Slab-Column Frame Building Damaged During Northridge Earthquake (sponsored by NSF)

Damage to RC structures during the recent earthquakes in Northridge, California and Kobe, Japan underscores the need to investigate the design of existing buildings in order to avoid future structural failures during strong seismic shaking. After the Northridge Earthquake of January 17, 1994, Professor Wight was able to obtain design drawings for a four story R/C building that was constructed in 1977. The building was located approximately 9 km southeast of the epicenter of the earthquake. The lateral force resisting system consisted of a continuous perimeter ductile moment resisting frame. The gravity load resisting system for the interior portion of the building was composed of slab-column frames with shear capitals at the connections.

The perimeter frames had no serious damage during the earthquake and appeared to have responded in a ductile manner, as designed. However, punching shear failures occurred at several of the interior slab-column connections. The most significant issue for this building, as was the case for several other concrete structures damaged during the Northridge Earthquake, was the importance of considering the compatibility of deformations between the "lateral load system" and the "gravity load system" in the building. Inelastic static and dynamic analyses of the building have indicated that lateral displacements in the slab-column frame system caused unbalanced moments to be transferred at the connections. These moments increased the shear stresses around the slab-column connection to a point where punching shear failures occurred. The participation of the slab-column system in resisting lateral loads was not anticipated by the designers of this structure.



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