Timber-sized rafters

[PA_Walnut]

I am building a large(ish) drying shed. Single slope roof, wanna go 20' wide. I got (paid) paid plans for a Timberframe, but decided to use steel beams in front so I could make big clear-span. Am going to saw rafters out of some nice/clear Eastern White pine I have.  Plans call for 5-1/2x7-1/2 rafters for the span.

Would thinner and taller rafters work for that kinda span also? Something like 2.5x10's? The pitch will be significant and covered with metal panels, so my primary concern is snow load.

Thanks

[Don P]

Do the plans specify what snow load they designed for? A pet peeve of mine, folks say follow the plan but if the plan was drawn by a numbnut who didn't specify snow load, species and grade they might as well have been sketched on a napkin by the kid asking if you want fries with that :D. So first question, what is your snow load?

Next, section modulus of a 5.5x7.5 is 51.56 in³
S_xx of a 2.5x9.25 is 35.65 in³
S_xx of a 2.5 x 11.25 is 52.73
of a 3.5 x 9.25 is 49.91

Section modulus=bd²/6

[PA_Walnut]

Thanks @Don P . They came from Timberframe HQ. I bought their plan in order to gain ideas, and better understanding for a TF structure I am gearing up to. Their fine print says that it's for "design use only..check with your local dept...blah, blah..."

I have some REALLY nice, big, 22' EWP that I am sawing into cants to get ready. Some will make 16x16's, maybe 20x20's.   (not that I NEED them, but they could).

My snow load is 30psf, I plan to use EWP and saw it myself. Not stamped (not required) and not inspected. I just want a roof that will not cave-in on my projected 30,000bf+ it will hold. I supposed the pretty steep pitch/slope of the single-slope roof will be a great help in not having snow piling up too much.

That plan I was casually referencing as a starting point, has them on 3' centers. My intent is to use EWP on top of the rafters also as a mounting point for metal. I could be persuaded to do something else if better. 

Any help or informed advice and size and placement is welcomed and appreciated.




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[Don P]

Assuming #1 EWP, I posted a grading card on Jeff's build thread in the first coupla pages, no knot whorls. design for the day the snow sticks.

Let's try a couple of spacing scenarios

First up 3' centers x 20' = 60 sf x 40 psf (30 snow+10 dead)=2400 lbs, 600 lbs is dead load. Using the initial 5.5 x 7.5" timbers
https://forestryforum.com/members/donp/beamclc06b.htm
Fail

Tighten up to 2' centers 2'x 20' x 40psf=1600 lbs, 400DL
Hit it again, slight fail, bump the depth up to 8" and it passes.

When you drop to 2-4" thick material use this calc, there are different, better, design values for smaller members.
https://forestryforum.com/members/donp/ddsimplebeam.html
Trying 2.5"x 11.25" members at 3' centers, snow duration, no on repetitive members (you can only click yes when they get to 2' centers or tighter) and it fails.

Tighten up to 2' spacing, yes on repetitive spacing, 3 or more, and it passes. so there's a couple of ways forward.

[PA_Walnut]

Great info. Thanks! I am just pouring slab, so have some time to figure this out.
I can move the front in 2-3 feet and overhang the roof in front, thus creating a span of more like 17-18'.

What is the repetitive spacing issue? Please explain or point me.
Thanks!

[Don P]

Don't forget the overhang is bearing on the beam, or rather, don't forget like I did to add the load from an overhang to the load on the beam :D. Recheck all that with your actual configuration.

Before pouring quantify the post loads and size the footings under those areas accordingly. Most of the time barring soil testing 2,000 psf bearing capacity for the soil is a safe presumption unless it is loose, boggy, etc. Figure out how many square feet of footing is needed and then assure that the footing depth is sufficient to draw a load path through the concrete at a 45 degree or steeper line between post and footing edge. For instance under a heavy chimney I might have a broad, thick, heavily reinforced footing to distribute the load over enough soil to prevent the chimney from settling more that the rest of the building. On heavily loaded posts the same thing may be going on. What the building code says is trace your load paths safely all the way to ground.

Repetitive member factor. When light framed members (2-4" thick) are spaced no more than 2' apart, (spaced rafters or joists or built up beam members) and there is a load bearing deck over them helping to distribute the load across several members at a time. Then the effect of a strength limiting defect at one place in one of those members is reduced, the multiple members by being closely spaced are considered to be helping one another. A 15% strength increase is factored into the bending strength equation in the calc when you click that button. In a seminar the engineer with awc and an engineering professor agreed that when 2 or more members are together such as in a built up 2 ply beam a 10% increase is appropriate, that is the middle choice in that button on the dimensional lumber calc. Since heavy timber is spaced wider that factor is not used there, each one stands alone. as an aside, when people say heavy timber is stronger, well, that redundancy increase in light frame is real and is not a bad thing.

[PA_Walnut]

Footings at the load points have 24" depth added and additional rebar layers, adding 1,000psi per location. Additionally poured 4,000 psi footings to give extra strength.

Interesting info on the repetitive member factor. Without extensive engineering info, but good sense and research, I clear-spanned my 20' kiln opening with oak 2x12's laminated 4 thick. (netting an 8x12). It sagged 1/8" in the middle, which I accounted for with my kiln door design and seals and has been holding up well.

Looks like 2' spacing is my path forward. 2.5x11.25 passes, if I am reading your note right?

Thanks.

[Don P]

Yup, you're reading right on the rafter size.
Substantial overkill on the footings, but nothing wrong with that. 4,000 psi concrete is capable of a compressive load of 576,000 lbs per square foot bearing on soil presumed to be capable of 2,000 lbs per square foot. A big snowshoe helps spread your load but it doesn't necessarily have to be a titanium snowshoe. But it should be there for awhile.

[PA_Walnut]

Great...I think I have a course plot for sawing now.
Footings are in and slab pour happening this week.

Next issue for me to grind upon: I am spanning 65' in front with two 10x7" I beams. Post on either end and one in the middle. I intend to fab-up a connector for the beams as in one of the below pics. And fab connectors for the posts to be bolted to the floor, similar to pic. I've saw 10x10 oak beams, but may reduce to 10x7" to match steel. I'll sit the post on some kinda synthetic sill seal or similar to keep off the floor, and wicking-up moisture.

Thoughts or concerns? Thx.

[Don P]

My main concern would be wind bracing and then post side load capacity if the braces do drive into them. Have you considered steel posts and connections on that side?

[PA_Walnut]

UPDATE: I have the 2.5x12's sawn and the slab is done.

On the connectors for the beam to floor. My fabricator whipped up a drawing (yes, he's not VanGogh or anything). Gonna increase the size of it, per Don's concern to help address lateral side load, making the 48" tall side to side. Not sure if I am going to lag into them or through-bolt. They will be attached to floor via Hilti Anchors and epoxy.

Also considering increase posts to 12x12 instead of 10x10. Again, I intend to fab brackets (also on drawing) to slip into a slot top of post and then through bolt them.

Any thoughts about this? Concerns? Ready to go up in a week or so, once I work on the details. Much thanks!

[PA_Walnut]

Calling @Don P Calling @Don P  ;D :D
Any thoughts on these brackets addressing the side-load issue(s)?
Fabricator is starting metal work today...these brackets plus beefy joist hangers.
Thx!

[Don P]

 :D I think the base is fine, I'd bore some holes or leave some drainage notches so it isn't a water catcher at the base. The top I'm trying to think of ways to get some bracing out of it without going too far. This was a quick thought last nite playing on a sketch I had open but not loving it yet. 


 

[PA_Walnut]

@Don P thanks for your efforts! Now I see what you mean.
I will extend out the flange on the piece that slips into the top of the post and then weld it to the steel, per your design...I like this thinking.

You feeling a 10x10 or 12x12 post would be good? I have a beast log up on the mill right now to finish up. Wrestled it yesterday for WAY too long...so heavy, the arms would not lift it, the claw would not turn it. Then, the obligatory metal to further then pain.  >:( :-\

Thanks!

[PA_Walnut]

@Don P finally got my oak posts sawed...what a wrestling match.  :(
BIG log...and HEAVY. (will snap a pic) It's on sticks and drying a bit before I start going vertical.

The pine 2.5x12's are stickered and drying ala a large fan. So, soon ready!!  8)

[addicted]

PaWalnut
 Great looking log on the mill. Sorry to hear about the metal. What tool are you using to pull what looks like fencing out of the log in the picture.
Rusty

[PA_Walnut]

PaWalnut
Great looking log on the mill. Sorry to hear about the metal. What tool are you using to pull what looks like fencing out of the log in the picture.
Rusty

Addicted, that tool is a Crescent nail puller....works like a champ. As long as you can grab the metal with the pliers, you rock forward onto the crescent for leverage and pulls it right out.
If you Google for " CODE RED Crescent" you will find it. 

Thanks for the kudos on that log. It was a BEAST. Hydraulics laughed at me when I tried to lift it and/or roll it with the claw. Not even close. Got'er done though!