Seismic assessment of concrete-filled steel frame substations

A study was conducted to understand the likely seismic performance of one-to-two story electricity distribution substations and to develop methodologies for their assessment. These structures were generally constructed early this century with frames consisting of riveted built up steel members and perimeter walls composed of cast-in-place reinforced concrete. They may be situated close to known seismic faults so their performance under severe ground motions is a concern. A 3-D elastic analysis of the full structure was performed and demand-to-capacity ratios were estimated using procedures outlined in FEMA 273. It was found that wall-foundation slip, shear slip at the wall-roof interface, out-of-plane wall flexure, and steel truss deformation were possible modes of non-linearity. Displacement-based-design and evaluation methods in which the structure is modeled as an equivalent single degree-of-freedom system were shown to be inappropriate for this type of structure since out-of-plane breathing of the walls provided significant contributions to the response. The multiplication of the lateral force demands by amplification factors to take into account short period effects and effects of hysteresis loop degradation was shown to have a large effect on the computed response, and the resulting predictions may be overly large in some cases. Finally, recommendations are made regarding improved models for frame assessment.