This paper presents the results of a study on the effect of prescribed delamination on the natural frequencies of laminated beam specimens. Experimental modal analysis was used to measure the effect of delamination length on the first four frequencies of the simply supported test specimens. The experimental results were corroborated with detailed finite element models as well as with simplified beam theory models. The presence of a delamination degraded the even numbered vibration modes much more rapidly than the odd numbered modes for a delamination centered at the specimen midspan. Delaminations which covered 1/3 of the specimen length had no more than a 20% effect on any of the first four frequencies.

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A study was conducted to quantify the ground motion ampliÿcation that takes place at the base of bushings mounted on electric substation transformers as a result of the exibility of the transformer tank and turrets to which they are connected. This study was undertaken to assess the adequacy of the ampliÿcation factor of 2.0 speciÿed by the Institute of Electrical and Electronic Engineers in Standard IEEE 693-1997 for the seismic qualiÿcation of transformer bushings. The study included ÿeld tests of typical 500-kV transformers to obtain experimentally their natural frequencies and damping ratios, the development of simple analytical models that closely matched the experimental data, and the calculation of the transformers' dynamic response under di erent earthquake excitations using these analytical models. Ampliÿcation factors were then computed for each of the excitations considered, deÿning these ampliÿcation factors as the peak shear force at the base of the bushings over the corresponding shear force obtained when the bushings are assumed mounted directly on the ground. It is found that the ampliÿcation factors for the 500-kV bushings are within the ampliÿcation factor of 2.0 speciÿed by Standard IEEE 693, but not those for the 230-kV bushings, which exceed the IEEE 693 ampliÿcation factor by almost a factor of two.

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