huangyhg

timber frame knee brace
considering a simple post and beam frame, can knee braces be used to reduce the clear span between the posts? currently, the beam is undersized for the uniform load and the distance between posts. is this an 'acceptable' solution?
could be. but you are putting a horizontal thrust into the post, as well as an axial force into the beam. these must be checked.
daveatkins
thanks dave
i was hoping for some more input from others or a link to an analysis.
i can understand the the horizontal thrust applied to the post. but, how much of the load from the beam does the brace transfer. it wouldn't necessarily be all the uniform load on the beam to the midspan. the question is - is how much? since the beam is supported on both ends with posts, seems that the brace starts taking up some load when the beam starts to deflect.
poseiden - here are a couple of links that may help. not quite what you are looking for, but a start...
thanks slideruleera-
asace pg.8 says that they don't address wood braced frames; and the other also deals with steel. thanks, but i'll keep looking. one of the wood associations had a pdf document with a section on knee braces. but, when i opened the pdf, all the sections just had headings with the remainder of the pages blank. when i searched the association's web site, i could't find it. seems there should be more on timber braced frames.
the load resisted by the brace will depend on the relative stiffnesses of the beam and columns (and also of the axial stiffness of the struts to a very minor degree). to determine this a frame analysis would have to be done. i don't think there is an easier answer.
with regard to the load taken if supporting a cantilever, i think you will find that the reaction is (w l^2)/(2 l), where;
w = udl
l = total length of beam
l = span between supports
this is not equal to half the distance from the post to the brace, plus any cantilever from the brace outward.
thanks apsix-
you're probably right about the reaction for the cantilever. i was simply trying to convey that a reaction for the brace can be easily found with a known uniform load, and only 1 post/column.
but, when the beam is supported at both ends, the reaction (for the brace) is more difficult. your suggestion with a frame analysis using the stiffness (modulus of elasticity-e) of the beam is probably necessary. i do have values(e) for southern yellow pine, as well as the uniform load,spans, and posts. unfortunately, i'm unfamiliar with the proceedure for a wood frame analysis.
any links would be helpful. thanks

you really need at least a 2d analysis program to do this correctly. you could use moment distribution, but i barely re
enercalc.com has a free 2d frame analyzer download called fastframe.
analyze the brace to see if it is stiff enough to act as a brace. i would only use the live load when calculating this, but make sure they installed the brace after all the dead load was in place.
if you're looking to change your unbraced length, then a simple brace such as this isn't effective, since it won't guard against lateral torsional buckling (no resistance in the out of plane direction).
like others have said, if you are trying to change the actual clear span length of the
wood knee braces are unreliable, due to cracking at connectors, etc. i would be very careful...using lag screws and steel gusset plates.

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