Table Tests: Gravity Load Collapse of Reinforced Concrete Frames
Experimental research and postearthquake reconnaissance have shown that reinforced
concrete buildings constructed before the introduction of modern seismic designs
and details are vulnerable to severe damage and collapse during earthquakes.
These older building frames typically have stiffer and stronger beams than columns,
resulting in a concentration of damage in the columns when the frame is subjected
to lateral seismic loads. Furthermore, the transverse column ties were typically
provided at a wide spacing with 90° hooks, making the columns vulnerable
to brittle shear failures. Such column damage can lead to a reduction in the
axial load capacity, forcing some or all of the gravity loads originally carried
by the damaged column to be transferred to neighboring elements. A rapid loss
of axial capacity would result in an impulse-type loading and the associated
dynamic amplification of load on the neighboring elements. This study investigates
how the load redistribution after shear and axial failure of a column can influence
the progressive collapse of a structure.
Fig. 1. Three-column test specimen on earthquake simulator
In March 2001, shake table tests were conducted on half-scale reinforced concrete frame specimens with three columns (fig. 1). The tests were conducted at the PEER Center, UC Berkeley Earthquake Simulation Lab by Kenneth Elwood, Ph. D. graduate student, and Jack Moehle, PEER Director. The center column, with ties at large spacing and 90° hooks, modeled a typical older reinforced concrete column from the first story of a seven-story building. The outside columns were circular in cross section with continuous tightly spaced spiral transverse reinforcement to ensure that those columns would not experience any loss of axial capacity. Two frames were tested, differing only by the axial load applied to the center column. Both frames were subjected to a scaled ground motion from the 1985 Ms 7.8 Chile earthquake. The results from these tests will provide data on the dynamic shear strength and the hysteretic behavior of columns failing in shear; the loss of axial load capacity after shear failure; the redistribution of loads in a frame after shear and axial failure of a single column; and the influence of axial load on each of the above mentioned variables.
Figures 2a and 2b show the damage to the center column after each of the tests. Figure 3 shows the hysteretic behavior of the shear-critical center column for the second test. A website (http://peer.berkeley.edu/~elwood/research), expected by June 2001, will provide more detailed information about the tests as well as several videos.
Fig. 3. Hysteresis for specimen #2 center column