DAMAGE STATES FOR REINFORCED CMU MASONRY SHEAR WALLS

Jia Li and Terence A. Weigel

Department of Civil Engineering, University of Louisville, Louisville, KY 40292, USA

Abstract

In order to assess the damage loss of reinforced masonry walls under earthquake loading, it is helpful to have a set of commonly accepted damage states. HAZUS gives detailed description for the qual­itative damage states and assigns threshold drift ratios for achievement of each damage state. The HAZUS damage states are assigned based on expert opinion and judgment, and performance and experience data. Unfortunately, application of the HAZUS damage states is limited by the fact that they do not differentiate failure mode: flexure, shear and possible mixed flexure/shear. Furthermore, drift ratios defined in HAZUS have not been fully verified by experiment or experience. As a step toward addressing these deficiencies, this paper examines experimental results from three experi­mental programs and assesses the accuracy of the HAZUS methodology. Drift ratios at achievement of defined damage states are compared with HAZUS definitions. Results show that the HAZUS methodology tends to overestimate the drift ratio achieved by a wall at a given level of damage. In this paper, only experimental results for concrete masonry unit (CMU) walls are considered.

Introduction

Shear walls are the primary lateral load-resisting elements in reinforced masonry structures. When excited seismically, they are usually subjected to simultaneous in-plane loads, out-of-plane loads, axial loads and overturning moments. Design of shear walls for in-plane loads, as well as estimation of damage and associated economic loss upon occurrence of a seismic event, are important issues to all stakeholders: architects, engineers, building officials, insurance companies and owners. A key requirement in establishing a basis for accurate design provisions and assessment of the economic impact of seismic events is having an accurate set of quantitative damage descriptions for critical wall conditions. Also important is a corresponding set of qualitative descriptors that can be related to the quantitative damage descriptions.

A damage state is defined as a specified level of damage under earthquake, corresponding to some critical condition of the wall. Damage states have been defined differently by different agencies / researchers. For example ATC-40 (1996) defines the performance levels (consistent with damage) of Operational, Immediate Occupancy, and Life Safety and Structural Stability. HAZUS (FEMA,

1999) defines four qualitative damage states: slight, moderate, extensive, collapse, as shown in Table 1. HAZUS is software that will estimate potential regional earthquake losses. Loss estimates are used to plan and assist in reduction of risks from earthquakes, and to prepare for pre-earthquake emergency response and post-earthquake recovery. In addition to qualitative damage state, HAZUS also provides quantitative assessment of damage based on drift ratio and a “code level”, as is shown in Table 2. Other quantitative damage indicators used by researchers (Hwang, 2001; Park, 1985; Mander, 1999) include displacement, ductility, energy, restoration time and money. In this paper, the HAZUS damage states are used. Much of this research is done for concrete and may not be directly applicable to the performance of masonry walls.

An issue not addressed by researchers and the HAZUS methodology, at least not directly, is the influence of the mode of response of the wall on the relationship between qualitative and quantitative damage measures. A reinforced masonry wall may respond to a seismic event in a shear critical or

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© 2006 Springer. Printed in the Netherlands.

Table 1. Qualitative definition of damage states in HAZUS.

Damage States

Building Type (RM1L/RM2L*)

Slight

Diagonal hairline cracks on wall surfaces; large cracks around door and window openings in walls with large proportion of openings; minor separation of walls from the floor and roof diaphragms.

Moderate

Most wall surfaces exhibit diagonal cracks; some of the shear walls have exceeded their yield capacities indicated by larger diagonal cracks. Some walls may have visibly pulled away from the roof.

Extensive

Most shear walls with large openings have exceeded their yield capacities and some of the walls have exceeded their ultimate capacities indicated by large, through-the-wall diagonal cracks and visibly buckled wall reinforcement. Partial collapse of the roof may result from failure of wall to diaphragm connections.

Complete

Structure has collapsed or in imminent danger of collapse due to failure of the wall an­chorages or the wall panels. Approximately 13(low-rise) of the total area of the building is expected to be collapsed.

*RM1L/RM2L is used to denote low-rise reinforced masonry bearing walls, generally ranging from 1-3 stories, with a total height less than 20 feet.

Table 2. Quantitative definition of damage states in HAZUS.

Seismic Design Level Building Type (Low-Rise) Drift Ratio at Damage State Threshold

Slight Moderate Extensive Complete

High-Code*

RM1L/RM2L

0.004

0.008

0.024

0.070

Moderate-Code

RM1L/RM2L

0.004

0.007

0.019

0.053

Low-Code

RM1L/RM2L

0.004

0.006

0.016

0.044

Pre-Code

RM1L/RM2L

0.003

0.005

0.013

0.035

* High-Code, Moderate-Code, Low-Code correspond to the “quality” of the design code to which the building was designed. Pre-Code is used to indicate that the building was not designed for seismic loading.

flexurally critical mode. Some walls may exhibit a mixed flexural/shear critical mode. There are other possible response modes (for example, base sliding), but only flexure and shear are considered in this paper.

Flexurally critical walls exhibit yielding of vertical reinforced and crushing of the compression toe as they experience cycles of large excursions of reversed cyclic deformation. Shear critical walls exhibit diagonal tensile cracking when they experience the same type of loading. Examination of Tables 1 and 2 shows that the HAZUS methodology does not differentiate damage states based on behavior mode.

The principal objective of this research is to develop realistic qualitative and quantitative damage states and relate these states to behavior modes. In this paper four damage states are used as found in HAZUS: Slight, Moderate, Extensive, Collapse. The assessment of damage states and their rela­tionship to behavior mode is based on experimental work for in-plane loading done by Shing (1988, 1990), Ibrahim (1999) and Eikanas (2003).

Damage States for Reinforced Masonry Walls