Maria Teresa De Risi

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.

RINTC PROJECT: NONLINEAR DYNAMIC ANALYSES OF ITALIAN CODE-CONFORMING REINFORCED CONCRETE BUILDINGS FOR RISK OF COLLAPSE ASSESSMENT

This paper reports on the results of an ongoing Research Project aimed at computing the risk of collapse in code-conforming new Italian buildings. A companion paper describe the overall Research Project ([11]), funded by the Italian Civil Protection Department (DPC), its different areas of application (reinforced concrete, masonry, steel buildings, etc), the overall seismic risk calculation procedure and the ground motion selection process for the recorded ground motions used for the multi-stripe analyses. This paper deals with different classes of reinforced concrete buildings (namely 3-, 6and 9story high moment resisting frame) designed in cities with increasing seismic hazard. For the sake of brevity, only the results of 6-story buildings are reported. First, the paper describes the geometry, material characteristics and main design properties of the buildings, including their elastic dynamic properties. Three different configurations are considered: Bare Frame (BF), Infill Frame (IF) and Pilotis Frame (PF, that is a building with infills at all levels except for the ground level). Design of the first two configurations is identical, while the PF requires an increase in the design forces at the ground level. Second, the paper introduces the nonlinear models used for the nonlinear analyses: lumped plasticity models for beams and columns, strut and tie models for the infills. Since the buildings are designed according to capacity design principles, the nonlinear behaviors of beams and columns in shear and of beam-column joints are not considered. Pushover analyses are used to estimate the EDP (top floor displacement) value used for the definition of the collapse limit state. Finally, the results of the multi-stripe analyses are presented for ten different ground motion intensities. 1474 Available online at www.eccomasproceedia.org Eccomas Proceedia COMPDYN (2017) 1474-1485

INFLUENCE OF JOINT RESPONSE IN THE ASSESSMENT OF SEISMIC PERFORMANCE OF EXISTING REINFORCED CONCRETE FRAMES

In the seismic performance assessment of existing Reinforced Concrete buildings, non-ductile failures related to beam-column joint regions can represent a critical issue. Therefore, within the context of Performance-Based Earthquake Engineering, a growing attention should be addressed to the behavior of these non-ductile elements, starting from the classification of their failure typology. In particular, in typical existing buildings, seismic performance might be significantly affected by the non-linear behavior of joints which are involved in the failure mechanisms because of poor structural detailing (e.g. lack of an adequate transverse reinforcement in joint panel, deficiencies in the anchorage or absence of any capacity design principle). Nevertheless, conventional modeling approaches consider only beam and column flexibility, although joints can provide a great contribution to the global deformability. In this study, a numerical investigation on the influence of joint response on the seismic behavior of a case study RC frame – designed for seismic loads according to an obsolete technical code – is performed. A preliminary classification of joint failure typology of the elements within the frames and the definition of the corresponding nonlinear behavior are carried out. Structural models that explicitly include beam-column joints are built. A probabilistic assessment based on nonlinear dynamic simulations of structural behavior is performed to evaluate the seismic response at different performance levels. Uncertainty in seismic ground-motion is accounted for.

Experimental Investigation of Exterior Unreinforced Beam-Column Joints with Plain and Deformed Bars

Experimental tests on four full-scale exterior unreinforced reinforced concrete (RC) beam-column joints, representative of the existing non-conforming RC frame buildings, are carried out. The specimens have different longitudinal reinforcements (plain or deformed) and they are designed in order to be representative of two typical design practices (for gravity loads only or according to an obsolete seismic code). Different failure modes are observed, namely joint failure with or without beam yielding. The local response of the joint panel is analyzed. The different joint deformation mechanisms and their contribution to the deformability and to the energy dissipation capacity of the sub-assemblages are evaluated.

Seismic Assessment of Existing Hollow Circular Reinforced Concrete Bridge Piers

ABSTRACT Experimental works in literature have paid proper attention to the seismic response of hollow circular piers only quite recently, despite their widespread use in existing bridges. Herein, experimental cyclic tests on two scaled reinforced concrete piers with hollow circular cross-section, representative of typical existing Italian bridges, are carried out. Design criteria, adopted setup, experimental response, and damage evolution are discussed. A focus on shear–critical piers is performed, collecting an experimental database of tests from literature. The more specific existing shear strength models are compared with the collected data. Model by Ranzo and Priestley [2001] has finally shown promising results.

A modelling approach for existing shear-critical RC bridge piers with hollow rectangular cross section under lateral loads

Most of the existing Reinforced Concrete (RC) bridges were designed before the recent advancements in earthquake engineering and seismic codes. The performance assessment of these bridges is, therefore, a crucial issue for seismic safety of bridge infrastructures and estimation of losses due to seismic events. Despite the seismic assessment of columns with solid cross-section in ordinary buildings may be considered as quite comprehensive, a similar conclusion cannot be drawn for shear-critical hollow core piers, widespread in existing bridge structures. The present work aims at contributing to the investigation about the response of RC piers with hollow rectangular cross-section under cyclic loading. The main goal of the study is the definition of a comprehensive and practice-oriented modelling approach for the assessment of seismic response of RC hollow rectangular piers, able to account for all the deformability contributions, and, particularly, able to reliably predict drift-capacity at shear failure and subsequent degrading stiffness. A three-component model, accounting for flexural flexibility, shear flexibility and slippage of rebars is adopted. The shear capacity assessment is dealt with more in details. A proper experimental database is collected, made up of cyclic tests on hollow rectangular piers failing in shear, with or without yielding of longitudinal reinforcing bars. A new empirical formulation for the assessment of the displacement capacity at shear failure, specifically for the investigated structural elements, is calibrated. The degrading stiffness also is empirically calibrated to completely define the degrading shear response. Finally, the proposed numerical model is validated through the comparison with the experimental results carried out by the Authors (also in terms of local deformability contributions) and with test results collected from literature, proving that it can be a simple and reliable tool for the seismic assessment of existing shear-critical bridge piers.

Empirical drift-fragility functions and loss estimation for infills in reinforced concrete frames under seismic loading

Earthquakes that have occurred in the last twenty years in the Mediterranean area have had significant economic and social impacts. Most of the economic losses of reinforced concrete (RC) frames was due to nonstructural component damage, particularly masonry infills and partitions. Therefore, the seismic behaviour of masonry infills should be reliably characterized. The main goals of this study for a more reliable loss estimation for infilled RC frames are: (i) the analysis of the inter-story drift ratio (IDR) capacity at given damage states (DSs) with the aim to define drift-based fragility functions and (ii) analyse direct losses due to infill damage following seismic events. First, a database of experimental tests performed on 1-bay, 1-story scaled RC frames infilled with clay bricks or concrete blocks is collected. Drift-based fragility curves are obtained, which depend on the infill brick materials and properties. Then, the drift capacity threshold at each DS is correlated to the in-plane response of the infill panel to directly quantify the relationship that exists among them. The influence of openings on drift capacities is also evaluated. Then, seismic losses related to infills are computed, providing expected monetary losses depending on the infill typology. The required reparation activities and their costs are also listed. The bearing of each activity and cost at each DS is explicitly evaluated. Additionally, loss functions that directly depend on IDR demand are provided, thus fusing together the damage analysis and loss analysis. Finally, a simplified formulation for loss functions is proposed for a simple, practice-oriented loss calculation.

CAPACITY MODELS FOR SHEAR-CRITICAL RC BRIDGE PIERS WITH HOLLOW CROSS-SECTION

The assessment of seismic performance of existing bridge structures that have been constructed without appropriate details to support seismic loadings is a primary issue for seismic prone areas. Hollow section piers represent a very common structural solution for reinforced concrete (RC) bridge structures, due to the economical convenience and the higher structural efficiency with respect to solid sections. Nevertheless, a quite limited number of experimental tests are present in literature. Moreover, despite their widespread use, none of the current codes addresses specific attention to define proper capacity models for RC hollow core members, both for design and assessment. In addition, proposals from literature are generally calibrated on columns with solid cross-section or on a very small amount of experimental data. An experimental program of cyclic tests on four 1:4-scale models of typical RC existing bridge piers with hollow rectangular cross-section, poor structural detailing and small web thickness, has been carried out at the Department of Structures for Engineering and Architecture of the University of Naples Federico II. Different failure modes have been observed: flexure and flexure-shear failure, basically depending on the specimens’ slenderness. On the basis of the major experimental results carried out by this experimental campaign, in this work, a database of similar tests from literature has been collected and presented. All tests exhibited a shear failure, with or without yielding of longitudinal bars. Collected experimental results are compared with capacity models existing in literature or codes, in terms of strength and displacement. Their predictive capability is analyzed and commented, and alternative proposals to improve their reliability are carried out. 675 Available online at www.eccomasproceedia.org Eccomas Proceedia COMPDYN (2017) 675-688

CYCLIC RESPONSE AND NONLINAR MODELING OF EXTERIOR UNREINFORCED RC BEAM-COLUMN JOINTS WITH PLAIN LONGITUDINAL BARS

Abstract. The seismic performance of existing Reinforced Concrete (RC) frames is significantly affected by the behaviour of beam-column intersections involved in the collapse mechanism, especially in non-conforming buildings with poor structural detailing or completely unreinforced joints. Even if a quite significant amount of studies on seismic performance of unreinforced joints has been carried out in last years, a very limited number of them deals with specimens reinforced with plain hook-ended longitudinal bars, widespread in Italian and Mediterranean building stock, or with the analysis of local aspects, such as the evaluation of joint shear strains. The majority of the models proposed in literature to simulate the cyclic behaviour of RC joints were developed and calibrated by means of tests performed on elements with deformed bars. Thus, these models may be not adequate for elements with hookended plain bars, especially due to the peculiarities in the interaction mechanisms between concrete and steel for this bar typology. This study first analyses the cyclic experimental tests of four full-scale exterior unreinforced RC beam-column joints with longitudinal plain bars in beams and columns carried out by the authors, in continuity with a previous experimental campaign. The specimens mainly differ for joint aspect ratio and beam longitudinal reinforcement ratio. Global and local responses of such tests are analyzed. The joint deformation mechanisms – rotation at the interface between beam/columns and joint, and shear deformation of the joint panel – are also analyzed. Then, a numerical modelling approach is carried out by OpenSees software to reproduce the experimental cyclic response. Bond-slip is particularly taken into account by introducing a slip spring whose properties are calculated using a bond-slip model properly proposed in literature for plain bars. The numerical results are presented and finally compared with the experimental results highlighting the influence of nonlinear response of joint panel and bondslip mechanism in the response of the sub-assemblage.

SEISMIC BEHAVIOUR OF POORLY DETAILED RC BRIDGE PIERS WITH HOLLOW CROSS-SECTION

Abstract. The assessment of seismic performance of existing bridge structures is a primary issue, especially in regions where most of bridges have been realized without proper details to support loadings due to earthquakes. Hollow section piers are a very common structural solution for reinforced concrete (RC) bridge structures, due to the economical convenience and the higher structural efficiency with respect to solid sections. Nevertheless, a quite limited number of experimental tests are present in literature about this structural typology. An experimental program of cyclic tests on four 1:4-scale models of typical RC existing bridge piers with hollow rectangular cross-section, poor structural detailing and small web thickness, has been carried out at the Department of Structures for Engineering and Architecture of the University of Naples Federico II. Different failure modes have been observed: flexure and flexure-shear, basically depending on the specimens’ slenderness. In this paper, major experimental results are described and discussed, also evaluating the deformability contributions to the overall applied displacement and highlighting the importance of shear deformation. Finally, numerical simulation analyses of the structural response of the tested piers have been performed in OpenSees in order to reproduce the above-mentioned deformability contributions by simulations. Main numerical-versus-experimental results are shown and discussed.