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.