Post-Earthquake Quick Damage Evaluation Based on Crack on the Concrete Foundation of Exposed Column Base

Abstract eng:
From lessons learned in the 2011 Tohoku earthquake, it becomes clear that the quick procurement of an emergent public shelter is very important in time of disaster. Structural engineers are required to estimate the seismic damage of structural components in buildings, and to judge whether these are able to be occupied or not. However, the visible damage can hardly be related with the damage to steel structural components, while crack width has been used as a clear damage index for reinforced concrete structures. It is very important to establish a visible damage index for quick damage estimation of steel structures. In the reconnaissance of the 2011 Tohoku earthquake, many cracks on the concrete foundation around exposed steel column base were observed. From the detailed investigations, it is pointed out that the crack width is strongly related with the seismic damage to the framing components covered by non-structural components. In the present paper, cyclic loading tests focusing on crack patterns and its width of concrete foundation were conducted to confirm the relationship between structural performances and the visible damage indices. The test specimen is a single span full scaled frame subassembly composing of a beam, a column, a brace, and an exposed column base with concrete foundation. All cracks on the concrete foundation are classified into the 2 major categories, that is, the cracks between concrete foundation and cover mortar, and those diagonally growing from anchor bolts in the compressive side to foundation beam. The cracks in the first category are a result of the column-base rotation together with the cover mortar. Therefore, on the assumption that the rotation center is located on the column flange in compression, the relationship between the crack width and column-base rotation is obtained. Meanwhile, because the residual crack width is roughly 90% of the maximum crack width after anchor bolts yielding, the maximum column base rotation can be estimated by measuring the residual crack width. The maximum story drift angle can in turn be estimated, since the column-base rotation constitutes most of the story drift ratio after the anchor bolts yield. The cracks in the second category can be attributed to the concrete edge failure due to shear forces on the column base. It was observed that the crack width is about half of the lateral deformation at the column base. Therefore, lateral deformation at the column base due to the concrete edge failure can be estimated by the width of the diagonal crack from anchor bolts in compression to foundation beam.

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