I'm not even sure what You are building from Your description...

My first thought was a roof for a porch, or pole barn, but why would You want connections that rotate. Wouldn't it be difficult to make a fold away temporary use structure with 2 X 8 lumber?

More dimensions are needed to determine what kind of structure You are designing, pictures also do wonders.

Yes, it is a porch roof.

Because classical mechanics views the assembly as an inderterminate one. Eliminating the moment, by providing a pinned connection, simplifies the solution.

You will need to put your collar ties at 32" oc., with glued and screwed 3/4" plywood gussets. Because of this you will be best with 'nominal' sized lumber, butted at an angle and gusseted both sides. This will be in tension. The alternate is to add the collar ties after the rafters are in place, lapped and glued, using 16D common nails (screws do not have enough sheer strength). Use the roof pitch to cut the ends of the ties for maximum surface and nailing. 2X6 nominal lumber will be sufficient. You must support the ridge beam until the ties are in place and the glue is set, otherwise the roof will collapse. If the ceiling is to be finished, you might as well use 2x4s at each rafter, same method.

Good luck

Good morning Tippycanoe.

I believe the client would sooner opt for the more rustic look of steel gussets. Other than my drawing up a custom design of say 3/16" plate, do you have any comment?

There will be no ceiling nor joists.

3/16" (?) I've used 1/4" plate gussets with rough sawn on a significantly bigger roof. They are unobtrusive, but look 'authentic', and within context when you do see them. Use square headed bolts, washers, and nuts if you can get them. Distress the edges, apply stove black, and you have 'the look'. Yours should be sandwitched between the rafter and tie.

Regards, Carl

Good morning Carl.

Excellent suggestions. However, I could not get the assembly to calc out unless I went for 4x8 rafters at the collar ties. Since this produces surfaces of the rafter and collar tie in different planes, I cannot use gussets.

I will recommend the use of black square headed bolts and nuts tho.

Hello FH,

Yeah, sometimes you have to 'trick' a cad program. I save what I have, copy, and rework the calcs so the program 'thinks' contact is made. Print a copy of the new calcs, so the client and building inspector know what you've done.

Actually, If you're sure the moisture content of you're lumber is below 14%, the joints could be epoxied, eliminating the gusset. Faying surfaces need to be freshened with coarse sandpaper. The joint then becomes stronger than the wood.

Stay with it. It's part of the fun!

flyinghigh,

Can you provide a cross section of the rafters and collar ties. If you have ceiling joists, please indicate.

I have found that the term "collar tie" as used by framers is frequently incorrect. In many instances, the collar tie is a strut (taking compression). Give us a sketch if you don't mind.

Good morning ba/ael.

I have not yet figured out how to make a drawing on CR4. A collar tie is a miniature joist, usually placed in addition to joists, and aligned horizontally connecting 2 rafters, just under the ridge beam.

So if you look at Cyberfool's sketch, with the rafter notched for the wall plate, and in your mind add a horizontal collar tie halfway vertically between the ridge beam and the top plate, without joists, you'll have the design the customer wants.

In this design, without joists, the collar ties are taking all the load, preventing the rafters from splaying out flat; therefore they are in tension, and some moment.

Quote: I have not yet figured out how to make a drawing on CR4.

I used my "Paint" program on my Computer to make the drawing, to insert it, click on the thing that looks like a small Camera in the message editing window, and search for where you put the file.

Quote: So if you look at Cyberfool's sketch, with the rafter notched for the wall plate, and in your mind add a horizontal collar tie halfway vertically between the ridge beam and the top plate, without joists, you'll have the design the customer wants.

Like this kind of bracket?

I just wish CR4 didn't shrink my drawings, oh sure they allow you to resize them up, but when you make them bigger, they get fuzzy.

Good morning Cyberfool. Thx. for the tip. I will try that the next time.

The moment is the controlling factor. Not the moment in the collar tie, but the moment in the tied rafter. Say the uniform roof load is w psf and the overall span of roof is L. The simple span moment is wL²/8. If you put a collar tie every foot, the tension in the tie would be wL²/(8*3.1). Since you want to use a collar tie every 4', the tension in the collar tie will be 4wL²/(8*3.1) = wL²/6.2.

Assuming w as 15 psf dead load plus 25 psf live load = 40 psf and L as 2*10.6 = 21.2, the tension in the tie will be 40(21.2)²/6.2 = 2900# (a horizontal force).

The bending moment in each tied rafter is Pab/L = 2900*3.1*2.9/6.0 = 4345'#.

The section modulus of the rafter is 1.5*7.25²/6 = 13.14 in³. The maximum fiber stress is 4345*12/13.14 = 3970 psi. That is too much, so the program is telling you the rafter is overstressed in bending.

Thx Bruce, GA.

I followed everything you detailed. I just want to confirm that in your 3rd paragraph, about the Moment in the tied rafter, the ratio 2.9/6 is the fraction of the maximum moment that occurs at the midpoint of the 20.2' L, based on the vertical distance from the wall top plate to the point at which the collar tie is exerting its force, iow, 2.9 ft up of a total distance of 6 ft. Correct?

Thx again.

flying high,

Consider a beam 6' long with a load P acting 2.9' from one end (and hence 3.1' from the other). The moment is Pab/L or P*2.9*3.1/6.0. In this case, the rafter is longer than 6', in fact it has a length of about 12.2'. Because the force is horizontal, we don't have to consider the sloping length of rafter, just the vertical length. Neglecting the 16" width of load on the tied rafter, the moment in the rafter varies from 0 at the top of wall to maximum at the tie, then back to 0 at the ridge line.

Got it. Thx for your expertise.

flyinghigh,

You're welcome. How are things in Arizona? I just heard on the TV that we are going to get 12 cm of snow tomorrow. What a ripoff...and just when everything was greening up.

It could be worse, You could be getting 12 inches.

Can't argue with that.