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Author Topic: Clear Span Roof Truss Calculator  (Read 4701 times)

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Offline Don P

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Re: Clear Span Roof Truss Calculator
« Reply #60 on: February 18, 2019, 07:04:06 AM »
What I drew above, the trusses were on 4' centers x 36' span x 30 psf=4320lbs each. Need to keep an eye on the actual dead weight of the steel though, it might be worth bumping that up to 35 psf.

For the bar joist "trusses" you showed, here's how I'm thinking about them and maybe one of the engineers will help me out here. They are designed as parallel chord trussed half beams, each one 18' long. Then at the peak the two halves are connected by a rigid moment resisting connection. If that is the case the stress on that peak connection, where you want the hole, is something like (4320lbs/2)x 18'=38,880 ft-lbs. Think about it just like a pair of torque wrenches, the handle is 18' long and you are pulling down with 2160 lbs of force, the "bolt"(connection) is seeing that torque (rotational moment). If you've ever tried to torque up to a few hundred ft-lbs and broken bolts you understand my concern with rednecking this. All that is simply my read of how that is working, not saying I'm reading the problem right, that is why I kick it upstairs.

I'm also wondering about the roof construction beyond this room, you've mentioned 2x8's. If 2x8 rafters are spanning 18' we need to talk about that :P.
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Offline Sedgehammer

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Re: Clear Span Roof Truss Calculator
« Reply #61 on: February 18, 2019, 09:19:11 AM »
Yeah, prolly right

Canceled the hole. If we add the LED, it'll be below the truss. Some of the manufacturers of those trusses bolt together at the ridge and these are just made from angle. Pretty sure these are just 2.5x2.5x3/16





What would you put under the plate for studs. Will be 2x8. I am thinking at least 3

Spans are 24' and 20'. Was going to add webs where needed, but hadn't really looked at it yet and since we're going with the 8" syp t&g for the wooden trusses now instead of 5/8 osb, there'll be more weight to consider

Offline Don P

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Re: Clear Span Roof Truss Calculator
« Reply #62 on: February 18, 2019, 09:47:23 PM »
Quote
What would you put under the plate for studs. Will be 2x8. I am thinking at least 3

Spans are 24' and 20'. Was going to add webs where needed, but hadn't really looked at it yet and since we're going with the 8" syp t&g for the wooden trusses now instead of 5/8 osb, there'll be more weight to consider
Lost me again :D. Draw me sumthin, I don't know where we are.
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Offline Hilltop366

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Re: Clear Span Roof Truss Calculator
« Reply #63 on: February 18, 2019, 10:26:35 PM »
Been following along but out of my league to comment on design but I'd price the weight of the same kind of steel out and compare it to the cost of the truss from the company in the link.

I'm thinking that if you can make a stock pre-made truss work for you verses a diy that the pre-made with engineering, cutting, welding, and priming all done might be a good deal.  

Offline Sedgehammer

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Re: Clear Span Roof Truss Calculator
« Reply #64 on: February 19, 2019, 01:33:54 AM »
Lost me again :D. Draw me sumthin, I don't know where we are.
Hopefully tomorrow, but it's just a vaulted parallel truss with a modified top plate mount

Offline Sedgehammer

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Re: Clear Span Roof Truss Calculator
« Reply #65 on: February 19, 2019, 01:43:48 AM »
Been following along but out of my league to comment on design but I'd price the weight of the same kind of steel out and compare it to the cost of the truss from the company in the link.

I'm thinking that if you can make a stock pre-made truss work for you verses a diy that the pre-made with engineering, cutting, welding, and priming all done might be a good deal.  
I know where you're coming from and I wish that'd work, but several issues
1. Those truss are made to standard specs and they build a ton of them, so costs are very low because of
2. Those are pole barn style trusses and while they would get the job done, doesn't look very nice and this is going in a house
3. Since we're making a known truss, engineering won't be much. Most likely not even needed
4. Can't be red primered anyways, so bare metal would be preferred
5. Sure I could buy those and then cut them to fit and re-fab them, but then we'd really have no idea where we were, as it has to be a vaulted parallel truss

Offline Sedgehammer

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Re: Clear Span Roof Truss Calculator
« Reply #66 on: February 19, 2019, 05:23:48 PM »
Decided to go with blown in fiberglass since I have the room with making a vaulted parallel truss as deep as I want basically. A fair amount cheaper than the polyiso





What I'm not showing is the 1/2 wall plate that goes under the 7" top cord to give me the 7 1/2" needed for the rafter tails

Also not showing the 3/8 wall plate that will go from the bottom of the beam down to 1/2" past the bottom cord





We could make the truss only 22.5" deep and run the 3x6 flush to the bottom cord. Trying to get some pictures to weight both options

Was thinking if we leave the 16" exposed, was to make some faux bolted plates at every exposed web connection that would look like it was bolted together. Kinda like bridges were built years ago

A friend just suggested to make the beams a bit bigger like a 3x10 and drop it down below the truss 2" or leave the truss only exposed it 2", so the truss would only be about 24.5" deep

Which ya guys think......teeter_totter

I also am wondering how far that 2x8 syp t&g (1.5x7.25 actual) could span? If 60" for the roof deck and 52" ceiling, we could lose 1 truss

Offline Don P

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Re: Clear Span Roof Truss Calculator
« Reply #67 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?
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Offline Sedgehammer

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Re: Clear Span Roof Truss Calculator
« Reply #68 on: February 19, 2019, 10:06:16 PM »
If the beam mid truss, would just weld a plate between webs flush with beam bottom

Would still set a beam at least 1 size deeper than beams sandwiching truss, between trusses at ridge. Would give impression that the 3x10's and 12x12's are doing the work. Would need to come up with a way to bolt angle on 3x10 ends at ridge where butts to the 12x12 though. Would a 6x6 angle work drilled at the 1.5" point from edge on both sides of the angle and bolted with 4,  1" bolts or would it put to much stress pulling in different directions

If the beam below truss, could weld 18" or so of the plasma cutouts to the bottom with round stock, small pipe or ? for a stiffener. Then could still add the circle at ridge

So the 60" works,  smiley_safety_glasses smiley_bandana 

Been there 10 years ago, walked along the river and had dinner I guess, but don't remember much (from accident). Wife said we're going again this summer, so   digin1 smiley_beertoast

So which ya guys think 

Offline Sedgehammer

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Re: Clear Span Roof Truss Calculator
« Reply #69 on: February 19, 2019, 10:20:26 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

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.

(Image hidden from quote, click to view.)
 
Just saw this after I posted, thanks!
If midtruss beam, just weld a plate between beams in between webs to seal 
Per your picture. Need area for insulation, so metal would need to extend 15" above beam. Then someway to fasten decking to the truss. I guess a 2x4 could be bolted to each side and then could screw decking into it
Kinda kool idea....... 🤔🤔🤔
The 4" is how thick, .375"? 
With plate being so deep, could be fairly thin I would think

Offline Don P

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Re: Clear Span Roof Truss Calculator
« Reply #70 on: February 19, 2019, 10:46:06 PM »
3/8 would be more than enough structurally for a tie, gotta think about stiffening side to side which is as much about width as thickness but it is a tension element in my way of thinking.

Here's what I was thinking, the wood on the sides of the steel is hidden, it can be local framing stock, I've got a 2x12 and a 2x4 drawn. Bolt through the lumber and steel sammich in a W pattern to form the flitch plate beam. The roof deck goes on top, the ceiling goes underneath and the insulation is in between.





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Re: Clear Span Roof Truss Calculator
« Reply #71 on: February 19, 2019, 11:52:14 PM »
Kinda liking this curved thin design.

Curveball......smile_juggle

Show the beam and screw 2x8 t&g to the top of it like before. Bolt a 2x6 on top of the upper tie to screw the decking to.

The metal that is sammiched can be just regular plate to cut down on cost, but a question on the beam to metal. Beam is may be wet (still waiting on kiln drying guy), does there need to be some separation between beam and metal? If so, prolly just an area 2"x3/16 wide where bolted to beam would suffice. There would be gap if I put the angle ledge to hold beam.

Just weld the deco metal to the bottom of the regular plate (but above beam line) with the deco circle in it to get desired depth and run down the wall about a 1' or so with a plate that bolts to the wall.

I guess this same curved deco idea could be also welded to the bottom of the tube truss.

Offline Don P

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Re: Clear Span Roof Truss Calculator
« Reply #72 on: February 20, 2019, 07:53:10 AM »
Yup, the way I'm looking at that is the upper section is designed as a rafter pair. The rafter pair can be wood, steel, bar joist, flitch plate, etc as long as it can take the load and deflection. The lower arched plate is the tie across the rafter bottom to restrain the rafter pair from spreading, that removes that peak moment resisting connection. The deco plate in between is just keeping that bottom chord from changing length. So it is really at its heart just a typical pair of rafters with a ceiling joist.
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Re: Clear Span Roof Truss Calculator
« Reply #73 on: February 20, 2019, 08:57:41 AM »
Question, if we put a top tie on it, why the 2, 2x12's & 2x4's?

Also, one could actually just use a 27" deep vaulted parallel I-beams and put the 3x10 on top of the bottom flange and a 2x6 lying flat on top of the top flange. Then once one knows what that thickness is, don't put the lower flange on at 27" point, put it on the bottom of the curved part. 

In this way one could get design values fairly easily I would think, but I haven't had any luck finding a calculator yet.

Offline Don P

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Re: Clear Span Roof Truss Calculator
« Reply #74 on: February 20, 2019, 09:38:12 AM »
I'm not following all of that but here is my thinking, the 2x12's are common rafters, there is our easy design value. They would pass as 2 #2 syp or dougfir 2x12 rafters spaced at up to 5' apart, I drew them at 4'9" spacing above.  The 2x4's were packed on just to give that 15" depth for blown in insulation. The top edge of the 2x12's is the nailing surface for the roof T&G decking. The bottom edge of the 2x4 is the nailing edge for the ceiling T&G. The other structural element needed in that is a bottom rafter tie to keep the rafters from spreading. The arched plate. It's length is fixed by the deco metal. By having the rafters bolted through the deep decorative plate, it is increasing the rafter strength and stiffness, I haven't counted that in the above. If the rafters deflect they may put some amount of buckle into that thin unstiffened plate, that is cosmetic not structural. If you want to add decorative beam work below the ceiling plane that is fine. If it begins to add up to much load then the # 2 rafters should be bumped up to #1, SS or LVL. The 2x12 or both 2x12's are ripped down to 2x8 as tails that extend over the wall.
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Re: Clear Span Roof Truss Calculator
« Reply #75 on: February 20, 2019, 01:16:18 PM »
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

Offline Don P

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Re: Clear Span Roof Truss Calculator
« Reply #76 on: February 20, 2019, 01:40:41 PM »
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.
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Re: Clear Span Roof Truss Calculator
« Reply #77 on: February 20, 2019, 03:06:27 PM »
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

Offline Don P

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Re: Clear Span Roof Truss Calculator
« Reply #78 on: February 20, 2019, 04:15:19 PM »
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.
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Re: Clear Span Roof Truss Calculator
« Reply #79 on: February 20, 2019, 05:45:20 PM »
🤔 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. 


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