529 Beam Bearing Plates

AISC ASD Approach. Beams may be supported directly on concrete or masonry if the bearing pressure is within the allowable. The flanges, however, will act as cantilevers, loaded by the bearing pressure fp (ksi). The maximum bending stress (ksi) may be computed from

where B = flange width, in k = distance from bottom of beam to web toe of fillet, in t = flange thickness, in

The allowable bending stress in this case is 0.75Fy, where Fy is the steel yield stress (ksi).

In the absence of building code regulations, the allowable bearing stress Fp (psi) may be taken as 400 for sandstone and limestone, 250 for brick, 0.35f for concrete when the full area of the support is covered by the bearing steel, and 0.35f VA2/Aj < 0.70f if the base plate covers less than the full area, where f is the 28-day compressive strength of the concrete.

When the bearing pressure under a beam flange exceeds the allowable, a bearing plate should be inserted under the flange to distribute the beam load over the concrete or masonry. The beam load may be assumed to be uniformly distributed to the bearing plate over an area of 2kN, where k is the distance (in) from bottom of beam to web toe of fillet and N is the length of plate (Fig. 5.40).

Steel Bearing Plate Design
FIGURE 5.40 Beam seated on bearing plate on concrete or masonry support.

Example—AISC ASD. A W12 X 26 beam of A36 steel with an end reaction of 19 kips rests on a brick wall (Fig. 5.40). Length of bearing is limited to 6 in. Design a bearing plate of A36 steel.

With allowable bearing pressure of 0.250 ksi, plate area required is 19/0.250 = 76 in2. Since the plate length is limited to 6 in, the plate width should be at least 7% = 12.7 in. Use a 6 X 13 in plate, area 78 in2. The actual bearing pressure then will be fp = 19/7s = 0.243 ksi.

For a W12 X 26 the distance k = 0.875 in (see beam dimensions in AISC manual). The plate projection acting as a cantilever then is l = B/2 - k = 13/2 - 0.875 = 5.62 in. As for a column base (Art. 5.28), the required thickness can be computed for a bearing pressure of 0.243 ksi from Eq. (5.8):

Use a 1-in-thick bearing plate.

The beam web must be checked for web yielding over the bearing plate, over a length of N + 2.5k = 8.188 in. The allowable bearing stress is 0.66Fy = 23.6 ksi for A36 steel. For a web thickness of 0.230 in, the web stress due to the 19-kip reaction of the beam is

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Renewable Energy 101

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