The paper describes the complexity of the seismic assessment and rehabilitation of three different existing buildings in New Zealand. The assessment was performed using Progressive Collapse Analysis. This method has been materialized into explicit requirements for redundancy in building codes. Conventionally, the engineering industry uses a simplistic procedure for most seismic assessments, which models only linear beam and column elements. This neglects the contribution of walls and slabs, leading to uneconomic solutions. Walls and slabs may be considered secondary members in other types of analysis but in progressive collapse analysis, walls and slabs often behave as primary members with slabs carrying load though membrane action and walls providing alternate load paths in case of loss or extensive damage of columns.
The buildings have been modelled using the “Applied Element Method” (AEM) [1, 2, 3]. This approach allows tracking of the structural collapse behavior passing through all stages of the application of loads including elastic stage, crack initiation and propagation in tension-weak materials, steel yielding, element separation and element collision. It has also the unique ability to accurately evaluate the dynamic Eigen modes accounting for the phenomenon of period elongation due to cracking of the structural elements during the ground excitation. Period elongation is a phenomenon that may alter significantly the response of the structures and the effects of the ground motions on the buildings. This is a significant breakthrough not only for the New Zealand industry but also for the international engineering community.
Extensive research was undertaken to overcome the modelling complexities to incorporate the specific building characteristics including riveted connections, slabs, infill panels, foundation and surrounding soil and to assess the performance of the structures using the state of the art methodology [4, 5, 6, 7]. A set of Numerical Integration Time History (NITH) analyses in compliance with AS/NZS 1170.5  recommendations was completed for the Progressive Collapse methodology. Various geotechnical and material testing was undertaken to confirm the parameters used in the analysis. The ground motions were selected and scaled in accordance with Site Specific Seismic Hazard Assessments.
To validate the accuracy of the models, the results were checked against ASCE41-13  acceptance criteria in conjunction with AS/NZS code requirements and limitations [7, 9]. The post-earthquake observation in one of the case studies were used to validate the results of our analysis. The results indicate the efficiency of the specific methodology to visualize the extent, magnitude and direction of any potential local or global collapse or crack occurrences within the structures and provide accurate insights on the performance of the buildings, leading to the most effective strengthening strategy.
This methodology also enables the engineers to safely design the egress routes away from falling debris, for the safe evacuation of the buildings during the earthquakes.
Prionas, I. (2016). Progressive Collapse Analysis of Existing Buildings – A Performance Based Approach, Congress on Earthquake Engineering (16WCEE). Key Words: Progressive Collapse Analysis; Applied Element Method; Performance Based Design; Period Elongation.