How to Design a Concrete Beam

Sketch out the span and general size of the beam(s) required for the project. While the ultimate load-bearing requirements will determine final dimensions and makeup of the beam, physical space limitations must also be considered. Define and draw the design in inches.

Research and analyze the maximum, single-occurrence load that the beam will need to withstand in lbs. If the beam is to provide vertical support only, this is a simple calculation and it applies to the entire exposed end surface. If it is a span load -- i.e. bridge construction -- then it is typically a mid-span vertical load-bearing requirement. Include all reasonable weight contribution in your calculations. As an example, a 3-axle truck weighing 4 tons with an 8-ton load would require a load-support rating of 24,000 lbs.

Convert the load-support weight calculation to the required stress that the beam must be engineered to withstand. (Note: the stress calculation in lbs/in^2 is based on the load-support weight requirement divided by the target space -- or footprint -- upon which the weight will be applied.). Determine the footprint of the target beam in square inches. A 10-by-20-inch footprint would yield a stress area of 200 square inches. Divide the load-support requirement by the number of square inches in the footprint (the stress area). A 24,000-lb load on a 200-in^2 footprint would yield a stress rating of 120 lbs/in^2. Un-reinforced concrete has a stress rating of 3000 lbs/in^2. In this example, pure, aggregate-containing but un-reinforced concrete would easily handle the load.

Take the total load support requirement and divide by the number of same-dimension concrete beams in the structure -- i.e. the load sharing -- to determine the final weight-support amount in lbs/in^2. Increase that number by 50 percent to apply a safety factor.

Determine if un-reinforced concrete will suffice for the structure of the beam. For most home and shop requirements, standard-grade concrete will suffice for the content structure of the beam.

For extreme load demands (bulldozers or tractors) use the following guidelines. Determine the load-requirement category of the subject beam from this range of categories: (Note: this set of categories assumes a 15-foot beam; 1:1.5:3 mix ratio for concrete/sand/aggregate; and a metallic strength additive.)
A-Class: 6 inches by 8 inches <= 28,000 lbs/in^2 rating (single-level rebar)
B-Class: 8 inches by 10 inches <= 48,000 lbs/in^2 rating (two-level rebar)
C-Class: 10 inches by 12 inches <= 66,000 lbs/in^2 rating (two-level rebar)
D-Class: 12 inches by 14 inches <= 90,000 lbs/in^2 rating (three-level rebar)

Lay out the final design of the concrete beam as a rectangle that is taller than wide. (Note: the beam's width-to-height ratio is never less than 1 for good structural design.) The placement of the rebar should be in the lower half of the beam only, 2 inches above its lowest edge, lying horizontal. (Note: the upper quadrant of the beam experiences compression but not stress. Stress occurs in the lower quadrant, and that is where the rebar becomes critical.) Draw the design with equally spaced steel bars, and note their thickness on the design. Rebar for most home and shop design requirements should be either #4 or #5 (.5-inch or .625-inch diameter, respectively).

Lay out the final design of the beam and the frame (pouring cavity) for the subject beam. Its interior dimensions should match the height and width of the beam layout.