Ridge beams and roof pitch 2

[Bammer25]

Silly question but do you guys take into account pitch when designing a ridge beam (no rafter ties or collar ties). I typically just look at it as a beam with tributary area halfway between rudge and bearing walls. Am I killing my customers when I have a higher pitched roof or just being conservative?

 

[ANE91]

The math will show you how much of a difference it makes for steeper roofs. I always account for it, because my spreadsheets are set up that way.

Labor is the premium, anyway. Contractor’s typically not passing off savings to the homeowner (in residential construction, where I see ridge beams most often).

Probably better to have a beefier beam, anyway, given how shoddily homes are put up in my market.

 

[RontheRedneck]

I think it's reasonable to design for more dead load on steeper roofs.

 

[Bammer25]

I guess I am saying, with a 3/12, the eye test makes it look more like a beam. On a 14/12, the eye test says they are columns and just bracing each other at the top. I always just take half the total downward dead and live load and apply it to the length of the beam as a plf. I can see how this could be an overdesign on a steep roof. I guess I could run it in STAAD and get reactions. Seems like the math would give the same vertical reactions regardless of pitch, but the horizontal would obviously be way different.

 

[jerseyshore]

We always use the full snow load (or slightly higher than code min.) and just use the plan view trib area on ridge beam.

 

[Greenalleycat]

My beam spreadsheet is setup to account for the roof pitch
Some of it comes out in the wash though

1/ Gravity loads are larger due to the inclined roof plane being a bigger area than the projected horizontal area
But when you decompose it back to the vertical/horizontal reaction the ridge beam it cancels
So it doesn't matter how you do it

2/ Our snow loadings are already calculated off a horizontal projected area, the reduction for roof pitch is handled separately
So I just use horizontal area for that

3/ Wind loading is perpendicular to the plane of the rafters so you need to calculate this properly
Roof pitch is factored in via the pressure coefficients - once you've worked those out, calculate it properly on the rafter -> ridge beam

You have to be careful if designing a rafter too as the wind load on the rafter will act on the inclined length which could be substantially longer than the projected length

My failsafe with steep pitch roofs is normally to sark them with 7mm plywood anyway (once they're up to ~35-40 degree mark or so) which removes a lot of concern anyway

 

[Eng16080]

I typically design it as though it were a horizontal beam, except that for steep pitch roofs I account for extra dead load since there's more material per foot (in plan view). Snow loads should be applied along a horizontal projection anyway so this approach works well with that. Good point above concerning wind loads, which you would need to adjust accordingly, otherwise this method would be non-conservative for wind.

In Design of Wood Structures (Breyer) there's a few pages that get into the horizontal beam versus sloped beam design approaches. I find the horizontal beam approach simpler and generally adequate for what I typically do. There is potentially a compression force acting on a sloped rafter which wouldn't be accounted for in designing it as flat. With a ridge beam, it's probably reasonable to neglect this though. For a tied roof, you would technically want to check combined bending and axial stress, although in practice, most engineers probably don't. In most cases the axial capacity likely surpasses the demand by a significant margin.

 

[XR250]

For me, 20 LL, 10 DL and trib regardless of roof pitch. The steeper the roof, the less chance of LL so it all works out!
Also, the steeper the roof, the more folded plate action takes over so I would not sweat it.

 

[Bammer25]

We always use the full snow load (or slightly higher than code min.) and just use the plan view trib area on ridge beam.

Same as me. Ok I feel better

 

[Bammer25]

At what point do you say it’s a nailer (ridge board) regardless of rafter ties? A frame homes the rafters are a serious design consideration for wind load but you can’t tell me there’s much vertical load on the ridge.

 

[jerseyshore]

At what point do you say it’s a nailer (ridge board) regardless of rafter ties? A frame homes the rafters are a serious design consideration for wind load but you can’t tell me there’s much vertical load on the ridge.

Rafter ties = nailer, no rafter ties = structural ridge. We've never designed it any other way unless it's a really low pitch and not practical to use an A-frame design.

 

[phamENG]

Once you get past 45degrees, then I would say you should starting looking more closely at the actually behavior of the structure. 14:12 probably won't be much different, but if you start getting into those super steep A frame "cabins", then yeah the beam isn't doing much.

Bammer25:

At what point do you say it’s a nailer (ridge board) regardless of rafter ties?

If there are rafter ties, it's a ridge board. Or, if the building is just a roof sitting on the ground, adequate thrust resistance at the foundation. If the roof isn't a vertical face and there's no load bearing element at the ridge, then there will be thrust. Even if there is a load bearing element, if it's too flexible you'll get some thrust. You need to either be able to accept the resulting movement or resist it.

 

[SE2607]

Silly question but do you guys take into account pitch when designing a ridge beam (no rafter ties or collar ties). I typically just look at it as a beam with tributary area halfway between ridge and bearing walls. Am I killing my customers when I have a higher pitched roof or just being conservative?

When you say "take into account", do you mean dividing the roof dead load by the cosine of the roof slope? If so, that's a 3% increase for 3:12 roofs, 12% for a 6:12 roof. If so, my answer is "yes" (in either case). My biggest concern is that, without rafter ties, collar ties or ceiling joists, the walls will bow outwards depending on the stiffness of the ridge beam. To find out how much, calculate the deflection of the ridge beam. Using geometry where the rafter length does not change appreciably, you can determine the maximum outward bow of the walls. Of course, this is restrained by the ability of the top plate to resist gain in length, but, again, you can determine the axial stresses in the top plate by determining the bowed length of the top plate (based on delta max), comparing it to the straight length and determining the axial stresses from delta = P/A. Of course, this is a conservative approach because the rafters do shorten under the compression loads which reduce the amount of bowing, but, without running a FEA on the 3D system, this approach will yield reasonable results.

 

[GeorgeTheCivilEngineer]

The pitch can introduce 15 to 25% extra dead load which can affect the long term deflection calculation. It's a simple calculation to do.