Methods to Fix High-Rise Steel Fail Stress Tests

Costly retrofitting methods already being used to fix steel-frame buildings cracked in the Northridge earthquake have failed strength tests, leaving officials baffled about how to mend broken welds and beams in dozens of Los Angeles high-rises.

Tests conducted at the University of Texas at Austin found that the repair methods recommended by Los Angeles city building officials failed when subjected to simulated 7.0 magnitude earthquakes, researchers said over the weekend.

The connections broke easily and were only one-sixth to one-third as strong and flexible as they should be, they said.

“There are some very fundamental things here that we don’t appear to understand,” said University of Texas professor Michael D. Engelhardt, a leading steel expert who conducted the study for the American Institute of Steel Construction. “We can now report what doesn’t work.

“But we still don’t know what does work,” he said. “Even with improvements, there were cracks early on in the tests. It’s very discouraging.”

The test results were so poor, he said, that five remaining simulations have been halted.

The initial results will be presented to Los Angeles building officials today, so they can discuss other possible retrofitting methods.

Steel-frame mid- and high-rise office buildings, designed to bend with enormous earthquake forces without breaking, were considered among the safest to ride out an earthquake.

None of these modern buildings came close to collapse in the Jan. 17 quake, engineers said. But in a development that rattled experts, widespread cracking has been discovered in the welded connections of the beams and columns of at least 90 buildings, most in the San Fernando Valley and on the Westside. Up to 90% of the connections in some buildings cracked.

Results from the Texas tests--the main effort to find a solution to the problem--are of particular concern because the welding methods and design changes tested are currently being used in the city’s high-rises. Officials are requiring their use in the retrofitting of quake-damaged buildings and in new construction.

“Construction is going on with processes that may not work,” said Los Angeles structural engineer Thomas Sabol, who serves on the city panel studying the problem with steel-frame buildings. “They are wasting their money.”

Such retrofits cost $800 to $2,000 per joint, so repairs could run into the millions of dollars in large buildings with hundreds of cracked joints.

The owners of 30 steel-frame commercial buildings already have taken out permits to conduct repairs, and about half of the repairs have been completed, said Richard Holguin, assistant chief of the city’s building bureau.

The city’s recommendations to steel-frame building owners after the Northridge quake were based on engineering simulations conducted before the temblor. These showed that joints reinforced in certain ways--usually with steel plates--performed better than those built to the current code.

“It appears that the fixes we have been proposing do not seem to work,” Holguin said. “We’re not sure what the next step is. At this point, we don’t have a clue.”

U.S. Borax Inc. is completing repairs on its four-story headquarters in Valencia, where 80% of the 700 building connections cracked or were damaged. The tab: $5 million.

Reconstruction supervisor Andrew Lisowski said the company used several retrofitting techniques in consultation with experts and conducted its own stress tests to find a solution. “I don’t think we will have to redo this,” he said.

Like many other businesses with cracked steel-frame buildings, U.S. Borax has had to operate in temporary quarters. The company has temporarily dispatched staff to work in offices from Wilmington to London.

Previous city building codes were based largely on specifications recommended by the Structural Engineering Assn. of California. Derived from the best available earthquake simulation test data and engineering calculations, these specifications were thought to be rigorous enough so that steel-frame buildings might sustain structural damage in the largest expected earthquake, but never collapse.

Some engineers now believe, however, that those tests may have been based on an underestimation of how flexible the building joints really are.

The Texas tests sought to measure how far steel could be stressed without snapping. Life-sized columns and horizontal beams were welded together in California. Then the mock-ups were shipped to the University of Texas, where a hydraulic arm was used to shake the beam up and down, simulating an earthquake.

The steel frames were strengthened in various ways. Welds were reinforced. Previously bolted parts were welded. And the connections were buttressed by additional steel plates welded to the top and bottom of beams.

Nonetheless, some connections broke after the beam had been shaken up and down just once.

“It just snapped. This was nothing less than shocking,” said Engelhardt, adding that such connections had held up in previous, less sophisticated tests.

Researchers, Engelhardt added, have not even begun examining why in some buildings the steel columns themselves cracked. “Finding answers to all this could take some time,” he said.

Engineers and city officials are still unclear as to what magnitude earthquake it takes for the connections to crack. But some say further study is needed to determine whether a limited number of these buildings could collapse in an earthquake stronger and longer-lasting than the Northridge temblor.

But engineers can point to only one steel-frame building that has collapsed in an earthquake--in Mexico City--and they emphasized that even now steel-frame buildings are considered safer in an earthquake than non-ductile, or brittle, concrete buildings.

Manufacturers of steel-frame homes say they have not had problems with connections cracking. Cracks in the steel frames of government and commercial buildings occurred as far from the epicenter of the Northridge quake as East Los Angeles. City officials estimate that nearly a quarter of the 400 steel-frame buildings in the Valley and Westside were affected. The tallest is an unidentified 23-story tower in the Valley, which is unoccupied.

City officials have been urging the owners of steel-frame buildings to conduct inspections for cracks. They also are pushing a proposal, now before the City Council, that would require inspections of the steel-frame buildings in the Valley and on the Westside.

The California Seismic Safety Commission in April issued a public advisory about steel-frame office buildings, urging that owners who observed even cosmetic damage to their buildings have them inspected by engineers and other experts.

“I really don’t have any instinct on how to solve this. This is a puzzle to us,” said Paul Fratessa, an Oakland structural engineer and member of the seismic commission.

“I still have a lot of confidence in steel,” he said. “I’m not panicked, but I am concerned that we move rapidly to get an answer.”