Q: I have heard that when a structure penetrates the building envelope, it can conduct heat and compromise the energy efficiency of the building. What can be done to tackle this? Are Manufactured Structural Thermal Break Assemblies available for use? How do you design with them?
A: In many buildings, projecting concrete- and steel-framed elements such as balconies or roof canopies often are not able to be enclosed in the thermal and moisture system of a building. Consequently, they “bridge” the building’s envelope and create a path for building heating or cooling to escape. But there now are proprietary systems available that allow structural engineers to maintain an effective load path for the projecting structure while greatly minimizing the energy loss. Such systems are called Manufactured Structural Thermal Break Assemblies (MSTBAs).
As energy codes get tighter and designers focus more on the need to reduce building energy use, such MSTBAs are becoming more appealing. It’s an opportunity to better integrate and elevate the structure’s performance design with architectural and mechanical systems. A more predictive energy model should result, as well.
The ongoing introduction of MSTBAs to the U.S. market affords solutions that can be more cost effective, consistently reliable, and less challenging to project schedules. Plus, it’s much easier for engineers to specify a pre-designed system than to attempt to design and detail one themselves.
Ideally, bridging connections can be part of the initial building systems or sustainability goal discussions. Even when a project does not include consideration of a high-performance building envelope, or when a detailed energy model will not be part of the project, MSTBAs may make sense.
Early and frequent communication of options is important in implementing relatively new technologies. In cases where the option of insulating around the outside or inside of projecting roof, canopy, or slab elements is not tenable, a sleek MSTBA can be an elegant solution.
MSBTAs can be included in the design by performance specification. However, as is common with fairly new systems, it is imperative to enlist the assistance of the manufacturer’s technical staff during design for detailing and specification work. Obviously, the connection reaction information will be critical. The manufacturer can work with cost providers and contractors, and can be instrumental in obtaining required fire and thermal ratings and approvals.
Require the MSTBA submittal to precede the structural components if possible, regardless of preliminary designs or supplier software-aided designs. The more involved the manufacturer is, the less likelihood of problems. By making acquisition and installation of the MSTBA part of the structural steel or concrete contractor’s scope, coordination issues such as adjustments to concrete reinforcing cover or steel bolt clearances can be simplified. Also, an “early” submittal can ensure satisfactory lead time with the supplier’s engineering, manufacturing, and onsite technical assistance schedules. We have not found the lead time to be problematic with advance, team-based planning.
Our experience is that while the manufacturers can prepare and submit engineered calculations, domestic code reports may not be readily available. Clearly, this affects structural engineering practices and jurisdictional reviews. While supportive track-record information is available, it seems that increased demand in the United States, really a developing market for these products, would compel more complete conformance documentation.
Currently, two companies have readily available products for use in the United States: Schoeck (www.schoeck-canada.com) and Halfen (www.halfenusa.com). Archtechnik (www.archtechnik.co.uk) actively is interested in the market.
Russ Miller-Johnson, P.E., is with Engineering Ventures, PC in Burlington, Vt. He can be contacted at email@example.com. Jim D’Aloisio, P.E., SECB, LEED AP BD+C, is with Klepper, Hahn & Hyatt in East Syracuse, N.Y. He can be contacted at firstname.lastname@example.org. Both authors serve on the SEI Sustainability Committee and its Thermal Bridging Work Group, and SEI’s Thermal Steel Bridging Task Committee.