huangyhg

lateral support of wood beam
hi,
i am evaluating the amount of lateral support that i can consider for the attached sketch. the beam will have joists hung from it at 400 mm o.c. (on the bottom) and will be connected to a roof diaphragm near the centre of the beam. my question is about the unsupported lenght of the beam. the joists hangers are near the bottom, and will likely not provide much out of plane support anyway when acting alone. the roof diaphragm is near the centre of the beam and thus does not provide much torsional rigidity to the beam. all of this being said, when i look at the beam i just can't see lateral support being an issue.
has anyone else come across this situation? was lateral support considered?
thanks
the load below the center will add to the stability of the beam. kickers may be added from top of beam at a diagonal down to the joists or roof diaphragm to fix the compresion flange. most timber beams have enough width to height that the radius of gyration is large, (i/a)<0.5.
how wide is the beam? per the 1997 nds, section 4.4.1.2.c, if d/b <= 4, no lateral support is required. so, if your beam is at least 5" wide, the full fb can be used, even if there is no bracing.
daveatkins
hello structfton,
the joist load is applied eccentrically to the beam so the beam is loaded in torsion. it might be a good idea to tie the bottom of beam with a steel strap to every third or fourth joist to prevent beam rotation. the top of beam is then effectively laterally braced by its own rigidity.

best regards,
ba
i agree with baretired, assuming the beam is at lease 5" wide.
mike mccann
mmc engineering
hi all,
thanks, yes the beam is more than 5" wide. this beam is designed to csa o86, and i cannot find where the depth/width ratio is called up....it is good to know that it is contained in other codes.
thanks all
you don't have to worry about torsion or buckling with the drawing you show. i wouldn't do anything. no strapping or kickers are needed. the joists provide the lateral restraint.
idecharlotte,
i agree that torsion should not be considered; however, what is your reasoning for considering the joists as lateral restraint?
short answer: the joists brace the side of the beam. they do not have to brace the top for the beam to be considered laterally braced. they just have to keep the beam from rotating, which in this case they do.
long answer: there is a sight moment connection in almost any joist to beam connection, which can be calculated with the nds (check toenail pull out capacities). that force couple, albeit small, is usually enough to provide the necessary restraint from ltb. i always assume the lateral force needed to restrain ltb is 5% of the gravity weight. i've also been told that is somewhat conservative. even in a pure pin connection, it would work through the diapghram action of the joists and shear walls. in most cases, and in this case, the joist bracing the lower half of the beam restrains the beam from ltb.
idecharlotte,
if the joists are considered to be supported at the shear center of the beam and if the beam is 5" wide and the length of joist shoe is 2", the eccentricity of reaction is 3.5". assuming the bottom two nails are installed 2" from the bottom edge of the beam and the height of joist is 9", the nails operate over a moment arm of about 7".
if 'r' is the joist reaction in pounds, the moment at the center of joist reaction is approximately 3.5r. the tension in the lower two nails is 3.5r/7 or 0.5r, considerably more than 5% of the reaction.
if you want to argue that the beam is capable of taking this small torsion, i won't argue too strenuously since it has a depth to width ratio of 4 or less. nevertheless, i would use a tension tie between the underside of beam and every third joist because (a) nails in tension should never be relied upon to provide stability to a structure, (b) i like to sleep at night and (c) the cost is trifling.

best regards,
ba

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