Modeling and Seismic Response Analysis of Italian Code-Conforming Reinforced Concrete Buildings

ABSTRACT

This study investigates the seismic response of reinforced concrete buildings designed according to the current Italian building code. Number of stories, site hazard, presence and distribution of masonry infill panels, and type of lateral resisting system are the key investigated parameters. The main issues related to design and modeling are discussed. Two Limit States are considered, namely Global Collapse and Usability-Preventing Damage. The main aim of the study is a comparison between the seismic response of the buildings, investigated through nonlinear static and dynamic analyses. Irregularity in the distribution of infill panels and site hazard emerge as the most influential parameters.     

Masonry Italian Code-Conforming Buildings. Part 2: Nonlinear Modelling and Time-History Analysis

ABSTRACT

The unreinforced masonry (URM) buildings designed to be conforming with the Italian building code, as illustrated in the companion paper, were analyzed by performing time-history analyses on models realized using an equivalent frame approach and by adopting two different constitutive laws. Both the effect of record-to-record variability and of epistemic and aleatory uncertainties in modelling were explored. The achieved results constitute the basis for the evaluation of the risk level implicit in Italian code-conforming buildings. Two main performance conditions are considered, namely usability-preventing damage and global collapse limit states.     

Review of Different Pushover Analysis Methods Applied to Masonry Buildings and Comparison with Nonlinear Dynamic Analysis

This article presents the comparison among different nonlinear seismic analysis methods applied to masonry buildings, i.e., pushover analyses with invariant lateral force distributions, adaptive pushover analysis and nonlinear dynamic analysis. The study focuses on the influence of lateral force distribution on the results of the pushover analysis. Two simple benchmark case studies are considered for the purpose of the research, i.e., a four-wall masonry building prototype without floor rigid diaphragms and a two-wall system with a cross-vault. The comparative study offers a useful review of pushover analysis methods for masonry structures and shows advantages and possible limitations of each approach.     

Simple Models for Inelastic Seismic Analysis of Asymmetric Multistory RC Buildings

A simple stick model is presented for the inelastic seismic analysis in 3D of two-way eccentric multistory RC buildings. It has 3 DoFs per floor, point hinges at the ends of the vertical elements connecting floors, elastic story stiffness derived from the corresponding story force-interstory deformation relations of the elastic 3D structure under inverted-triangular floor loading (by torques for torsional stiffness, by horizontal forces for the lateral ones), story yield forces derived from the total resistant shear of the story vertical elements, but no coupling between lateral and torsional inelasticity. It is evaluated on the basis of comparisons of response histories of floor displacements to those from full nonlinear models in 3D of four actual buildings. Alternative locations of the story vertical element with respect to the floor mass center are examined: (a) the floor “center of twist” of the elastic 3D building under inverted-triangular floor torques; (b) the story “effective center of rigidity,” through which application of inverted triangular lateral forces does not induce twisting of floors; (c) the centroid of the secant stiffness of the story vertical members at yielding and (d) the centroid of the lateral force resistance of story vertical elements. Among alternatives (a)–(d), the floor “center of twist” provides the best agreement with floor displacement response-histories from full 3D nonlinear models. This means that the static eccentricity that matters for torsional response may be taken as that of the floor “center of twist.” The center of resistance comes up as the second-best choice.     

Distribution of Lateral Forces in Base-Isolated Buildings Considering Isolation System Nonlinearity

The ASCE 7 equivalent lateral force method for base-isolated buildings applies a triangular distribution of forces to the superstructure. This distribution attempts to approximately account for the observed effects of isolation system nonlinearity on the superstructure response, but a more rational approximation is needed. Using nonlinear regression analysis of median response data from nonlinear response history analysis of representative systems, improved equations are developed to estimate the lateral force distribution in the superstructure. The ASCE 7 distribution, a revision considered by a SEAONC committee, and the improved distribution developed here are evaluated. Only the improved equations are accurate over many system parameters.     

Simplified Seismic Sidesway Collapse Capacity-Based Evaluation and Design of Frame Buildings with Linear Viscous Dampers

This article describes a simplified procedure for estimating the seismic sidesway collapse capacity of frame building structures incorporating linear viscous dampers. The proposed procedure is based on a robust database of seismic peak displacement responses of viscously damped nonlinear single-degree-of-freedom systems for various seismic intensities and uses nonlinear static (pushover) analysis without the need for nonlinear time history dynamic analysis. The proposed procedure is assessed by comparing its collapse capacity predictions on 272 different building models with those obtained from incremental dynamic analyses. A straightforward collapse capacity-based design procedure is also introduced for structures without extreme soft story irregularities.     

Controlled Rocking CLT Walls for Buildings in Regions of Moderate Seismicity: Design Procedure and Numerical Collapse Assessment

Controlled rocking heavy timber walls are designed to rock on their foundations in response to earthquakes. For regions of moderate seismicity, it is proposed that this rocking behaviour can be adequately controlled using only post-tensioning, even with a large force-reduction factor and no supplemental energy dissipation. This article presents a force-based design procedure for controlled rocking cross-laminated timber walls without supplemental energy dissipation, including a method for estimating higher mode effects. Fragility analyses of three prototype walls demonstrate that the procedure can limit the probability of collapse to <10% during a maximum considered earthquake in a region of moderate seismicity.     

Optimum Design Approaches for Improving the Seismic Performance of 3D RC Buildings

In this article, a number of design approaches for 3D reinforced concrete (RC) buildings are formulated in the framework of structural optimization problems and are assessed in terms of their performance under earthquake loading. In particular, three design approaches for RC buildings are considered in this study. In the first, the initial construction cost is considered as the objective function to be minimized. The second one is formulated as a minimization problem of the torsional response, while a combined formulation is also examined as the third design approach. The third approach is considered with two distinctive formulations. According to the first approach, the torsional behavior is minimized by minimizing the eccentricity between the mass and rigidity centers, while the second one is achieved by minimizing the eccentricity between the mass and strength centers. It is shown that the optimized designs obtained according to the minimum eccentricity of the rigidity center behave better in frequent (50/50 hazard level) and occasional (10/50 hazard level) earthquakes, while the designs obtained according to the minimum eccentricity of the strength center formulation was found better in rare (2/50 hazard level) events. Designs obtained through a combined formulation seem to behave equally well in the three hazard levels examined.     

Analytical Seismic Assessment of a Tall Long-Span Curved Reinforced-Concrete Bridge. Part II: Structural Response

The seismic assessment of special bridges, even under the hypothesis of full knowledge of site conditions, structural characteristics, and seismic activity at their location, is not an easy and straightforward task due to the complexities and uncertainties related to the finite-element modeling approaches, structural loading scenarios, and seismic analysis methodologies. In this article, a series of nonlinear static and dynamic finite-element analyses on the Mogollon Rim Viaduct are performed with consideration of both uniform and conditionally simulated non-uniform seismic motions. The failure modes of the bridge using different numerical modeling approaches are discussed, and the degree of sensitivity of its response to the different seismic assessment strategies is evaluated. The effect of the multi-component, multi-support and multi-directional excitations of ground motions on the design and response are studied, and the pros and cons of the commonly used structural analysis methodologies of bridges are also addressed. The numerical results of the present study provide a deeper insight into the nonlinear behavior of curved reinforced-concrete bridges, and suggest practice-oriented approaches for their seismic assessment.     

Simulation Of Shake Table Tests on Out-of-Plane Masonry Buildings. Part (II): Combined Finite-Discrete Elements

ABSTRACT

Out-of-plane response of unreinforced masonry elements is frequently the most critical aspect of the seismic performance of existing masonry buildings. The response of such elements is usually governed by equilibrium rather than strength. Hence, it is customary to resort to rigid-body models, accounting for possible rotations, and/or sliding. However, the results of such analyses depend on the initial choice of the mechanism. In this article, the shaking-table experiments on a brick-masonry specimen, and on a stone-masonry specimen have been modeled by resorting to a combined finite-discrete element strategy. Despite the coarse discretization of both discrete and finite elements, the three-dimensional models are able to capture the experimentally observed multi-degree-of-freedom mechanisms, without any a priori assumption on the mechanism. A sensitivity analysis is carried out, addressing eight different parameters. The identification of the mechanism is sufficiently robust, but the assessment of its activation and failure is best done by combining the finite-discrete element model with a simplified model of the recognised mechanism.     

Simulation of Shake Table Tests on Out-of-Plane Masonry Buildings. Part (I): Displacement-based Approach Using Simple Failure Mechanisms

ABSTRACT

A displacement-based (DB) assessment procedure was used to predict the results of shake table testing of two unreinforced masonry buildings, one made of clay bricks and the other of stone masonry. The simple buildings were subject to an acceleration history, with the maximum acceleration incrementally increased until a collapse mechanism formed. Using the test data, the accuracy and limitations of a displacement-based procedure to predict the maximum building displacements are studied. In particular, the displacement demand was calculated using the displacement response spectrum corresponding to the actual shake table earthquake motion that caused wall collapse (or near collapse). This approach was found to give displacements in reasonable agreement with the wall’s displacement capacity.     

Jet-Pacs Project: Dynamic Experimental Tests and Numerical Results Obtained for a Steel Frame Equipped with Hysteretic Damped Chevron Braces

The experimental and numerical results obtained by Research Units of the University of Basilicata and University of Calabria for a steel frame, bare or equipped with metallic yielding hysteretic dampers (HYDs), are compared. The shaking table tests were performed at the Structural Laboratory of the University of Basilicata within a wide research program, named JETPACS (“Joint Experimental Testing on Passive and semiActive Control Systems”), which involved many Research Units working for the Research Line 7 of the ReLUIS (Italian Network of University Laboratories of Earthquake Engineering) 2005–2008 project. The project was entirely founded by the Italian Department of Civil Protection. The test structure is a 1/1.5 scaled two-story, single-bay, three-dimensional steel frame. Four HYDs, two for each story, are inserted at the top of chevron braces installed within the bays of two parallel plane frames along the test direction. The HYDs, constituted of a low-carbon U-shaped steel plate, were designed with the performance objective of limiting the inter-story drifts so that the frame yielding is prevented. Two design solutions are considered, assuming the same stiffness of the chevron braces with HYDs, but different values of both ductility demand and yield strength of the HYDs. Seven recorded accelerograms matching on average the response spectrum of Eurocode 8 for a high-risk seismic region and a medium subsoil class are considered as seismic input. The experimental results are compared with the numerical ones obtained considering an elastic-linear law for the chevron braces (in tension and compression), providing that the buckling be prevented, and the Bouc-Wen model to simulate the response of HYDs.     

Protective Structures for the Preservation of Medieval Buildings and their Remains: Case Studies from Monastery Hilandar and Mount Athos

The objective of this study is to establish a methodology for temporary protective structures, that is, the guidelines for planning, design, and implementation of future protective structures. The basic method of this paper was the observation and analyses of temporary protective structures as well as the evaluation of the results achieved in implementation of these rules in completed structures, within the Hilandar Monastery complex on Mount Athos, in Greece. Mount Athos was inscribed on the UNESCO World Heritage List for its cultural and natural values. The Holy Mount Athos is a unique monastic complex in the world and only males are allowed past the entrance. Having performed the evaluation of certain types of temporary protective structures, the given results were offered as recommendation for their design and implementation. The most significant accomplishment of this paper is verification of the designed, calculated, and completed protective structures through practical application within the Hilandar Monastery complex, in accordance with all the factors of site management.     

Analytical Seismic Assessment of a Tall Long-Span Curved Reinforced-Concrete Bridge. Part I: Numerical Modeling and Input Excitation

The seismic response of bridges is affected by a number of modeling considerations, such as pier embedment, buried pile caps, seat-type abutments, pounding, bond slip and architecturally flared part of piers, and loading considerations, such as non-uniform ground excitations and orientation of ground motion components, which are not readily addressed by design codes. This article addresses a methodology for the nonlinear static and dynamic analysis of a tall, long-span, curved, reinforced-concrete bridge, the Mogollon Rim Viaduct. Various modeling scenarios are considered for the bridge components, soil-structure interaction system, and materials, i.e., concrete and reinforcing steel, covering all its geotechnical and structural aspects based on recent advances in bridge engineering. Various analysis methodologies (nonlinear static pushover, time history response to uniform and spatially variable seismic excitations, and incremental dynamic analyses) are performed. For the dynamic analyses, a suite of nine earthquake accelerograms are selected and their characteristics are investigated using seismic intensity parameters. A recently developed approach for the generation of non-uniform seismic excitations, i.e., spatially variable simulations conditioned on the recorded time series, is used. Methods for the evaluation of structural performance are discussed and their limitations addressed. The numerical results of the seismic assessment of the Mogollon Rim Viaduct are presented in the companion article (Part II). The sensitivity of the bridge response to the adopted modeling, loading and analyzing strategies, as well as the correlation between structural damage and seismic intensity parameters are examined in detail.