EARTHQUAKE / LIFELINES OF L.A. : THE INSPECTORS : Southland’s Nightmare Is an Engineer’s Dream : Experts came running to see what structures were safe--and what happened to those that weren’t. Lessons from the real-life laboratory will shape future construction.

Although for many a horrific tragedy, the largest earthquake ever recorded in the Los Angeles Basin was also a marvelous opportunity, “a full-scale laboratory,” in the words of a New Zealand structural engineer.

Just as volcanologists consider getting a close-up look at the cone of a boiling mountain the scientific thrill of a lifetime, the world’s foremost structural, seismic and civil engineers swarm to earthquakes for the rare opportunity to see in the rubble the intersection of theory and practice, of a computer’s calculations and the sudden, undeniable force of a heaving fault line.

Other engineers and technicians by the hundreds have a more immediate concern: determining as quickly as possible which structures are safe. But all have to work hurriedly, because some of the best evidence is cleared away quickly in the rush to rebuild.

From their initial battle-scene observations come rapid conclusions about why buildings or freeway bridges buckled or fell, as well as the seeds of change. If the tale told by failed structures is heeded, it can affect future designs and save lives.

“There’s so much terrible damage that we have to turn this into something to work with,” said Susan Tubbesing, executive director of the Earthquake Engineering Research Institute. “Otherwise, it’s just a greater loss.”

Within hours of the Northridge quake, the EERI, an Oakland-based professional organization partially funded by the National Science Foundation, dispatched a reconnaissance team that eventually included about 100 experts in buildings, freeways, geology, utilities and emergency response.

They and hundreds of engineers and scientists from universities, governmental agencies and nations including Mexico, Canada, Japan and New Zealand, videotaped, photographed, measured and analyzed all aspects of the quake. Japan alone sent more than a dozen delegations, some with as many as 50 members.

So many sought to view the highest-profile destruction, such as the interchange of the Antelope Valley and Golden State freeways, that engineers sometimes outnumbered cleanup workers.

“You learn something from every earthquake, because no two spots on the earth’s surface are completely alike,” said J. David Rogers, a geotechnical engineer from Pleasant Hill who was in Los Angeles when the earthquake occurred. Rogers studied the collapse of the Cypress roadway structure in Oakland in the 1989 Loma Prieta quake and prepared a report on it for the state Assembly Transportation Committee.

The aspect of the Northridge quake getting the greatest attention from engineers and seismologists is the strength of the shaking, in some cases at locations far from the epicenter. That movement was captured on hundreds of sensitive but relatively simple recording devices known as accelerographs, installed by the state and federal governments and property owners at more than 500 buildings and open sites around the Los Angeles Basin.

The intensity of those readings, which included a vertical ground movement in Tarzana that was measured at nearly twice the strength of gravity and was among the most violent such forces ever recorded, stunned the experts and will be studied for years.

Any vertical force in excess of gravity is sufficient to simply toss structures into the air, were they not anchored. Because they are, however, structures resist, magnifying the destruction. With each such cycle of movement, walls and columns and roadbeds weaken and, eventually, fail.

The records of that motion could lead to tougher requirements for the construction of freeways, bridges, apartment buildings, dams and utility lines, Rogers said. The Antelope Valley-Golden State freeway interchange that collapsed was designed to withstand a shake of 15% of gravity, or 2 1/2 times as powerful as the jolt recorded in the 1971 Sylmar earthquake in the San Fernando Valley. Today, all new Los Angeles County freeways and bridges are designed to withstand jolts 70% as strong as gravity.

John F. Hall, an associate professor of civil engineering at Caltech and an expert in concrete dams, said an accelerograph at the Pacoima Dam in the San Gabriel Mountains, several miles southeast of that interchange, registered a sideways jolt that was more than twice as strong as gravity and opened up a gap of several inches between the dam and the rock abutment.

The strength of the movement was “higher than we would usually assume when we do design and safety assessments,” said Hall, who headed the EERI reconnaissance team for the quake. “It’s an important event and it will change the way we do things.”

*

Lessons are learned in every earthquake. After the Sylmar quake, the state began retrofitting freeway overpasses with massive cables to hold overpass sections together. Then, after the 1987 Whittier quake, Caltrans began jacketing the most vulnerable single-column freeway structures in steel to make them stronger. No retrofitted columns failed in the Northridge quake. The 1989 Loma Prieta quake focused attention on multicolumned structures, on freeway overpasses in areas of unstable soil and on the weakness of double-deck freeways.

The first Caltrans personnel into the field after any major quake are area maintenance supervisors assigned to check certain stretches of freeway and radio damage reports to the department’s emergency operations center. On their heels are bridge experts and other engineers heading to the most badly damaged areas. Because of the severity of the Northridge quake, 24 additional engineers were dispatched from Sacramento.

Some of the bridges still standing were inspected with an $18,000 bore scope, a much larger version of the devices that doctors use to look inside the human body. Caltrans had one scope, and two more were purchased the day after the quake for inspecting roadbeds and connection joints.

The department also sent from Northern California two “snooper” trucks equipped with buckets that can be raised to get a close look at overpasses.

In addition to dispatching structural engineers, the department sent teams of seismic experts to document the forces that struck bridges and determine how the structures performed, Caltrans spokesman Jim Drago said. Those experts compile “the official engineering record of the event” that will then be reviewed and studied for years to come, he said.

Engineers “like to do fancy math . . . to calculate” what would happen to structures subjected to great stress, said Hall, of the EERI. But, he said, “most of what we learn comes from experience. We build something, an earthquake comes along and shakes it and we correct it the next time we build.”

Roland Sharpe, a consulting structural engineer based in Cupertino, said, however, that it sometimes takes several years before knowledge gained about how structures performed in an earthquake is put into practice.

Sharpe, who headed an effort by the engineering profession to develop seismic design guidelines for highway bridges, said the pressure to quickly reopen downed overpasses often means that new knowledge does not get incorporated into the rebuilt structures.

As an example, he cited the reconstruction of the Antelope Valley-Golden State freeway interchange after it was damaged in the Sylmar quake.

“They certainly improved the design but it certainly wasn’t up to what we knew could have been done two or three years later,” he said.