Modelling and Seismic Response Analysis of Italian Code-Conforming Base-Isolated Buildings

ABSTRACT

This paper reports on results of nonlinear analyses performed within the RINTC project on an RC building isolated with different systems (High Damping Rubber Bearings, High Damping Rubber Bearings and Flat Sliding Bearings, Friction Pendulum System) and designed according to the Italian design code. The seismic response has been evaluated under different seismic input levels of two sites with different hazard and by considering two Limit States: Global Collapse and Usability-Preventing Damage. The influence of seismic stoppers and modelling uncertainties is also evaluated. Results permit to compute the implicit collapse risk and to identify critical aspects of current design procedures.     

Modeling and Seismic Response Analysis of RC Precast Italian Code-Conforming Buildings

ABSTRACT

In this study, industrial single-story RC precast buildings are investigated. Twenty-four case studies have been considered, in which the column height, the beam spans and the seismic hazard level are varied. The seismic design of the selected case studies is performed according to the Italian building code and additional technical documentation. Three-dimensional nonlinear models are defined to perform static and dynamic analyses for the seismic assessment of the selected case studies. Demand/capacity ratios in terms of the selected engineering demand parameters are computed for ten increasing values of the seismic input return period.     

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.     

Modeling and Seismic Response Analysis of Italian Code-Conforming Single-Storey Steel Buildings

ABSTRACT

This article describes the structural design, nonlinear modeling, and seismic analysis of prototype single-storey non-residential steel buildings made of moment-resisting portal frames in the transverse direction and concentric braces in the longitudinal direction. Various design parameters (building geometry, seismic hazard, foundation soil category) and different modeling assumptions (bare frame model, model including cladding elements, ground motions including vertical accelerations, and modeling uncertainties) were considered to investigate their effects on the simulated seismic performance.     

Seismic Reliability of Code-Conforming Italian Buildings

ABSTRACT

This paper presents and discusses some research results related to the seismic failure risk of standard, residential and industrial, buildings designed for damage, and life-safety according to the Italian seismic code, which is somewhat similar to Eurocode 8. The five considered structural typologies are as follows: masonry, cast-in-place reinforced concrete, precast reinforced concrete, steel, and base-isolated buildings. The archetype structures have been designed according to standard practice at three sites, representative of the seismic hazard across the country. Seismic risk is defined here as the annual rate of earthquakes able to cause structural failure in terms of usability-preventing damage and global collapse. For each structure, the failure rates have been evaluated in the framework of performance-based earthquake engineering, that is, via integration of site’s probabilistic hazard and structural fragility. The former has been computed consistently with the official hazard model for Italy that is also used to define design actions in the code. The latter has been addressed via nonlinear dynamic analysis of three-dimensional numerical structural models. Results indicate that, generally, design procedures are such that seismic structural reliability tends to decrease with increasing seismic hazard of the building site, despite the homogeneous return period of exceedance of the design seismic ground-motion.     

A Seismic Hazard Study through the Comparison of Ground Motion Prediction Equations Using the Weighting Factor of Logic Tree

Toward the assistance on selection of ground motion prediction models for seismic assessment, this article presents a seismic hazard study (compared to the viewpoint of attenuation equations), using a recent tool based on engineering judgment, called “weighting factor,” through a procedure similar to logic tree. For this purpose, the weighting factors were incorporated with a Venn diagram of attenuation models regarding experimenter’s concern and expert’s knowledge. It is found that the attenuation equations of the newer and intersection ones could be considered to estimate plausible and reasonable accelerations. The results indicate that the weighting factors could beneficially assist for suitability of attenuation models. This work is a novel for the region (Gaziantep, Turkey), thus it could complement expert’s knowledge about the attenuation models for future studies.     

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.     

Predicting the Seismic Response of Capacity-Designed Structures by Equivalent Linearization

An equivalent linearization procedure is developed for predicting the inelastic deformations and internal forces of capacity-designed structures under earthquake excitations. The procedure employs response spectrum analysis, and mainly consists of the construction of an equivalent linear system by reducing the stiffness of structural members that are expected to respond in the inelastic range. These members are well defined in structures designed with capacity principles. Maximum modal displacement demands of the equivalent linear system are determined either from the equal displacement rule, or from independent nonlinear response history analysis of SDOF systems representing inelastic modes.

Predictions obtained from the proposed equivalent linearization procedure are evaluated comparatively by using the results of nonlinear response history analysis as benchmark, linear elastic response spectrum analysis and conventional pushover analysis. The deformations and capacity controlled actions of a 12-story symmetrical plan concrete frame and a 6-story unsymmetrical plan concrete frame are obtained by each method under 96 strong ground motions. It is observed that the proposed procedure results in better accuracy in estimating the inelastic seismic displacement response parameters and capacity controlled forces than the other two approximate methods.     

Critical Assessment of Intensity Measures for Seismic Response of Italian RC Bridge Portfolios

The seismic assessment of a road network depends largely on the characterization of the fragility of its bridge components. The accuracy of bridge seismic demand estimates and the use of proper intensity measures (IM) will significantly influence such task. The available literature has mainly focused on buildings or a limited number of bridge configurations and IMs, which may not be representative for bridge portfolio assessment studies. In this paper, the correlation quality between a larger pool of traditional and innovative IMs and the nonlinear dynamic response of typical Italian RC bridges is investigated to identify the best-performing IMs.     

A Microscale Approach for the Seismic Response of MDOF Structures Including Soil-Foundation-Structure Interaction

In this article, a three-dimensional microscale computational framework utilizing the discrete element method is presented to analyze the seismic response of soil-foundation- MDOF structure systems. The proposed approach is used to explore the response of MDOF structures on a square embedded footing founded on a dry granular deposit. Computational simulations were conducted to investigate the response of the system to several base excitations. The impact of replacing a MDOF structure with its equivalent single degree of freedom (ESDOF) structure while accounting for soil-foundation-structure interaction (SFSI) is investigated. Detrimental or beneficial effect of SFSI on the response is also examined.     

Optimum Design of Passive Control Devices for Reducing the Seismic Response of Twin-Tower-Connected Structures

Based on the 3-single-degree-of-freedom (SDOF) model of twin-tower structures linked by the sky-bridge and passive control devices, the frequency functions and the vibration energy expressions of the structures are derived by using the stationary white noise as the seismic excitation. The analytical formulas for determining the connecting optimum parameters of viscoelastic damper (VED) represented by the Kelvin model and the viscous fluid damper (VFD) represented by Maxwell model are proposed using the principle of minimizing the average vibration energy of either the single tower or the twin tower. Three pairs of representative numerical examples of twin-tower-connected structures are used to verify the correctness of the theoretical approach. The optimum parametric analysis demonstrates that the control performance is not sensitive to damper damping ratio of VED and relaxation time of VFD. The effectiveness of the proposed control strategies based on the 3-SDOF models is also proved to be applicable to multi-degree-of-freedom systems. The theoretical analysis and numerical results indicate that the seismic response and vibration energy of the twin-tower-connected structures are mitigated greatly under the two types of dampers. The presented control strategies of VED and VFD can help engineers in application of coupled structures.     

Methods and Challenges for the Seismic Assessment of Historic Masonry Structures

ABSTRACT

Despite the high vulnerability of historic structures to earthquakes, the approaches for evaluating seismic demand and capacity still appear inadequate and there is little consensus on the most appropriate assessment methods to use. To develop an improved knowledge on the seismic behavior of masonry structures and the reliability of analysis tools, two real-scale specimens were tested on a shake table, and several experts were invited to foresee failure mechanism and seismic capacity within a blind prediction test. Once unveiled, experimental results were simulated using multi-block dynamics, finite elements, or discrete elements. This article gathers the lessons learned and identifies issues requiring further attention. A combination of engineering judgment and numerical models may help to identify the collapse mechanism, which is as essential as it is challenging for the seismic assessment. To this purpose, discrete modeling approaches may lead to more reliable results than continuous ones. Even when the correct mechanism is identified, estimating the seismic capacity remains difficult, due to the complexity and randomness of the seismic response, and to the sensitivity of numerical tools to input variables. Simplified approaches based on rigid body dynamics, despite the considerable experience and engineering judgment required, provide as good results as do advanced simulations.     

Estimating the Higher-Mode Response of Ductile Structures

While the importance of higher-mode actions is appreciated within the engineering community, the affect that ductile nonlinear response has on higher-mode characteristics and the subsequent implications this has for design has received little attention. In this article, the manner in which the higher-mode response of frame-wall structures is affected by inelastic behavior is closely examined and a means of accounting for this in design is proposed. The work focuses firstly on the characteristics of the higher modes present at the development of peak response and then considers how these characteristics would affect the total forces in the building. The study utilizes a series of nonlinear time-history analyses of two different groups of RC frame-wall structures subject to a suite of real records. It is shown that a new modal analysis approach that incorporates transitory inelastic modal characteristics gives significantly improved predictions of peak base shear in frame-wall structures than more traditional modal analysis methods which use elastic higher-mode characteristics. The issues associated with the use of transitory inelastic modal characteristics are discussed and various challenges that would need addressing for the prediction of other response parameters and structural types are identified.     

Use of Pushover Analysis for Predicting Seismic Response of Irregular Buildings: A Case Study

Structural irregularity undermines capability of conventional methods for 2D pushover analysis to closely approximate results from inelastic dynamic analysis. In recent years, different methods have been developed to overcome such limitation and their suitability has been checked with reference either to idealized building models or to geometrically simple tested structures. In this paper, suitability of one such method, proposed by Fajfar et al. [2005] Fajfar, P., Maru?i?, D. and Perus, I. 2005. Torsional effects in the pushover-based seismic analysis of buildings. Journal of Earthquake Engineering, 9(6): 831–854. [Taylor & Francis Online], [Web of Science ®] , [Google Scholar], is evaluated considering an existing school building which presents both vertical and plan irregularities. Types of irregularity encompass not only those usually considered by seismic codes but also those deriving from a bad conceptual design and construction inaccuracies, very frequent at the year of construction (1974). It is found that, even under such complex irregularity conditions, this ‘modified’ pushover analysis correlates well results from inelastic dynamic analysis almost up to failure, since, in most cases, its predictions of interstorey drifts and plastic rotations are conservatively close to values from inelastic dynamic analysis. Even failure mechanism, consisting of a floor mechanism at the third level, is correctly predicted, thus demonstrating adequacy of such method for actual framed structures.     

Seismic Response of the Central Part of Indo-Gangetic Plain

In spite of high seismic risk, there is no numerical model of Indo-Gangetic Plain. In this paper, seismic response of the central part of Indo-Gangetic Plain, i.e., Ganga Plain is studied using a two-dimensional plane strain finite element model. Seismic source is assumed to be located beneath the Himalayas near Himalayan Frontal Thrust. Basin response is simulated for a hypothetical M_w 8.0 Himalayan earthquake. Ground motion amplification and sensitive frequency band of the basin, obtained from the numerical simulation, show that the sediment depth and epicentral distance at a site play significant role in the seismic response of the site.     

Investigation Techniques for the Seismic Response Assessment of Buildings Located in Historical Centers

ABSTRACT

It is an acknowledged fact that historical centers, given its significance from the cultural and architectural viewpoint, bring further challenges in terms of maintenance planning, survey, and safety assessment. The preparation of an adequate investigation plan and the extent of data to be collected is highly reliant on many aspects, such as the category of the architectonic asset, the importance of the heritage site, or the resources available, for example. In what regards the seismic response assessment of urban cultural heritage assets located in historical centers, the amount and detail of data also depend on this article, scale of assessment, and current state of conservation and occupation. Within this framework, this article provides an overview of the state of the art of investigation techniques currently used in survey operations, which are currently available for improving the knowledge level of urban cultural heritage assets within historical centers, as a supporting tool for the seismic response assessment of such singular assets. Finally, acknowledging the lack of accuracy when evaluating the seismic response of an asset enclosed in aggregate as an isolated structure, this article also focuses on the identification of the main particularities inherent to buildings enclosed in aggregate.     

Physical and Numerical Approaches for the Optimal Insertion of Seismic Viscous Dampers in Shear-Type Structures

This article presents the results of an exhaustive parametric analysis which compares the performances offered by various systems (which lead to both classical and non classical damping matrices) of added viscous dampers in shear-type structures. The aim of the research work here presented is the identification of the system of added viscous dampers which maximizes the dissipative properties under an equal “total size” constraint. The choice of the systems of added viscous dampers considered in the comparison is carried out both using a numerical approach (based upon the use of genetic algorithms) and a physically based approach (based upon the properties of classically damped systems). The comparison is carried out through the numerical evaluation of the dynamic response of representative shear-type structures to both stochastic and recorded earthquake inputs. The results obtained using both approaches indicate that a damping system based upon the mass proportional damping component of the Rayleigh viscous damping matrix (referred to as MPD system) is capable of optimizing simultaneously a number of different performance indexes, providing the best “overall” damping performances. The MPD system is characterised by viscous dampers (a) which connect each floor to a fixed point and (b) which are sized proportionally to the corresponding floor mass.     

Development of Probabilistic Framework for Performance-Based Seismic Assessment of Structures Considering Residual Deformations

Recently, the importance of considering residual (permanent) deformations in the performance assessment of structures has been recognized. Advanced structural systems with re-centering properties as those based on unbonded post-tensioning tendons are capable of controlling or completely eliminating residual deformations. However, for more traditional systems, which count for the vast majority of buildings, residual deformations are currently considered an unavoidable result of structural inelastic response under severe seismic shaking.

In this article, a probabilistic framework for a performance-based seismic assessment of structures considering residual deformations is proposed. The development of a probabilistic formulation of a combined three-dimensional performance matrix, where maximum and residual deformations are combined to define the performance level corresponding to various damage states for a given seismic intensity levels, is first presented. Combined fragility curves expressing the probability of exceedence of performance levels defined by pairs of maximum-residual deformations are then derived using bivariate probability distributions. The significance of evaluating and accounting for residual deformations within a Performance-based Earthquake Engineering (PBEE) approach is further confirmed via numerical examples on the response of Single Degree of Freedom (SDOF) systems, with different hysteretic behavior, under a selected suite of earthquake records. Joined fragility curves corresponding to various performance levels, defined as a combination of maximum and residual response parameters, are derived while investigating the effects of hysteretic systems and strength ratios. It is observed that stiffness degrading Takeda systems result in lower residual deformations than elasto-plastic systems and show lower probability of exceeding a jointed maximum-residual performance level. For a chosen performance level, Takeda systems with higher strength ratios show better performance, particularly with lower intensity of excitations.