Vulnerability analysis of unreinforced masonry churches

Abstract

We undertake the first (to our knowledge) seismic vulnerability method specifically designed for New Zealand Unreinforced Masonry (URM) churches. The vulnerability index (VI) methodology developed by Lagomarsino et al. (2003) for European churches and other monumental buildings has been the basis for our work. The technique is based a macro-seismic approach, which obtains the expected seismic damage to the churches given a certain seismic scenario. These scenarios are created using macroseismic intensities, in the New Zealand Modified Mercalli intensity (MMI) scale. The method has been redefined, with a new set of parameters and modifiers specifically created for New Zealand URM churches. This has been done by analysing the damage caused to 48 URM churches in the Canterbury region during the 2010-2011 Canterbury earthquake sequence.

The main achievements obtained during this project, funded by the Earthquake Commission (reference 14/660, January 2014-June 2016), include: (a) the structural data compilation of a wider stock of 297 URM churches spread within New Zealand; (b) a specific typological classification for New Zealand unreinforced masonry (URM) churches; (c) a damage survey form for URM churches; (d) a method to obtain the seismic vulnerability of URM churches specifically developed for New Zealand URM churches, using the damage data from the Canterbury earthquakes; and (e) the development of seismic scenarios for the URM churches in Wellington, Auckland and Dunedin, using the new parameters developed within this project. These scenarios provide expected damage levels in case of a rupture of the fault with the highest contribution to the seismic hazard in each of the three cities.

Analysis of the typologies of URM churches in Europe and New Zealand justified the need to develop a method specifically created for this country, as results show the great differences in typologies to European churches, with very simple architectural designs and a majority of one nave churches in New Zealand. The method has been applied to three cities in New Zealand, with very different seismic activity, from low (Auckland) to intermediate (Dunedin) and high (Wellington). Differences in the results show the need to develop specific scenarios for each  city / region.

This project is seen as a first step towards the qualification of all the historical buildings in the country, in order to preserve New Zealand’s cultural and historical heritage. Future work identified includes (a) the development of seismic scenarios for the URM churches in the rest of New Zealand, (b) the inclusion of “site effects” to the seismic scenarios, to account for local differences in intensities experienced in each church, (c) development of a more sophisticated method based on the structural behaviour of individual components of the building called “macroelements” (such as the bell tower, the nave, the apse, etc.) and (d) the addition of other buildings part of the cultural heritage in New Zealand, such as palaces.