That looks like a birdsmouth to me. What's confusing is the board at the eave in the plane of the roof. I think it's just a 1x to conceal the lighting that points up onto the ceiling. CB.

What is the name of the book written by Mr. Chapell that explains the calculations needed for design?

I am brand new to timber framing (I have yet to make even a single cut) but am a software engineer by trade so this sound like it would be right up my alley!

A Timber Framer's Workshop

Thanks! I saw that foxmaple.com site a few days ago but I must have missed the book and I didn't make the name connection.

Is this step lap or just birdsmouth as well? Is the principal rafter joint the same as the common rafter joint?
http://www.lancotf.com/windows/kronenberg_1.html

I am very intrigued by the many ways to create a frame.

Chris

Not much detail available there.

When calculating the loads on a tie beam to post joint how do you take into account the outward thrust on the plates from the rafters if you have kingposts and a ridge that is tenoned into the kingposts for the common rafters to rest on. Does the outward thrust completely go away or is it just a certain fraction of the total thrust. The tie beams in my frame are going to be 8"X16" white oak into 8"x8" white oak posts. I am going to use a through half dovetail tenon and am trying to calculate how many pegs I need.

Any help would be appreciated

cbecker940,

You should really call an engineer, as this free carpenter's opinion will be worth what you pay for it.

first you need to know the load on the truss. Generally this is the design load for roof area multiplied times the square footage of area the truss is holding up. Say the trusses are 15' apart and span 20'. The common rafters span from the plate to the ridge and the ridge is supported by the kingpost. Assume the design load is 100 pounds per square foot (A nice round number -- it depends completely on where you are as snow loads vary drastically and I am not in your area, so don't know what value to guess)
take half the width of the truss, because the rafters are dumping half their load on the ridge and half on the plate, and multiply that times the spacing between trusses. that is 10' times 15' or 150 sq ft. multiply that times 100 psf and you get a vertical point load of 15000 lbs. Pitch is still 8 in 12? then your thrust is 12/8 or 1.5 times 15000. I think that is 22,500 lbs. in this scenario.

That is my understanding of it. I am not an engineer and I made up the load numbers, so ignore the results. Hopefully that sheds some light on it for you,though.

Feel free to correct or add to the above in any way.


I feel obliged to say that a better (and traditional) arrangement is to have the top chord land on the bottom chord and resolve the thrust in a compression joint, rather than a tension joint as you have with a dropped tie. With your suggested arrangement, you will need to make sure your post can handle being pushed on at the top by the rafter and pulled on by the tie beam -- that is not a particularly good arrangement.

There is a photo here that was posted by northern hewer which shows a failure in a dropped tie frame.

With such a large ridge beam and top plate, I wonder if you use a rafter with seat cuts at the ridge and plate. So the rafters are sitting on top of each member. I am led to believe there is no rafter thrust with this configuration. More comments....Next. Tim

Oh, also don't let the rafter peaks touch or they will thrust. Tim