"Fenestration Beyond Standards
Successfully integrating windows, storefronts, and curtain walls
by David W. Altenhofen, CSI, CCS, AIA
Reprinted with permission of The Construction Specifications Institute
99 Canal Center Plaza, Suite 300, Alexandria, VA 22314, from The Construction Specifier

System selection

Selecting window, storefront, and curtain wall systems is a far cry from just reviewing standard products in catalogs and selecting the desired profile. The first step involves determining all the criteria affecting the project.

The most basic of criteria is the wind speed and resulting structural loading. Multiple standards exist and more than one may be necessary on a project. Building codes typically establish loading. International Building Code (IBC) requires compliance with American Society of Civil Engineers (ASCE) 7-98, Minimum Design Loads for Buildings and Other Structures, but allows for compliance with internal requirements, as well as use of Southern Building Code Congress International (SBCCI) SSTD 10, For Standard Hurricane Resistant Construction. FMG Property Loss Prevention Data Sheet 1-28, Design Wind Loads, applies when the project is covered by Factory Mutual (it is generally more stringent than building code).

AAMA also publishes standards for wind loads. Technical Information Report (TIR) A10, Wind Loads on Components and Cladding for Buildings Less than 90 Feet Tall, is based on ASCE 7, Minimum Design Loads for Buildings and Other Structures. For tall, large, or complicated buildings, it may be prudent to use wind tunnel testing to determine design loads as described in AAMA CW-11, Design Wind Loads for Buildings and Boundary Layer Wind Tunnel Testing. Wind tunnel testing will likely reveal some areas of higher and lower wind loads than those indicated by code. When it comes to windows, it is important to remember smaller tributary areas require a higher structural load, while corner zones are higher than in the field. Finally, seismic loads, building movement caused by loading, temperature and wind, and deflection of adjacent building structures and materials all affect the window, storefront, or curtain wall system.

After determining the design wind pressure, the next step is to determine the test pressure for air and water penetration. AAMA recommends 20 percent of the wind load for "most parts of the country under the normally prevailing weather conditions." However, AAMA goes on to warn higher loads may be necessary in areas of sustained high winds and rain. Again, performance criteria need to be adjusted to suit the particular project situation.

Thermal performance criteria must also be established. U-value and glass shading coefficient are typically set by an energy code and by their impact on the HVAC load. Aesthetic issues may influence reflectivity and the color of glass, both of which affect U-value and shading coefficient. A minimum condensation resistance factor (CRF) needs to be established based on local weather, interior temperature, and relative humidity (RH). A proper CRF is crucial for eliminating condensation on the interior surface of window frames and glass.

Other criteria that could be important to the project are acoustic performance, fire ratings, hurricane resistance, security, bullet or blast resistance, and sustainable/green issues. Each has associated standards and guidelines, but it is not enough to instruct builders to just follow the standards. They must be interpreted by the architect and reduced to clear and measurable performance criteria. All of these issues need to have clear criteria established to select window, storefront, and curtain wall systems that will be able to adequately perform.