UC Berkeley

Earthquakes that have occurred recently across the globe in various countries including United States, Japan, New Zealand, Haiti, Turkey and Italy have brought into light the poor seismic performance of older-type, non-ductile concrete buildings. These buildings, mainly designed and constructed prior to 1980s, lack proper seismic detailing and may pose an unacceptably high seismic risk.

Non-ductile concrete buildings pose one of the greatest seismic safety problems in the world due to the large amount of old buildings constructed in earthquake prone regions. It is indicative that according to the Concrete Coalition and the California inventory project there are 16,000-17,000 of these buildings in high earthquake risk counties of California. Many of these buildings have high occupancies, including residential, commercial and critical services. In case of a severe earthquake, the severe damage or even collapse that could occur in these buildings could result in large number of casualties.

While engineers generally recognize that proactive steps are required to address the risk posed by these buildings, mitigation efforts are largely stymied by insufficient knowledge about the scale of the problem, insufficient tools to identify the truly dangerous buildings, high costs of strengthening, and owner resistance to pay for the strengthening with uncertain benefits. This study constitutes an effort to identify seismically hazardous concrete frame buildings through simplified methods that do not require complicated analysis.

Three idealized concrete frame buildings with different heights are used as archetypes. The study attempts to link the collapse performance of these buildings with various structural deficiencies that appear commonly in older construction practice. To evaluate the performance of these buildings non-linear dynamic analysis for several far-fault ground motions is performed. The analysis considers nonlinearities associated with flexural yielding, shear and axial failure. The main deficiencies explored are development of weak story mechanisms due to strong column-weak beam designs, brittle shear or axial failure modes associated with inadequate column shear reinforcement detailing, and splicing and connectivity weaknesses between structural members.

The results indicate that the suggested methods can be used to assess the collapse risk of older-type concrete buildings. The methods developed in the current study use simple engineering parameters such as column-to-beam strength ratio and column flexural to shear strength ratio to estimate the collapse risk of older type concrete buildings. A probabilistic approach is suggested that takes into account record-to-record variability and could accommodate as well uncertainty associated with structural properties and collapse modeling.

In Chapter 7 the proposed methodology is evaluated by applying it to the three idealized buildings developed. The estimated probabilities of collapse calculated for each of the buildings according to the proposed methodology are compared with the values provided by sophisticated non-linear dynamic analyses. The results suggest that the proposed methodology successfully identifies deficiencies that are leading to high collapse potential and provides an effective tool in classifying collapse prone concrete frame buildings.