Earthquake risk associated with 1935-1975 reinforced concrete buildings in New Zealand

Technical Abstract

This report gives the results of an investigation into the earthquake risk associated with New Zealand's 1935-1975 reinforced concrete building stock.

Current New Zealand legislation on earthquake risk buildings (Local Government Act, 1974, Section 624) covers only unreinforced masonry and unreinforced concrete buildings. This type of construction was not permitted in New Zealand for main structural elements after the introduction of the 1935 building code.

However buildings constructed using reinforced concrete have collapsed in previous earthquakes. This was highlighted recently by the collapse of the Cypress Street Viaduct which killed 42 people during the Loma Prieta earthquake. The viaduct was a two level, elevated freeway structure. It was designed in 1951 and completed in 1957 and collapsed due to brittle behaviour of the columns supporting the upper level of the highway. This demonstrated once again the potential seismic vulnerability of early reinforced concrete structures.

The objective of this research project was to obtain an assessment of the type and extent of problems associated with reinforced concrete buildings of the 1935 to 1975 era and hence to obtain a more realistic assessment of their vulnerability to earthquake damage.

It is expected that the results of this investigation will assist with a more rational planning approach to the use of these buildings and a more accurate assessment of their seismic risk for insurance purposes.

The report is presented in three sections:

 Section 1 details the results of a survey of the buildings constructed within the non residential part of Wellington City between 1935 and 1975. Wellington was chosen for the survey because its associated seismic risk is greater than that for any other New Zealand city.

 The characteristics of the buildings were identified in the survey as the first step that is required to establish the size and nature of the earthquake risk associated with this group of buildings.

 The survey indicated that the floor area of 1935 to 1975 reinforced concrete buildings in the surveyed area constitutes approximately 10% of the total floor area of all buildings in Wellington City.

 The survey also indicated that 78% of the floor area of the 1935 to 1975 reinforced concrete buildings is concentrated in buildings with four or more storeys and that 86% of this floor area was constructed during the 1960's and early 1970's.

 Section 2 of the report gives the results of a preliminary evaluation of a range of typical 1935 to 1975 Wellington buildings. The evaluation identifies common potential deficiencies in the buildings.

 An assessment was then made of the risk damage or collapse associated with the potential deficiencies. This assessment was based on a review of the performance of reinforced concrete buildings in previous earthquakes. It was concluded that many of the potential deficiencies identified would result in strength and stiffness degradation. This aspect of the buildings likely behaviour was then examines by a review of pervious research on the effects of strength and stiffness degradation.

 Extensive research indicates that the effects of stiffness degradation are not as significant as they were once thought to be. However more research is required to clarify the effects that strength degradation has on the response of structures.

 A tentative method for evaluating structures exhibiting strength and stiffness degradation is proposed for future development.

 Section 3 for the report details an investigation of two reinforced concrete shear wall buildings built in the late 1950 and early 1960. The walls were modelled so that they could yield in both shear and flexure at the base of the wall and analyzed using inelastic dynamic analysis.

 Current New Zealand design standards are intended to preclude significant shear yielding.

 The analysis indicated that earthquakes could impose large inelastic shear displacement demands on walls if they are not designed to these standards.

 A tentative procedure for evaluating the seismic performance of walls that are likely to be subjected to significant inelastic shear displacement demand is proposed for future development.