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How much bearing surface is required to support a load bearing beam in a single-family residence? Are there tables? Are end gaps ok or required? A 1/4" to 3/8" end gap on each end (1/2" to 3/4" short overall) would be really helpful for getting it into place.

In particular, I have a 16' long 4x12 solid fir beam, with one end to be buried in a 2x6 exterior wall and the other end into a 2x4 interior wall.

Instead of square end cuts, I could make a tapered end cut of a couple degrees on one end to get a little more bearing. I suppose I could fir-out the interior wall if more bearing than 3.25" is needed.

Bill
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3 Answers3

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You’re smarter than you know. Most people don’t check for “end bearing “.

Beams can fail by1) bending (extreme fiber in bending), 2) shear (horizontal and vertical), and 3) deflection. But seldom does anyone check for bearing.

Once a beam is installed, we worry about “crushing”. Crushing from 1) the applied load (usually as a point load), and 2) crushing from the reaction (at the bearing point).

Each species has a different value of fiber stress to resist this crushing...both compression perpendicular to grain and compression parallel to grain. You’re concerned about Douglas Fir (probably Larch) at the bearing points in perpendicular to grain. (Parallel to grain loading will always have a higher value than perpendicular, so we only check for perpendicular loading stress.)

So, assuming you sized the beam properly, the maximum reaction at each end for a 1) Standard and Better grade is about 2,000 lbs, 2) Select Structural grade is about 2,600 lbs., and 3) Dense Select Structural grade is about 3,000 lbs.

Douglas Fir has a compression stress value of 385 lbs. per square inch for Select Structural grade.

Therefore, the required bearing for MAXIMUM capacity for a Douglas Fir -Larch 4x12 beam spanning 16’ is: 2,600 / 385 x 3.5” (width of beam) = about 2”.

And yes, you can “back cut” the end as long as you have the required bearing, and no, the beam does not need to fit tight to the wall, as long as it has the required bearing surface.

Lee Sam
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The answer for bearing length is a little more complicated than most people would assume it would be. I'm going to assume it is supporting uniform loads and no point loads.

1.5" of bearing length is all you need for this douglas-fir beam, but that's not the limiting factor here. Beam bearing lengths don't take into account the supporting member's ability to carry the loads. So as long as your wall is less than 10' tall, give the beam 3" of bearing with a 2 ply 2x4 built-up column under it. Nail the two plies together following NDS standards. If your supporting member was a steel beam, then that's when 1.5" bearing length would be enough.

It seems counter intuitive, but with dimensional lumber the longer the span the less bearing length you typically need because the end reactions are lower.

For example, your 16' long 4x12 can support a max uniform load of 150 PLF which means you have 1200 lbs at each end. If the same beam was only 5' long, it could support a max uniform load of 1900 PLF which means you have 4750 lbs at each end.

I don't know the answer to your end gap question. I have a feeling that 1/4" is fine on either end, but I don't know for sure.

Dotes
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The American Wood Council maintains a document (the National Design Specification, or NDS) as a standard for the engineering methods and properties used to calculate wood beam limits (as well as a number of other engineering calculations with respect to wood construction).

More specific to your question, they also maintain an on-line calculator for 2x wood beam span calculations - including Bearing Length. This calculator allows you to select from among almost all available species and grades of 2x lumber found in North America, include European species found in North America. You can find this calculator here: https://awc.org/calculators/span-options-calculator-for-wood-joists-and-rafters/.

Edit: Per Chapter 5, R502.6, of the 2018 Residential IBC, the IBC code required minimums are:

  • 1- ½" wood on wood;
  • 3" wood on concrete;
  • 1"x4" strip (if joist is nailed to adjoining studs)
baskren
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