Christ the Redeemer Church's Unique Structural System
April 16, 2009
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In my last article, "From the Architect: Design Considerations for Christ the Redeemer Church," I mentioned briefly the unique structural system employed in the building of the new church for Christ the Redeemer Parish. The design of the roof was to allow for a large volume worship space, uninterrupted by structural elements, within a building framework that historically includes large columns to carry significant roof loads at the points where they concentrate. In this article, I will elaborate on the roof structural frame and illustrate the system with graphics from the design and engineering phases of the project.
Vision to Reality
When all of the work on the Master Plan and the Conceptual Designs for a new church project are complete and hundreds of parishioners have studied the artist’s renderings and committed their emotional and financial resources to the vision, the real work then begins. The work to turn the vision into reality starts with initial meetings between the architects and those consultant engineers responsible for the design of increasingly complex building systems. I distinctly recall the initial meeting with Don Greive and Adam Cryer of Pinnacle Structural Engineers. We presented in detail and to document for construction the initial concept and schematic drawings of the church. We reviewed the artist’s rendering showing the 100-foot tall bell tower and the 40-foot diameter dome rising over the crossing of two pitched roofs above the nave and transepts. Next we discussed the schematic floor plan drawing that illustrated the rooms in the new church, as well as the interior rendering of the 14,400 square-foot seating area for nearly 1,700 people.
With a nervous laugh, the engineers pointed out that we had inadvertently omitted the four columns needed to support the weight of the structural members and roofing materials (part of what is referred to as “dead load”) of the main roof sections and the large dome. Equally nervous, we responded that we had not omitted the columns; we simply did not want structural elements in the nave that would obstruct the view from seating areas and break the volume of the space. After all, the pretty rendering did not have any columns under the dome…it must be possible. Of course, it was not quite that easy, but we did eventually convince the engineers to work with the roof design and within the exterior wall configurations we had designed to carry the significant loads and distribute those loads to columns that were placed at the back of seating areas and outside the lines-of-sight to the sanctuary and tabernacle.
The Challenge Defined
To remain true to the historic examples that served as inspiration for the church design, the roof was laid out in the shape of a cross (cruciform) with a large, round dome at the crossing, or intersection, of the post and beam members of the cross form. These high roof sections are pitched at 22½ degrees, constructed of glue-laminated timber beams supporting 3-inch thick tongue and groove wood deck, and covered with clay tile roofing. To make matters of roof loads even worse, the four roughly square inside corner sections of the cross, each covering a significant area of seating below, add their weight to the structure of the high roof areas as they have perimeter support on only two sides. These low-slope roof areas, commonly and inaccurately called “flat roofs,” also carry the four large roof-top air-conditioning units that cool and heat the church below, each weighing nearly 8,700 lbs.
When the engineers tallied-up all of the dead loads for the dome, the four gabled high-roof sections, the four low-slope roof areas, the mechanical equipment, plaster exterior walls, and secondary load considerations (lighting fixtures, fire sprinkler pipes, sound system equipment, etc.), structural analysis indicated that the 120-foot clear-span structural elements must support loads equivalent to approximately 2,500 pounds per foot. With the material code prescribed safety factors combining to 140% to allow for variations in load, materials, and construction methods, the total “factored” load on the entire clear span structural system was finally calculated at some 600 tons.
The task that appeared so impossible in that structural kick-off meeting slowly evolved into a solution that featured “parallel chord” or “common box” steel trusses nearly 18’ tall in their final design. This composite steel truss design, where the top and bottom members are horizontal and connected by diagonal and vertical intermediate bracing members is a common element in long span structural engineering, such as bridge design. Camber, defined as “a slight convexity, arching, or curvature,” is often pre-designed into these trusses to ensure horizontal positioning of the top and bottom chords after the forces of gravity, in the form of permanent dead loads, have been applied. Long spans and high load carrying requirements for steel trusses are nothing new to engineers. Where top and bottom chords can be continuous between bearing points, all members of the composite truss can be sized appropriately and will efficiently do the work required.
The distinct difficulty presented by the design at Christ the Redeemer came in the form of the 42-foot diameter central dome. More specifically, the arched base of the dome’s supporting walls interrupt the path of the bottom chord of the long-span trusses, carrying the weight of the dome and the upper and lower roofs. This break in the continuity of the bottom chord creates even larger forces in the truss system and requires a different geometry to resolve those forces. In short, the inherent simplicity of the truss form would have to suffer so that the volume and continuity of the worship space would not.
To allow for the spatial requirements we designed and to accomplish the interruption necessary for the arched base walls under the dome, the engineers designed an “inflection” of the bottom chord. This design essentially transfers the work of the bottom members of the truss to the center point of span, above the top chord. This point is connected diagonally in both directions to the break points in the bottom chord and resolves the high tension forces attempting to fold the truss at the center. The 2 inches of upward arching camber designed into each truss was monitored closely during construction, with readings taken several times as the dead loads were applied to the structure. To allow for the high clerestory windows that flood the interior of the church with natural light the design team worked closely together to coordinate the placement of intermediate vertical and diagonal “web” bracing members, further complicating the truss design.
Visitors to Christ the Redeemer Parish are immediately impressed with the size of the newly completed church building. Upon entering the narthex (or entry foyer), the size of the room and height of the ceiling above anticipates the worship space ahead. When one walks through the large portal doors into the church, the volume of the room opens in all directions. The view sweeps in broad arcs to each side and forward, following the radius of curved pew sections and sloping down gently toward the sanctuary platform. The 52-foot high ceiling over the nave follows the center aisle toward the altar, crossing the transept ceilings at the central dome which stands 80 feet above the floor. All areas of the church are visible at once: the whole assembly, the devotional chapels, the choir, and sanctuary. The church at Christ the Redeemer is a historically inspired worship space with sight-lines to rival the most modern performance halls. The building framework that allows for this generous expression of space is a collaboration of architectural and engineering designs, sharing a common creative inspiration.
Photos provided by Stephen Lucchesi
Stephen A. Lucchesi, AIA is an architect with HBL Architects, Houston, Texas, USA.
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