Design of Purlins with metal Roofing Sheet
Design of Purlins with metal Roofing Sheet
(OP)
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!!






RE: Design of Purlins with metal Roofing Sheet
RE: Design of Purlins with metal Roofing Sheet
RE: Design of Purlins with metal Roofing Sheet
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.
RE: Design of Purlins with metal Roofing Sheet
Can u please give me the website where I can look at the proprietary bridging proudct. Thank you!!
RE: Design of Purlins with metal Roofing Sheet
http:/
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
RE: Design of Purlins with metal Roofing Sheet
Don't forget that for the net uplift pressure on the purlins they would be fully unbraced unless supplementary bracing is provided.
RE: Design of Purlins with metal Roofing Sheet
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.
RE: Design of Purlins with metal Roofing Sheet
hokie66, can u find any website for any proprietary bridging details being used in Australia?
Thanks!!!
RE: Design of Purlins with metal Roofing Sheet
Have a look at the following website.
h
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.
RE: Design of Purlins with metal Roofing Sheet
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.
RE: Design of Purlins with metal Roofing Sheet
RE: Design of Purlins with metal Roofing Sheet
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.