Q: I’m curious about the basis for the requirement of an interior wall uniform load of 5 psf. Can you provide some background information?
A: The first step to understanding where this requirement comes from is to provide some background information that puts the answer in its proper context. From a structural standpoint there are two types of interior partitions. One is considered structural in that it has the capacity and is designed to carry the weight of the occupants in the building plus the weight of the structure. The second wall type is considered as non-structural, or non-load-bearing, for it does not carry any of the structure or occupant load. These structural loads are sometimes termed as axial loads because they are transferred through the stud along its vertical axis.
Both types of walls must be able to carry lateral loads, or the loads that are applied perpendicular to the vertical axis of the wall stud. The most common example of a lateral load is wind on the exterior of a building. These lateral loads are considered to be uniform and, therefore, are measured in pounds per square foot, or psf, of wall surface. On the exterior of a building it is fairly easy to visualize the applied wind load and convert the anticipated wind into pounds per square foot. But what is the source for the design lateral load for an interior wall where, in theory, there is no wind?
This now leads to a brief history on the development of the minimum 5 psf design load requirement for interior partitions, which is the widely accepted standard by industry and codes.
Original studies were carried out in the 1940s by the Building Research Advisory Board. This advisory board was a federally funded organization and part of the U.S. National Research Council. The one principle developed by their study was that a uniform load is imparted on interior partitions by internal pressures created when windows on the exterior wall are open. Using the BRAB study as a basis, the Housing & Home Finance Agency introduced proposed performance standards in 1947. As the name implies, HHFA was a federal agency related to the financing side of homes; it was later replaced by the U.S. Department of Housing and Development. HHFA proposed a wind load of 15 psf with a deflection limit criterion of l/240 (gypsum board) and 1/360 (plaster). In an earlier Wachuwannano, there was a discussion on deflection limits, but for here, the limits are included as a basis of design to minimize the potential for cracking of the finish material. Also included in HHFA specification were requirements for the ultimate load, concentrated and impact loads, but these items were later dropped.
Moving forward to the mid-1960s, the 15 psf wind load had become a Federal Housing Authority requirement as contained in the HUD Minimum Property Standards for both single and multifamily dwellings. The FHA falls under HUD. In the 1977 revision of the MPS, the wind load requirement was reduced to 10 psf while the deflection criteria remained at l/240 (gypsum board) and 1/360 (plaster). During the same time, the U.S. Army Corps of Engineers used the identical requirements.
By the late 1960s, two of the three model codes embraced a lateral load requirement of 10 psf with a deflection criteria of l/120 (flexible) and l/240 (plaster). These two model codes were the Uniform Building Code, which was drafted by the International Conference of Building Officials, and the Basic National Building Code, which was administered by the Building Officials and Code Administrators International. The first reduction to 5 psf was in November 1967 by ICBO, which approved a change in the UBC based on code change proposal from the Gypsum Association. This proposal was based on tests conducted by Wiss, Janney, Elstner & Assoc., and documented in their WJE October 1966 report. (WJE is a well-recognized consulting engineering firm that provides research, testing and design services for the construction industry.)
To summarize, the 5 psf requirement is based on work that was done back in the 1940s by a department of the National Research Council. Their recommendations have been reduced to the current level through testing by a recognized engineering consulting firm. However, the building codes establish only minimum requirements, and designers have the right to increase to the design loads to meet their project requirements. The tables cover conditions where the design load can be 5, 7 1/2, 10 or 15 psf. The design load requirements should be included in the project contract documents. If not found in those documents, it is advised to formally ask what the loads are and not assume they would be the minimum 5psf requirements.
Robert Grupe is AWCI’s director of technical services. Send your questions to [email protected], or call him directly at (703) 538.1611.
