Choosing appropriate UL designs      

Architect designing on drafting table
Photo courtesy Bigstock

by Eric Montplaisir, PE
Many UL designs have load restrictions—a matter of great importance and potential liability for engineers of record (EORs), who, in accordance with several building codes and the UL Fire-Resistance Rating Directory, are responsible for identifying and approving the use of such designs on a project. Understanding which designs are restricted, and calculating the load restrictions, can be a complex process.

Background
The International Building Code (IBC) requires the fire-resistance rating of building elements, components, or assemblies be determined by the test procedures set forth in ASTM E 119, Standard Test Methods for Fire Tests of Building Construction and Materials or UL 263, Standard for Fire Tests of Building Construction and Materials. Under these standards, the members being tested must be loaded as near as practicable to the maximum allowed under nationally recognized structural design criteria.

The American Institute of Steel Construction (AISC) provides two methods for calculating the load: Limit States Design method (LSD) and Working Stress Design method (WSD). Until 2006, these methods yielded different results under identical conditions; however, AISC changed the calculation process for WSD (i.e. allowable stress design method) in the 2005 American Institute of Steel Construction (AISC) Specification for Structural Steel Buildings. This change resulted in the two methods generally ending in the same design loads. This change was first adopted in the 2006 IBC.

However, several UL designs used the pre-2006 WSD Method—where structural members were tested with lower loads than those called for by the LSD or post-2006 WSD methods. Thus, according to UL, assemblies tested using the latter methods would have a shorter fire-endurance period. To address this discrepancy, load restrictions are imposed on UL designs evaluated under the pre-2006 WSD method, as stated in the disclosure at the front of the design:

This design was evaluated using a load design method other than the Limit States Design Method (e.g., Working Stress Design Method). For jurisdictions employing the Limit States Design Method, such as Canada, a load restriction factor shall be used – See Guide BXUV

The above disclosure states “for jurisdiction employing the Limit States Design Method,” when in fact, a more appropriate statement is “for projects employing the Limit States Design Method.” The design method is, in fact, the choice of the project team. While the US does not mandate the use of the LSD Method as Canada does—it is widely used throughout the US design community. Even if the post-2006 WSD Method is used, the result is the same.

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