Assessment of Existing In-Plane Unreinforced Masonry Walls in New Zealand

Student: Mr Alistair Russell
Principal Supervisor: Associate Professor Jason Ingham
Co-Supervisors: Associate Professor John Butterworth

This project is a part of the Seismic Retrofit Solutions project

Project Overview

Unreinforced masonry (URM) buildings have been shown countless times to perform very poorly in earthquakes.  This became apparent especially after the 1931 Napier earthquake in New Zealand.  However the construction of unreinforced masonry buildings in New Zealand remained in-use till the late 1940s, and was finally outlawed in 1965.  There are, therefore still a number of existing unreinforced masonry buildings today.

Buildings which are seismically at risk can either be demolished and a new structure built in its place, or else the original structure can be retrofitted to ensure it aligns with today's current performance criteria.  As unreinforced masonry was a very common building material in early New Zealand (particularly 1860 - 1930), a significant proportion of the URM building stock falls into the category of 'heritage architecture'.  Moreover, many of these buildings are protected by The Historic Places Trust of New Zealand, and may not be demolished.  Arguably, URM buildings in this country is the most important category of buildings which must be given attention for preservation.

Before a building can be appropriately retrofitted, it is first of all necessary to provide an accurate assessment of the structure's inherent capacity to withstand an earthquake, and what parts in particular are most at risk.  Without accuracy at the assessment stage, overly conservative and hence more costly retrofit interventions are required.  To provide cost-effective retrofit solutions and to encourage building owners to implement them, this excessive conservatism associated with current assessment methods must be removed. 

Some steps have been taken towards producing methods of assessment for URM buildings which are seismically at risk.  Research has been undertaken in the Unites States, Italy and Australia towards analysing the behaviour of URM buildings under earthquake loading.  Little research has been undertaken in New Zealand to find out how our particular URM building stock will perform in an earthquake. 

For practitioners in New Zealand two methods of seismic assessment exist; that outlined in FEMA 356 (American), and that outlined in the document produced by NZSEE, "Assessment and Improvement of the Structural Performance of Buildings in Earthquake," which is largely based on research undertaken in Italy. 

The aim of this research project is to complement existing research with data and procedures particular to that of New Zealand?s unique building stock. 

The main outcomes of this research are listed below:

1. Characterisation of New Zealand's URM buildings stock.

• URM buildings fall into categories depending on building geometry and other relevant parameters, and within each category are typical defective details which are of interest. 
• This characterisation will allow a basic understanding of the building's earthquake performance before any analysis is undertaken.
• This will also assist in facilitating life cycle costing of URM retrofits.
• This work is being undertaken in conjunction with the Schools of Architecture at the University of Auckland and Victoria University, and currently ongoing.

2. Material testing of URM structures in New Zealand.

• Currently little data is available on the material properties of URM buildings in New Zealand.  Italy has a comprehensive database with important parameters and material properties well documented, and available to apply to new assessments.  It is proposed to build a database of material properties which are unique to New Zealand, and will include regional variation in those properties.

3. Refinement of URM assessment techniques.

• As outlined above, there exists currently assessment techniques for URM buildings, but not which have been derived or validated for the New Zealand context, with our particular building stock.
• With particular emphasis on New Zealand relevance, both simple and comprehensive assessment methods will be presented, with direct applicability to the current building stock.

4. Testing of walls/subassemblies

• Laboratory testing will consist of both element and subassembly structures, and will be conducted pseudo-statically and dynamically, in order to best appreciate the structure?s response when subjected to earthquake loading.
• This testing will illuminate relevant parameters to be used as inputs for computer modelling.

5. Modelling of URM structures

• Data obtained from laboratory testing will be used as appropriate inputs for structural modelling using common computer analysis programmes.

The models analysed and the relevant parameters used in modelling, as well as the other outputs outlined above will be communicated to practising structural engineering consultants through conference and journal papers, in particular through the journals of both NZSEE and SESOC.