SAC Phase 1 Analytical Studies of Building Performance
SAC Survey of Steel Moment-Resisting Frame Buildings Affected by the 1994 Northridge Earthquake
David Bonowitz,S.E.; Nabih Youssef & Associates
Nabih Youssef, S.E.; Nabih Youssef & Associates
This report presents summaries and preliminary interpretations of data from steel
moment-resisting frame buildings shaken by the Northrige earthquake of January, 1994. The
subject buildings were investigated for earthquake damage to various degrees at various times
since the event. Data was collected for this report between August, 1994 and March, 1995.
In general, two patterns of unacceptable conditions are discernible. One pattern is limited to
weld root discontinuities that may represent poor construction quality these show little
relation to structural demand. The other pattern involves full-blown weld or base metal
fractures and does appear related to locations of critical structural demand. A remaining
question is whether the two patterns are linked, that is, whether poor initial quality leads
to fracture during earthquakes. Survey data suggest that poor quality, while it may contribute to
damage, is not by itself sufficient to cause damage, even under relatively high stress.
Post-Northridge laboratory testing has shown that weld discontinuities are also not necessary
Some broad findings regarding general damage patterns and quantities include:
1. About 20% of surveyed buildings reported no damage at all. About 30% of surveyed
buildings reported weld damage only.
2. Over 40% of investigated floor-frames were undamaged, and another 40% had weld
damage only. Among case study buildings, which are somewhat more damaged than average, over
70% of individual connections were undamaged, and another 17% had weld damage only. Of all the
weld damage reported, about half was limited to root cracks detected by ultrasonic testing.
3. Weld cracks (frequently not confirmed by visual inspection) were reported two to
three times as often as base metal fractures. (Cracks in the weld-column fusion zone are
generally considered as weld damage.) Weld cracks were reported at the beam bottom flange
about three times as often as at the beam top flange. Base metal fractures at the top of the
connection were extremely rare. Top inspection was substantially incomplete compared to bottom
4. Overall, about 16% of floor-frames had some base metal fracture at the bottom of the
connection, about 33% had either base metal fracture or significant weld cracks.
5. About 80% of all floor-frames had less than one third fo their connections damaged.