Design of Purlins with metal Roofing Sheet 4

[dcnnng]

Any expert please advise, in accordance with BS 5950-1:2000, whether purlins supporting ordinary metal roofing sheets is considered adequately restrained to its compression flange so that no lateral torsional buckling check is required. Thank you!!

 

[hokie66]

Not familiar with the British standard, but this is the typical roof system in Australia. It is complex, and engineers here generally depend on published tables. Bridging members are used between the purlins, and these have been shown by full scale testing to provide support for the flanges. For the top flange, most of the bracing is by the sheeting which is screwed to the purlins, and for the bottom flange, all of the bracing is by the bridging.

 

[dcnnng]

TQ Hokie66 for your response. If I read and understand corrctly, according to your practice in Australia, the top flange is considered adequtely restrained but however the bottom flange which will be in compression under wind suction is to be restrained by other mean. We normally use sag rod.

 

[hokie66]

Yes, the sheeting restrains the top flange.
Sag rods are not commonly used in Australia, although I believe they were in the past. The bridging used now consists of light gauge channels, with connections near the top and bottom of the purlins. These are proprietary items, and differ a bit from manufacturer to manufacturer.

 

[dcnnng]

Hi Hokie66,
Can u please give me the website where I can look at the proprietary bridging proudct. Thank you!!

 

[csd72]

Heres one more appropriate to the UK.

http://www.kingspanstructural.com/multibeam/rp/index.htm

I think you will find that the load tables for most purlin manufacturers are based on the use of their proprietary bridgings - sag rods just are not sufficient as they do not provide torsional restraint.

csd

 

[MarcbSE]

With a screw down metal roof deck the purlins would be considered fully braced.

Don't forget that for the net uplift pressure on the purlins they would be fully unbraced unless supplementary bracing is provided.

 

[hokie66]

csd,

Interesting site. UK take on purlins is certainly different. But I couldn't see any bridging which looks like it would brace the bottom flange.

MarchSE,

That is correct. That is the purpose of the bridging we are discussing.

 

[dcnnng]

csd, I couldn't find the bridging details as brought up by hokie66!!

hokie66, can u find any website for any proprietary bridging details being used in Australia?

Thanks!!!

 

[BarryEng]

Have a look at the following website.

http://www.bluescopesteel.com.au/go/product/lysaght-zed-and-cee-purlins-and-girts

Go to Lysaght zed & cee purlins & girts user manual (top RH corner) & hit the download key (not for the cee purlin C7510 manual).

Sag rods are sometimes used with wall girts & usually with connections over the ridge (see manual).

I designed a 50,000 m2 roof (15 odd years ago) & a part of the speci was to test two complete bays of roofing for dead loading (down) & suction (wind loading) up. The results (then) indicated that sag rods for roof purlins were not sufficient to prevent torsional distress.

There has been several large scale tests done since then, that have only verified the original conclusions. I think that all of the purlin manufacturers in Oz, base their safe load tables on similar testing. However, there has been a lot of theoretical analysis (I understand) to confirm the testing data.

The roof sheeting is generally a ridge fixing (valley fixing in rare cases) with some flexibility from the tilt of the fixings subject to a lateral loading. However, testing has indicated that, at failure, this tilt is only a consideration in the very early stages of loading (movement) of the sheeting. The fixing of the sheeting generally creates sufficient friction (sheeting to purlin) to allow an effective lateral restraint of the purlin flange at the design loading.

 

[BarryEng]

I meant to mention - as hokie66 said above, the whole reason for the bridging, is to ensure that the purlin deflects vertically under ALL types of loading.

If the bridging does not keep the purlin vertical, the deflection distortion is a torsional rotation (due to the fact that the purlin is not axisymetric such as an I section) with a very large reduction in the (effective) I & Z value of the purlin, with a consequent reduction in load carrying capacity. A tie rod cannot develop this type of torsional restraint.

Essentially, bridging is also req'd for C sections but for a different reason. If the C is loaded by the sheeting on the top flange, the loading is (generally) directed thru the neutral axis & NOT thru the shear centre of the C section. The load mutiplied by the distance to the shear centre, will create a torsional moment that will result in a rotation of the C section (under loading). This rotation will result in a reduction in the effective I & Z values.

All of the above conclusions, have been verified by both theoretical studies & testing of 'real life' structures - usually by purlin manufacturers.

 

[BarryEng]

For the development of the purlin safe load tables in Oz, read page 3 of the zed & cee purlin manual (mentioned in the thread above) "Introduction - background"

 

[hokie66]

dcnng,

Sorry I didn't get back to you, but Barry has done a better job than I would have anyway.

We commonly use zed sections for roof purlins, as they can be lapped at supports, thus making more efficient use of the material where the moments are greatest. The top flange goes up the slope, thus the vertical component of load is applied near the shear centre.