Masonry Italian Code-Conforming Buildings. Part 1: Case Studies and Design Methods

ABSTRACT

Several architectural configurations of unreinforced masonry residential buildings are designed according to the different methods proposed in the Italian code: rules for the so-called simple masonry buildings, linear and nonlinear static analyses. Always complying with code requirements, for each building-site combination, the design was made, as much as possible, without an excessive margin of safety. The different design methods provided buildings with very different levels of safety, being linear static analysis largely overconservative with respect to the nonlinear static approach. These buildings were then analyzed in the companion paper.     

Effects of Geometrical Features on the Seismic Response of Historical Masonry Towers

ABSTRACT

Historical masonry structures are often located in earthquake-prone regions and the majority of them are considered to be seismically vulnerable and unsafe. Historical masonry towers are slender structures that exhibit unique architectural features and may present many inadequacies in terms of seismic performance. The seismic protection of such typologies of structures and the design of effective retrofitting interventions require a deep understanding of their behavior under horizontal loads. This paper presents the results of the seismic performance evaluation of historical masonry towers located in Northern Italy. A large set of case studies is considered, comprising a significant number of towers with high slenderness and marked inclination. First, a preliminary assessment of the dynamic behavior of the different towers is carried out through eigenfrequency analyses. Then, non-linear dynamic simulations are performed using a real accelerogram with different peak ground accelerations. A damage plasticity material model, exhibiting softening in both tension and compression, is adopted for masonry. The huge amount of results obtained from the non-linear dynamic simulations allows a comparative analysis of the towers to be performed in order to assess their seismic vulnerability and to show the dependence of their structural behavior on some geometrical characteristics, such as slenderness, inclination, and presence of openings and belfry. The evaluation of different response parameters and the examination of tensile damage distributions show the high vulnerability of historical masonry towers under horizontal loads, mainly in the presence of marked inclination and high slenderness. Some general trends of the seismic behavior of the towers are deduced as a function of the main typological features.     

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.     

Theoretical and Numerical Seismic Analysis of Masonry Building Aggregates: Case Studies in San Pio Delle Camere (L’Aquila,Italy)

Masonry building aggregates are large parts of the Italian building heritage often designed without respecting seismic criteria. The current seismic Italian code does not foresee a clear calculation method to predict their static nonlinear behavior. For this reason, in this article a simple methodology to forecast the masonry aggregate seismic response has been set up. The implemented procedure has been calibrated on the results of two FEM structural analysis programs used to investigate three masonry building compounds. As a result, a design chart used to correctly predict the base shear of aggregate masonry units starting from code provisions has been set up.     

Identification of Suitable Limit States from Nonlinear Dynamic Analyses of Masonry Structures

Performance-based earthquake engineering, developed over the last decades for the design and assessment of other structures, can also be applied for masonry structures if the particularities of masonry are incorporated into the procedure. According to this methodology, structural performance can be assessed according to damage states which are identified through displacement/damage indicators. While various methods for the identification of limit states from the results of nonlinear static analyses exist, the identification of damage states from the results of nonlinear dynamic analyses is still uncertain. This article investigates a number of criteria allowing to identify the attainment of significant limit states from the results of time history analyses, in terms of appropriately identified response quantities. These criteria are applied to five building prototypes and their results are compared. A comparison with the limit states derived from nonlinear static analyses is also made.     

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.     

A Multi-Mode Method for Estimation of Floor Response Spectra

A new simplified procedure for estimation of floor response spectra (FRS) is proposed. This methodology enriches the most common procedures using nonlinear response-history analysis to predict FRS by including a direct multi-mode technique to estimate FRS. A novel feature of the procedure is that the coupling effect is considered to establish equivalent modal systems and the FRS are developed by incorporating capacity spectrum method in conjunction with ductility-based FRS for each modal system. Both the proposed method and the traditional method are applied to three steel moment frame structures, and a reasonable accuracy is demonstrated.