Sawmill Shelter

[chet]

The outside top cord is 2x12, the inside top cord is a 2x10 and the 2 bottom ones are 2x8's. The cross braces are 2x6's, all are full dimention Red Pine. I'm spanning 20' 8 "

[brdmkr]

Thanks Chet!

[Don P]

Let's see if I can really confuse this. Chet's is a parallell chord truss. One more critical dimension is the truss depth.
I drew a couple on the Johns Hopkins truss calc to show how the internal loads on the members change as the truss is built shallower. The connections and the members get better the deeper you can make it.

Its structurally a kingpost truss, the same load is on both, 6000 lbs, only the depth changes





I'm still looking Warren, haven't found it yet and must be having oldtimers. If anyone can point me back to that link I'd appreciate it. I did find a section on a nail/glue built up beam with plywood and discontinuous chords. I'm still reading it but basically the lumber chords are designed to handle the bending loads, the plywood takes the shear loads. Butt joints must be staggered 50 thicknesses apart and the bending is calculated based on what looks like 80% of half the lumber size. I think its all there but I'm still trying to wrap my head around it. If anyone wants a copy of that pm me your email.

[Don_Papenburg]

If you plan on cutting a 20' log then you want a door that is 21or 22' wide .  One thing I dislike is twisting  things into a opening that is just right to silightly small.

[BBTom]

Don,  Chet has a cross on each side.  Wouldn't that other cross piece take a portion of the load?

[Don P]

Yes I saw them, I wasn't sure whether to call them additional or gravy so I opted for gravy. I wouldn't count on them taking load the same way so wouldn't calculate member or connection sizes on them as "helpers". Bridge designers used to call them "counters". My fear when trying to add things that "share" a load is if I'm wrong one item after another fails, each incapable of taking the entire load. I do like them in there though, they are also forming a kingpost truss, one in tension, here's a pic, same loads. The angled webs there are in tension, steel takes tension real well. You'll see overstressed trusses and beams "bowstringed" sometimes like this. The same depth concerns come into it, deeper is cheaper if possible. Notice the stresses are the same the "sense" reversed, compression is now tension, tension members are now in compression but the magnitudes remain the same. Counters are good to have if there is a stress reversal, on a bridge that can be a rolling load on a roof a big wind maybe.




While I had the program open I drew a brace and put a 100 pound load on it to show the axial load down the brace, notice the axial load goes up to 141 lbs, this is another example of something we talked about a few months ago;