Rafter unbraced length
Rafter unbraced length
(OP)
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






RE: Rafter unbraced length
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.
RE: Rafter unbraced length
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).
RE: Rafter unbraced length
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).
RE: Rafter unbraced length
RE: Rafter unbraced length
That is news to me. What is the basis of your statements?
RE: Rafter unbraced length
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...
RE: Rafter unbraced length
I imagine we are talking about the same paper.
RE: Rafter unbraced length
Don't worry, I think the silent majority believed you.
RE: Rafter unbraced length
I anxiously await the article that discredits the notion that tension flange loading tends to stabilize a beam.
RE: Rafter unbraced length
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.
RE: Rafter unbraced length
Thanks for clearing that up. After reading the repsonses to civipersons post, which (I believe) said that tension flange loading tends to stabilize a beam, I expected to see recent research that discredits that notion. I guess I'm reading them wrong.
RE: Rafter unbraced length
Unfortunately I cannot find the paper, however there is a FAQ on CISC (Canadian Institute for Steel Construction) which summarizes this issue with regard to the Girber system I have mentioned; The key line reads: "Kirby and Nethercot, among others, have pointed out that a point of inflection, or zero moment, cannot be taken as a lateral support (Design for Structural Stability, 1979)". Take a look at the FAQ at http://www.cisc.ca/content/technical/faq.aspx, there are a number of useful tips and tid-bits to be read.
Now if this situation hasn't already been confusing enough, you really need to keep in mind the fact that if you have a relatively still rotational point of influence (ie: able to sustain an applied MOMENT) restraining your rafter, you DO get a point of buckling restraint. However, this has nothing to do with the point of counterflexure, and csd72's post was spot on.
The HERA paper dealing with the issue is Appendix 5 of HERA Design Guide Volume 1 (R4-80). I would strongly encourage any designers involved in the construction of portal frame buildings to review the paper.
One caveat: I have never really wrapped my head around what would happen with sustained vertical loading (like a good coat of winter snow back in Canada). Typically this wouldn't be of issue in any case, as we'd normally use OWSJ since CFS rafters just won't do the job under good old Canadian roof loads...
Cheers,
YS
B.Eng (Carleton)
Working in New Zealand, thinking of my snow covered home...
RE: Rafter unbraced length
RE: Rafter unbraced length
RE: Rafter unbraced length
RE: Rafter unbraced length
RE: Rafter unbraced length
I'm also suprised by the claim that load application on the bottom flange (for gravity loads) eliminates LTB concerns.
I don't think AS4100 will allow us to ignore it.
I have just found that App'x H2 & H3 can be used to obtain bending capacities for load application below the centroid. I'll have to run a few checks to see what the effect of bottom flange loading is, when time permits.
RE: Rafter unbraced length
Sorry to continue to pester you, but in that issue, there are some interesting articles, but none seem to be on point.
RE: Rafter unbraced length
1. When I mentioned the bending effective length will be between inflexion points for uplift, I was referring to the purlins near those inflexion points that will provide the rafter with lateral restraint, not purely the inflexion point by themselves.
2. Additional allowances must be taken into consideration on whether the load is acting on the critical flange and therefore contributing to lateral-torsional buckling. According to my SpaceGass help doc, the Australia code multiplies the effective length between restraints by a factor of 1.4, which is a conservative factor when compared with other design codes.
3. AS4100 takes the critical flange of a cantilever beam to be the tension flange. Refer Clause 5.5.3,
Segments with one end restrained:
When gravity loads are dominant, the critical flange of a segment with one end unrestrained shall be the top flange.
RE: Rafter unbraced length
I dont know about the current version of AS4100, but previous versions used specific factors for the location of the loading in the section giving a lower (better) factor for bottom flange loading.
asixth,
Doesnt the 1.4 factor reduce to 1 for loading at or below the centroid?
It has been a few years but I am sure these are correct.
RE: Rafter unbraced length
From the "Steel Interchange" 05/01/1997:
"Does an unbraced trolley beam that is loaded on the bottom flange have the same buckling characteristics as an unbraced beam loaded on the top flange?"
Answers:
Recommended approximate solutions to estimate a beam's critical capacity under concentrated loads have been presented in a July 1971 issue of the Structural Engineer in Nethhercot and Rockey's A Unified Approach to the Elastic Buckling of Beams. The content of this article was later referenced in the text of Chen and Lui's Theory and Implementation, 1987, Elsevier with comparison to theoretical solutions of Timoshenko and Gere. The approximate solutions for centrally loaded simple beams with tip flange, shear center and bottom flange loading shows close agreement using Cb values.
........
Barry P. Gahagan, PE
Forte and Tablada, Inc.
Baton Rouge, LA
___________________________________
07/01/1997
An unbraced trolley beam that is loaded on the bottom flange has increased resistance to buckling compared to a similar beam loaded on the top flange. When a load on the bottom flange moves with the beam during buckling, it causes an additional moment about the shear center of the beam, and therfore, resists the tendency of the beam to buckle. ........
Brian J. Bidonde
Baker & Associates
Pittsburg, PA
_________________
reference www
RE: Rafter unbraced length
Bottom flange loading helps with the torsional but not the lateral.
RE: Rafter unbraced length
Thanks for the further information. I understand that tension flange loading reduces the tendency to buckle, but in the real world, I will continue not to depend on it. Reduces is not eliminates, and I am no hero.
RE: Rafter unbraced length
RE: Rafter unbraced length
How did you calculate the beneficial effects?
RE: Rafter unbraced length
That's correct, kl=1.4 above the centroid, kl=1 below the centroid. Good desciption of lateral-torsional buckling. It's always nice when someone brings some simplicity to engineering.
Clansman,
I ended up using three fly braces in my end span, located atound the quarter points. I'll have to wait for the tech to build up the model before I specify which purlins these are.
RE: Rafter unbraced length
At the time, I was using the LRFD 3rd Edition. Formula F1-6 has an X2 term that applies to tension flange loading. That is, the commentary to F1 implies that X2 = 0 for "top" flange loading. I merely cranked X2 through formula F1-6, which increased Lr. Looking back, I see that it had a minimal effect, increasing my Lr from 14.0 feet to 14.9 feet.
My old Guide to Stability Design Criteria for Metal Structures has the derivation of the critical load formulas that appeared in the 9th Ed. AISC Codes. The formulas for "Bottom Flange Load" and "Top Flange Load" were identical, except for a C2 term that was negative for top flange loading, positive for bottom flange loading, and zero for load at the centroid. The Guide explained that load placed on the top flange had a tipping effect that reduces the critical load; and conversely, if the load is suspended from the bottom flange, there is a stabilizing effect that increases the critical load. I believe the Guide formulas were simplified for the ASD Specification by eliminating the C2 term. With the dawn of the LRFD Specs, the C2 term seems to have been resurrected.