Preventing design derailments
by Helen Sanders and Chad Ricker
Railroad tracks can buckle in the heat of summer when joints are not effectively designed to manage the thermal expansion of the metal (see image). This phenomenon can also occur in aluminum windows and doors if they are not designed effectively, causing a condition known as “thermal bow."
When metals become hot, they expand. The extent of expansion depends on the metal and is determined by its coefficient of thermal expansion (CTE). The higher the CTE, the more the metal will expand for a given change in temperature. The higher the temperature of the metal, the more it will expand.
The impacts of thermal expansion are most noticeable in larger metal-framed operable windows and sliding doors. In these large units, if designs do not mitigate expansion, bow in the framing members may prevent the fenestration from opening. This is especially true of systems with 45-degree mitered corners. In this case, the framing members can’t expand longitudinally because its ends are fixed, and so it manages the expansion by bowing outwards.
As the thermal performance of fenestration ratchets up, thermal expansion is an even greater factor in design. Exterior extrusions can become very hot due to solar absorption. Since thermal barriers are very effective at insulating outer framing members from interior members, they reduce the flow of absorbed heat to the interior. Because of the temperature differences developed across the thermal barrier, the outer member will expand significantly more than the internal member, creating a situation akin to a “bimetal strip.” Thermal barrier systems, unless designed appropriately, do not facilitate longitudinal expansion of the exterior aluminum, thus the exterior member will preferentially bow out laterally.
The key to designing around thermal expansion is to understand the drivers in the fenestration system. Here are several strategies that can be used alone or in combination.
- Stiffen the extrusions: If stiffer and larger, they are less likely to bow.
- Allow for expansion: Use butt joints rather than mitered corners.
- Reduce thermal absorption: Use lighter colors on the frame to reduce maximum operating temperature.
- Design special thermal barriers: Special polyamide thermal barriers (often referred to as “shear-less” or “anti-bimetal” strips) can be designed to facilitate longitudinal movement for the exterior extrusion, thus preventing bowing laterally.
Because it is difficult to verify efficacy of these strategies through modeling, it is important to physically test systems. The European test method (EN1121) and specification (EN 12219) are useful references.
While it is preferable to design for thermal expansion, if a problem with an already commercialized design is encountered, there are processing changes that can be evaluated. To reduce the risk of bowing, a lighter crimp around the polyamide strip or eliminating knurling of the aluminum could be implemented to provide some additional movement to allow for longitudinal expansion. The downside is that structural performance is reduced, but if there is already sufficient design margin, it may be an appropriate trade-off.
The message is clear: Thermal expansion must be a key design consideration for high-performance aluminum fenestration.
(image credit: U.S. Dept. of Transportation Public Domain)