Bosnian Mosque Design 2 of 3
Triangulating a Dome
The first step for Ray, Jim and Rick Parrino, Plum’s general manager, was to bring the design and production departments together to discuss the whole project and come up with a reasonable bid. “Since we had such a good relationship with the builder, we knew it was worth the time and effort to get a good handle on the amount of man hours it would take to design and build the truss arches,” says Jim. “It was an unusual application; we couldn’t just throw a square-footage number at it.” In the end, the bid they came up with was warmly accepted by the ICCB board
The dome itself is 20 feet tall and 40 feet in diameter. The dome frame were still constructed out of steel beams, with a tension ring at the base and a compression ring at the top. The trusses were affixed to the steel rings. “The loading conditions didn’t present a challenge, but I spent a majority of the time trying to optimize the chord sections,” says Ken. “I had two goals. One was, to the greatest extent possible, make as many of the chord and web pieces the same.” In talking with production, he knew they would have to juggle a lot of individual pieces, which created a lot of opportunity for error. Ken added, “it wouldn’t be hard to put a similar looking piece in the wrong spot and not notice until they were trying to affix the sheathing and something didn’t line up.”
“My second goal was to minimize the number of pieces so the software would stop crashing,” says Ken. In his final design, the dome was built using 32, 20-foot tall arch trusses. Each truss had 16 top chord and 16 bottom chord sections, along with 33-34 webs, for a total of 65-66 pieces each! That one-web range is thanks to what Jim described as the one curve ball they were thrown during the design process. “While it wasn’t in the original design, they added a pitch to the tension ring in order to slope the roof it sat on for bulk water management,” says Jim. Over the 40-foot diameter of the dome, that pitch resulted in a little over a one foot drop from back to front.
Since the dome couldn’t pitch forward, the arches needed to be altered slightly to accommodate the change in the base ring elevation. “So, instead of having 32 identical arch trusses, I ended up designing 16 pairs of trusses,” says Ken. “Over 90 percent of each truss stayed the same and I made all the changes at the bottom leg of each truss.”
We can clean these up and then highlight how the base of both of these are different.