Eccentric Roof Bracing (Purlin to bracing offset)

[youngstructural]

Hello All;

I have a bit of a problem and I wonder if I can get a few opinions. Please consider:

- Structure is a 21mx15m steel portal-frame building (360UB45) with CFS light gauge metal purlins (Dimond DHS 200/18).

- Because of client's specfic requirements, the purlins are being set inside (but not fully inside) of the steel flanges (see image) so I am having to brace at the bottom chord. Centre line of purlin to bracing distance is 260mm; Beam is 352mm deep. Peak load in bracing is ~100kN and causes an ~65kN compression in the purlin.
See:

http://www.engsoc.org/~leito/PurlinCleatDetail.jpg

- Roof bracing consists of a proprietary steel rod system, attaching to the BOTTOM flanges of the portal frame rafters.

I have some concern regarding the eccentricity between the bracing line and, what is in effect, the compression strut (purlins) for the latteral load resisting system. My gut says the effect is negligible, but I am hoping to get a couple of opinions.

Should I be worried about this eccentricity?

Thanks in advance,

YS

B.Eng (Carleton)
Working in New Zealand, thinking of my snow covered home...

 

[Tomfh]

How do you transfer the load through the eccentricity?

 

[MikeE55]

I don't think it is any worse than a normal MBS situation where the purlin is on top of the beam, and the bracing is below. However, calculating the axial load capacity of a purlin with a non-diaphragm roof is beyond my ability. If I were designing this type of building, I would add a HSS strut or something that could be calculated and place the strut in the plane of the bracing. I know that's not what the MBS boys do, so maybe one of them can explain how they calculate the capacity.

 

[Tomfh]

<i> so maybe one of them can explain how they calculate the capacity </i>

Several of the NZ companies have this info in their purlin manuals. Compression capacity is presented in tables. Instructions for bending/axial interaction is also given.

Purlins are pretty good compression members because they are deep and they are laterally restrained by the roof sheeting.

 

[apsix]

A flybrace at that location would certainly ensure you don't have a problem, but I suspect the rafter would have enough torsional rigidity as detailed to be OK.

If relying on purlins as bracing struts I would normally use a double purlin, even if a single one is shown to be OK.

 

[youngstructural]

Thank you all for your help; I'm sure I have it sorted now. One further piece of information I received from HERA (the New Zealand Heavy Engineering Research Association) steel solutions centre:

- If you're considering loading a beam in torsion, you need not consider the resulting moment and torsion effect if you are within a zone of torsional stiffness defined by:

a= SQRT((EIw)/GJ)

This would apply up to any position of full or partial lateral restraint. In my case, I am well within this distance from full depth stiffeners, and as such the eccentricity is not of concern.

Thanks again everyone...

YS

B.Eng (Carleton)
Working in New Zealand, thinking of my snow covered home...