THE LOS ANGELES EARTHQUAKE : High-Rise Design Gets Good Marks in 6.1 Test

As far as Los Angeles’ skyline was concerned, it was no big deal. The modern skyscrapers trembled a bit during Thursday’s 6.1 earthquake, then went on about the business of being there. The people inside, the engineers suggest, felt more strain than the buildings.

The structural engineers expected nothing less. And when the San Andreas Fault unleashes its monster 8.0-plus quake, as seismologists predict, more glass will shatter, more bookcases will tumble, more ceilings will crumble. But just like this time, the engineers insist, the skyscrapers will not come down.

This is true, they say, among the modern, steel-frame and steel-reinforced high-rises of downtown, Wilshire Boulevard, Century City and elsewhere.

What separates buildings that stand through earthquakes from those that fall is a matter of structural engineering, an applied science built upon a basic lesson of Newton: Force equals mass times acceleration.

Endure the Force

The earthquake provides the acceleration; the building itself is the mass. It is up to the engineers, working with architects and contractors, to make sure the structure can endure the force.

Although the skyscrapers should survive, the cataclysm that seismologists say is sure to come could bring down a large number of Southland buildings. Mostly built in the 1950s and 1960s, these are relatively brittle structures made of pre-poured concrete that lack steel reinforcing bars. Another source of trouble could be so-called “soft story” buildings--structures with weak ground floors.

For structural engineers, “ductile” is the key word in seismic safety. It refers to a combination of flexibility and strength, the ability to bend but not break.

“We want to make our buildings more flexible. Allowing the building to move with the earthquake is really the concept we should be designing our buildings with,” said John Kariotis, who heads a Pasadena structural engineering firm.

“You make them tough, like a coat hanger,” agreed Lee Benuska, a structural engineer and general manager of Kinemetrics, a Pasadena firm that manufactures seismic instruments to measure the movements of buildings. “You can bend it, but you have to bend it back and forth a lot of times before it breaks.”

It is the steel frames of skyscrapers, and steel reinforcing bars--or “re-bars”--placed in masonry buildings that provide the ductility that enables the structures to hold together. Steel forms a three-dimensional network that stretches, bends and absorbs the energy; concrete compresses and breaks.

13-Story Limit Until 1957

Such principles may seem elementary, but only 30 years ago the City of Los Angeles thought it implausible to build earthquake-safe high-rises. Until 1957, because of the fear of temblors, city codes limited structures to 150 feet, or about 13 stories. The only exceptions were City Hall at 27 stories and the Los Angeles County-USC Medical Center at 19.

Today, the city’s tallest building is the 856-foot, 60-story First Interstate Bank Tower. Construction recently began on a tower that will be taller still.

Despite the scenes of devastation from the 1985 Mexico City quake--of buildings that “pancaked” in a heap--engineers stress that their surveys showed that the structures built to modern standards there survived, damaged but still standing.

2% Have Major Damage

Benuska said 2% of the buildings in Mexico City incurred major damage. The quake “showed there were enlightened engineers and enlightened contractors whose buildings did OK because of the knowledge gained in the last three decades,” he said.

Kariotis, who surveyed the Mexico City damage, said design errors, construction errors and material shortcuts collaborated to cause the structure failures. To make matters worse, Mexico City is built upon a dry lake bed, an unstable foundation that served to amplify the to-and-fro movements of the quake.

To build a quake-resistant building (despite their confidence, structural engineers balk at the phrase “quake-proof”), one begins a strong foundation on solid earth. Builders should avoid soft soils that have the potential of “liquefaction”--a rare phenomenon in earthquakes in which the jostling of soil and groundwater turns the ground into a jelly-like substance.

The building’s structural system, through its design and materials, must then be able to absorb the quake’s energy. The energy first enters through the foundation, then gets transmitted up through the building, before it turns back toward to the foundation. Although the top of a building, like the end of a switch, experiences the most movement, the bottom endures the greatest force. Therefore, the bottom must be structurally stronger.

A case in point is the Whittier Hilton, a new, nine-story concrete block structure in the heart of Uptown Whittier, the old business district that sustained the most damage in Thursday’s quake.

Although the hotel incurred breakage to contents--such as lamps and glassware--”not a single concrete block was broken. Not even chipped,” said structural engineer James Amrheim, a structural engineer who inspected the building. After cleanup, the hotel was opened for business and ran almost to capacity Thursday night, catering to the out-of-town media reporting on the quake.

Use of Steel ‘Re-Bars’

Reviewing architectural plans, Amrheim, who is executive director of the Masonry Institute of America, pointed out how during construction, steel “re-bars” were placed vertically and horizontally throughout the building. Considerably more steel was placed in the lower floors than the upper floors, essentially forming a pyramid of strength within the box-like structure.

The fact that the greatest forces accumulate at the bottom is the reason that “soft story” structures cause such worry. “Soft story” buildings typically feature a ground floor with setbacks and glass expanses that may be aesthetically pleasing but seismically unsound.

Olive View Hospital, which was badly damaged in the 1971 Sylmar quake, was such a structure. After the quake, which measured 6.5, it had to be razed, redesigned and rebuilt.

Kariotis, who also works in a seismic-safety program funded by the National Science Foundation, suggests that soft-story buildings and structures built with “non-ductile” concrete frames are the greatest source of worry in the event of a major San Andreas quake. Even more so, he said, than the unreinforced masonry.

This is due to the “harmonic” qualities of vibrations. Like a tuning fork, each structure has an inherent frequency and is thus more stimulated as the vibration approximates that inherent, rhythmic frequency. And each quake triggers its own kind of vibrations.

A San Andreas quake--not unlike the Mexico City quake--is expected to unleash a long, rhythmic to-and-fro motion that will stimulate greater reactions in taller buildings. Thursday’s quake, by contrast, produced short, random vibrations more likely to affect shorter, squat structures.

The masonry structures, with generally squatter build, are more apt to “ride” the longer ground motions. “Hopefully, we’ll have them all strengthened by then,” Kariotis said.

According to the California Seismic Safety Commission, there are about 50,000 unreinforced masonry buildings statewide, including more than 7,500 in Los Angeles County. In Los Angeles, which enacted a stringent seismic strengthening code in 1981, about 1,000 have been brought up to code, while the remaining 6,000 are on varying schedules that require upgrading or demolition within three years.

Moreover, a state law requires local jurisdictions to survey all of their unreinforced masonry buildings by 1991 and develop a plan to mitigate the hazard. However, the plan may range from reinforcement to the posting of warning signs.

Reinforcing existing masonry buildings is a relatively simple process, said Ray Steinberg, a Van Nuys structural engineer who has worked with city officials in its program. The most essential element is using bolts and plates to strengthen the ties between the walls and floors and roof. Other procedure are used to buttress sheer walls. Seismic strengthening costs in the range of $3.50 to $10 per square foot, Steinberg said.

Open warehouse and factory structures are at greatest risk, and thus more expensive, Steinberg said. It is less expensive to reinforce an apartment building because internal walls already help distribute the stresses.