Ridge Pole and Purlin Loads

[aaroncgi]

Hi All,


I have some hopefully quick questions about loads on roof members.

I understand that if you have a structural ridge pole and your rafters clear span between the ridge pole and the plate logs on the walls, then your ridge pole is supporting 50% of the roof, and your plate logs support the other 50% (or 25% the weight on each wall). That seems clear enough.

What happens if you start adding intermediate purlins between the ridge pole and plate logs?

For example, say you add a single purlin on each side, halfway between the plate and ridge. Does your ridge pole still support half the roof, while the remaining half is split equally between the purlins and plate logs (aka 12.5% each). Or, is the load split evenly between all five members - 20% to each? Or something more convoluted?


Cheers!

[rocklock]

I understand that if you have a structural ridge pole and your rafters clear span between the ridge pole and the plate logs on the walls, then your ridge pole is supporting 50% of the roof, and your plate logs support the other 50% (or 25% the weight on each wall). That seems clear enough.

I don't think its that simple...If I remember some of my engineering courses about forces and stuff, but then again its been 40 years... Which is why a wet stamp is required on our plans...

[mudflap]

You have to do some vector calculations, along with figuring the weight of each purlin. The angle of the forces butting against the ridge pole vs the forces of a relatively non-loaded center purlin add some complexity as well.

My old math professor, when asked "what could you do with a Phd in math besides teach?" said he had a friend in California that worked for the highway department making $400/hr "thinking" about highway bridge design. Now THERE'S a government job for ya...

blog: https://loghomejourney.wordpress.com/

[BigD]

My faulty logic tells me that if adding purlins decreased the load on the ridge and plate logs, then all one would have to do is keep adding enough purlins before their log home started to hover. i.e. adding purlins won't change the load. What the purlins would do is to help with the loads, mid span.....but the same weight is going to be on the ridge and plate logs.
This is what I remember from my engineering classes (simple Free Body Diagrams).....but I could be off base, entirely.

[Mountain Lion]

Let's say you add 2 purlins on each side. If you set them low such that the rafters don't touch, clearly the ridge and cap continue to carry all the load. If you set them high enough, you can get them to take all the rafter load off the ridge and cap. If you do the normal thing and put tops of ridge, cap and purlins on a plane, the load will distribute. Math left as exercise for the reader

Purlin load is carried by gable walls and purlin support vertical logs.


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[rreidnauer]

It isn't split equally exactly. It's the area of tributary width, with tributary width being half the span between two supporting members. So:
Ridge pole carries half span to each purlin.
Each purlin carries half span to Ridge pole and to cap log.
Each cap log carries half span to purlin.

A standard ridge pole/cap log arrangement is 25%-50%-25%.
With purlins, it's 12.5%-25%-25%-25%-12.5%.
As you can see, the cap logs always carry less load being they only have a single side of tributary width. (in theory, as they still need to address the weight on the cantilevered eves)

[Arrowman]

Hi All,


I have some hopefully quick questions about loads on roof members.

I understand that if you have a structural ridge pole and your rafters clear span between the ridge pole and the plate logs on the walls, then your ridge pole is supporting 50% of the roof, and your plate logs support the other 50% (or 25% the weight on each wall). That seems clear enough.

What happens if you start adding intermediate purlins between the ridge pole and plate logs?

For example, say you add a single purlin on each side, halfway between the plate and ridge. Does your ridge pole still support half the roof, while the remaining half is split equally between the purlins and plate logs (aka 12.5% each). Or, is the load split evenly between all five members - 20% to each? Or something more convoluted?


Cheers!

Depending on what you are trying to use for a ridge pole, purlins are primarily used to span longer lengths without having to go to extremely large rafter sizes. I'm not sure I would design a house using purlins to meet a load requirement on a smaller ridge pole. Put another way, I would two piece a ridge pole before I butchered my floor plan by dropping purlin supports into it. You may be able to design a floor plan that integrates the purlin supports, but good house design is tough enough dropping three RPSLs into the mix, not to mention designing around an addition 2-6 purlin support point loads.

[Mountain Lion]

Intuitively thinking, the cantilevered weight hanging over the cap log (eaves) will first unload the purlin and then eventually the ridgepole. This depending on the rigidity of the rafters and the rest of the roof system. The unlikely case of a perfectly rigid roof is an exception where the ridgepole would unload at twice the rate of the purlin. I suspect that any math done on this will make a lot of assumptions (which will be close enough, given our propensity to overbuild).

It isn't split equally exactly. It's the area of tributary width, with tributary width being half the span between two supporting members. So:
Ridge pole carries half span to each purlin.
Each purlin carries half span to Ridge pole and to cap log.
Each cap log carries half span to purlin.

A standard ridge pole/cap log arrangement is 25%-50%-25%.
With purlins, it's 12.5%-25%-25%-25%-12.5%.
As you can see, the cap logs always carry less load being they only have a single side of tributary width. (in theory, as they still need to address the weight on the cantilevered eves)

[rreidnauer]

You fail to take into account the area of load on the interior side of the cap logs far exceeds that of the eaves. There cannot be the uplift you describe. Any forces associated to uplift end when your interior side tributary width exceeds the eve cantilever distance.

However eve overhangs do, as I previously mentioned, increase bearing to cap logs, that the span and live load calculations do not take into account when designing a roof.

[Mountain Lion]

I'll be the first to admit that I could be thinking this wrong.

However, I mentioned "unload", not "uplift". I meant that the forces on the non-cantilevered portion of the roof are reduced by the cantilever weight. You can think of this in the limit. There is a force that you can apply on the lever of the cantilevered roof that will lift the rest of the roof. When you apply that force, there will be no force on the ridgepole or purlin. When you apply no force on the cantilever, the ridgepole and purlin will be fully loaded. At all intermediate cantilever force levels, the ridgepole and purlin will be partially loaded and there should be a continuous function that describes the relationship. This is the case for a perfectly rigid roof structure. Flexibility of the roof structure does not negate the effect of the fulcrum, only where the loads ultimately fall.

This is how I thought of it, but like I said, I could be all wrong.