Volume 2   Issue 2               Summer 2001

a l u m i n u m   a d v a n t a g e s

One Size Does Not Fit All
by Rand Baldwin

“Of course this will fit you, it adjusts and stretches automatically to the circumference of your upper body.” I listened to this as the salesman was attempting to sell his line of T-shirts to a rather husky customer. In a manner of speaking he was correct. The shirt did stretch to fit the gentleman. However, in my eyes and by the chuckles of the others in earshot, something in the product was going to be sacrificed. Looks, wearability, comfort and, most importantly, product life definitely would suffer if this customer wore this T-shirt for any length of time.

Throughout history product manufacturers have attempted to push the limits of their products by putting them into applications for which they simply are not designed. Window applications are no different. 

Manufacturers design and construct their products for specific applications and per specific industry guidelines. When these boundaries are stretched, as in the above example, something suffers. The same is true in material selection for window application. 

Window manufacturers have several choices of materials to choose from when producing their products. All have their limitations. Concern arises when a material is applied to an application beyond its performance capabilities. In the application of windows, consideration is given to areas of colors, price, availability, etc.—all features that require careful thought. Some design specifications, such as wind loads, stress, structural integrity, effects of heat and cold on the expansion and contraction of the materials and the resultant consequences are overlooked. When these considerations are neglected, problems can occur.

Consider Deflection Limits

To ensure long-term performance, prime consideration should be given to deflection limits on window framing systems. There are very few framing materials that can withstand the performance requirements in buildings of more than three stories or large openings in building envelopes. The American Architectural Manu-facturers Association (AAMA) has classified windows into five categories: residential (R), light commercial (LC), commercial (C), heavy commercial (HC) and architectural (AW) (see table 1).

Don’t let the classification categories on the labels mislead you. Only the AW and HC categories have deflection limit testing and design load testing. The designation of commercial in this instance does not signify long-term structural performance. The requirements in this window class are not stringent enough to ensure that the fenestration product withstand the expansion and contraction and the resultant air infiltration that is essential in a multi-story structure. Studies done for the Canada Centre for Mineral and Energy Technology call into question claims made by some materials as they apply to window performance. The study, Long Term Performance of Operating Windows Subjected to Motion Cycling, concluded that after motion cycling tests, most window framing materials allowed as much as 136 percent more air and water leakage than aluminum.

Aluminum has long been utilized when structural performance is required. There is a reason that jumbo jets are manufactured with this material. The strength-to-weight ratio has no competition from other materials. The strength of this material allows it to pass the stringent test requirements of AAMA for the heavy commercial and architectural categories with ease. Add recyclability, multitudes of finishes, thermal performance and sustainability and aluminum have no equal.

Don’t simply look at categories and labels. Look closely to the performance that is required to achieve a certain category. You will find often, and especially in this case, that aluminum is the one and only material that can be recommended adequately for use in a “commercial” building.

Rand Baldwin serves as president of the Aluminum Extruders Council.

© Copyright 2001 Key Communications Inc. All rights reserved. No reproduction of any type without expressed written permission.