This Rapid Response Research (RAPID) award will collect data to document the performance of unreinforced masonry (URM) construction during the February 2011 Christchurch, New Zealand earthquake. Specifically, this award will document the ability of originally constructed and retrofitted floor and roof diaphragms to efficiently distribute loads to all walls and other lateral-load resisting elements. NZ URM construction from 1880 to about 1970 typically used timber joists, rafters, and trusses, very similar to U.S. construction from the same era. The project will catalog seismic performance at both the qualitative and quantitative levels. At the qualitative level, the project will identify weak or poor detailing typical in URMs and effective mitigating technologies. At the quantitative level, the project will develop guidelines for the type and detailing of retrofit measures to be used in strengthening projects in the United States. These guidelines will be developed in close collaboration with the U.S. masonry industry and practitioners to ensure that results from this research will be transferred quickly to American practice.
Building stock in large parts of the United States, including most of the east coast and the Midwest, consists of URM construction. This type of building is known to be very susceptible to collapse under earthquake loading; therefore, effective and economical retrofit techniques are needed to mitigate damage and ensure human safety. The Christchurch event provides unique and exceptional data to calibrate current U.S. practices for seismic evaluation and retrofit of these systems, including retrofits that did not bring the structures to 100% of the strength required by current codes. This latter data is important for the case where seismic retrofits will not be carried as a single project, but rather as a series of smaller projects spread over a number of years as other improvements are made to existing URM buildings. This staged approach has been proposed to spread the cost over a longer period and make the retrofits more feasible. The NZ experience will clarify which vulnerabilities need to be addressed first and which techniques should be used.
The most common New Zealand construction from 1880 to about 1950 was primarily unreinforced brick masonry with timber floors and roof framing. The timber framing consists of joists, rafters and trusses, and is very similar to the United States construction from the same era. The primary difference with US construction is the type of wood. Unreinforced masonry (URM) construction is very common in large parts of the USA and is well-known to be very susceptible to damage from even small earthquakes due to the lack of steel reinforcement in the walls, the relatively weak mortars typically used in this type of construction, the large masses associated with this type of construction, and the poor connections between the walls and the floors or roof (often referred to as diaphragms). This report briefly describes the retrofits and performance of 8 URM structures during the Canterbury Earthquake swarm of 2009-2011. The February 22, 2010 earthquake that devastated the city of Christchurch, in particular, provided a rich opportunity to develop information on the performance of URM retrofit techniques that is directly applicable to US practice. The case studies indicate that historic unretrofitted URM structures will perform poorly if subjected to accelerations consistent with current design codes. Poor performance is primarily the result of (a) inadequate connections between the walls and diaphragms, (b) the presence of long and tall unsupported masonry walls, and (c) the generally poor quality of mortar and deteriorated timbers. However, the report documents that even small efforts at retrofit can be very successful with respect to life safety concerns if key vulnerabilities in the design are addressed in the retrofit. The most successful projects were those that those with a two-step approach. The first step was to provide additional highly deformable walls and bracing elements with rigidities similar to those of the existing structure. The second step was to tightly connect these new elements to the existing structures in order to provide additional load paths for both gravity and lateral loads. The case studies demonstrate that the conception of a global retrofit scheme is the key to a successful outcome, but also that the proper execution of the strengthening details at the job site is essential. The 8 cases studies address a wide variety of construction types and materials. The consistent observations made with respect to outcomes indicates that it is possible to develop generic retrofit strategies for older URM structures and that these strategies need not be expensive and can be carried out with the building in service. However, they also indicate that careful analysis of the stiffness of the existing structure is necessary in order to match the deformation and strength characteristics of the older and new parts of the structure. In addition, improving the connections in the existing structure and careful consideration of the connection design between the older and newer elements require considerable effort and experience from the designers.
