As the toll of earthquake damage to steel-frame buildings mounts, the state has issued an unprecedented advisory urging owners of buildings that suffered cosmetic damage to conduct thorough inspections to check for cracks in supporting columns.
Steel-frame office buildings have long been considered invulnerable to collapse, but the California Seismic Safety Commission issued its advisory this week because engineers have uncovered widespread evidence that steel beams and welds cracked in the Jan. 17 Northridge quake.
Los Angeles city officials said that cracking has been found in the steel frames of at least 50 buildings, constituting at least 10% of the low- to mid-rise structures built in the area since 1970.
Most of the failures occurred in mid-rise buildings of less than 10 stories, but an 18-story twin tower in Warner Center and an unidentified 23-story high-rise elsewhere in the San Fernando Valley also had structural problems.
The failures already have prompted city officials to require more rigorous welding procedures in new steel-frame buildings.
On Thursday, city officials said they plan to announce within two weeks that owners of all steel-frame buildings near the quake's epicenter will be required to conduct inspections for hidden structural damage.
Richard Holguin, assistant chief of the building bureau of the city's building and safety department, said officials are also considering whether to require citywide inspections of such buildings, which made up half of all buildings constructed last year in Los Angeles.
Meanwhile, the mounting damage tally has sparked concerns about the safety of the cloud-piercing towers in Downtown, Century City and elsewhere, even though engineers emphasize that no steel-frame buildings came close to collapsing during the temblor.
"The world is watching," said L. Tom Tobin, executive director of the California Seismic Commission. "There is no question that this has been a wake-up call for the engineering and construction community world wide."
Steel-frame buildings, designed to bend with the enormous forces of an earthquake without breaking, have been considered among the safest to ride out an earthquake. But the commission noted: "Damage was reported to a significant number of steel structures shaken by the Northridge earthquake, raising concerns about the extent and implications of this damage."
The advisory issued Wednesday said damage to some types of steel buildings has never before been observed. "This is a new concern that can be serious, but should not be exaggerated," the commission said.
Noting that "damage can weaken a building's ability to resist future earthquakes," the commission urged building owners who observed damage to contact architects or engineers to investigate.
City officials fear that because steel columns are covered by walls and thick fireproofing material, damage is not readily apparent to building owners or even to engineers, and that the number with cracked frames may escalate further. By late February, only a dozen buildings with steel-frame damage had been reported.
So far, about 90% of the steel-frame buildings where cracks have been reported to the city are in the Valley; most of the others are on the Westside. All are believed to have been built in the last decade.
Failures among these relatively new structures may have been the result of a different welding method used in recent years, said Los Angeles structural engineer Nabih Youssef, who is the head of a city blue-ribbon panel on seismic hazards.
Ten percent of the joints in the Warner Center's 18-story twin towers cracked; other buildings had breaks in up to 90% of the welds that hold vertical columns and horizontal beams, Holguin said.
"Building owners should take this seriously," he added. "There is a concern that buildings out there that don't appear to have damage do."
Steel industry officials emphasized that steel buildings are safe. "These structures found a way to survive despite these problems," said Nestor Iwankiw, director of research at the American Institute of Steel Construction.
Ray Remy, president of the Los Angeles Area Chamber of Commerce, said: "I think any property owner, particularly one hit hard by the earthquake, is well advised to follow the advice of the committee."
Remy said that having buildings inspected now improves chances that at least some of the costs will be defrayed by government programs.
Engineers have suggested that low-quality steel, badly designed beam-to-column connections or poorly executed welds could have led to the cracks. About a dozen of the 50 buildings required temporary or partial evacuations for repairs; the remainder are being fixed on weekends, Youssef said.
Meanwhile, at the annual meeting of the Seismological Society of America in Pasadena, one expert estimated that some steel-frame buildings would collapse if a magnitude 7 earthquake struck directly underneath Downtown Los Angeles, which is a remote possibility.