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Evaluation of the ASCE 43-05 approximate method for estimating the peak response of rocking objects in nuclear facilities

Unanchored components, such as spent-fuel dry storage casks, portable standby generators , portable transformers, tool cabinets, unreinforced masonry radiation shielding walls, etc., can be found at various locations within nuclear facilities. While the ASCE 43-05 standard states that it is generally preferable to anchor components in order to avoid rocking and sliding during earthquake shaking, it permits unanchored objects as long as requirements provided therein are satisfied. Even if these components are not critical to safety, their interaction with safety-critical systems and components may result in functionality disruptions or even catastrophic damage. Design criteria for rocking objects in ASCE 43-05 are based on a best-estimate value for rotation angle. This rocking angle is to be used with a safety factor of 2.0 to ensure it does not exceed the overturning limit state. According to ASCE 43-05, the best estimate of the maximum rocking angle may be computed either through nonlinear time history analysis or through an approximate method that is prescribed in detail in the standard. The approximate method leans on the assumption that a rocking block can be represented by an ‘equivalent’ SDOF oscillator with constant damping, but whose period depends on the amplitude of the rocking. This paper compares the peak response of a rocking block as estimated by ASCE 43-05 approximate method with the peak response obtained by directly integrating the nonlinear equations of motion of a rocking block. The paper concludes that the approximate method in ASCE 43-05 provides poor approximations for the peak rotation angle of a rocking object in a nuclear facility.

Author(s):

Amitabh Dar    
Bruce Power
Canada

Dimitrios Konstantinidis    
McMaster University
Canada

Wael El-Dakhakhni    
McMaster University
Canada