In the Middle Ages, a cathedral took shape after decades of grueling labor by hundreds of craftsmen and laborers using hammers, chisels and wooden scaffolding.

The Cathedral of Our Lady of the Angels in downtown Los Angeles is going up in three years using the most advanced computer modeling and laser technology.

"This is a large, great city and we don't really have a great church," Mahony said last week during a site tour.

But that's changing quickly, as a forest of rebar rises skyward from the floor of the 5.5-acre site. Two cranes tower overhead while workers busily pour concrete in the early-morning hours. The cathedral walls are almost halfway to their ultimate height of 130 feet.

This is not your typical concrete building, either by church or construction standards. For starters, it is designed to last 300 years and withstand a major earthquake. By comparison, the typical office high-rise is designed to last 50 to 100 years.

Secrets beneath the walls

To add to the cathedral's longevity, 200 rubber pads called base isolators, imported from Great Britain, have been installed under the sanctuary and a separate campanile. The structures are encircled by moats covered with steel that can pop up, allowing a horizontal shift of up to two feet in any direction during a quake.

"Not one inch of the cathedral touches the ground," Mahony noted.

The cathedral's unusual shape, angled roof and curved walls also pose an architectural and engineering challenge. There are no right angles in the structure, and each of the 10 chapels lining the nave has a different shape, making the placement of all the building components much trickier than in a standard, symmetrical space. Lasers are used to direct that placement, based on coordinates supplied by a 3D AutoCAD program.

"It's very complicated, but that's also part of the beauty of the design," Mahony said. "You have to build a separate form for every single wall."

The roof will be supported by steel trusses at the top, above a cedar ceiling that will bow out slightly, like the bottom of a boat.

"That's where we've used computer modeling a lot, to make sure the roof trusses fit," said Nicholas Roberts, project manager with Leo A. Daly, executive architect for the cathedral.

Slanted shafts of light will flood the interior through windows of delicately veined Spanish alabaster. More alabaster will be used here than in any other church in the world. Mahony said stained glass was nixed because the windows are situated so high on the walls that visitors would have difficulty seeing the images, and because stained glass would make the sanctuary too dark.

"Natural light will play off the walls and tapestries and floors," he said. "It will be rich in light and color."

Because alabaster is heat-sensitive, it will be used only on the interior windows, with a layer of patterned, laminated glass on the exterior and a space in between that can be lit at night and where air can circulate to hold down the temperature.

Tapestries depicting saints will line the nave, serving not only an aesthetic purpose but an acoustical one. The 3D models revealed several "echo locations" and pointed up the need for tapestries to dampen the sound.

"It's important the cathedral sound like a great building," Roberts said. "You know that sensation when you walk into a cathedral in Europe, and you can hear footsteps 200 feet away and the sacristan putting his key in the lock?"

Technology is also playing a major role in the creation of the tapestries. John Nava, an artist who lives in Ojai, scans his tapestry designs into a computer and then e-mails them to looms in Belgium and North Carolina.

Wind from below

There are even interesting touches underfoot. Numerous holes vents for the air conditioning and heating systems have been punched n the concrete where future pews will be placed. The abundance of vents will allow a gentle air flow rather than a rush of air, Mahony said. Eventually, the floor will be covered with Spanish tile.

Mahony himself designed the altar, which will be made of burgundy-hued Turkish marble. Sculptor Robert Graham designed the great bronze doors for the south entrance, which will include an image of Our Lady of the Angels, another name for St. Vibiana.

The surface effect of the exterior walls also was carefully thought out. The walls will be shingled to create a play of light and shadows. "If we had one sheer wall, it would look very plain and would have no character to it," Mahony explained.

One of the most complex challenges was devising just the right composition of the concrete. It took two years and scores of tests, mock-ups and input from international concrete experts to develop just the right recipe to meet Our Lady of the Angels' particular aesthetic and durability requirements.

Mahony and the design architect, Jos & #233; Rafael Moneo of Madrid, were very exacting about the color of the concrete a warm adobe hue reminiscent of California mission architecture.

The ultimate solution turns out to be a mixture of a white cement from Denmark, sand and gravel from the San Gabriel Valley and a byproduct from coal-burning power plants. The copper roof will also protect most horizontal surfaces from airborne pollutants that could wash into the concrete by rain.

"We were trying to juggle between cost, aesthetics and longevity," said Senior Project Manager Audre Kleven with Morley Construction Co. "This is one of a kind."

And its challenges are monumental seemingly without end.

"Once you have these five things solved, you walk around the corner and there are another five things," she sighed.

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