Volume 35, Number 7, July 2000
The Perfect Match
storefront will not do as
substitution by any stretch
by Steve Green
Few, if any of us in the glass industry would knowingly violate project specifications. However, increasingly smaller margins of a highly competitive market might tempt us to economize, even cut corners. One area in which we should never allow this to happen lies in the decision of whether to utilize storefront or curtainwall. The consequences in terms of structural integrity, performance and safety are simply too dire and potentially catastrophic. And of course, mis-application can result in possible adverse consequences, such as air or water infiltration, glass breakage or structural failure. But the storefront versus curtainwall decision is not always clear-cut.
We have to admit, sometimes the comparative costs of storefront can be mighty appealing. Given the gravity of the issue plus the enticements, its worth spending a few minutes reminding ourselves of the crucial distinction between these products.
Going into the Grey
to Avoid the Red
Of course, at the most basic level, storefront is designed for single-span, single-story installations; curtainwall, for multi-span, multi-story installations. As Ira Field, vice president/architectural manager with Steel Encounters of Salt Lake City, reminds us, such measures would seem to justify the rule of thumb many of us go by: storefront is for 8 to 9-foot heights and curtainwall for 10 to 12-foot heights. Field also says that such means assume a certain maximum windload (specifically, 25 p.s.f.) and mullion spacing (5-feet). If we exceed either, that famous rule of thumb, excuse the expression, goes right out the window.
Sometimes the use of storefront, even in single-story applications, is prohibited, says Field. You cant state a specific allowable number of feet of height at all, because you always have to factor in windload and spacing of mullions.
Dont Over-extend Yourself or Your Aluminum
In general, curtainwall is designed for greater spans than storefront because of its greater strength and ability to handle deflection, keeping in mind that the maximum allowable deflection (flex) for load-bearing members is 1/175 the span (length of frame).
Curtainwall also provides a large surface area from which to drain water. When water is trapped, it immediately flows to the outside of the structure from weep holes drilled into the horizontal pressure plate. Storefront, as a lighter-weight extrusion, is only appropriate for low-rise applications, which do not have a danger of buckling or collapse. Like curtainwall, storefront also weeps water at each horizontal pressure plate. If it is misused, water infiltration to the interior can occur. This is due to the method of drainage diversion of water from the horizontal pressure plate to the vertical, then to a sill member, where flashing prevents penetration to the interior. Simply put, this method simply is not designed to handle water in higher-rise applications.
Initially, it is less expensive to use storefront rather than curtainwall in marginal situations, but only until problems occur. Eventually they almost inevitably will. The most important issue, as always, is to cover your bases: check the structural requirements of the building and verify that the specifications of the project match the capabilities of the product being used. And when it comes to framing systems, never stretch the truth.
Steve Green is vice president of client relations for Tubelite Inc. in Reed City, Mich. Glaziers Guild appears monthly with rotating authors.
© Copyright 2000 Key Communications Inc. All rights reserved. No reproduction of any type without expressed written permission.