Stony Gorge Dam is concrete slab-and-buttress (Ambursen-type) dam built in 1928. The existing dam, which has a crest length of 868 feet and a structural height of 140 feet, consists of three components: buttresses, slabs, and struts. Special care was taken during construction to ensure that each slab rests simply supported on corbels located on the buttresses. The cast-in-place struts, located on a 24-foot grid between the buttresses, provide lateral support to the structure and are simply-supported, resting in tapered recesses. Following a non-linear explicit finite element analysis of the existing structure which indicated a deficit under seismic loading, analysis of a modification to the structure was undertaken. The proposed modification consisted of diaphragm walls and compression-only struts between existing buttresses to provide lateral support only without transmitting shear loads into the existing structure. The finite element model of the structure and the proposed modification consisted of: 1) 42 existing buttresses with slabs between them; 2) 254 struts providing lateral support between buttresses; 3) 32 diaphragm walls; 4) 39 struts across the top of the dam and 22 struts at mid-height; and 5) hydrodynamic loads applied to the sloping upstream face using a directional application of Westergaard's added mass modified by Zangar's research. Ground motions representing a return interval of 25,000 years were used in the analyses and for the designs. The modification of the structure will conserve the nature of the existing structure while increasing the load capacity.
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