Enduring Entity

The 1994 Northridge Earthquake was the precipitating event that led to the completion last year of a new cathedral for the Archdiocese of Los Angeles. St. Vibiana's Cathedral, a 127-year-old unreinforced masonry structure, was damaged by the earthquake and declared unsafe by the city. When new construction on its site proved not feasible, the archdiocese purchased a 5.5-acre site at the edge of downtown.

This series of events strengthened Cardinal Roger Mahony's resolve that St. Vibiana's replacement should be designed and constructed for a service life of at least 500 years. That was the daunting assignment presented to the Building Team for the Cathedral of Our Lady of the Angels as it executed the design of Madrid-based architect Jose Rafael Moneo.

The effectiveness of the Building Team's work was apparent to the Building Team Awards judges, who voted the project a Grand Award.

Taming tremors

Our Lady of the Angels is the world's only cathedral that incorporates a base isolation system to mitigate the impact of earthquakes. The building is supported on 149 high-damping rubber isolators, and 47 slider bearings that accommodate rotation as well as lateral motion. In addition to mitigating major structural damage, base isolation will minimize earthquake damage to glazing systems and other architectural features.

The controlling seismic criteria for the cathedral are based on the nearby Elysian Park Fault, which has the potential to deliver a 7.1 magnitude earthquake. Base isolation is designed to accommodate a lateral movement of as much as 27 inches in the 28-in.-wide gap that inconspicuously surrounds the cathedral's perimeter. This dry moat, an essential element of a base isolation system, has a 6-ft.-wide concrete cover with a chamfered edge that would allow the cover to slide onto the adjoining plaza area in the event of a major earthquake.

The lateral acceleration expected from Elysian Park activity is estimated at 1.2 G. As required by code, the cathedral's walls are designed to withstand a lateral force that is one-fourth that magnitude, or 0.3 G. If the cathedral had been constructed as a fixed-base building, seismic forces transmitted to it would be at least three times greater.

The Cardinal's durability directive also had major implications from the outset of the project. "From a technical point of view, it resulted in doing things very differently from the way we normally do projects," says Nicholas Roberts, project manager in the Los Angeles office of the project's executive architect, Leo A. Daly. In particular, it led to extensive investigation of concrete mixes and of the use of alabaster, a compact form of gypsum, as a glazing material.

Research on concrete mix designs, including the use of fly ash to make a denser mix, was performed jointly with structural engineer Nabih Youssef & Associates, Los Angeles, and general contractor Morley Construction Co., Santa Monica, Calif. Mix designs were tested initially in a laboratory and then in full-sized mockups. "We probably tried two dozen different designs, somewhat in a trial-and-error fashion, covering durability, workability, and aesthetic issues," says Mark Benjamin, CEO of Morley. "That took a lot of collaboration."

Finding the optimum mix

"The real advancement in the state of the art provided by this project was the design of architecturally exposed concrete for extreme qualities of shape and color across large exterior and interior surfaces, and durability and longevity of performance when subjected to seismic activity," says structural engineer Nabih Youssef. Innovative concrete technology, coupled with the use of a base isolation system beneath the building, was the means by which these objectives were applied to the cathedral's 450,000-sq.-ft. of architectural concrete, with some walls as thick as 5 ft.

Specific measures also were taken to prevent rusting of steel reinforcing within the concrete, including the placement of this rebar at least 3 inches from the exterior surface of the concrete. Additionally, stainless steel rebar was placed adjacent to horizontal surfaces where water might stand for extended periods. Stainless steel tie wire was used for all rebar applications.

The cathedral's base isolation system will also mitigate concrete cracking, Youssef says. "Even a hairline crack attracts humidity," he adds. "When this occurs over an extended period, corrosion results, leading to in staining, volume expansion and spalling."

Youssef says the base isolation system and "rational code interpretation" permitted a substantial reduction in the amount of rebar that normally would have been required. (He explains that the code requirement covering the ends of a shear wall envisions a multiple-story structure, and not a 120-ft.-tall structure like the cathedral). This reduction, amounting to 20% for the shear wall and pier system, made concrete placement substantially easier.

The water content of the concrete mix was minimized to reduce the potential for cracking, and measures were taken to control the heat generated by the mixing of water and cement. Concrete placement was targeted to be complete by 9 a.m., with the maximum concrete temperature at 75 F, or 65 F for the thickest walls, which are more subject to heat buildup as the concrete cures.

The Cathedral of Our Lady of the Angels is a complex, non-orthogonal structure. "Its geometry posed tremendous challenges for all of us," says Roberts. Youssef nevertheless points to the architectural/engineering integration implicit in building features such as buttresses that also have a shear wall function. "Except for very minor locations, I can't recall where we asked [the architects] for something a little more" in order to satisfy structural requirements, he adds.

This integration of structure and architecture prompted judge Larry Griffis, president, Structures Division, Walter P. Moore & Associates, Houston, to note, "The beauty is hiding the structure in architecture."

Youssef finds an interesting contrast between the exoskeleton-framed cathedral and the nearby Frank Gehry-designed Disney Concert Hall, another large-volume, high-design structure, which will open this fall. The concert hall is designed with a more typical differentiation between skin and structural elements. Youssef believes the two buildings also reflect differences in the relationship between their architects and structural engineers.

Cooling the alabaster

The large-scale use of alabaster as a glazing material has been limited. But this semitransparent variety of calcite is known to deteriorate under prolonged exposure to heat. Accelerated life-cycle testing determined that the material should not be subjected to a temperature greater than 120 F, which would have been easily surpassed if placed in direct sunlight.

The solution that was developed consists of an inner lite of alabaster panels and an outer lite of laminated glass with a 50% ceramic frit. After air is exhausted from the nave of the cathedral, it circulates in this cavity, flowing past both sides of the 5/8-in.-thick alabaster panels. "We had to make sure there would be enough cooling to take the heat away," says Andy Howard, project principal with Arup, the team's mechanical engineer.

The glazing cavity varies in width from 6 ft. at its base to 3 ft. at the top. It is sized to permit the insertion of a boatswain's chair to enable cleaning of both the exterior glass and the alabaster panels. The temperature in the glazing cavity is monitored and has never exceeded 105 F, according to Howard.

The cathedral has one of world's largest installations of alabaster. It utilizes a total of 27,000 sq. ft. of reddish, brown, and gray panels, which were obtained from three Spanish quarries. Major applications include the 100-ft.-long, 60-ft.-high windows on the building's north and south elevations.

Low operating costs as well as occupant comfort were key requirements of the HVAC system. A displacement ventilation system — unusual for a structure with a volume as large as the cathedral's 3.3 million cu. ft. main space — was used. It delivers air to the nave at a low velocity of 40 ft./ minute through floor-mounted diffusers spaced approximately 10 ft. apart and located below the pews. The air is exhausted at the ceiling.

"You just want to control the temperature of a zone up to about 10 ft.," Howard says. "If you're conditioning anything above that, you're pumping energy and money into the atmosphere."

A traditional overhead distribution system requires a higher velocity fan system to drive the air down to the occupied level, Howard says. The displacement ventilation system allows air to exit the floor diffusers at about 65 F, or some 10 degrees higher than would be required with a conventional system. And an overhead system would require cooling to dissipate heat from sources above the occupied level, such as lights. "We just exhaust that out," Howard says.

None of the HVAC system's elements are exposed. Ductwork is suspended from the ceiling of the cathedral's basement, which primarily houses a 45,000-sq.-ft mausoleum. Since M/E system components obviously will not last 500 years, the building design incorporates enough space to facilitate their removal and replacement. Because the cathedral uses steam and chilled water produced by a city-owned central plant, it required a smaller mechanical room.

When the outside temperature is the same or lower than the supply air temperature required, as is often the case in Los Angeles, the air can be delivered without the need to cool it. Sensors regulate the cathedral's supply air intake to insure that carbon dioxide concentrations remain below 800 parts per million. This is now a California requirement for public assembly buildings, although it was not in effect when the cathedral was designed.

The HVAC system passed its first major test on the cathedral's opening day, last Sept. 2, when the temperature hit 90 F, the humidity 80%, and 3,000 people packed into the building. The interior temperature rose by only 2 degrees.

In addition to minimizing earthquake-related damage to its physical structure, the archdiocese wanted the cathedral to continue to be operational after a major earthquake. "The idea of the building as a refuge was very important," Roberts says. "Symbolically, the church is a place of spiritual refuge. The idea was that it would be a physical place of refuge as well."

Acoustical matters were among the most vexing issues faced by the Building Team. The biggest challenge was combining the reverberation time necessary for organ and choral music with a short enough reverberation time for speech intelligibility. "The Cardinal told us that the Word [the Scriptural text] had to be absolutely intelligible," Roberts says. "With the building untreated, reverb time was about 9 seconds, which would have made speech completely unintelligible. After performing 3-D computer modeling, acoustical consultant Shen Milson Wilke Paoletti of San Francisco recommended a series of absorptive panels to reduce reverb time to about 3 seconds. The result was spectacularly successful, according to Roberts. "It sounds great for organ music and choral singing, but speech is also completely clear."

Acoustical considerations, as well as sometimes conflicting issues such as aesthetics and fire safety, also weighed heavily in the selection of wood for the ceiling of the nave. "From the time we started thinking about the ceiling until we had something everybody agreed to probably took a year," Benjamin says.

Moneo began the cathedral design in August 1996, and Leo A. Daly came on board in January 1997. Morley was hired when the project consisted of six sketch sheets.

On-site precision

Precision in the field was critical to the successful execution of the project. "When we were pouring concrete, it was vital that everybody know what was going into that wall in terms of outlets, conduit, inserts, and plumbing," Benjamin says. Morley hired Los Angeles-based architect SPF:a to convert two-dimensional drawings of every concrete wall into 3-D drawings, showing every concrete pour. The 3-D drawings served as shop drawings for building wall forms, as well as field verification for the contractor and the architect that imbeds and other inclusions were properly placed. "Everybody had to sign off on a sheet stapled to every wall form indicating that the wall was ready to pour," Benjamin says. Roberts characterized the 3-D drawings as "very technical, but also very collaborative."

Morley built a cathedral model that was large enough for tradesmen to stand inside. This enabled them to obtain a sense of Moneo's design concept. The model was used to get subcontractor supervisors "as familiar and up-to-date as possible on what the drawings were trying to describe," Benjamin says.

A scale model kept in Morley's field office enabled surveyors to visualize the cathedral plan, which incorporated 850 individual corner conditions on hundreds of document sheets, and its complex angled wall intersections. "The layout man would look at the model once in a while to get a dose of reality," Benjamin says. "The plans just showed cuts every 8 or 10 ft., and he had to extrapolate from that."

"Everybody gave everything they had, because they knew it was such an extraordinary opportunity," says Roberts.

Construction Costs