Finally, the proposed fragility curves are compared with main empirical fragility curves for RC buildings from literature studies.

INFLUENCE OF INFILL PANELS AND THEIR DISTRIBUTION ON SEISMIC BEHAVIOR OF EXISTING REINFORCED CONCRETE BUILDINGS

Infills can highly influence the seismic response of a Reinforced Concrete (RC) building: generally speaking, their presence leads to an increase in global stiffness and strength, but their brittle behavior can result in an increase of displacement demand if a certain threshold of seismic intensity is overcome. Moreover, presence of infills often leads to a change in the collapse mechanism compared with the bare structure, leading, for instance, to column-sway storey mecha- nisms characterized by a detrimental localization of inelastic displacement demand. In this paper, a numerical investiga- tion of the influence of infills on the seismic behavior of a case-study existing gravity load designed RC building is car- ried out. Different infill configurations are considered (Bare, Uniformly infilled and Soft-storey infilled). Seismic capacity assessment is carried out by means of Static Push-Over analyses, within the N2 spectral assessment framework. A sensi- tivity analysis is carried out, thus evaluating the influence of main material and model parameters on seismic response at different Limit States, namely Damage Limitation and Near Collapse, mainly due to the change in parameters as effective period of vibration, base shear and displacement capacity.