Determining Unbraced Length of Compression Flange

[P1ENG]

I have a situation where I have a W8x15 rafter beam and purlins attach to the rafter at various locations. The purlins are bolted to a clip through the web, and then the clip is bolted to the web of the rafter.

The purlins have different cold-formed shapes, some where the flanges of the purlins are flush with the flanges of the rafters, but the flanges don't touch. Some of the bolts in the rafter web are ~2" from the top/bottom of the beam.

I have a feeling that I don't have any lateral bracing for the compression flange of the rafter. Perhaps someone can answer whether the bolts used that are ~2" from top/bottom of the flanges can be considered lateral bracing. If so, where do you draw the line? (say that lateral web bracing within the depth/4, or some other ratio, from the compression flange is sufficient for compression flange bracing?)

Also, there is a gap between the purlin and rafter flanges, but since the purlins are located on both sides of the rafter, the lateral displacement of the rafter's compression flange would have to be limited to the gap between the flanges. Once the deflection of the gap is reached, I would then have bracing against lateral buckling. Could I allow this?

Finally, the roof rib decking might provided the bracing that I need for gravity loads, but I am unsure right now of the fastening locations of the roof decking, so please stick to answering the (2) hypotheticals above.

Thanks

Juston Fluckey, E.I.
Engineering Consultant

 

[Lion06]

It sounds like the purlins have single angle connections to the W8. The "confinement" of the top flange of the W8 is not what provides lateral bracing. No floor systtems are built this way - there is always a gap. AISC just says that the bracing needs to be near the top flange. I think there was a provision in the green book that allowed for the bracing to be within the top 1/3 of the beam. If you have a clip angle with a bolt 2" from the top of the beam, I'd say it's braced for positive bending.

 

[P1ENG]

I was not able to find that provision in the 9th Ed. If you know where to find it, maybe you could provide it?
Thanks,

Juston Fluckey, E.I.
Engineering Consultant

 

[Lion06]

I'm not positive it's there, but I haven't looked at my green book in a LONG time. Either way, if your clip angle comes to within 2" of the compression flange, your braced.

 

[BAretired]

Without doing any calculations, I would agree with StructuralEIT that a bolt 2" from top and 2" from bottom of a W8x15 constitutes lateral bracing for either flange provided that the clip angle is thick enough.

A brace is deemed adequate if it can provide 2% of the compression in the flange to be braced without excessive deflection.

You could ensure that the clip angle has the required resistance and you could estimate the deflection of the clip angle and the beam web by making a few simplifying assumptions.

BA

 

[P1ENG]

I appreciate your answers, and I don't disagree with either of you. I am really looking for a reference to a code that says this is o.k. Vague words are being used such as "excessive deflection" and "near the top flange", and I have no way of quantifying these. This is no longer a "can I do this", but a quest for knowledge on the subject.

I know the flange cannot move laterally without taking a portion of the web with it, but where to draw that line? Sorry, I was that kid in class that always asked "why?".

Juston Fluckey, E.I.
Engineering Consultant

 

[nutte]

Look in Appendix 6 of the 2005 AISC steel specification for equations that will give you a more precise answer. Also, read "Fundamentals of Beam Bracing" by Yura from AISC's Engineering Journal, 2001 Q1. These will tell you how stiff your brace needs to be and how much strength it needs to be able to withstand.

 

[JAE]

agree with nutte.

 

[weab]

P1,

This link has a similar thread. Again, it's not in the code, but it has a link to AISC. Check it out.

http://www.eng-tips.com/viewthread.cfm?qid=246172&page=123

 

[BAretired]

I am not familiar with AISC but CSA S16 (Canadian Code) specifies three methods to determine bracing requirements, i.e. Simplified Analysis, Detailed Analysis and Direct Method.

In the Simplified Analysis, an initial misalignment, [Δ]₀ is assumed. The brace is required to resist a lateral force of 2% of the factored compression in the element being braced. Moreover, the displacement, [Δ]_b resulting from that force must not exceed [Δ]₀. For simplicity, members may be considered hinged at brace points.

In the present case, the deflection of the clip angle must be considered as part of the displacement. If it is light gage material, it is likely to be the most flexible part of the connection.

BA

 

[RFreund]

I know this is not a direct reference but I would think since lateral torsional buckling is more of a global failure state that 2" from the top flange in this situation would be adequate to prevent this type of failure.

 

[ToadJones]

Answers wont always be in the code, that's why you have to be an engineer. If they were all in the code, then cooks could design buildings .
I say so with a light heart.
In general, it appears that you have a very light roof system with a light rafter and light gage pulins. I'd agree with others that your comp. flange is braced.

I noticed that you are an E.I. but call yourself an "Engineering consultant". You sound on the up & up, but be careful with titles, they can get you in trouble.

 

[BAretired]

The compression flange of a beam or the compression chord of a truss can be adequately braced by a properly designed brace anchored to any point including the other flange or chord.

Sorry, I was that kid in class that always asked "why?".

Perhaps you should have been that kid in class who asked "why not?"

BA

 

[P1ENG]

Thanks to all for the input over the weekend.

-Toad, my company put "Engineering Consultant" as my email signature and on my business cards. :-D I'm not too concerned with titles except for P.E. Next month will be the end of my (4) years as an intern, and I hope to take that E.I. off soon!

Juston Fluckey, E.I.
Engineering Consultant

 

[ToadJones]

Good Luck Juston...I hope you pass!!

 

[BAretired]

All the best, Juston. Or as they say in show biz...break a leg.

BA