Test Protocols for ASCE 41 Backbones in Concrete Building Evaluation

Abstract

Backbone curves were generated from previous physical lab tests conducted on identical full-scale RC components utilizing different loading protocols (slab–beam–column and column specimens). These curves were subsequently compared with those prescribed in ASCE 41. It was observed that there was a substantial difference between the two, with ASCE 41 backbones (based on standard tests) showing significantly lower ductility capacity compared to backbones from tests using a loading pattern representative of earthquake demands, leading to a near-collapse condition. A four-story archetype RC building was analyzed to assess the influence of different backbones on its seismic response. The analysis revealed that using backbones representative of earthquake demands produced a median spectral capacity somewhere from 25% to 50% greater than that obtained using ASCE 41 upper bound backbones. Similarly, for drift capacities at incipient collapse, it was found that using backbones representative of earthquake demands resulted in a median capacity somewhere between 1.6 and 2.9 times that of using ASCE 41 upper bound backbones. These differences are significant, implying that ASCE 41 can be overly conservative by rejecting buildings that would otherwise be acceptable if ASCE 41 criteria were based on tests using protocols representative of earthquake demands. Examination of the time-history responses of the study building revealed that earthquake demands were predominantly characterized by relatively few large drift excursions having a one-direction bias. Standard protocols differ significantly from responses observed during actual earthquakes, as they feature numerous symmetrical fully reversed cycles of drifts with progressively increasing amplitudes that are much more demanding. This leads to overly conservative ASCE 41 criteria (too pessimistic). It is concluded that future component lab tests must include protocols representative of earthquake demands to achieve more accurate estimates of component behaviors. Use of such protocols will lead to more appropriate (less conservative) ASCE 41 building evaluations, aligning more consistently with the philosophy of performance-based earthquake engineering.