of structure on the roof we hadn’t accounted for that was a serious thermal bridge at a load bearing condition,” Wilkes said. The resulting rooftop structure is a composite construction in which the steel truss frame is overlaid with poured concrete and topped by a membrane and insulation. The building’s cooling towers are supported by steel posts penetrating through the roof’s insulation. It is at these penetration points where the building is most vulnerable to thermal bridging. Exhibit space lies directly below the roof’s cooling towers, so any po- tential for thermal bridging had to be eliminated to protect the collec- tion. STBs provided the solution to the problem. “They [STBs] were the only option,” Wilkes said. SmithGroupJJR specified Isokorb STBs from Schöck to simultaneous- ly insulate and support the cooling tower support columns where they penetrate the building envelope. Wilkes had first heard about STBs at a conference several months prior to the NMAAHC commission. “I kept the product at the back of my mind, and when this unique situation surfaced, the STB seemed like a happy marriage,” he said. Insulating while supporting loads Schöck engineers assessed factored loads from the self-weight of the cooling towers and the wind at roof level to determine the number of STBs required to attain continuous insulation and support throughout the roof structure. The Isokorb Type S22 specified is a load-bearing thermal insulation el- ement for steel structures that accommodates normal and shear forces. A block of BASF Neopor insulation foam, 2 to 3 inches thick forming the thermal break is held under compression with high-strength bolted stainless steel rods between two end plates. Steel STBs are oriented horizontally for canopy and balcony penetrations, but vertically in this rooftop application. Four STBs at each of four steel rooftop penetrations per cooling tower prevent thermal bridging between exterior and interior environments while supporting loads. Photo: courtesy of SmithGroupJJR
Isokörb Type S22 structural thermal break is a load-bearing thermal insulation element for steel structures consisting of insulation foam held under compression with bolted stainless steel rods between two end plates.
Diagram (right) shows vertical orientation of steel structural thermal breaks at the NMAAHC rooftop as opposed to typical horizontal orientation (left).
The structural moments at the rooftop are transferred and the load of the cooling tower is transmitted by the STBs, which are arranged in a quad pattern of four modules at each of four penetrations through the roof, for a total of 16 STBs per cooling tower. By reducing thermal bridging, STBs prevent the building’s roof columns and beams from chilling interior cavities, which could cre- ate condensation and encourage mold growth that would ultimately threaten the museum’s collection. The energy savings also afforded by the thermal breaks reduce heat loss at each point of penetration by up to 50 percent, contributing to sustainable design and NMAAHC’s LEED certification goals. Pushing the insulation envelope In addition to the significant challenges of designing to LEED Gold standards, the unprecedented physical form and performance demands of the NMAAHC required designers to maintain the humidity required to preserve artifacts, while preventing associated condensation and mold. By incorporating STB technology as part of its conservation strategy, the Smithsonian created a signature work of international ac- claim that meets specialized requirements without compromise.
Information provided by Schöck (www.schock-na.com/en-us/home).
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february 2018
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