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earthquake. Project leaders wanted the new structure to have a design life of 300 years."To an engineer, locating a 110 foot high cathedral made of delicate materials so close to an active fault line and expecting it to survive an earthquake like the 1906 temblor—that is the ultimate challenge," said Mark Sarkisian, S.E., Director of Structural Engineering at Skidmore, Owings & Merrill (SOM), San Francisco. Yet, this is precisely what the new cathedral achieves. Set for a September 2008 opening, the 21,660 square foot, 1500 seat, $80 million cathedral is an exemplary structure that utilizes traditional building materials in modern ways.
Structural system
The Cathedral’s strength is achieved through the creation of glued-laminated timber beams (glulam) and steel rod space frames. The frame is constructed with 26, 10-3/4 inch wide by 99 foot -9 inch long glulam ribs that vary in depth from 30 inches at the base to 19-1/2 inches at the top. Between each rib are 32, 5-1/8-inch-wide glulam louvers varying in depth from 22-1/2 inches to 39 inches. The louvers are installed at 7 different angles to optimize the light effects. The roof of the cathedral is composed of a tension-free glass oculus supported by a steel compression ring which resists the horizontal thrust of the glulam ribs. Parallel to each rib is a glulam mullion 10-3/4 inches wide by 15 inches deep and 103 feet long. The mullions are installed 80 degrees from horizontal and are connected to the wooden vaults by turned glulam struts with tapered ends of lengths varying from 2 feet to 15 feet.
The space frame’s diagonal members are made with pre-tensioned high strength steel rods installed such that in an earthquake they will always be in tension. The building is subdivided into five levels where fixed connections tie the louvers to the ribs completing the structural frame.
Given the cathedral’s proximity to fault zones (4.7 km from Hayward and 25km from San Andreas) and its nonconformance to a standard California Building Code lateral system, the City of Oakland hired a peer review committee, composed of 3 university professors and one industry expert, to establish the required toughness and ductility requirements.
Load testing
First, the committee determined that the glulam timbers must remain elastic under cyclic load conditions. Second, all of the ductility of the system was required to come from the pre-tensioned steel rods. This required ductility testing of the tension members to demonstrate that they could achieve 2.1 percent elongation over the entire length of the rod, not just at the threaded ends. The rod manufacturer, Halfen Anchoring Systems, tested all five rod diameters. The initial testing pointed out that the two largest rod diameters did not meet this requirement as the elongation was limited to the threaded portion of the rod. Halfen therefore re-tooled their machinery to upsize the threads on these rods and achieved the required elongation. The resulting stress strain curves of the testing were input into the SAP2000 computer model to define non-linear behavior of the structure.
Third, the rods were required to be pre-tensioned to between 3 percent and 10 percent of their yield stress so that they were never loose and would be in tension immediately when loaded with seismic forces. The glulam supplier/erector, Western Wood Structures, located in Tualatin, Ore., (WWSI) developed an ingenious tightening sequence to eliminate force interference between rods. Without this sequence, tightening one rod would affect the forces already applied to all the other rods, tightening some and loosening others. WWSI also developed the required tightening torques, which were calibrated to the desired pretension and ambient rod temperatures. Fourth, the criteria specified a non-linear push-over analysis. This analysis required a progressive failure model which recalculated the stiffness based on the surviving structural elements to determine structure viability along the way. The final requirement was a Time History Seismic Analysis essentially scaling the Loma Prieta earthquake to a 1000 year event.
Specification of glulam beams
Given the architectural significance of the glulam timbers, their appearance is crucial to the aesthetics of the structure. SOM and WWSI collaborated to develop a customized appearance specification that provided a more appropriate finish than the standard premium appearance grade. WWSI worked with the glulam manufacturers to hand select the lumber used in the laminations to minimize knots and voids on the faces of the members. The few remaining voids were left unfilled. The louvers were originally intended to be covered with an acoustical material. In the end, it was decided to leave them exposed to view saving the project nearly a million dollars.
Conclusion
In the process of designing the cathedral, engineers at SOM were able to achieve appropriate structural strength and toughness for this building using a structural system not recognized by the building codes. This was accomplished by carefully defining the ductility requirements of the structure, modeling its non-linear behavior, testing the components which were relied on for ductility and field verifying the installation of these components.
The design team at SOM worked closely with the glulam supplier to achieve appropriate finishes of the various glulam members. The use of a full scale mock-up was instrumental in allowing the architects and engineers to see how the structure would appear when completed.
The design and erection of the Cathedral of Christ the Light demonstrated that modern glulam construction could be used to build a significant building intended to be architecturally worthy and structurally capable of lasting 300 years.
Paul C. Gilham, P.E., S.E., has been designing engineered timber structures at Western Wood Structures, Inc. of Tualatin, Ore., for 26 years. He has extensive experience in the design of timber bridges and building structures. He also is involved in the inspection and rehabilitation of existing timber structures. He can be reached at paulg@westernwoodstructures.com. Karyn Beebe, P.E., is an engineered wood specialist with APA. She has written, lectured and consulted on residential and commercial wood-frame construction for the past 8years. She can be reached at Karyn.beebe@apawood.org.
