The Weight Bearing Capacity of Floors
By Will Gish
, last updated March 13, 2012
The weight bearing capacity of floors, more commonly referred to as load bearing capacity, refers to how much weight a floor can support before it bends or breaks. No uniform measurement exists for floor load bearing capacity because this calculation depends on far too many factors to generalize. The most important factors in weight bearing capacity are materials, support and distribution. Experts measure weight bearing capacity in pounds per square foot, abbreviated PSF.
The load bearing capacity of floors depends heavily on the material used. When it comes to wood, for instance, each type of wood possesses different tensile strength measurements. Tensile strength refers to how much weight a material can support before it breaks. This holds true for engineered wood types such as plywood or laminate wood. Concrete floors exhibit a very different weight bearing capacity than wood floors, and also vary greatly amongst themselves. For instance, according to a sheet produced by textbook publisher Cengage, a reinforced concrete floor can support 12.5 PSF per inch of concrete thickness. A 4-inch thick floor, therefore, can support 50 PSF, while a 10-inch thick floor can support 125 PSF.
The support mechanisms for a floor dramatically impact how much weight the floor can support. For instance, most wooden floors possess joists as a support mechanisms. Joists are wood beams that run in rows below the entire surface of a floor. Builders install joists between the primary floor framing beams, perpendicular to the frame. How much load bearing capacity joists add to a floor depends on the type of material used in the joist, the thickness of the joist, joist construction methods and how builders secure the joists. A floor with inadequate support may possess a lower weight capacity rating than the raw materials used on the floor -- without support, the floor can collapse under its own weight.
Like support, distribution plays a large role in a floor's load bearing capacity. When it comes to structural engineering, distribution refers to the pathways along which surfaces distribute weight to relieve stress. For instance, a second story floor passes weight onto trusses or joists located between the first story ceiling and second story floor. The trusses or joists in turn pass the weight to the wall, which distributes the weight downward to the foundation and ground. The more effective and structurally sound a weight support system, the greater the load bearing capacity of the floor.
Determining the actual load bearing capacity of a floor requires the help of a structural engineer. This engineer must consult charts on the load bearing capacity of various materials and do a variety of mathematical equations based on weight distribution and support networks. Alternatively, you can contact the contractor who built your home for information. Contractors generally design homes to meet local building code standards. For instance, if your city requires a residential floor load support of at least 40 PSF, the contractor designs floors to meet these requirements. Find the minimum load bearing capacity of your floors by checking local building codes. With prefabricated concrete floors, you can contact the manufacturer directly for PSF specifications.
Understanding PSF Ratings
PSF ratings for floor load bearing capacity refer to capacity across an entire floor. For instance, assume a floor possesses a weight bearing capacity of 50 PSF. In order to collapse this floor, you need to place 50 pounds of pressure per each square foot on the floor.