Can fly brace use as a support for purlin?

[Arcanisez]

Hi, guys

I'm new and this is my first post here.

I have to analyze the purlin with double fly bracing system.

For long span purlin let say about 5m. Normally I will look it as simple beam and analyze loading full span.

But other designer opinion takes the fly bracing as a support of purlin and then moment will decrease significantly compared to analyze as simple beam.

The question is it appropriate to take the fly bracing as a support of purlin?

If appropriate what design basis of bracing shall take in account compression, tension, torsion?

 

[Tomfh]

Not appropriate.

 

[KootK]

Probably inappropriate. As you've rightly identified, the trusses will have very little torsional stiffness so the fly braces won't offer much support stiffness for any unbalanced wind loads.

 

[hokie66]

Don't even think about it. Would you use fly braces on every purlin or different purlin sizes? You are designing a building, not a sewing machine or airplane.

 

[Arcanisez]

I have asked the designer who detailed about this,

He said this fly brace can use to support the purlin then the purlin span be reduced with purlin section also. (maybe for economic purpose)

I am thinking about that diagonal pipe, shall we say that is a fly brace or a part of structural system?

because that diagonal pipe seems big and brace at L/3 of purlin span. Theses is not similar with ordinary fly brace that brace near a truss only.

Do you have any opinion on this point?

 

[canwesteng]

It's essentially making the purlin continuous, but should be analyzed as a system to properly capture the effect of unbalanced load. The benefits are negligible though, since you shouldn't have fly braces at every purlin as already pointed out, and mixing and matching purlins is not common. A better layout is to have a single fly brace only - in which case the amount of work connecting fly braces at heights is reduced by half, and you have a true simple span condition, instead of this one, where it is pseudo fixed over the support.

 

[human909]

Not appropriate.

I agree.

The question is it appropriate to take the fly bracing as a support of purlin?

You aren't the first person on this forum to ask this question.

Like most here I don't believe it is something that should be done. But yes, the fly braces could be considered supports if detailed and analysed correctly. But to do so is quite a complicated task for a whole host of reasons.

In fact ignoring the fly brace as a 'point of support' is simplifying the actual behaviour of the structure, whereas considering as a point of support is more accurately modelling the structure. HOWEVER the first simplification is almost certainly conservative in most if not all respects. Modelling fly braces as points of support would make you life MUCH more difficult. IMO if you are asking the question you aren't immediately aware of the complications it induces. (eg shear loads on purlins are now in 3 place, twist is potentially being INDUCED in the truss/beam which should be checked, etc...)

 

[hetgen]

You got all appropriate responses here, but check this thread for additional responses:

https://www.eng-tips.com/viewthread.cfm?qid=497580

 

[driftLimiter]

For long span purlin let say about 5m. Normally I will look it as simple beam and analyze loading full span

I have also struggled with purlins modeled as a simple beam.

The trick to light purlins is to splice them partial fixity. The purlins can be slapped at the support which can be included in strength calculation.

The other (in the US at least) is there is a system factor that applies built up purlins /roofing systems that is like repetitive member factor basically.