LATERAL LOAD TESTS ON EARTHQUAKE DAMAGED HOUSES IN CHRISTCHURCH

Thousands of houses in Christchurch were damaged in the Christchurch earthquakes of 2010 and 2011. Uni-directional lateral load tests were undertaken on two houses in the residential red zone in Christchurch in June 2012. The aim of the tests was to measure the lateral load stiffness of typical moderately damaged houses to identify the change in stiffness due to the seismic load history. The wall bracing systems of the houses are assumed to satisfy the regulations in force at the time, which are outlined. Several relevant tests from the literature are briefly reviewed. A diagonal tension load system was designed that provided a near horizontal load of 130kN at the ceiling level. A number of electronic measurements were recorded with dial gauges and simple measurements to provide backup and to check uplift or slip from the foundation. The minimally damaged house tested on Wairoa St included fibre cement weatherboard, internal gypsum plasterboard, with a heavy tile gable roof. It was built in 1983 and had a timber pile foundation. The narrow weatherboard house in Bexley Rd was built in 1947 with a heavy tile hip roof with a pile and ring beam foundation. This house had pre-existing damage due to differential settlement, movement relative to the foundation and damage at the internal fibrous plaster wall to ceiling interface. Under the same maximum 130kN test loading, maximum deflection of the 1983 house was approximately half that of the 1947 house. Both houses exhibited considerably more stiffness than anticipated, with the 1980’s gypsum and diagonal steel brace system approximately twice as stiff as the 1940’s. The expected seismic softening was not detected so the earthquake softened deformations are likely to be less than 1mm and of modest serviceability concern. The damage indicated the applied load was in excess of the earthquake loads but the pattern of damage was typical of earthquake damage observed in many houses. The residual stiffness of timber framed houses, even after significant shaking, is considerable and a future test load system needs to be stiffer with a much higher load capacity to determine the residual strength.