rafter unbraced length

huangyhg

rafter unbraced length
i have rafters spanning approximately 15 meters (50 ft) and was wondering if i should have bracing (i believe they're called kicker bracing, or kick bracing, or something like that) between the rafter bottom flange and the purlin to reduce the unbraced length when the rafter is subject to uplift. now, typically, would this bracing connection be specified for each rafter to purlin connection, or every other rafter to purlin connection, or sometimes none at all? i suppose what i'm wondering is when do you decide to use this particular bracing connection? thanks
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clansman,
if the rafter is sufficient to span the whole length unbraced then you do not need any fly braces (or knee braces or kicker braces).
if it doesnt then you need to supply as many as required to get the effective length down to an acceptable length for the moment requirements. usually you would do it one at the center or 2 at thirds.
if you find that you need to do more than one every 3rd purlin then you would be more economical upping the beam size.
clansman,
in australia we call them fly braces and they are typically used in end bays where uplift forces are the greatest.
when considering the rafter under pure compression, the effective length in-plane (lx) we equal the span between vetical supports, the effective length out-of-plane (ly) will equal the purlin spacing.
when considering the rafter under uplift.  the bending effective length (le) will be from point of inflexion to point of inflexion, this will be around 60% of the total length.  the load is applied above the shear centre of the section so you won't need to make any additional allowances to the bending length (a load height fudge factor).  
if your rafter has insufficient capacity for the design actions then you will need to shorten the effective length in bending, which is done by the use of a fly brace. providing a fly brace to one side of the rafter should provide enought rotational restraint, however, it is common to provide fly braces to both sides of the rafters.
i would look to use as few fly braces as possible.  i am also doing a rafter design at the moment spanning 15m continuous, and i will be looking to use a rafter depth around span/35 (a 460ub australian), and two fly braces (around 1/3 and 2/3 of the span).
asixth,
you are usually spot on but...
it has been shown by research that the point of infection does not act as a braced point and should not be used as such. there are moment factors in as4600 that allow for the stress reversal.
i suggest you do a bit of research on this (i believe it was published in australian papers).

loads applied below the shear center downward and loads applied above the shear center upward reduce the lateral torsional buckling action by straightening the twist of the beam.  asixth is correct is his statements.  points of inflection do not act as a brace, but the load application upward above the shear center does eliminate the ltb concerns.
civilperson,
that is news to me.  what is the basis of your statements?
civilperson: i'm pretty sure that's not correct, even though there are many older papers advocating that design approach... was often used in the design of girber (partially cantilevered floor systems with drop in spans), and was a popular approach in canada, however it has been recently discredited.  i'll try to lay my hand on the article for you.
on topic: i posted a detailed summary of when you need to add fly braces, as you often do not.  essentially if you have purlins deeper than half of your rafter depth and they are attached to the top of your rafter by a welded cleat and 2 or more bolts each, you can use the rafter as a point of support against rotation, thus reducing your segment length.  there's a good paper out of hera (heavy engineering research association of new zealand) that explains this in detail.
cheers,
ys
b.eng (carleton)
working in new zealand, thinking of my snow covered home...
thanks ys, i needed some support as no-one believed me.
i imagine we are talking about the same paper.
csd72
don't worry, i think the silent majority believed you.
youngstructural-
i anxiously await the article that discredits the notion that tension flange loading tends to stabilize a beam.
miecz,
i think there are 2 separate issues that are getting mixed up here:
1. does the location of the applied load stabilize a beam under that load - the answer is yes.
2. does that load act as a point of buckling restraint at contraflexure - the answer is no.
the paper will confirm point number 2.
each section needs to have a point of restraint at each end of the effective length.