Timber frame barn span design

[Gfeucht]

Hello. New to the site but have been reading lots of posts and info. I have been preparing for a timber frame barn with a sawmill that I have and have cut some timbers and have been air drying for a while. The span is 38 ft wide x 90 ft long. Looking to do a clear span in the "middle area"  with a king truss design with a 8/12 pitch. The timbers are 7" x 14" x 23 ft long for the design but have been looking for some help with the specifics. Most of the posts/forums of info I run into are smaller spans. No issue down south here in Louisiana with snow loads and no ceiling loads in the barn will be done. Roof is to be with cross purlins and bead board pine as ceiling with metal roof. Was thinking 10 to 12 ft bays of truss spacing. Lots of info here but wanted to throw this out there.  Barn will be enclosed with walls on ends and sides with 2 x 6 framing minus 2 open 12 ft openings at end for boat/tractor. Any help would be great. Thanks. 

[Don P]

Welcome to the forum.
Are you saying you want to design a kingpost truss that spans 23'? ... There are lean to's for the rest of the 38? Sketch please

But, yes 23' is fine for a kingpost truss.

[Gfeucht]

The span of 38 ft is for the "king truss". There will be no lean to involved    So I would have 2 bottom cords at 19 ft each connecting in middle at the bottom of this truss type design. Not sure how to draw sketch on here? help.  Thank you. 

[Don P]

Gotcha,
This is ~24' with a 2 piece bottom chord splined through the kingpost.


 

The spline was black gum for the interlocked grain, elm might be another. If things start to split or shear there, and it is a high tension joint, you want it where you see it, not in the spline. You have 2 shoulders and one spline, choose good spline stock.

The heel has equally high tension creating a shear problem (in red), and a localized crushing check on the bearing faces of the joint;


 

This is one way to restrain that heel thrust as the span and thrust increase, the steel angle iron stops are welded to a strap with enough bolts at spacing to develop tensile strength. With what you are doing I would plow a trench for a steel rod from this angle through the king and the far angle. Thread the ends and tune the truss. And do build with some camber in the bottom chord.


 

[Don P]

This is crude but a starting point.
Untitled

[Gfeucht]

Thank you.  I have cut all southern yellow pine for the project. Is this ok?  Also is the size of the timbers sufficient for this span? I have 7 x 14 x 23.   Did you have a calculation for sizing somewhere in this forum? I read that somewhere. Sorry for all the questions. Thank you.

[Don P]

Click the untitled link above and let's walk and talk;
Total load on the truss, wind rather than snow controls, minimum 20 lbs per square foot + dead load (material weight) of 10 psf = 30 psf load.

How many square feet are loading the truss, the tributary area..
38' span x 12' spacing =456 sf
area x load=30psf x 456sf=13,680lbs total load on each truss

From there fill in the particulars, span is 456", pitch is 8, species is #2 SYP, click "show result" and it should fill in. I plugged in 7x7 web members

The shearing force on the truss heel is about 7600 lbs, notice the compression in the joints, they need enough area to resist that. This is where I would use steel for the bottom chord and the wood there is mostly decoration and stabilizing the steel chord.

It's looking buildable to me. The combined axial and bending load is only using 46% of top chord capacity, nothing is wanting to buckle. The wood is sufficient, the joinery holding those forces together is the key.

I'm not sensing an understanding at this point. Do not rush or push until that happens. This is the deep end of the pool and your first posts. What you are creating is potentially a large deadfall trap. If you do not thoroughly understand what you are doing this will collapse on everything underneath. What I have done is shown that it probably can be built, what I don't know is whether you should be building it. The final design should be checked by an engineer and there should be some real honest introspection before you commit. None of that is meant in a mean or bad way. I want us all to make it home at the end of every day.

[Gfeucht]

thank you. been working on this for a couple of years cutting timber and air drying. Will try to get a sketch of my design and send over.  Problem I have is finding an engineer in this field around here. Any list of engineers would help . Thanks a bunch.

[Don P]

There is a list of engineers on the TF Guild website @ Jim_Rogers may have a better link. A recent project here was done by FireTower.

[Jim_Rogers]

Look here:
Find An Engineer | Timber Framers Guild (tfguild.org)

There are four listed for your state.
As mentioned, I work with Firetower, but all the others are good as well.

Jim Rogers