Impact of slenderness on the seismic response of rocking frames in ontario nuclear power plants

Canadian Nuclear Power Plants (NPPs) are located on the eastern side of the North American continent, with the majority of them in Ontario. The Design Basis Earthquake (DBE), based on West Coast records, is prescribed in the Canadian nuclear standards. Seismic Probabilistic Risk Assessment studies of the existing plants consider time histories obtained from the latest research on the East Coast earthquakes as seismic input to the analysis.Although Canadian standards are silent about rocking response of unanchored objects, various industry guidelines and the standard ASCE 43-05 prescribe methodologies in this regard. Applications of a rocking frame in a NPP may vary from squat piers supporting a heavy rigid object to a slender masonry frame consisting of two concrete block walls and a rigid diaphragm on top. The methods of analysis prevalent in the nuclear industry recommend obtaining the response of an individual pier of a rocking frame, rather than an equivalent pier representing the rocking frame. Methods of obtaining an equivalent pier, whose response is the same as that ofa rocking frame, have been detailed in the literature where it has been emphasized that rocking frames are more stable than an individual rocking pier. However, it is noticed that the response of rocking frames is influenced by their slenderness and also by the boundary condition at the contact between the piers and the top mass. The support boundary conditions are bounded by two extremes: the full top width of a pier, or a point support at its top center. This paper compares the equivalent block parameters of rocking frames for these two extreme boundary conditions. Also, it presents the seismic response of a slender rocking frame subjected toearthquake records compatible with the DBE spectra of Ontario NPPs, as well as spectra used in risk analysis.