Earthquake vulnerability of existing unreinforced masonry buildings

Abstract

New Zealand still has a large stock of low rise unreinforced masonry (URM) buildings. The performance of this type of building in past earthquakes has sometimes been poor. This has resulted in increasing pressure to strengthen or demolish these buildings. However, despite their poor reputation, many URM buildings have survived strong shaking with relatively little damage. This includes most of the URM buildings in Dannevirke during the May 1990 Weber earthquake and large numbers of domestic buildings in the 1985 Chilean earthquake.

This would indicate that only modest amounts of strengthening may be required for many URM buildings in low and moderate seismicity zones to produce an acceptable level of risk. 

Unfortunately the response of URM buildings to earthquake motions is not well understood. This makes it difficult to predict the likely seismic performance of a particular URM building and identify those features of the building that require strengthening.

In this project, damage reports for URM buildings in seven earthquakes are reviewed. Statistical damage data is summarised and building features that have resulted in damage or collapse are identified.

A computer model of a face loaded URM wall is then analysed using inelastic dynamic analysis. Predicted behaviour is compared with test results and the computer model is used to develop a methodology that can be used to assess the seismic resistance of a face loaded URM wall. In this methodology the effective period of the face loaded wall motion is computed using semi-empirical formulae and an elastic displacement response spectrum is used to predict the earthquake magnitude required to cause wall collapse. The proposed methodology is expected to provide more realistic assessments of face loaded wall stability than currently used procedures.