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Clear Span Roof Truss Calculator

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

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Don P

Happily we were above freezing, hmm, that could go either way... mud :D

Hanging the overhangs, not a good idea, it's gonna sag over time or depart in high wind, keep thinking on that.

On steel trusses, I'm just reading one chapter ahead of you, in other words keep tuning the design but it does need to go to an engineer. I've been reading through the steel construction manual and really the way to design this is not as a truss but as a tapered plate girder. This is an older stand alone section which no longer appears in the manual but is more in line with what you are wanting to do, also notice the section and examples on access holes;
https://www.aisc.org/globalassets/aisc/publications/out-of-print/welded-tapered-girders.pdf


Sedgehammer

It made the table on the compound miter saw really slippery today. Them 2x4x12's moved like they was on ice......  smiley_headscratch

Yup, I would generally concur

Yap, that would describe it better, but the manual it's got lots of funny looking characters along with numbers in it......  electricuted-smiley

What's the loads we're looking at for it do you figure :P
Necessity is the engine of drive

Don P

It looks to me like the web works at the quarter point but is possibly in buckling trouble at the midpoint at 11 gauge. What you are really doing is marrying 2 tapered plate girders at the ridge with a hole in the spliced section. There is a rotational moment connection at the peak right there that needs serious consideration. I believe it is quite do-able, the loads are light, the details are up to the engineer. Work out the parts of the design that you can do before going there though, the overhangs still need work. I understand that you would like to avoid that cost but it is money well spent... the role of the engineer is to do with one dollar what any damfool could do with two. The shop where I buy steel and have fabrication done has a couple of engineers they like to work with who do good work at reasonable prices.

 What I would try is lifting up the section you drew and extend the top chord out to support the eave overhang and bury it in a notched out rafter tail. The 2x8 T&G then forms the soffit. On the gable ends also extend the T&G out to support those overhangs. One way to lower the overall height is to try an upturned channel as the top chord with a 2x T&G nailer laying flat in the channel, if that works that would lower the overall overhang thickness about 3". Then remember that is just an idea, don't become too invested in it. The engineer might come in out of left field with a better idea.

I'll tell a little story from this week as to why I like to consider things before bringing in the design professional. We had thought through several options for plumbing and HVAC with those trades before bringing in the architect's engineer. The ideas coming out of that engineer, well, sucked. Because we had other ideas to offer the ideas were out there for all the parties to consider. I think we redirected what would have been an ill considered plan, and if not I will listen and possibly redirect again. Hopefully it saved the time of going down an undesired path. On our own projects we have more time to do some of the legwork and time to consider things.

Sedgehammer

Any idea on what an engineer would charge?

The place you work with, even though I'm out of state, you think they'd be usable?

Notched rafter tail......... They's only gonna be 2x8's. Not much meat to be anotchin on.....

Thought about the upturned channel. Abandoned it for the look of the beam. Removing it still leaves me short of going over the plate with any metal to extend out for the rafter tail. I'd still need to make that wall shorter. Looks like 6.5" total. Then there's the roof thickness issue. We're still at a thickness of 9". Plus the thickness of the T&G is about 1.125" thicker, soooooooooo electricuted-smiley

A few thoughtin's.....

1. Make girder deeper. Bolt 3x3 wood to both sides down 7.5" to the top of the 3x3. Nail T&G to top of the 3x3 and then polyiso on top of that. Have truss/girder 7.5" above the wall. Bolt 2x8x4' rafter tail 2' up onto the girder

2. Build webbed truss and put cut outs into it or just leave them as is or redesign it to look more rugged/deco/I don't know what to call it. These here are very popular here, but too pole barnish looking. 10' oc and span 40'. https://wheelermetals.com/store/products/d/19948/trusses/40-truss Drop the T&G down same as above

3. Scrap metal truss and figure something else out. Any idea's  ;D

4. Build a tipi and convince the wife we're the long last tribe of the Mohicans  smiley_indianchief

Update; Had an epiphany at church smiley_angel02_wings
Developing pc_smiley
Necessity is the engine of drive

Sedgehammer

Build a truss similar to these 2;





The above is how it will fit on the wall with modifications. 

Will be 7.5" above the wall, which is like the rest of the house

Plate on the wall will be 12" wide x 1/4" plate and 32" deep and will extend over to the top plate 8". Will bolt through the top like in my previous drawing and through the wall with corresponding plate on the exterior





It will be a web truss, but made with 2.5"x1/8" sq tubing

Depth will be 38"

Will use 8" syp t&g for the entire roof system, so as all roofs will match. Will add approximately $1,300 to budget, but makes layout simpler, thus faster, so a savings there

From the top of the truss will drop down 13.5" and weld a 2x3x1/4" angle ledge on both sides for 3x6 timber to set on. 3x6 timber will be bolted to & through the truss and to each other. 8" t&g will nail to top of 3x6 and 6" of polyiso on top of that, which will be to the top of the truss. Screw 8" t&g to the truss. At rafter tails, bolt 2x8x4' rafter tail to truss

Between truss at ridge place a 7x7 timber cut to fit between trusses. Will sit on 2x3x1/4" ledge welded to both sides of each truss and will weld to the angle that is welded on for the 3x6. Will cut 18.5° angle on edge of the 7x7 timber to match roof slope. Will bolt to the 3x6 timber where they meet with a 6x6x1/4" angle

I think this is a good blend of both metal and wood. If we want we can add a collar brace and add the arsty fartsy stuff there, but collar brace won't be needed
Necessity is the engine of drive

Don P

The shop that makes those trusses more than likely has an engineer on staff that can design and fabricate what we were talking about earlier.

Sedgehammer

Going to build it ourselves or if not, have a guy locally that's only $25 an hour in his shop. Just depends how busy we are and how I feel. 
Necessity is the engine of drive

Don P

That one link to a 40' tube steel truss... was it local? that was cheap

Sedgehammer

104 minutes 

That's a standard size, so they build a gazillion of them, but I will give them a look see. Not sure if they do custom or not. 

Necessity is the engine of drive

Sedgehammer

What kind of loads you think we're looking at?

Thanks
Necessity is the engine of drive

Don P

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.

Sedgehammer

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
Necessity is the engine of drive

Don P

QuoteWhat 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.

Hilltop366

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.  

Sedgehammer

Quote from: Don P on February 18, 2019, 09:47:23 PMLost 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
Necessity is the engine of drive

Sedgehammer

Quote from: Hilltop366 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.  
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
Necessity is the engine of drive

Sedgehammer

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
Necessity is the engine of drive

Don P

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?

Sedgehammer

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 
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

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.


 
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
Necessity is the engine of drive

Don P

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.






Sedgehammer

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.
Necessity is the engine of drive

Don P

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.

Sedgehammer

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.
Necessity is the engine of drive

Don P

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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