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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

Section 2

HORIZONTAL GYPSUM SHAFT WALL APPLICATIONS

In recent years, vertical gypsum shaft wall applications have been turned on their sides and are now being used for a whole series of new, but equally critical, applications. When applied horizontally, gypsum shaft walls serve as thin, fire-resistant assemblies that are ideal for use under ducts, on stair soffits or for corridor ceilings that require fire protection.

A key to these applications lies in the C-H-stud, which enables application from one side only. Stud depth and gauge are determined by stud span, which is calculated by using the manufacturer's limiting spans table (not to be confused with a limiting heights table used for vertical shaft wall applications).

On the finished side of the system, over the 1-1/2 inch flange of the C-H stud, one to three layers of fire-rated gypsum board can be attached over the 1-1/2 inch flange of the C-H-stud. The thickness of panels - either 1/2 inch or 5/8 inch - and the number of layers determine the fire endurance rating of the wall.

The System

When a gypsum shaft wall system is installed vertically, the studs do not have to be screw-attached to the runner. However, in the horizontal mode, pan-head screws are required to transfer the loads from the system back to the structure. This should be designed prior to the release of the contract documents. The gypsum face panels are installed with a suitable length bugle-head screw. The correct screw spacing is determined by the corresponding fire test. Appropriate corner bead and trim are used with a joint finishing system to complete the installation. The proper location of any control joints should be predetermined.

System Limitations

Horizontal gypsum shaft walls are intended to carry dead load only. They are not designed to carry any live load. They are not a floor-ceiling system. Manufacturers' limiting spans tables only consider the dead load of the assembly, and the weight of the gypsum panels and the various steel components. They do not include any concentrated loads from fixtures that might, (but should not) be supported by the C-H stud. The shaft wall cannot be used for storage. Further, manufacturers' limiting span information does not factor in the added load of a mechanic installing the system.

System Application

The fire endurance testing procedures for horizontal gypsum shaft walls are specifically detailed per ASTM E119. By definition these assemblies are called either floor/ceiling or roof/ceiling systems. As the definition implies, a floor/ceiling system must have the ability to carry all the live and dead loads it was designed for. Stair soffits, corridor ceilings, and horizontal metal duct enclosures only serve to provide fire separation, and are not intended to carry any live loads.

The Council of American Building Officials (CABO) National Evaluation Service Report Number 258 covers gypsum shaft wall systems in general, and horizontal applications specifically. Per this model code document, if the intended application is specific to stair soffit or corridor ceiling, a single-layer (1-hour) or double-layer (2-hour) system is appropriate. All other installations require a triple-layer, fire-tested system.

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