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20ft span what size lumber

Started by 1953belair, July 05, 2019, 12:14:39 PM

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1953belair

20ft span what size lumber.i am building a carport I have my 6x6 post and side beams up already 2x8 tripled up...what size beam will I need to go across the front,back and one in the middle without a post in the middle of it

1953belair

I have 2x6x14ft long for the rafters and 2x8x24ft long for the ridge beam I'm going to double them up I just need a beam that will go all the way across so I can put a 4x6 standing up on them with the ridge beam on top on the 4x6 I just don't want the beam to be bowing to bad.....the post are 20ft apart so minus the 6x6 post it's 18ft 11" span I thought a 4x10 beam would work good but just wanted to make sure 

Jim_Rogers

No one can help you without knowing where you are and what loads will be on the roof.

Jim Rogers
Whatever you do, have fun doing it!
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1953belair

I am in North Alabama we get very little snow here and im just putting 1xs down and metal roofing so I don't think there will be much weight on the roof....I'm trying to upload a picture of what I want to build  and what I already have done 

chestnut

  As a reference, I live in central pa and built a shed roof off my work shop that is 14ft out from building and has a 4.5/12 pitch with steel roof. The beam holding rafters is a 24ft span made up of three 2x12x24ft. Specked by a local truss company. The post are set in 2ft on each end giving the beam a 20ft span. It has handled a fair amount of snow but I don't get the wind you do Alabama. 

1953belair

If I can't get that beam in 4x10 cut out of cedar or pine.....do you think double 2x10s 20ft long with plywood or osb in the middle would do the job? I'm trying to find someone here to cut that big beam so I don't have to put anything together 

Southside

You definitely don't want that out of cedar, and I don't see a pine beam in that size holding up either.  The easiest thing would be to pick up a glue lam spec'd to the span you need, they are a lot more affordable than you would think.  
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Mike W

I'm with Southside on this, based on the span a glue lam or LVL, the cost is minimal and even if your not permitting the structure, you'd want the piece of mind to some extent should the unforeseen happen and need to tap into your insurance for recovery of whatever is under the cover, having a pre-engineered beam in lieu of something made up will help in such a situation.  Albeit, I typically design and construct pole barns and structures on my property with milled wood from the property, knock on wood (pun intended) to date all has worked out well as we typically over-design our builds anyway, and we do get a good bit of snow here.

Don P

The other guys type faster, and we got a storm blackout blip partway through my post dang it.

Cedar isn't beam material. Ply and osb don't really make a beam stronger, they might add a tad of stiffness but I just neglect them in any strength calcs. I'm assuming we're talking about southern yellow pine.

Let's see what you've got. 19' span center point loaded, calc here;
http://forestryforum.com/members/donp/beamclc_ctrpointload.htm
the ridge is supporting half of each rafter pair so 9.5' of width. The post is supporting half the ridge so 12'. 9.5'x12'=114 square feet. Code minimum live load is 20 lbs per square foot. If built VERY light call the dead load 5psf so 25psf x 114sf=2850 lbs
Adjusted for wind, Fb for SYP is 1536 psi, E is 1.4, Fv is 280psi

Enter all that into the calc, nope. try 3 full cut 2"x10's
Fb goes to 1619 the rest are the same, passes barely in bending, deflection at full load is 1".

I suspect the ridge is waaay too light. It is uniformly loaded, calc here;
http://forestryforum.com/members/donp/ddsimplebeam.html
9.5' of tributary width x 24' long x 25 psf=5700 lbs load. Not even close, deflection 13", its headin for the ground. 3 full dimension 2x12's in #1 passes, 1-1/4" deflection at full load, it's probably not going to feel real good walking on the roof at midspan. The ridge should be an LVL, the supplier will spec it for you, prolly a triple 14" (if you want to play with it lvl Fb 2800 psi, E 2.0, Fv 280)

This isn't good let's keep checking. Side girders, uniformly loaded, same calc as just above.
Trib width 5'+1' overhang=6' x 24'x25 psf=3600lbs
triple full cut 2x10's

Rafters, same calc
span 10'x2' on center x 25psf=500lbs per rafter
They look good.

1953belair

I think the ridge will  be fine I'm building a trusses like beam in the front back and middle so the max the ridge beam will be spanning is 10ft from the centre to the front or back 

1953belair

Not sure on what the pitch will end up being but It will have a good pitch on it.i don't want it to be flat.so should take some of the gravity force down


1953belair

Do I have to upload pictures to --Photos MUST be in the Forestry Forum gallery!!!!!-- or something before I post them on here cause I can't seem to figure it out from my phone

Don P

There's the problem with "word pictures" :D. This is sounding better.
Photo tutorial is here;
http://forestryforum.com/board/index.php?topic=100194.0

1953belair


1953belair

Ok so I got some photos on here now the middle photo is were I am at right now and the other too are one that I found that I would like to make similar too I'm not building the enclosed building part just the carport 

Don P

That appears to be a truss, hard to say without seeing the joinery. In that case the kingpost is trapped between the rafter tops, dangles down and actually holds the bottom chord up from sagging, kinda counterintuitive.
This is a pic from my gallery showing that setup, the bottom chord here is actually 2 pieces splined together at the kingpost. The bottom chord is a tension member in a truss as is the kingpost, they could both be a cable.



What you are describing is a crown post standing at midpoint on a tie beam and supporting the ridge beam, correct? Post spacing is 2 rows of posts 20' apart, the plates on the rows are 24' long and post spacing is 10' in the row?

1953belair


Don P

OK, let's play with it.
The middle bent is bearing the most load so we'll check it.
The crown post is supporting 1/2 the 19' width side to side- 9.5' X halfway towards the front and halfway towards the rear-10' so 95 square feet x 25 lbs per square foot= 2375 lbs

http://forestryforum.com/members/donp/beamclc_ctrpointload.htm
#2 SYP
enter load 2375, span 240, width 4, depth 10, Fb 1536, E 1.4, Fv 280
Nope.
Try 3 2x12's
width 4.5, depth 11.25, Fb1380, the rest the same ... Nope but closer
bump width to 6,  that's a 4 ply 2x12 built up beam, Bingo, about 11/16" deflection at full load, good.

Lets back up and try a 3 ply 2x12 using #1 SYP
width 4.5, Fb-1840, E 1.6 ... That also works, deflection is around 13/16"

------------------------------------
Ridge check;
A double #2 2x8 SYP,
Tributary area is 9.5' x 10', from crownpost to crownpost 2375 lbs
uniformly distributed load so this calc;
http://forestryforum.com/members/donp/ddsimplebeam.html
load 2375, span 120, width 3, depth 7.25, #2 syp 2x8, 10 min (wind), yes 2 members, 2&3x8, no,no, passes, the fail in deflection is for a floor, it passes for a roof at about 7/16" deflection

The plates will have less load so should be good, the rafters already checked.
That thing is a sail, brace it well, probably even the crown posts to ridge, hurricane ties rafters to plate, flat straps on top of the rafters over the ridge to the opposite rafter to keep it from inflating and blowing apart up top (lumber band strapping works)
 All you lack is finishing if that all works.

Disclaimer, I'm just a nail banger, use all that at your own risk.

What is the old car in the pic?

1953belair

Nice thank you....so your saying use 2x12's tripled up to go all the way across? And that 2x10s doesn't pass right?     Ridge being 2x8 double up works and the rafters are ok? The old car is a 48chevy fleetline I'm going to put air ride on and turn into a rat rod I believe,it is a yard ordiment right now we dress it up for Christmas and Halloween have another 53 Chevy I've had for about 8 years I've restored and is also on air,I just won 3rd place with it at a big car show in Biloxi Mississippi

1953belair

2x10's. 20ft long are $18 here and 2x12's 20ft long are $34         so the price is a big difference but I also don't want to have to redo it if I have to spend more money I will,is a solid 4x10 or 4x12 stronger? 

Don P

Right, the 10" isn't deep enough. Depth creates bending strength faster (and cheaper) than anything. The old carpenters saying is "deeper is cheaper". In the math where you check the section modulus, depth is squared, bd2/6. That reflects how a rectangular beam behaves in bending whether wood, steel ,etc, sort of down in the weeds, that is just the geometry part of the strength of materials. or another way, if the depth has to be shallower the strength really needs to go up.

A solid timber is less strong because the built up beam distributes the defects better. You get a 15% bump in bending strength for a built up beam of 3 or more plies, they call that the repetitive member increase. you get to take that increase when there are 3 or more members within 2' of each other so we also take that bump for rafters or floor joists in typical light frame construction.

Look at the grade stamp on the lumber, if it is #2 you need a 4 ply beam, if it is #1 a 3 ply will work. If you see #2 Dense we can look at it. SS (Select Structural) is the highest grade you'll probably see, it would work fine. Occasionally you'll see machine graded lumber, it will have the Fb and E numbers included on the stamp, as long as they are higher than the numbers we were using above you're good.

1953belair

What is the difference between what I'm doing and a kingpost truss I'm just thinking the 2x12's are going to look overbig with my 2x6 rafters witch I might put a 1x8 over for the front and back end rafters just to give it a better look.......if needed to I can put a steel plate connected my 4x6 to the tie beam and put my rafters on both sides of the 4x6 kingpost tieing everything I together.the bottom beam can't sag down unless the top moves right?

Don P

I've spent the last 3 days with 10 jacks, various chains, binders, come-alongs and straps lifting and pulling on a barn and its roof trying to reverse the effects of time and gravity on a poorly tied frame. I'll walk you through some roof thoughts.

Take a greeting card and put it on a slick table tent fashion, there's a roof. Push down on the ridge and the feet slide out and the ridge drops. It'll do the same thing on walls or posts.

Stick a pencil under the ridge as a beam and support it on a short glass at each end and you have a structural ridge with the roof hanging from the ridge beam. When you push down on the ridge the feet cannot splay unless the ridge sags. You are supporting the posts under the ridge on a tie beam, it also needs to be stiff enough not to allow the ridge to sag and have the rafters thrust against the post plates.

Remove the ridge pencil and support posts and tape a piece of string across the roof feet and push down on the ridge. If the feet cannot slide outward the roof stays up. Notice the failure modes, the string can come untapped and the feet slide out. The rafters can buckle at midspan if too light as with a greeting card.

A rigid triangle cannot change shape by rotating about its joints. A 4 or more sided structure can. The simplest truss is a conventional roof formed by 2 rafters and a ceiling joist. As long as the connections at the joints hold that shape is rigid. The ceiling joist could be replaced by a cable or chain and perform the same function. I'm using that example to help visualize that the force in the bottom chord of the truss is tension. The chain is keeping the rafter feet from splaying out under load. The roof load is travelling down the rafters, the bottom chord is restraining the horizontal force produced by that vertical load travelling at an angle.

How much horizontal force is the chain restraining? At a 12/12 pitch, 45 degrees, a 100 lb vertical load on the ridge produces 50 lb of tension in the chain and at its connections. At 6/12 100 lbs on the ridge produces 100 lbs of tension in the chain. At 4/12 that same load produces 150lbs of tension in the bottom chord. Roof pitch plays an important role in the force in the bottom chord and in those heeljoint connections.
This is a typical heeljoint in a timber truss, be sure there is enough meat beyond the notch in the bottom chord that it cannot shear;




There's more than one way to skin a cat though. Below, the angle iron is keeping the top chords from sliding out, it is welded to the bar that extends under the rafter and is lagged to the bottom chord with enough lags of sufficient size to resist the load. In the barn I'm working on we are going to weld angle iron end restraints to rods that extend all the way across the building, we are tieing the feet together across the building with steel rods, no one will see them.
 



As the bottom chord becomes longer and more slender and particularly if web members spring up from the bottom chord to reinforce the top chords, the rafters, you need to keep the bottom chord from sagging. Going back to the bottom chord being a chain in your mind, it begins to sag. Well, throw a rope over the peak and drop it down to the chain and cinch it back up. The kingpost in this scenario is another tension member. The brits sometimes call it a kingrod and that is probably a better term.  The kingpost is pinched between the top chords. These kingposts support a built up ridgebeam that in turn supports common rafters.




This is what the kingposts looked like;
 

 It's not real intuitive but those posts do not sit on the bottom chord, they hold it up, in fact you could cut them off above the bottom chord and they would still support the ridge.

With the kingposts now supporting the bottom chord, web members can spring back up and support the rafter chords in their midspan area helping to keep them from buckling if they are light or need reinforcement. This is a discontinuous bottom chord, splined through and tabled into the kingpost with webs springing from that point back up to support the top chords. Those webs are compression members, canted posts;




Whoa, I just wrote a chapter, hope its clearer than mud :D

1953belair

Thank you there is a lot to take in lol her is a picture of my back porch roof I built and how I did it just to share will probably be about the same setup how the rafters screw in to both sides of the post

 

 

1953belair

Would a metal plate help and All the way across the bottom tie 9" or so x 20' I'd probably have to weld some together but that's no big deal if it would help 


Don P

 It's easier to get a solid connection in steel for that heeljoint and steel is good in tension.
What is the roof pitch going to be?

Don P

Here's one way of looking at it. The peak of the truss needs to support around 3000 lbs. This is a pic of the forces at a 4/12 pitch, probably the lowest pitch you would use, add 2 zeros to all the numbers, load is the 3000 lbs in black hanging from the peak, which becomes 4700 lbs compression heading down each leg of the toggle and produces 4400 lbs of tension in the bottom chord and at the heeljoint connections, kingpost is carrying zero load;



If simple is the main goal and given that you can weld, that bottom tension chord could be a piece of pipe or rod welded to angles at each end. One angle clips leg down down over the post plate beams. Another angle is back to back with that, leg up and pockets the end of a pair of rafters. The pipe is welded to the angles across the building, a rigid triangle. The ridge beam is secured well between the rafter tops.  There would be a rafter on each side of the pipe and a filler block between the rafter pair to make it a rigid assembly, the canted post heading up to support the ridge beam. This is similar to what we're getting ready to do today on the barn repair. I'm going to weld a turnbuckle in the middle of the bottom chord so we can pull that puppy back together while we're at it. Not high art but it takes care of the connections well. A 20' one inch pipe is going to sag in the middle, there's where the kingpost dangles down and holds it back up.
 




1953belair

The pitch will probably be alittlem than what's on drawing you posted.....so a pipe would help alot? I also have some 3"inch angle iron to.at first I thought about doing angle and sandwiching it between two wood beams and hiding it would this be a better route? 

1953belair

QuoteI'm going to put as much pitch on the roof as the 14ft rafters will let me with about a 1ft of overhang past the side beams maybe a 8/12 

Don P

That would work fine and would be easier to hide. With a steeper pitch the tension in the bottom chord and at the connections decreases. Notice how much the forces drop at 8/12.





 The trick with most trusses is keeping that low end of the rafters, the heeljoint, from drifting outward under the load from above. It is hard to simply pin or bolt the bottom and top chords together adequately to resist that thrust in wood. That is the reason for that upturned angle in my last pic or for burying the top chord into a large notch like in my timberframe pics above. This just got a lot better at that pitch though, a 3/4" bolt through a SYPine 3 piece joint, double rafters sandwiching a single bottom chord is good for about 1500 lbs so a 2 bolt connection is starting to work without steel, but the steel heel is a more rigid connection. Just stuff to think about as you design it. I'm off to load and hit the steel shop, I'll check on you tonite.

1953belair

Well I doubt that I will be able to get that much pitch im just going to get theses beams up after I figure out how I want to build them then mut my 4x6 in the middle of the beam and get the most I can out of the 2x6x14 rafter

1953belair

Would a small cable also work In between 2x10 if so what is the best method to connect it and get it nice and tight..the angle iron I have the most of is almost 1/4" think and 3".it is pretty heavy I can get different stuff tho my uncle works at a steel plant

1953belair

Also I do plan on doing a birds mounts notch on the rafters and maybe some collar ties to keep rafters from trying to spread apart.on my porch roof I did collar ties on every other rafter but it's also alot heavyer roof with plywood and shingles but it's not as wide either.....sorry for all the post just been thinking about it in between work breaks 

Don P

Deducting 1-1/2" from the 1' overhang and another 1-1/2" for half the doubled ridge I'm at 12'11-1/16" at 8/12, so that should work with 14' material.

The level seat cut on your birdsmouth should be no more than 2-1/2" to stay within the max 1/4 rafter depth notching rule.

They've modernized roof terms, or defined them more clearly in the last 20 years or so. A collar tie is in the upper third of the roof height and its job is to keep the ridge from blowing apart in high wind. We are supposed to either put at least a 1x4 across every other rafter pair in that zone or run a metal strap up and over the ridge nailed to the opposing rafters to hold the rafters and ridge together in high wind. If you remember the films of the roofs flying apart during Andrew it is to restrain that. A collar tie is too high to restrain the spreading forces at the rafter feet.

A rafter tie is a ceiling joist or tie across the rafters in the lower third of roof height. It's job is to restrain the thrust we have been talking about. If those are not on every rafter pair then a ridge beam is required. That's what you are doing between your 3 trusses.

Just as an aside, doesn't matter here but for others reading along, in a conventional roof as a rafter tie is raised off the plate to that 1/3 height limit, the rafters need to get deeper and the heeljoint connection needs to get better, that is all spelled out in the footnotes in the span tables in the rafter chapter of the building codebook.

Securing and tensioning a cable would be a job. I'm having a hard time right now visualizing hiding steel in this, long day ::)

Drawing it out and then trying to lay out bolts, there isn't sufficient room once you allow for sufficient edge and end distances and spacing between. Nails would work, a .131x3" nail in SYP is good for 106 lbs in single shear in 1-1/2" material, that is a common size gun nail, its the diameter of an 8d common hand drive but in 3" length. It would take about 12 from each rafter into the tie if it is assembled like this;

 


I drew that without fill between the rafters, I would fill it to make it stiffer and to keep the bees out of there.

Supporting the ridge beams is another detail to think about, one way could be hangers nailed to the trusses and drop in 10' sections of beam between trusses.

1953belair

With that pitch will 2x10s on the outside of the 4x6 middle post carriage bolted to the 2x10s work if I box it in with 2x4s top and bottom inside/in-between the 2x10s or can I put 10ft 2x10s together with  plywood to make up the Gap and put them in between the 2x10s on each side of the 4x6 post so that it makes it a solid beam.....that way I can leave a hole in the middle for the 4x6 beam to go In and would make a good connection with the carriage bolt.


Don P

 Basically like so?
I drew the bottom chord as 2x8 it can be 2x10 if you like, the key is to get the number of nails in that heeljoint connection. put ply or 1/2" material between the rafters to get them to the 4x6 thickness and pinch the top of the 4x6 as in the pic. The bottom connection of the kingpost is not that critical here so carriage bolts would work, they are really not considered a structural bolt because they are threaded all the way, but as we talked about the bottom chord is really a tension element here and the king isn't really doing much work at the bottom chord. You can pack in whatever fill between the bottom chords. In that situation I would probably stop the king short of the bottom about 1" and slip in a flat 2x4 hanging it down about 1/2" rather than trying to make it flush make it a stepped trim detail.





1953belair

That's pretty much what I was thinking I have got 2x10s yesterday evening...the bottom cord the 2x10s I haven't decided if I want to sister them together with plywood or do them like your drawing on each side of the 4x6...I like that because the 4x6 has a pocket to set in-between but I feel like the 2 2x10s butted together with plywood wood make a stronger beam and would resist warping better, just don't have the connection to the king 4x6 in the middle of the bottom cord,if I did it that way I would probably make a metal T bracket to bolt the bottom cord to the king post better,witch do you think is better

Don P

The critical connections are first the bottom chord to rafter heeljoint, then the top chord rafters to kingpost, then the ridgebeam support and finally the bottom of the kingpost to bottom chord.

The kingpost top the way it is drawn requires some fussing but it is then pretty easy to nail a concealed flange double joist hanger to it for the ridgebeams to drop into. All just stuff to roll around in your head.

1953belair

Yes I no it lots of different ways to do it....thanks for all the help I'm going to work on it this weekend hopefully il get alot done

Don P

Good luck, supposed to be even hotter over the weekend. When I built my shop I figured you should be able to hang a truck underneath a truss, but nobody would loan me their truck :).

1953belair


1953belair

Haven't got as much done as I would like to but I thought I'd post a picture of the progress...the 4x6 in middle of beams is notched for the double 2x8 ridge beam to sit inside of it and on the other end the bottom it slides in-between the 2x10s the thickness of the osb I put in between the two beams.... should work out good 

Southside

What kind of wood is that in the roof structure?
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1953belair

It is all pine I just been staining it to give it a better look

Don P

Looking good. Don't forget to brace well in the other direction from the posts to bottom chords as well.

1953belair

Yea I've got them cut out already....I'd like to do a arch on one side of the braces they will be 4x6 post .might be to much Time cutting them out though.going to try to cut one on a scrap piece and see how it goes

Hilltop366

Nice work, it always seems to take longer than you think it well.

I would be tempted to add X bracing in between the trusses along the centre line as well.(from king post to king post).

Weekend_Sawyer

Don, It's a pleasure to watch you cipher!

Mr Belair, that's going to be a nice looking structure.

Jon
Imagine, Me a Tree Farmer.
Jon, Appalachian American Wannabe.

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