Rafter unbraced length

[GalileoG]

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

 

[csd72]

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.

 

[asixth]

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).

 

[csd72]

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).

 

[civilperson]

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.

 

[hokie66]

civilperson,

That is news to me. What is the basis of your statements?

 

[youngstructural]

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...

 

[csd72]

Thanks YS, I needed some support as no-one believed me.

I imagine we are talking about the same paper.

 

[apsix]

csd72
Don't worry, I think the silent majority believed you.

 

[miecz]

youngstructural-

I anxiously await the article that discredits the notion that tension flange loading tends to stabilize a beam.

 

[csd72]

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.

 

[miecz]

csd72,

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.

 

[youngstructural]

Hello All;

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...

 

[hokie66]

I still would like to know the basis of civilperson's statement that load application on the tension flange "does eliminate the LTB concerns". Does that mean that I can stop bracing monorail beams?

 

[civilperson]

Yes, if the monorail hangs below the bottom flange of a S beam.

 

[hokie66]

Then you are discounting LTB as a concern. I don't understand why. Perhaps someone can elaborate rather than just tell me not to worry.

 

[civilperson]

See Modern Steel Construction, May 1997 issue.

 

[apsix]

hokie66
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.

 

[hokie66]

civilperson,

Sorry to continue to pester you, but in that issue, there are some interesting articles, but none seem to be on point.

 

[asixth]

A few comments on the issues.

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.