Faith in Engineering

The Cathedral of Our Lady of the Angels, now going up downtown, is expected to be a gleaming new jewel of the Catholic Church. But scientists and structural engineers also have great hopes for the building.

The cathedral is emerging as one of California’s leading examples of elaborate, built-in seismic protection. The idea is to keep the structure functional in case a major earthquake strikes during its projected life of 500 years.

Cardinal Roger M. Mahony’s plan is that the cathedral would remain usable as a shelter and sanctuary, even if an earthquake devastated much of central Los Angeles. His vision is that refugees from the quake would sleep, eat and pray in the nave or plaza of the building in the days after a temblor.

The construction is similar in some ways to that of Los Angeles City Hall, just down the street, which is undergoing a major seismic retrofit.

Both projects will utilize base isolation systems and reinforced concrete shear walls.

The idea of base isolation is that the vital superstructures are cushioned against the shaking of the ground by the configuration of bearings called isolators, which act as shock absorbers. In the event of a quake they move, but not as much as the ground does.

A moat surrounds each of the two main buildings, and the movement would occur within that space. A quake that generated displacement beyond the expected maximum range of 28 inches for the cathedral building would send the structure completely across the moat to collide with walls on the other side, and the system would fail. However, that would require a quake larger than any the Los Angeles Basin’s relatively short faults are believed to be capable of generating.

The lead seismic designer at both City Hall and the cathedral is Nabih Youssef, a Los Angeles structural engineer. His mission at the cathedral is to bring the design of Spanish architect Jose Rafael Moneo into accord with safeguards against California’s earthquake danger.

That danger may be particularly acute downtown, because scientists believe the Elysian Park fault is capable of a quake in the magnitude 7 range, far stronger than the 1994 Northridge temblor.

Although the intervals between such large earthquakes on the Elysian Park fault might be 1,000 to 2,000 years, and one probably will not occur in the next 500 years, odds are that less powerful, magnitude 6 quakes will occur within the life of the cathedral. This would necessitate major repairs unless the strongest seismic safeguards are built in now.

A huge quake on the San Andreas fault near Palmdale, or on such dangerous Los Angeles Basin faults as the Sierra Madre or the Newport-Inglewood, would probably not endanger lives at the cathedral, but would still cause cracking of the concrete surface, necessitating expensive repairs, Youssef warns.

Youssef, in a recent essay, said that “seismic isolation was determined to be the most effective design strategy for achieving the desired seismic performance” for two key structures in the cathedral complex: the two-story, 150-foot-tall main church building and the adjacent campanile, a 160-foot-tall bell tower.

Three other structures on the site--a residence for the cardinal and distinguished visitors, a conference center and an underground parking facility--will not contain base isolation devices.

Construction of the cathedral began May 17, 1999, and the goal is to finish in the spring of 2002. Construction costs are expected to run $110 million and the cost of the entire project, including furnishings and art, is expected to hit $163 million. The building is being constructed to exceed seismic code requirements.

In the cathedral building, base isolators between the foundation and the superstructure include approximately 150 rubber bearings and 50 flat sliding bearings. The bearings, according to Youssef, have been designed to be stable under shear strains in excess of 250% of normal, and the amount of shaking at upper levels will be reduced to one-fourth of what it would be without the isolators.

The isolator bearings are designed to be durable over long periods, so that even if an earthquake were to occur centuries from now, they would be instantly functional.

Under the campanile, different isolation devices are being used: four friction/sliding pendulum bearings accommodating a maximum displacement of 29 inches.

Without the isolators, engineers believe, the reinforcement of the walls would have had to be doubled, costing millions more.

Even so, as Youssef says in his essay, “The architectural features of the building’s design--exposed concrete, tall slender walls and an abundance of alabaster glass--placed exceptional demands on the structural design and construction.”

He notes that “the floor plan is irregular in shape, with many insets and nonparallel walls. . . . Although the building is essentially two stories tall, it has multiple roof levels.”

All of these features necessitated extremely precise building techniques--such as carefully controlled pourings of concrete--and other quality controls to make sure the shear walls were properly installed.

“Special measures were adopted and implemented in the design to address these concerns,” Youssef said. “A special concrete mix design was developed which minimizes cracking and preserves the aesthetics of the building without sacrificing durability.”

Durable materials and configurations were employed, including above-code top steel for beams and slabs to retard long-term creep. Floor slabs were also thickened above code minimums to mitigate long-term deflections and increase durability.

“Stainless steel reinforcement is being used for exterior exposed flat concrete surfaces,” Youssef observes. “Stainless steel tie wire is also being used for all reinforcement in the building to reduce propagation of rust from the surface of concrete.”

C. Terry Dooley, senior vice president of Morley Construction, a general contracting firm responsible for erecting the cathedral, said: “The cardinal does not want this building red- or yellow-tagged.”

*

Reich can be reached at ken.reich@latimes.com