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Author Topic: Ridge beam needed with 8x10 top plate beam  (Read 556 times)

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Offline northernlights

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Ridge beam needed with 8x10 top plate beam
« on: August 23, 2019, 10:34:21 PM »
Hi,

First time posting. I am designing a post and beam front entry to a house. This will be a gable entry with a simple frame that has (2) 8x10 top plate beams projecting from and perpendicular to the house. one side of each of these beams will be nested in a beam pocket in the exterior wall of the house and the other side of these beams (each one) will sit on an 8x8 column. Between and perpendicular to these beams will be an 8x10 collar tie beam butting into each top plate beam (above the 8x8 columns) (all beams will be at the same height). Above these beams and running paralell to the top plate beams will be an 8x10 ridge beam. The ridge beam and top plate beams will support regular 2x6 rafters. The ridge beam will also be nested in a pocket in the exterior house wall on one side and will sit on a post that sits on the centered on the

 collar tie beam on the other end.

The more I think about this the more I wonder if a ridge beam is even needed. Are the 8x10 top plates adequate in resisting the rafter thrust, such that only a ridge board is needed? interested in general thoughts, equations to calculate thrust etc.

I have attached a picture that basically depicts what I am doing. there will be no struts and there will only be 2 columns.


Online Don P

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Re: Ridge beam needed with 8x10 top plate beam
« Reply #1 on: August 24, 2019, 09:07:16 AM »
Hey northernlights, welcome.
Just general thoughts. That can work, you'll need to turn your brain sort of sideways, hmm, at different pitches :D. I'm just a carpenter so this is worth exactly what you are paying for it. If you parse the axial force of the rafters into their vertical and horizontal components, the 8x8 columns support the vertical load. You also need very good connections between the ties and plates to restrain the horizontal thrust, steel tension rods embedded in the top side of the ties and over the plates would accomplish that, I've also seen large L brackets lagged down to both timbers. The toolbox on the lower left column has my calcs in it, there are a couple of thrust calcs and multiple beam calcs, I'm not sure anything covers exactly what you are proposing, thoughts below;

The 8x10 plates are bending in both planes, from gravity vertically and thrust horizontally. It would probably be more conservative to look into the section modulus of a rotated rectangle and apply the rafters axial load to that section rather than simply checking the plates in each direction independently, make sense? If so that rotated section is a little smaller than the rectangular section viewed from either 90░ direction and the axial load coming down the rafter and hitting it is higher than either parsed load. This might take a little more thinking.

The pic posted is picking up lateral restraint from the roof and plates tied back into the building. That might be unconservative all depending. Some bracing on those outboard posts wouldn't be a bad thing.
The future is a foreign country, they will do things differently there - Simon Winchester


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