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Clear Span Roof Truss Calculator

Started by Sedgehammer, February 05, 2019, 12:00:58 PM

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Sedgehammer

Kind like this





This way don't need to double up on joists & timbers

and spacing I think is 60" based on 20' with 3 trusses
Necessity is the engine of drive

Don P

The section where you have the arrow that says 12 gauge will buckle from the roof load, it can't handle the compression. That is where the structural 2x12's and any 2x spacing you need go, which renders that flat 2x6 and welding up top unnecessary... more good for less work and cost. the 3x12's under that are not a bad thing atall, they stiffen the plate but are part of the trim.

Sedgehammer

I gotcha.

Could we use 2, 2x8's instead of a 2x12 and a 2x4? Part of the reason is the 2x8 can go right out for the rafter tail

Also, what does eastern red cedar compare to in using a span calculation 

Thanks
Necessity is the engine of drive

Don P

No it has to be a pair of nice 2x12's, rip the 2x12 down to 2x8 as it passes over the wall, make sure there are no other defects in that ripped down area.
ERC is not really a good structural wood which is why you don't see it listed in span tables and in the design value tables. It is very nice for paneling and trim.

Sedgehammer

🤔 won't the metal also be picking up some of the load since it's sammiched and can't buckle?

It might not matter, just not sure what I'll be using fir rafter tails yet. Waiting on the RCDF sample  

On the erc (eastern red cedar), before you posted i remembered about the wood website wood database and it lists it stronger than western red cedar https://www.wood-database.com/western-red-cedar/  and https://www.wood-database.com/aromatic-red-cedar/

With that said,  can it be used for porch rafters. 2x8x10' (1.5x7.5 actual) with a 8' span, 2' tail 2' oc. Will have the 2x8 t&g fir decking. Metal roof. 
Necessity is the engine of drive

Don P

I'm using the KISS method there, 2x12's work, 2x8's is an engineer's playground.

Wood database is reprinting data from the USFPL's "Wood Handbook". Those are ultimate average strengths based on break tests of small clear samples. Go carefully trying to apply that to allowables. WRC has different growth characteristics and so a higher likelihood of getting higher grade material in the field. That said, for small short spans, pay attention to grade. ERC is slightly stronger than northern white cedar which is in the drop down calc in the toolbox.




Sedgehammer

I hear ya on ther KISS method.

ERC passes at 1.5x7.5x8' actual at 15lbs per sq ft = 240 lbs.

Would you have an issue using ERC 1.5x7.5x4', 21" (exposed) for the rafter tails?

I can get it cheaper than the WRC and don't know how the RCDF will stain, as to match all the porches using ERC.

Don't really want to use RCDF fir the porches due to shrinkage, but if I dried it first and it stained well, it's a duh.
Necessity is the engine of drive

Don P

Run it again. We've been using 30 psf for total roof load 20LL+10DL Even if snow is below 20 psf wind then controls, never use less than 20psf Live Load, about never use less than 10 psf Dead Load. IF you do a skip sheathing and tin pole barn maybe, but even then I don't. Be conservative.

Not really a problem IMO on the tails however the usual rule of thumb on something like that is 2/3 inboard 1/3 outboard and nail the heck out of it.
The real issue is going to be finding enough 2x8 ERC that makes #2 unless there are much better trees out there than here, it is a small very knotty tree to me, most would be in the #3 category, porch roof sheathing maybe.

The shrinkage on a 2x8 is not a problem IMO... really if it's stickered under tin by the time you get to framing porches it'll be dry. West coast framers have put up a lot of green DF.

The poplar I'm sawing for framing now, well, whenever this ice and sleet stops, I can about guarantee by the time he gets the site and foundation ready its going to be mighty close to lumber store KD. We'll plane and straight line it right before use. The heavy timber is the slow poke and if anything, on something like an exposed heavy timber porch post, I'll put an oil type finish on it to slow down the surface drying to try to reduce checking in the sun and wind. That outer shell is drying and shrinking way faster than the fat green core, it wants to pop its buttons.

I saw a comment you made on borate and I'll give you a heads up there. If I treat green right off the saw, above fiber saturation point, when you can see the wet in the wood, Timbor or Solubor gives penetration as deep as Bora-care or Solubor + antifreeze. If the wood is dry, below FSP, then use Bora-care or add glycol to Solubor. The only difference there is Bora-care has ethylene glycol, antifreeze, in it, glycol dries very slowly, borate diffuses in on the wet.

Sedgehammer

Yeah, I messed up. The 15 lbs psf was the ceiling load. It passes at 30 lbs psf

Ok, so  5' one is pretty close. 

Can get 12x12 timbers here.

🤔🤔🤔  So maybe only dry the 3x timbers and just air dry the 2x8 stuff. That's a thought, would there be a price break for getting them kiln dried or not? They quoted $0.35 a BF.

So would you oil coat the big stuff as soon as we get it to slow drying?

Gotcha on the Timbor. I'm thinking that once we get the wood, we'll coat it as we unload what isn't getting kilned

Speaking of big stuff, have you used big timbers for door framing on a stick frame and if so, how'd you do the timbers and the door?
Necessity is the engine of drive

Don P

There would be a price increase for having someone kiln dry, not sure I'm understanding the question, either way .35 for material or drying sounds too cheap.

I've used Seasonite in the past with really good results, it came from Flood back then, it looks to be a Spanish company now. Other than that I've used boiled linseed oil or even just a first coat of the final finish if breathable with the understanding it will fail quickly on green wood. Never use Sikkens or a film forming finish on green wood. Borate before any finish. Price Solubor at the local real farm supply against Timbor, they are chemically identical down to the same CAS number.

I can't recall setting a door directly in between large vertical timbers although, probably have, lotsa houses and oldtimers :D. We talked about shrinkage movement and how to do it earlier.

Don P

Thinking about what I think you are asking re the door. If you are proposing having the timbers extend through the wall with the stick framing infilling between timbers, that is a very tough detail. As the timbers shrink it is difficult to maintain air seal. What I've seen happen is warmer moist air begins to pass out through the shrinkage gap, hits the cold dew point in the gap and drops its moisture in that zone between timber and stud, rot begins. Try to detail with the timbers either inside or outside of the envelope.

Sedgehammer

Yeah, been thinking the same thing. I was just tossing the idea around and was coming up with the same reasons why it won't work.

On those timbers used inside, would you rabbit them at floor level to tuck tile under them or would you just run the tile and other flooring up to and then trim on top of the flooring to cover any gap?

The $0.35 is the price per board ft to dry the df, so is that cheap enough that you'd dry the 2x8 also instead of air drying them. I didn't word it very well in my previous post. 

The stain we are planning on using is ready seal and here is from their website;

QuoteConventional wood stains typically form a rigid film on the surface of wood, rather than penetrating into the wood fibers themselves. This rigid film cannot flex with the expansion and contraction of the wood through the seasons and is prone to cracking, and peeling which eventually leads to coating failures.


All Ready Seal wood stain and sealers penetrate deep into wood fibers, moisturizing the wood with water-proofing oils and creating a flexible barrier that keeps moisture out. Because no rigid film is formed on the surface, our wood stains can flex with the woods' expansion and contraction and will never crack, chip, flake or peel, another reason Ready Seal wood stain is the best stain!

Necessity is the engine of drive

Sedgehammer

Quote from: Don P on February 19, 2019, 08:47:35 PM
Interesting. My first thought was that would be tough to insulate between and around the web work. Then the thought of lumber bolted to the sides of the flat plate girder type truss would help resolve the buckling issue and make a smooth easily air sealed ceiling plane. Then listening to your friends suggestion and combining that it sounds like a flitch plate beam, the beams made by sandwiching lumber on each side of steel plate. Some neat ideas floating around.

Looking at it like a flitch plate with the rest of the steel just dangling below. The arched tie is a piece of something like 4" wide flat plate welded to the bottom. Just throwing the thought of the night out there, if the flitch plate passes and the tie is there, poke the rest as full of holes as you want.


 

From the building code (table R803.1) #2 syp in 2x material would be good up to 72" span for roof sheathing.

Ever wander around under the bridges in San Antonio?
I have changed this up a little. Please look at my drawing. 

Drawing is incorrect. There will not be a 2x8 that runs the full length plus the rafter tail. The trusses do not come out in place to do this. I'll put a cross nailer in place to attach it to 3' up from outside edge that attaches to the 3x10 and I'll weld a bracket on the C purlin. I'll now be putting a 3x10 for the nailer and to help support truss. 3x10 will be bolted through metal for truss and the 12" C purlin. This 3x10 could be a 3x12 if needed.

I'm also attaching the load carrying of the 12" purlin. Can either get 12 gauge or 14 gauge.






what you think?
Thanks
Necessity is the engine of drive

Don P

Draw a plan view of the roof... in something darker. Maybe it will start to click.
I'm assuming the C channel is to create insulation space?

Sedgehammer

I'll try and draw something later today. not feeling good right now.

The C purlin replaces the2, 2x12's & the 2, 2x4's that you had sammiching the top of the metal arch in your sketch up picture. Then i added back a 3x for the nailer for the roof. I just didn't know how deep of a nailer i needed if we need more strength.
Necessity is the engine of drive

Don P

I think I'm following now. The trusses are on 5' centers, so 5'x30 psf= 150 lbs per foot. Try sending the manufacturer a sketch and see if they can provide any assistance, some of the engineered products will help with design and I think you've just come up with something they might want to showcase in one of their trade magazines. I've run into a truss guide for cold formed steel online before as well but this is outside of what they were doing, there are always details worth noting in those though.

Sedgehammer

Just getting the c purlins from our steel supplier that we use for some other projects. They get it straight from the mill. 
Necessity is the engine of drive

Sedgehammer

Here's a truss that my brother drew up. What do you think of it?

The metal legs would bolt to the wall and the plate between them would extend up and over the top plate of the wall. 

Since it's green wood and will shrink, one cannot box in the wood in the bottoms and sides and then bolt through the beam and the 2 opposing metal sides?



Necessity is the engine of drive

Don P

Technically that is not a truss since it has a tie that is raised and puts the rafters into bending.
Take a look at this calc,
http://forestryforum.com/members/donp/raisedtiethrust.htm
Play with lowering the tie as much as possible to lower the thrust but also bear in mind as you lower the thrust you are also lowering the bending point load on the rafters caused by the raised tie. I do like it just drop the tie as much as possible.
Also go to the rafter span tables in chapter 8 of the codebook here;
https://codes.iccsafe.org/content/VRC2012/chapter-8-roof-ceiling-construction
This is for dimensional lumber but gives a sense of what you are dealing with. Read the footnotes and notice the rafter span reduction as the tie is raised, this is due to that tie bending moment. Then go to the end of the span tables and read the heel joint connection table footnotes, that is the connections required to resist the thrust you figured in my calc above.

Backing up to the last design. I had a pm conversation with a forum member and he made a comment that bears bringing up. Unprotected steel does not perform well in a fire. I did a little checking in the code, you can scroll through that rafter chapter above and get to the cold formed steel laws. They don't specifically prohibit exposed steel there but it is clear they are assuming it is protected. In the main building code book it does call for 1 hr protection in the roof assembly. Then I walk into a walmart or Lowes and look up at exposed steel, hmmm. If you or anyone reading is considering this method it needs more fleshing out, engineering and a conversation with the insurance company would be good.

Sedgehammer

Yup, already discussed it with him before that it needs to be lowered all the way. he's working on a new one

If we go this route, it'll be 6x12's

I'll look through that code book

Once he's done with the changed design and you see it, I have questions about bolting the plates and timbers together that I raised earlier

Thanks
Necessity is the engine of drive

Don P

Make it easier, post his sketchup file, its the tab right above the "add photos to post" tab. I'll be out late again tomorrow nite, but the welds are consistently looking about ready to test 8). Can you get 32' material for the tie?

Sedgehammer

If I can copy it from what he sends me, you betchya

Awesome! Being a craftsman with wood is harder I think and you seem like you are all of that and more, so you should do fine..... boxingsmiley

It'd be 36' 5.5"....... Maybe, I'll check.
Necessity is the engine of drive

Don P

The truss is then dropped down enough to get the ceiling deck on top of the trusses, then the 12" insulation space and 8" tails out. The bottom chord extends into the walls for support.



 

The heel joint is the only tough joint here, it needs to resist a lot of toggle, thrust, from the rafters as they try to slide outward. This is sort of what I've done before, plenty of ways to play with it. Basically capture the end of the rafter chords and hold that well and bolt a strap to the tie with enough connection capacity in shear to resist the thrust. That could also be your threaded rod running from heel to heel buried in the upper surface of the bottom chord. There is a timberframe pavilion nearby that has that detail. The rest of the metal plating the sides of that joint is for pretty or doesn't need to be there structurally, you'll still need something top and bottom of the kingpost to make those tension connections. Something along these lines secures the heel better than side plates IMO.



 

Sedgehammer

The mill got back to me today. Can get a 38' pine. No DF. Longest they can get a DF is 24'. The pine is stable. It's beetle kill. 

If the bottom cord is of in pine, I'm assuming the top cords should also be pine?

Here is the truss wall metal bracket drawing I sent my brother. Red is the metal. It has a 1/2" welded in heel stop. The sections will be laser cut and the heel plate will go through each side plate and will be welded on the outside. He's working on it now. Any suggestions on it? 

The wall section isn't correct, as these will go in between the studs, so it will vary in exact location. I do plan on putting at least 4 studs under each and might even go with 5. What do you think?



Necessity is the engine of drive

Don P

That looks fine. The pine would work for the bottom chord that is in tension, The top chord I'd keep in DF if possible, it'll be stronger and stiffer.
Getting 6x6 post material from them might be worth thinking about, then the thickness under the truss will be the sam as the truss thickness and the studs up either side will be tight to the sides of the truss and can be nailed in easily. You might have to plane down the 5.5" dimension to match your 2x6's if you go that route, there's several ways to skin that detail.

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