Experimental Studies of Reinforced Concrete Structures Under Multi-Directional Earthquakes and Design Implications (NUREG/CR-7119)
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Manuscript Completed: September 2011
Date Published: July 2013
N. Simos and C. H. Hofmayer
Brookhaven National Laboratory
P.O. Box 5000
Upton, NY 11973-5000
A. Murphy, NRC Program Manager
NRC Job Code N6510
Office of Nuclear Regulatory Research
U.S. Nuclear Regulatory Commission
Washington, DC 20555-0001
This report describes the results of a multi-phase study focusing on the SMART2008 (Seismic design and best-estimate Methods Assessment for Reinforced concrete buildings subjected to Torsion and non-linear effects) shaking table experiment. The aim of the project was to enable an international benchmarking study to take place where different methodologies, modeling, and numerical approaches are used to study and predict the non-linear behavior and damage of reinforced concrete structures. Specifically, a 1/4th scale model of a nuclear reinforced concrete structure designed according to the French nuclear practices was tested on the shaking table at Commissart a' l'Energie Atomique Saclay, France using thirteen multidimensional earthquakes ranging from low seismic intensities to five times the design level. Three numerical prediction phases were executed which included (a) a "blind" prediction phase based on using best-estimate data for the structural properties and the induced seismic loads, (b) an "updated" prediction phase where best estimates were improved using some of the initial shaking table test results enabling the studies of higher loadings, and (c) a sensitivity and vulnerability analysis phase based on the SMART2008 specimen numerical model used in the prediction phase enabling the generation of damage fragility curves for the structure.
The report provides a detailed description of the SMART2008 experiments, an assessment of the test results, the structural response predictions using modeling and numerical techniques, and a damage fragility assessment. The report also discusses potential design implications of addressing the non-linear structural behavior during seismic events exceeding the design values or Design Basis Earthquake (DBE). The design implications are derived primarily from the ability of the numerical approaches adopted in this effort to predict highly non-linear structural behavior under seismic loading.
While the 1/4th scale SMART2008 test structure was designed and constructed following the French nuclear design code, the observations and assessments stemming from the test results should not be considered directly applicable to the seismic behavior of US nuclear structures due to differences in the details in the two national codes. However, this report summarizes the various aspects of the benchmark and the findings that were drawn from the shaking table tests and the numerical analyses. Based on this study, the following specific observations were noted and may warrant further consideration, as consensus codes are updated and the NRC revises regulatory guidance:
For the model used in the SMART2008 experiment, the capacity of the model, built per the French code, was higher that the design capacity, i.e., the model was designed for a 0.2g peak ground acceleration and withstood shaking table induced ground motions of about 1.1 g with only concrete cracking and minor localized steel reinforcement yielding.
The observed reductions in stiffness at lower seismic intensities are attributed to micro-cracking within the concrete.
The use of non-linear treatment of real concrete structures should be considered by regulatory authorities and it may be appropriate to allow it to influence the design through consensus codes.