28 x 44 Timber Frame/Post and Beam Pavilion

[dollar]

I am looking to build a 28 x 44 Pavilion very similar to this pavilion that we just completed.
This structure is 17' x 19' Under Roof and the OD of Posts are 15' x 17' x 10' High.

On the 28 x 44 Pavilion the Post will be 26' Wide so I need help to know if I can span with a Beam that is 6" Thick x 12" Wide x 25' 4" Length?  No center posts.
There will be four of these Gable Bents.  This will be an 8 Post design.

Mostly mechanical Fasteners. Headloc Screws and Galvanized Bolts 
Southern Yellow Pine
6/12 Pitch
8" x 8" x 15' 6" Posts
6" x 8" Ridge Beam-Scarf Joints to make 44'
6" x 6" Gable parts and corner braces
4" x 6" Rafters on 4' centers or close to it
1" x 10" Pine roof sheathing with metal roof.

Also does the 15' 6" Height of Posts cause any additional problems? I was going to make my Angle Braces long-80" on outside point.

Thank you in advance for any help and ideas.  I have already learned so much from the Forum.

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[Don P]

There are 2 ways to achieve a similar looking "kingpost truss", however they behave entirely differently.
What you have built is one of those ways. It is not a truss but it works fine if the spans and loads are fairly small. Easier construction. It is a simply supported beam with a post in the center of the span supporting a ridge beam that in turn supports a number of rafters. At 28' span, I think we can blow it out of the water right there at the tie beam. Start checking there.

Looks like the bents are 14.66'apart x half the roof width is resting on the ridge, 14'= 205 square feet
I'm going to guess wind controls live load rather than snow, 20lbs per square foot + 10psf dead load (weight of materials.

205 square feet x 30 lbs per square foot combined load =6150 lbs landing in the center of the tie.
Go here;
Midspan Point Loaded Simple Beam (forestryforum.com)
Span-336, 6x12 beam Fb850, E 1.2, Fv 165

Not even close.

For fun and my own edification lets see how big a beam it would take, and then how big a round log
A 12x18 or 16x16 in a sawn timber, or it looks like about an 18" dia round log. It takes serious meat to hold up a roof from a post in the middle of a 28 foot beam.

You need a true kingpost truss unless real beef is looking good to you. Which, that would look cool.

[dollar]

Thank you sir.

I will start right away finding a true kingpost truss and incorporating in my design.

Do you think I am ok with the four bent design for this 44' length?

With my design in the pics, can I feel comfortable in this design if I keep it to OD of Posts at a Maximum of 16' x 18' with a 6/12 Pitch with this six post design? 

I am currently marketing this very same design and have sold two additional structures so far that we are currently milling.
This structure was one of the sturdiest and seemingly strongest things I've built-even before we put the pine roof sheathing on. 
And yes Sir, we are in Georgia so no snow. 

[Don P]

My gut feeling is every bending member in the larger design is probably too light/overspanned. In other words, my gut feeling is that you used your gut feeling to size stuff?

If you get a class action on a design problem on a hundred structures, getting my okey dokey over the internet was no bargain. I did look at the picture as a one off, and, I'm not here to dive into everything I see every time, that would suck. But if you're manufacturing, do da due diligence and get a stamped design.

 All the life advice aside :. I was playing in the shop today. This is the important stuff, why does it do what it does. I like to make simple models to figure out stuff like this. In a truss every member spans from one panel point to the next, that is all. Every connection is a single pin, free to rotate.

This is a kingpost truss;


 


In the truss, the role of the kingpost is to hang down and hold a long bottom chord up. This is exactly opposite of what your build is doing. We have no load on that tie (bottom chord in truss lingo).


 

If the bottom chord is cut, the roof is not stable till the ridge is on the ground;





The bottom chord is a tension member, you can prove that in a model by replacing a stick with a string, or in real life it could be a cable, rod, chain, etc.
 

 


Proving the kingpost is also a tension member in a true kingpost truss. You can use the kingpost to camber the bottom chord.


 

The key to this is solid connections at the heeljoint and the ridge.



 

 


The kingpost gets pinched between the rafters (top chords). The harder the load pushes down the tighter the king is pinched.




I'm jacking the trusses up under the ridgebeam and then installing posts in the frame walls here but that is one way to go, put the ridgebeam above the trusses and hang common rafters from that properly sized beam. Another is to figure out how to drop the ridge between trusses and then plane the common rafters in with the trusses all in one plane.
 

 


Or you can run purlins above or dropped in on the trusses.

[dollar]

Thank you for the Advise-I'm taking it.  

I reached out today to a well known Timber Frame plan Company in the hopes of getting a Stamped Design of a King Post Truss for the two structures I have sold.  

I am hoping they will provide a Stamped Design for just the trusses but if I have to do the entire structure I will.

Tomorrow I am going to send out some emails to engineers listed on The Timber Frame Guild as well.

I am going to use the Purlins and Rafters like you have in your last picture, but if I have to purchase the entire plan to get a Stamped Design I would like to plane the Rafters in one plane with the trusses.

Any direction on where to get these stamped designs would be great and thank you.

[Don P]

Actually, I'm quite comfortable building roofs. Holding that sail up on legs is where I'm uncomfortable. I would be asking for the design from ground up. To be honest I've never thought much about it, or whether an engineer would be comfortable with it, it is not a one off. Uncharted waters, I'm curious what you find out.

[dollar]

This company would only design the entire structure from the ground up like you suggested.

They have a rack plan that will work for my size.  I am going to purchase it and go from there.

It will not be a Stamped Design.

They are 8 weeks out on this service and its a significant cost.

[Don P]

I wonder if that would be acceptable to the building department, everyone involved's insurance, etc. Just thinking through it like a product liability issue. This is really busting my desired weekend hobby money plans  :D. For instance in my case I think the BO would be fine with my design on something like this. Under 256sf accessory structures here he doesn't inspect, that can be as low as 150 sf other states. My insurance company on the other hand would want a stamp I'd bet to cover their liability if I am manufacturing product. Anyway, back to the flowers on your wall, see what those folks need to make everyone happy? Is all that over the top, I don't know anymore.

[mapleack]

This company would only design the entire structure from the ground up like you suggested.

They have a rack plan that will work for my size.  I am going to purchase it and go from there.

It will not be a Stamped Design.

They are 8 weeks out on this service and its a significant cost

If it were me and there was enough demand in your area to sell multiple gazebos / pavilions I'd get sealed plans for each size offered.   Offer cosmetic customizations but build the structure to the sealed plan.  Any future structural liabilities wouldn't be yours to carry alone.  If a customer wants a custom structural change include the cost for custom sealed plans in the bid.