Top of Column, Bottom of Beam Bracing, Wood Construction Standard Conventional Detail 1

[kgengr]

For conventional wood framing with simple span glulam beams, wood columns, and I-Joists, plywood diaphragm. Beams frame over top of columns with 12" tall side plates, deepest beam is 25 1/2" (6 3/4" width). Do you provide a brace at top of column/bottom of beam to diaphragm for column bracing? (Using reco of AISC of 0.01Pr even though a wood column.) I have received this requirement as a plan check item. Your insight is much appreciated.

 

[jayrod12]

Sketch your beam to column connection and post for us to review.

I see this more as you need to ensure the top of your column is restrained from displacement rather than bracing the bottom flange of the beam.

Flush framed joists or are the beams dropped below?

 

[KootK]

It's a joint that needs lateral bracing of some kind. There are a number of options for accomplishing that, however.

1) Discrete brace. Angle etc. Ugly if exposed.

2) Use flush joists to torsionally brace the beams and thereby restrain the bottom of the beams and top of the column against lateral movement. The extent of the torsional restraint at each joist is always a question, however, especially if the joists are less than 60% as deep as the beams. You're also kind of relying on some cross grain beam bending which is always a bit scary.

3) Run the column all the way up and brace it at the diaphragm level. Tie the beams into the sides of the columns.

4) Detail and design the saddle to function as an extension of the column and run that up to the diaphragm to be braved there. A vertical stiffener in the saddle between beams might be a way to do this if aesthetics is a problem. With TJI's I'm guessing that you've got a ceiling however.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[kgengr]

Thank you KootK.
The detail we have is the 2nd condition, but with consideration of the beam saddle also providing column extension to reduce length of beam which sees cross-grain bending up to joists (keeping this stress below 15 psi). However, it seems discreet braces may need to be added where the beam depth is large, or saddle lengths increased. Thanks again.

 

[KootK]

You're quite welcome kgengr. Maybe you can help me out with something.

keeping this stress below 15 psi

Are you using this as an allowable cross grain bending stress? Is it a limit that you devised or something that originates from a code clause or wood design text? I want to know so that I can perform that trick too. You know, assuming that your explanation doesn't lead me to question your sanity/judgement.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[kgengr]

Haha -- we shall see
Yes, allowable - and borrowed/extrapolated from cross-grain tension relating to radial stresses in curved beams - so Frt from NDS. Though I am told there is a reference to 15 psi and cross-grain bending/tension in Breyer 2nd Edition.

Here's an excerpt referencing Faherty/Williamson and Breyer, which is a bit convoluted - there is a better excerpt in the Timber Construction Manual.

"Radial Tension or Compression. The radial stress induced by a bending moment in a member of constant cross section may be computed from...
"When M is in the direction tending to decrease curvature (increase the radius), tensile stresses occur across the grain. For this condition, the allowable tensile stress across the grain is limited to one-third the allowable unit stress in horizontal shear for southern pine for all load conditions, and for Douglas fir and larch for wind or earthquake loadings. The limit is 15 psi for Douglas fir and larch for other types of loading. These values are subject to modification for duration of load. If these values are exceeded, mechanical reinforcement sufficient to resist all radial tensile stresses is required."