Steel-Frame Building Cracks Spur Wider Damage Fears

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.

Experts said they believe far more steel-frame buildings suffered damage than the 12 or so identified. They said reinspections are turning up additional buildings with potential problems.

“All of these buildings appeared on first inspection to not be terribly damaged . . . which raises the specter that there could be a significant number of other buildings with similar problems,” said Ron Hamburger, a San Francisco-based structural engineer hired to examine one of the damaged steel-frame buildings.

Los Angeles structural engineer Nabih Youssef said: “The list of buildings affected is growing.”

Although damage was not evident from the exterior of the U.S. Borax headquarters, ultrasonic tests and X-rays showed that more than half of the beam-to-column connections in the building needed repairs, said spokesman Will Browne. The work will cost $5 million and take until July to complete, relegating 200 workers to temporary quarters, Browne said.

The most common failure among steel buildings was at the welded connections binding the horizontal beams to vertical steel columns. The metal-alloy welds are supposed to be stronger than the girders they connect and thus are less likely to break.

As the welds hold, the beams and columns bend or twist, absorbing the earthquake’s energy.

In the affected buildings, however, some connections became brittle and broke. In some cases, only the welds cracked; in others, the weld fractures continued into the steel, deeply gouging columns or splitting them.

Thomas Sabol, president of Englekirk & Sabol structural engineers of Los Angeles, said the Northridge quake was the first recent earthquake to be centered in a U.S. urban area and therefore truly test steel-frame buildings. He also said minor cracks caused by previous quakes might have gone unnoticed because they were thought so unlikely that no one looked for them.

Some engineers said one cause for the unexpected failure of steel-frame buildings was the sheer violence of the Northridge quake, which produced unusually high vertical ground motion. Other engineers said state and local building codes focus on containing the damaging effects of side-to-side movement rather than up-and-down forces.

“We have to tighten up the codes,” said Egor Popov, professor emeritus of civil engineering at UC Berkeley.

Popov said California buildings ought to be more like Japanese buildings, which are designed to resist greater earthquake forces. That could boost the cost of construction by requiring thicker columns and beams.

He also said weld inspection techniques ought to be more exacting to reduce shoddy craftsmanship, and building designers should install more state-of-the-art shock-absorbing systems at the base of buildings.

Most experts interviewed by The Times said that steel quality did not appear to be a problem. They tended to blame the frame failures on poorly designed and executed welding.

Engelhardt, who tests steel-frame buildings at the University of Texas, Austin, had warned in a controversial paper published in December that current weld designs leave little margin for error and make them susceptible to failure in quakes.

Engelhardt, who has made two trips to visit four damaged buildings, said his testing showed that the performance of steel frames is “highly sensitive to the quality of the welding, which is difficult to control in the field.”

Duane K. Miller, an engineer with Lincoln Electric Co. in Cleveland, the nation’s largest supplier of welding material and equipment, said the welds themselves were less of a factor in the building failures than overall design. He said the welding and steel industries are searching for solutions. “We want to preclude this from happening in the future,” he said.

Researchers are readying applications to the National Science Foundation for grants to study the shortcomings of steel-frame buildings. But conclusions could be years in coming.

Meanwhile, the owners of damaged buildings are trying to get them repaired and reopened as quickly as possible. In the process, some structural engineers said, they are destroying valuable evidence by grinding or melting away welds.

MCA Inc., the Universal City entertainment giant, is repairing damage to a three-story steel-frame building under construction atop a parking structure. The building is scheduled to be completed by midsummer.

“It is not economically feasible to make any building a research project,” said Manuel Morden, senior associate of Brandow & Johnston & Associates, engineer for the MCA building. “We cannot wait for the research to come in.”

Santa Clarita has embarked on a $4.5-million repair job at City Hall, and officials hope to reopen the building in April.

Also under way are repairs to damaged columns and welds in a six-story office building under construction and scheduled for completion next year at the Getty Center in Brentwood. All connections similar to those that were damaged will be retrofitted in five other unfinished buildings at the center, said Getty Trust spokeswoman Ruth Goldway.

“We will be confident that the buildings will be safer than ever,” she said.

At least one building, however, probably will be torn down rather than repaired. The two-story, 160,000-square-foot Automobile Club of Southern California building in Santa Clarita now leans several inches to one side and toward the back because most of the first-floor columns were severed.

“I’ve never seen a failure of this type . . . and it’s going to be quite a task to work out just what went wrong with this one,” said David J. Gowers, a Rancho Cucamonga civil engineer hired to examine the structure.

Most structural engineers never thought they would be pondering such problems.

“Look at the old Superman movies,” said Scott Melnick, editor of Modern Steel Construction magazine, an arm of the American Institute of Steel Construction. “Steel isn’t supposed to crack.”

Cracked Frames

Buildings with cracked steel frames are being discovered throughout much of Los Angeles County, some far from the epicenter of the Northridge quake. Here are some of the dozen buildings where engineers have observed problems:

* AUTOMOBILE CLUB OF SOUTHERN CALIFORNIA

Design: Two-story steel-frame building

Where: Valencia Boulevard, Santa Clarita

Year built: 1992

Damage: General column failure. Building may be torn down.

Status: Damage still being analyzed by engineers. Unoccupied.

* THE GETTY CENTER

Design: Six-story building, one of six being built to house a museum addition and the administrative offices for the Getty Trust.

Where: Getty Center Drive, west of the 405 freeway in Brentwood

Year built: Under construction.

Damage: Six of 80 joints in the steel-framed building were damaged, one cracked in two. One steel column cracked.

Status: Damaged welds and columns are being retrofitted. Building scheduled to be completed next year.

* MCA INC.

Design: Three-story building under construction atop a parking structure.

Where: 100 Universal City Plaza, Universal City

Year built: Under construction.

Damage: Many of the joints cracks in the steel frame.

Status: Scheduled to be completed midsummer.

* OVIATT LIBRARY

Design: Four-story braced steel-frame building

Where: Cal State Northridge campus

Year built: 1973, expanded in 1991

Damage: Joints between supporting columns and foundation base plates cracked.

Status: Building is unoccupied. Reopening date undetermined.

* SANTA CLARITA CITY HALL

Design: Three-story office building

Where: 23920 Valencia Blvd., Santa Clarita

Year built: 1987

Damage: $4.5 million. Failed welds connecting supporting beams to columns, mostly on the second floor. Sheared connecting bolts. Twisted girders.

Status: Building is unoccupied. Due to reopen in April.

* U.S. BORAX, INC.

Design: Four-story corporate headquarters

Where: 26877 Tourney Rd., Valencia

Year built: 1993

Damage: More than half the 300 joints in the building’s steel frame failed, including 80% of those supporting the second-story. Second floor displaced more than an inch. $5 million estimated damage.

Status: Building is unoccupied. Due to reopen in July.

Sources: Structural engineers familiar with the buildings

Quake Cracks Steel Buildings

In past earthquakes, unreinforced brick and stiffly designed concrete buildings were considered most vulnerable to collapse. Buildings made of steel were deemed safer, because they tend to bend but not break. The Northridge earthquake shattered those assumptions. Engineers have identified a dozen or more steel buildings as high as 10 stories with badly cracked welds and supporting steel columns. Although they did not collapse, they were seriously weakened.

Two types of cracks

Fracture near interface of weld and column flange

Fracture through column

Moment resisting frame

A steel moment resisting frame is a rectangular assemblage of beams and columns. The beams are welded and bolted to the columns.

Source: Michael D. Engelhardt, assistant professor of civil engineering at University of Texas, Austin