At least a dozen steel-framed office buildings--a type of construction long considered invulnerable to collapse--sustained deep cracks in their supporting columns during the Northridge earthquake, raising new and troubling safety concerns.
The scope and the unprecedented nature of the failures has shaken the confidence of many engineers and prompted building owners to search for hidden damage in other buildings. The discovery also is expected to trigger a long and costly process of analysis and possibly retrofitting.
Steel-frame buildings are designed to bend with the enormous forces of an earthquake without breaking.
But just as earlier temblors exposed the weaknesses of unreinforced masonry buildings and stiffly designed reinforced concrete structures, the engineering hallmark of the Northridge earthquake may be that steel-frame buildings, which include the majority of the nation's commercial and high-rise structures built in recent decades, are vulnerable as well, even in moderate earthquakes.
"I think it is the issue which potentially could have the most engineering significance from the earthquake," said John Hall, an associate professor of civil engineering at Caltech who headed one of several teams studying the impact of the quake.
Worried about alarming people, many structural engineers said it is too early to draw broad conclusions about the relative safety or danger of steel-frame structures. None of the buildings with damage, they emphasize, were anywhere near collapse. They contrast this with reinforced concrete parking structures, such as the 18-month-old facility at Cal State Northridge that crumpled into rubble.
While noting that all of the damaged steel buildings are less than 10 stories tall, some engineers believe that the cloud-piercing towers in Downtown Los Angeles and Century City "are not as immune to damage as we once thought," Hall said.
But others stress that steel-frame skyscrapers are still thought to be among those least likely to fall, even in a massive earthquake.
"We need improvement but the fundamental system is very sound," said Michael Engelhardt, a nationally known expert on steel-frame buildings. "We can still have a tremendous amount of confidence in steel-framed construction."
Still, structural engineers were troubled to find that the damaged buildings, many of which had crucial welds that fractured and columns that cracked or broke, were generally less than five years old. Several failures occurred in the steel frames of buildings under construction.
Damage was found in steel buildings as far as 20 miles from the Northridge quake epicenter and in the types of modern steel-frame designs thought to be safest--those with so-called moment-resistant frames that are lighter and considered most flexible. About half of all buildings completed last year were of steel-frame construction.
At U.S. Borax's four-story headquarters in Valencia, 80% of the connections between the second floor's beams and supporting columns were weakened or pulled apart. Breaks in the frame of the three-story City Hall in Santa Clarita halved its resistance to sideways forces, engineers said. In one West Los Angeles building, which engineers declined to identify, steel supporting columns were reported to have cracked all the way through.
News of steel's problems in the quake has sent engineers rushing to Los Angeles to view the damage and stirred a hot phone and fax debate over what went wrong. The state's Seismic Safety Commission will discuss steel buildings at a Burbank meeting Thursday.
"If we have finally found the limits of steel, then this will be significant to us," said Paul Fratessa, an Oakland-based structural engineer and commission member.
Los Angeles city building officials said they had learned of the problem Monday. Jim Usui, a structural engineer with the city, said some buildings will be reinspected, and officials will work with the steel industry to develop any needed code changes.
Several steel, engineering and welding groups tentatively have scheduled conferences to examine changes in building codes, steel fabrication and welding and inspection practices. "Obviously, there is a problem. This wasn't supposed to happen," said Nestor Iwankiw, research director for the American Institute of Steel Construction, a trade group that plans a mid-March meeting on the issue. "This is the first time this type of failure has been seen."
Engineers in California had dismissed the collapse of a steel frame building in the 1985 Mexico City earthquake, citing generally poor construction methods there.
Now a few Los Angeles-area engineering firms are sending letters to clients urging them to look for fissures in the frames of their steel buildings. Engineers said some owners may be reluctant to look closely for damage--which may require cutting holes in walls and fire-proofing around steel members--because it is costly and could alarm tenants.