Ridge Beam Design

The link to the picture is here---

http://tuckertucker.fileave.com/Span%202%20forum%20purpose.jpg

The frame is complete, except for rafters and hip rafters.
The span of the tie beam will be 24', and the ridge beam will be 30' long.
I was wondering if anyone has used or seen this methode, (king post truss?) to support a ridge beam over this type of span. Ive taken TIMBEAL's advice on another post to extend the king struts beyond the brace, close to the ends of the tie. I have acces to large diameter Sitka Spruce so cutting large diameters on the mill would be no problem. Any comments would be appreciated, as I am a new Timber Framer and have lots to learn.
Cheers

I think in general the design is sound, but I would not proceed until a professional engineer had a crack at it, taking into account your exact species, snow loads, local codes, etc.

You'll need to check and be sure that the end distance from the strut to the end of the tie beam is sufficient for the anticipated load and species, dependent on joinery choice here.

And, as you'll be removing material here for probably a girt shoulder style joint, you'll need to be sure your tie beam is tall enough in this area to take the down pressure from the strut and the reaction from the post holding it up after you remove the material for the joinery.

I'm with Clark on this one. I've done this many times (support a structural ridge with kingpost trusses). It's one of my favorite structural approaches.

Specific to your frame, the heel joint where the top chord of the kp truss joins the bottom chord is the Achilles heel of the truss design. With such a low pitch, the thrust is magnified. It will take a lot of bearing area at that joint to transfer the load, and it will take a lot of shear plane in the relish between the bearing area and the end of the bottom chord to keep the force from splitting off the end of the bottom chord. And at that point you've taken a bit of meat out of the BC and it needs to be able to take the downforce of the truss and transfer it into the post without breakig. This is what Mike is talking about in the post above. You'll need a stout bottom chord for this reason.


But back to Clark's recommendation -- have an engineer work with you to design the connections. I always do.

I'm with Gabel, I've seen this failure more than once, apparently it was a popular truss here in southern nh for a while. Easily fixed by adding a post under the kingpost down to the floor....

Thank you for such a quick response, the detail and information was very helpful and Im exited to hear this design could work. I guess the only question I have now is at what point do you get an engineer involved?
If we consider the truss, do I need to design and finalize all the frame's plans before I can get the engineer to advise on the specifics of the truss and its joinery. Or should I draw the plans with approximate sizes pertaining to the truss and its associated members, allowing them to be altered and modified according to his results.
Im aware this might be a question for the engineer , I just want to know beforehand what to expect.
Cheers

A post under the kingpost is an option, but as the building will serve as a carport, one would benifit from the open space and not have two interior posts to navigate around. Do you know of any text or image pertaining to these old buildings you mention of southern nh. Im wondering if these failures are meant to tell me something, or just to be aware of its complexity.
cheers

You can draw it all up... but things may change a bit. The overall scheme should work - you'll just need to adjust the specifics.

Be sure to choose an engineer with TF experience. The TF Business Council has a list of recommendations.

tucker,

In my work, I send something like what you have now to my engineer. He uses sketchup, so a basic model with the right building size and shape and the pieces where i want them is all i need to work it all out with him. That is of course after I have the job. Which means, usually, that I have already priced it without his input. Not a good way to do things if you don't know what you're getting into. I have a good enough relationship with him that if something really weird comes up that I'm pricing, I'll run it by him and he will give me some input off the clock to help work out the tricky parts so I can estimate it accurately.

So if you have the job or it's your carport, now isn't too soon as long as the size and shape of the building are pretty well determined.

I completely agree with Mike on this one -- hire an engineer who knows TF.

The issue which I see is the same as all the others, the low pitch. I wonder if you flared the top of the king post getting the struts up as far as you can to the ridge, in effect increasing the pitch a little. In Historic American Roof Trusses there is an example of a 32' king post, set up the same way, with a 12/6.64 pitch roof. I can't find a pdf for it, but in the book it is on page 38, fig.4-6 In other examples metal straps were utilized.

What is the pitch on this car park?

I agree with the others about the weak link at the bottom chord and top chord joint being so close to the tenon into post.

Tucker, Have you thought about placing your bottom chords on top of the posts? You could shoot past the post with bottom chord and place a purlin out there. Plenty of room for all the forces in the truss to work themselves out.

It just so happens I designed something similar for a pick em up truck park. Mine has a slightly greater pitch but the design sits low.

Wall raising....

Very nice! Notice how the lower truss rafter lands on the chord at a point very close to the post. If the landing point of this lower rafter is within a 45 degree line across the beam from the top of the post there is actually no vertical shear. (i.e. the horizontal members at stonehenge are so deep that their entire length falls within a 45 degree line of the verticals resulting in zero bending in the very brittle stone, similar to a brick arch.. but different...) anyway.... this design is not delivering an undo bending moment in the chord... nor is it delivering an unfortunate shear load across the truss connection. Nicely done!

Thanks for all the help!
I sent what I have to an TF engineer and I will let him look at the truss design and see what he suggests. The brainstorming that takes place on this forum website is truly an asset to the guild and its members!
cheers