best purlines orientation
best purlines orientation
(OP)
Hi
I would like to find out what is the best orientation for Purlin fixing?
lets say for example lipped channels fixed on a roof with 20 degree slope
should they be open to the top or bottom? see attached image
I would like to find out what is the best orientation for Purlin fixing?
lets say for example lipped channels fixed on a roof with 20 degree slope
should they be open to the top or bottom? see attached image






RE: best purlines orientation
My personal oppinion:
"B" is better than "A".
RE: best purlines orientation
Structurally, I don't think it makes much difference, other than with A, the connection to the vertical strut will be in compression rather than in tension with B. Uplift will still control the fasteners for both.
Mike McCann
MMC Engineering
RE: best purlines orientation
RE: best purlines orientation
Option B is easier to clean out the bottom flange, if this is a factor.
In-service wind loads will act normal to the sheeting, so under these actions A = B.
RE: best purlines orientation
RE: best purlines orientation
Produces less twist because the twisting moment (vertical load times the eccentricity of shear centre) is less for position A.
RE: best purlines orientation
RE: best purlines orientation
RE: best purlines orientation
RE: best purlines orientation
The greatest trick that bond stress ever pulled was convincing the world it didn't exist.
RE: best purlines orientation
BA
RE: best purlines orientation
In reality they sometimes just end up orientated whichever way the unaware architect first drew them!
In this part of the world sag rods haven't been used for years. Purlin are always detailed with channels between which helps to deal with any twist.
RE: best purlines orientation
The greatest trick that bond stress ever pulled was convincing the world it didn't exist.
RE: best purlines orientation
RE: best purlines orientation
I would disagree with your statement about orientation being a bigger deal with Z sections. It is all about eccentricity of gravity loading to shear centre. With a Z section, the shear centre is at the web, while for the C section, it is outside the web. Therefore, the C has greater eccentricity with either orientation. With the Z section oriented correctly, top flange uphill, there is no eccentricity with vertical loading.
RE: best purlines orientation
1) With a C-girt, there's one benefit to proper orientation: the shear centre eccentricity thing.
2) With a Z-girt, there are two benefits to proper orientation: shear centre eccentricity & better alignment between load and principal strong axis.
Whether or not the two benefits with the Z-girt weigh more or less than the one benefit on the C-section probably varies based on the situation.
From a practical member selection standpoint, I suspect that the shear centre eccentricity won't mater at all. We orient the purlins correctly because it's good practice to minimize torsion. However, I've yet to see anybody actually design their open section Z & C girts for the inevitable torsion. Taking advantage of the principal axis orientation may well yield a lighter section.
The greatest trick that bond stress ever pulled was convincing the world it didn't exist.
RE: best purlines orientation
I do know that for gravity loading during construction, stepping on a Z section is more comfortable than a C of the same span and braced in the same manner.
RE: best purlines orientation
Mike McCann
MMC Engineering
RE: best purlines orientation
The greatest trick that bond stress ever pulled was convincing the world it didn't exist.
RE: best purlines orientation
That would be true for you as well, but bent you are.
RE: best purlines orientation
See attached link for a picture labelled fastbrace
Link