best purlines orientation 1

[lolobau]

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

 

[chrislaope]

My personal oppinion:

"B" is better than "A".

 

[msquared48]

From the standpoint of ease of erection for the guy in the field, I would prefer A over B.

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

 

[EngineeringEric]

I would think B > A. but that is only an opinion. I suppose if you wanted to get technical the best system may be a combination, have each other one face opposite direction to reduce the torsional effects from the cross section... but the slope and its a roof not a floor makes me think i would just specify B... but i'd accept A.

 

[Ingenuity]

During erection, option A will result in less propensity to twisting due to the reduced eccentricity from the shear centre. This is called Standard Mode in Australia.

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.

 

[JRGse]

Cee sections like theses should be alternated back/back, front/front as one proceeds up the slope. These will cancel out the roll forces that develop.

 

[civeng80]

A is the best option.
Produces less twist because the twisting moment (vertical load times the eccentricity of shear centre) is less for position A.

 

[hokie66]

civeng80 is correct. Always orient then so that the gravity loading is closest to the shear centre. Better yet, use zed shaped purlins, with the top flange upslope, which also deals better with the debris accumulation issue.

 

[jgailla]

From a constructability point of view, A is better because it's easier to carry.

 

[dcarr82775]

Agree with civeng80. I have not run the numbers on C shapes, but it makes a huge difference on Z purlins.

 

[KootK]

It's worth noting that orientation is a bigger deal with a Z-girt because, in addition to the shear centre issue, a Z-girt's principal axes are rotated at angle relative to the web. The top flange uphill orientation better aligns your maximum moments with your strong axis section properties. This doesn't come into play for C-girts. Old news for most, no doubt.

The greatest trick that bond stress ever pulled was convincing the world it didn't exist.

 

[BAretired]

In either case, you need sag rods tied to the ridge beam to carry the minor axis bending of the purlin.

BA

 

[Agent666]

A is preferred. But have seen and used both.

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.

 

[KootK]

The channels basically serve as improved sag rods, right?

The greatest trick that bond stress ever pulled was convincing the world it didn't exist.

 

[hokie66]

In Australia, the purlin manufacturers have proprietary bridging systems to suit their purlins and girts.

 

[hokie66]

KootK,
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.

 

[KootK]

I'm holding firm on this one. Let's try this:

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.

 

[hokie66]

I agree that bending and buckling in light gage sections is very complex. That is why we use load tables, which are based more on full scale testing than on calculation.

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.

 

[msquared48]

The best orientation for a purlin is straight.

Mike McCann
MMC Engineering

 

[KootK]

It's a lifestyle choice.

The greatest trick that bond stress ever pulled was convincing the world it didn't exist.