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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 


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