Structural detailing solutions to avoid thermal bridging

April 2010 » Columns » SUSTAINABLE DESIGN
Jim D’Aloisio, P.E., SECB, LEED AP

In February’s column, we discussed how structural elements typically “bridge” across a building’s thermal enclosure. Because many of the structural components used are made of highly thermally conductive steel, modifying these details can significantly reduce energy usage in the completed facility. By considering the thermal properties of the materials as well as strength and stiffness parameters, we can achieve a more integrated design as illustrated in the following examples.

Column base — Columns often extend through the envelope at their base. An insulated base plate assembly may be effective for controlling large differences in temperature. Analyzing insulated column bases involves balancing load and displacement with thermal parameters. Enhancing details at the exterior columns, where supporting pilasters are monolithic with the exposed exterior wall, outside of the sub-slab insulation, will have dramatic effects. For example, detailing a column isolation block comprised of closed-cell, insulating, flame-retardant, rigid polyurethane foam with a specified density and compressive strength to be placed between the top of concrete and the column’s base plate will result in observable improvements.

Steel lintel — Fiberglass Reinforced Plastic (FRP) has good thermal resistivity and well-known structural properties. The incorporation of a fairly thin section (1/2 inch) of FRP in a detail can dramatically reduce the thermal loss potential of a section. This concept has been used with success in the detailing of steel lintels for exterior masonry walls and other masonry support details. The photo shows that the vertical leg FRP angle is only acting as a shim in bearing, not resisting bending loads, which could result in long-term plastic deformation. Another advantage to this detail is that the continuity of the frequently overlooked building envelope’s air barrier, which is critical for energy performance, is maintained from the window across the lintel assembly.

Ideally the FRP is located within the insulation plane, but it is still fairly effective at reducing the conductance of heat or cooling from the conditioned building assembly to the unconditioned exterior wherever it is placed. It is especially effective when there is a heat-dissipating system, such as a brick façade in contact with the steel support angle, breaking the pull of building energy through the building envelope.

Exterior cantilever — Projecting steel elements such as balconies, canopies, and architecturally exposed cantilever steel assemblies are a particular problem. In addition to high potential energy loss, the cantilever stresses are usually greatest right across the building envelope. For these conditions, the use of alternative detailing (such as creating supports completely outside the envelope) might be best. Alternatively, there are proprietary structural elements by manufacturers such as Schock Isokorb, Farrat, or General Plastics that can be integrated into the structural detail.

Hung steel lintel assembly with FRP angle used as a thermal break

As we modify our traditional details to accommodate building energy concerns, we must pay attention to compliance with building code requirements, durability, and more. We need to share with each other.

Authors will be presenting on this topic at the ASCE/SEI Structures Congress in Orlando in May. Bring your ideas and participate in a discussion on this topic.

Jim D’Aloisio, P.E., SECB, LEED AP, is a principal with Klepper, Hahn & Hyatt of East Syracuse, N.Y., and is on the Advisory Board of the NY Upstate Chapter of the USGBC. He can be reached at jad@khhpc.com. Russ Miller-Johnson, P.E., is with Engineering Ventures, PC, in Burlington, Vt. He is a member of the Vermont Green Building Network. He can be reached at russmj@engineeringventures.com. Both are members of the Structural Engineering Institute’s Sustainability Committee. The committee’s website is www.seinstitute.org/committees/sustainable.cfm


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