Inventory and Seismic Fragility of New Zealand Masonry Infilled Reinforced Concrete Framed Buildings

Abstract

The out-of-plane fragility of masonry infilled Reinforced Concrete Framed (RCF) buildings in New Zealand was assessed using force-based analytical methods. The out-of-plane capacity and seismic demand on clay brick and concrete block masonry were evaluated for buildings from different construction periods, of different heights, and with different ductility capacities. Monte Carlo analysis was used to examine any possibility that affected the out-of-plane capacity and seismic demand. In order to do so, five thousand random values were generated for each parameter that had an influence on the response. Nine parameters were set as random variables in order to predict masonry infill capacity, namely, masonry compression strength (f’m); thickness of the infill panels (t); height of the infill panels (H); length of the infill panels (L); compression strength of the concrete frames (f’c); column depth (hc); column width (wc); beam depth (hb) and beam width (wb). Similarly, for seismic demand, variables were generated for masonry density (ρm) along with soil types, locations and return period factors - with the probability of the last three parameters being expressed in terms of the Peak Ground Acceleration (PGA). To avoid the analysis of unnecessary possibilities and to ensure that the results were representative of New Zealand data, the random values generated were set to follow a distinct distribution which was obtained from field investigations of 86 buildings (35% of the total number of masonry infilled RCF buildings in the Auckland region) in the Auckland Central Business District (CBD). Three types of goodness of fit test (Kolmogorov Smirnov, Anderson Darling and Chi-squared) were then used to determine which distribution best fitted the samples in order to ensure the maximum precision. The mean values and the distributions derived from the field investigations were adjusted for buildings from different construction periods. Twenty-seven sets of fragility curves were produced for clay brick and concrete block masonry infill panels from particular periods with different locations, different ductility capacities, and at various possible PGA values that could be encountered throughout New Zealand. The results showed that, in spite of the use of masonry with higher compression strengths, the thin panels used in new buildings had significantly reduced the capacity, making new masonry infilled RCF buildings in New Zealand surprisingly vulnerable to out-of-plane collapse compared to older buildings. The higher compression strengths used in new buildings, associated with higher density, also lead to higher seismic demands. Thus, it was also found that concrete masonry infilled RCF buildings in New Zealand are more vulnerable to out-of-plane failure compared to brick masonry infilled RCF buildings. The analysis showed, as expected, that up to the 4th storey the higher the infill panel position from the ground, the higher the out-of-plane collapse probability. However, the collapse probability at the 4th storey or higher remained the same. This is because the floor acceleration coefficient that determines the seismic demand remains the same when the value of hi ≥ 0.2 hn, which was almost always met at the 4th storey. It was also observed that buildings with a ductility factor μ ≤ 1 had a collapse probability of up to 25% and 24% (for brick and concrete panels, respectively) higher than buildings with a ductility factor μ = 2 under the same seismic load, and 31% and 33% higher than buildings with a ductility factor of μ ≥ 3.