Joist Loading Lingo
Description of Floor Loading
Typically, the loading on a floor system is broken down into three classifications. These are Top-live, Top-dead, and Bottom-dead. Thus, when looking at our Uniform Load Span Charts, a loading of, say, 55 psf (pounds per square foot) equates to 40 psf Top-live, 10 psf Top-dead, and 5 psf Bottom-dead. Thus the designation of (40-10-5). There may be other loads, such as rain, wind, snow, and ice, when our products are used in roof applications.
Dead loads are the weights of the building materials used in the structure itself. This might include such things as the joists, subfloor, carpet/padding, flooring tile, acoustic tile, and Sheetrock.
Since a joist can have flooring material on its top and ceiling materials on the bottom, a Top- or Bottom-dead load can be specified. In our example, we have 10 psf for the subfloor and floor covering, and 5 psf for Sheetrock below. This is typical of most floor systems. However, should you want a heavy tile floor (15 psf) and two layers of Sheetrock below (10 psf), a loading of 65 (40-15-10) would be more in order. Commercial applications and custom homes should go this route.
Live Loads are those loads that can come and go over the life of the structure. This would typically include people and furniture, but may include such things as wind, snow, and ice when our products are used in roof applications. Most building codes require a floor system to be able to withstand 40 psf live loading. Typically, a floor system will never see loading equivalent to the full live load rating unless it’s flooded 8 inches deep in water! Also, since people and pets usually don’t walk on the ceiling, there is no Bottom-live designation.
Seismic Loads are those loads resulting from ground motion during an earthquake. Seismic loading is site-specific, and is determined by the building designer.
Other loads, such as material stacking, ponding, moving, impact, etc., may also be applicable in certain cases as determined by the building designer.
Stiffness ratios are also important. These are the L numbers in our charts. It’s not only important to know that a joist will span the distance per code, we need to know how stiff (or firm) the resulting floor system will be at that span. All building codes require that the Live Load stiffness factor be at least L/360. These numbers are linear ratios for comparison purposes. The bigger the number, the better. For example, an L/720 floor system is twice as stiff as an L/360 system for a given span.
Camber is the slight curvature of a beam or joist needed to compensate for the dead loads as described above. Many builders will refer to this as a crown. Most other engineered joists are manufactured with no camber (i.e., flat). This means they will have a positive (downward) deflection under dead-load-only conditions. A fact easily missed when reading other engineers’ span charts since they only show live load deflections. They tell you in the small print to expect additional deflection due to the long-term dead loading. All TrimJoist® joist products have a camber (about 1/4 inch in 30 feet) to compensate for dead loads. This camber is removed as dead loads are applied and the joist settles during the construction phase of the job. Also, the TrimJoist® is unique in the floor truss industry as the camber manufactured into the truss is depth-dependent