MSTBAs help save building energy in colder climates

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    The degrading effects that thermal bridging can bring about in colder regions, where uncomfortably cold rooms are avoided and condensation gets mopped, are easily understood.

    The team-wide acceptance of incorporating Manufactured Structural Thermal Break Assemblies (MSTBAs, see the Sustainable Design column in the April issue of Structural Engineering & Design) in both case studies presented here involved straightforward discussions focused on their merits and on how to get them built before snow arrived.

    At a hospital addition in upstate New York, a cantilevered projection from a flat plate roof system was used to emulate the adjacent, existing entry canopy. The thin canopy structure was designed to extend out beyond the building envelope and would have efficiently radiated building heat. A concrete-to-concrete MSTBA was proposed to “break” the contiguous concrete framing, aligned with the insulated wall below. The design team reviewed cost and schedule implications of implementing MSTBAs on the project and found that the products could be incorporated within the project budget, and that they could be delivered to the site within a reasonable timeframe. The contract documents showed details, loading and performance requirements, a product specification, and manufacturers’ contact information.

    The MSTBA work was on the agenda for the specified onsite pre-construction concrete work conference. Discussions focused on the importance of coordinating the reinforcing with the pre-fabricated configuration of the assembly, and subsequent adjustments were made to the reinforcing. The concrete contractor received reassurance from the manufacturer’s detailed installation instructions. The submittal process went without a hitch, though it’s probably worth considering that only imperial units be shown for most projects (see Figure 1). The assembly placement was seamless and quickly concealed by the first snow — a positive sign that a thermal break was achieved!

    Figure 1: At a hospital addition in New York, a concrete-to-concrete MSTBA successfully “broke” the contiguous concrete framing, aligned with the insulated wall below, improving building performance.

    On another project, a so-called “snow roof” at a food-processing office and industrial facility in central Vermont was required to alleviate drift loading over an existing roof that would result from the new design of a higher adjacent addition. The projected framing was supported from the new edge columns with a steel system of beams and diagonal tie rods that penetrated the envelope 36 times. The owner had indicated an interest in cost-effective features that would save building energy consumption. For these conditions, MSTBAs were a good match.

    A fast-track project delivery required issuing the main superstructure prior to the snow roof in order to speed the building’s enclosure. The steel-to-steel MSTBAs were indicated in plans and sections, with a load schedule to define varying conditions. The CM and assembly manufacturer made contact, and the assemblies were assigned to the steel fabricator’s scope. However, when the snow roof framing submittal arrived later in the project, standard mild steel end plate connections were detailed. It appears that the time lag and unfamiliarity with the specialized product were factors in the contractor’s shying away. The detailer’s expedited attempt to design the assemblies by using the manufacturer’s software for procurement was not acceptable due to the risk of dimensional assumptions not matching the fabricated steel.

    The turn-around time for the MSTBA detailing submittal processing was eclipsed by the construction schedule, driven by the oncoming winter that would limit crane access. In an effort to minimize heat loss through the connections, stainless steel end plates within the building envelope were substituted for the more common and much more highly conductive carbon steel elements.

    In addition to saving building energy in cold-weather climates, MSTBAs can help reduce the amount of conductive heat gain in the summertime caused by projecting structural elements. As energy codes become stricter, the use of these proven systems can help ensure code compliance, as well as save clients energy costs during the service lives of their buildings.

    Russ Miller-Johnson, P.E., is with Engineering Ventures, PC, in Burlington, Vt. He can be contacted at russmj@engineeringventures.com.