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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

When it comes to pressure loading, it is important to differentiate between intermittent and constant load tables when reviewing manufacturers’ limiting heights tables. These tables evaluate the various combinations of gypsum panel thicknesses and layers with stud gauge and depth. Because the gypsum panels and steel studs behave compositely, the relative stiffness of the system is greater than the stud properties of the C-H-stud alone.

The intermittent load table is appropriate when designing an elevator shaft. If the shaft is to be used for HVAC and there is no sheet metal ductwork (e.g. the gypsum will serve as the duct liner) the constant load table should be used. This differentiation is important because AISI (the American Iron and Steel Institute) allows engineers to reduce the stress by one-third if the loading is considered intermittent. This reduction is not allowed for mechanical heating and cooling shaft walls where the pressure is constant.

It should also be noted that while codes require specifiers to assume a 5-pound per square foot uniform load for interior partitions, this may not be sufficient for elevator shaft walls, where loading is a function of the speed of the elevator and the number of elevators per shaft. (See Recommended Elevator Shaft Pressure Load table included in the additional online reading materials.)

These criteria are among the factors that define essential gypsum shaft wall performance. However, additional criteria such as life cycle costs and security considerations continue to shape how well the shaft walls function within the building as a whole. (See page 200 for more information about abuse resistance.)

Specification Guidelines

Following are a number of key issues that need to be considered when creating gypsum shaft wall specifications:

1. Design the system to meet anticipated elevator shaft pressures. Pressures will vary depending on the speed of the elevators and the number of elevators per shaft.

2. Maximum partition heights are determined by intermittent air pressure loads and allowable deflections. The applied pressure load is selected based on the elevator cab speed and the number of elevators per shaft. Specifiers should consider three factors to determine maximum partition heights:

• Bending stress – the unit force exerted that will break or distort the stud;
• End reaction shear – determined by the amount of force applied to the stud that will bend or shear the J-runner or cripple the stud;
• Deflection – determined by the amount of bending under a load that a wall can experience without exceeding a prescribed ratio related to partition height.

3. An important, but often neglected, design consideration is the interface of the elevator shaft wall and the B-labeled elevator doorframes. Be sure that the cavity shaft wall system has been tested by UL with the elevator door and frames that will be installed in the building. Not all systems have been evaluated for this critical juncture, and there are some specific details that should be followed.

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