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By: Robert Grupe
Director, Architectural and Technical Solutions, United States Gypsum Company

Phil Shaeffer
Manager, Codes and Technical Support, United States Gypsum Company

Dean Updegrove
Product Marketing Manager, United States Gypsum Company

Cut-away views of various gypsum shaft wall systems: an elevator shaft a mechanical shaft an air return shaft a stairwell

Design Parameters

The first definitive design parameters for shaft wall systems were developed more than 30 years ago by the structural engineering firm of Skilling, Helle, Christiansen and Robertson, working in association with Minoru Yamasaki and Associates and Emory Roth & Sons. Their study was prompted by a reported shaft wall failure in a prominent New York City office building. (A similar shaft wall failure had also been reported in a Cleveland building.)

The study revealed that the walls in the New York office building had failed due to elevator cyclic loading. As elevators move through a building, they create a piston-type action that induces alternating positive and negative loads in the shaft walls.

Based on these finding, Skilling, Helle, Christiansen and Robertson established the following criteria:

1. The shaft wall system should be capable of being built from one side only.

2. The assembly must obtain a two-hour fire endurance rating.

3. The assembly should have a minimum STC (Sound Transmission Classification) of 40.

4. The maximum dead load of the partition should be 15 pounds per square foot.

5. The shaft wall must withstand a 7.5 psf loading while limiting deflection to L/240.

6. The shaft wall must withstand an instantaneous load of 25 psf without structural failure.

7. The assembly must be cycled through its maximum deflection without failure for 1 million cycles.

8. The assembly must be airtight.

Gypsum cavity shaft wall assemblies meet all of these design requirements. They have been designed and tested using accepted engineering practices with deflection criteria of L/120, L/240 and L/360 clear partition heights. Limiting height tables account for flexural and shear forces, and a wide range of product and installation combinations is available to meet performance requirements.

The walls have been tested (per ASTM E119) to achieve fire ratings of one to four hours. The UL design numbers for the most commonly specified fire-rated gypsum shaft wall systems are:

• one-hour system: UL Design Numbers U415 and U469
• two-hour system: UL Design Numbers U 415, U438 and U467
• two-hour system (alternate): UL Design Number U492
• three-hour system: UL Design Number U415
• four-hour system: UL Design Number U415

In terms of sound transmission performance, gypsum assemblies using 21/2-inch studs have been tested to achieve an STC of 39. The rating can be increased to 47 by adding a 1-inch-thick layer of sound attenuation fire blankets (SAFB) within the partition cavity, while an STC of 52 can be achieved by using 4-inch studs with 3 inches of SAFB.

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