Seismic Design of Box-Type Unreinforced Masonry Buildings Through Direct Displacement-Based Approach

Fulvio Parisi

doi:10.2174/1874836801610010293

Abstract

In the last decade, displacement-based seismic design procedures have been recognised to be effective alternatives to force-based design (FBD) methods. Indeed, displacement based design (DBD) may allow the structural engineer to get more realistic predictions of local and global deformations of the structure, and hence damage, under design earthquakes. This facilitates the achievement of performance objectives and loss mitigation in the lifetime of the structure. Nonetheless, DBD needs further investigation for some structural types such as masonry buildings. In this paper, a direct displacement based design (DDBD) procedure for unreinforced masonry (URM) buildings is presented and critically compared to FBD. The procedure is proposed for box-type URM buildings with reinforced concrete slabs, bond beams and lintels above openings, which have shown acceptable seismic performance in severe earthquakes preventing out-of-plane failure modes. Seismic design of a three storey brick masonry building in a high seismicity region is discussed as a case study. The effects of ordinary and near-field design earthquakes, as well as load combinations and accidental eccentricity prescribed by current codes, were investigated. Finally, design solutions provided by FBD and DDBD were optimised and their construction costs were estimated. It was found that, particularly at small epicentral distances, neglecting the combination of horizontal seismic actions and accidental eccentricity may induce significant underestimation and an ideally more uniform distribution of strength demands on URM walls. In addition, construction costs resulting from DDBD may be significantly lower than those related to code based FBD procedures.

Highlights

In recent years, seismic design codes have been significantly revised to address the main concepts of performancebased earthquake engineering (PBEE)
It is stressed that: (1) wrong assumptions for lateral stiffness may result in unlikely distributions of seismic demand on walls, as shown by non-linear numerical simulations of laterally loaded masonry walls [21]; and (2) the stiffness based distribution of seismic floor forces according to force-based design (FBD) induces strength demand concentrations on stiffer macro elements, resulting in an available displacement ductility of the structure which may be notably lower than that associated with the assumed value of behaviour factor
The effects of the combination of horizontal seismic actions, accidental eccentricity, and type of design earthquake differentiated in terms of epicentral distance, may be taken into account and were assessed in the case of a three storey brick masonry building located in a high seismicity region

Summary

INTRODUCTION

Theoretical and experimental studies have shown that deformations rather than forces are typically a good proxy of seismic damage, delineating drifts as engineering demand parameters well correlated with losses. To account for this consideration, in the 1970’s and 1980’s, earlier force based design (FBD) procedures dating back to the 1930’s were improved by introducing the concepts of ductility and strength reduction factor. Displacement based seismic design procedures have received great interest from the 1990’s, as they are more effective than FBD approaches in predicting/controlling both local and global deformations of structures [3 - 5]. The majority of modern seismic codes allows engineers to design URM buildings according to the FBD approach. To really detect the main limitations of that design approach as applied to masonry buildings, the FBD assumptions are briefly reviewed in the following, making reference to the aforementioned building codes

Force-Based Design Procedure

Force-Based Design Limitations

DIRECT DISPLACEMENT-BASED DESIGN PROCEDURE FOR UNREINFORCED MASONRY BUILDINGS

Findings

CONCLUSION