Rafter strength

hopm · July 23, 2020, 01:41:44 PM

Which has the better strength to weight ratio for rafters....2x8 or 6x6 pine?

woodybutcher · July 23, 2020, 02:30:05 PM

In my humble opinion the 2x8.

Don P · July 23, 2020, 04:22:32 PM

The section modulus of a 2x8 is 13.14"³
Sxx of a 6x6 is 27.73in³

The higher the number the stronger that shape is. Depth does buy you strength at a square compared to width, "Deeper is cheaper". The difference in width here more than made up that difference.

But we're not done yet. The strength of the particular species and grade of each choice would be the next thing to look at. At "5x5 and larger" you go to a different strength table than for Dimensional lumber 2-4" thick".

A #2 2x8 in SYP has an adjusted bending strength of 1467 psi and stiffness 1.4
A #2 6x6 in SYP has a bending strength of 850psi, stiffness 1.2

Deflection, stiffness often controls.

Where I'm going with all this is, it is kind of complicated. The easiest way to answer it is to try your example in both calcs.
Heavy timber calc
Dimensional Lumber calc

SpaceBus · July 23, 2020, 05:30:28 PM

Quote from: Don P on July 23, 2020, 04:22:32 PM
Its actually a pretty easy equation to burn into your brain for the basic geometry behind beam strength bd²/6, bee dee squared over six. breadth x (depth x depth)/6

That is the section modulus of a rectangular beam, the higher the number the stronger that shape is.

That and strength of the particular wood line up on their side of the ring when you're figuring out a rafter, joist or beam. Load and span are tag teaming it out of their corner. If you do it right the first team wins ;D

Am I doing something wrong, my math ends up with the 6x6 being stronger?

hopm · July 23, 2020, 05:40:22 PM

Me too....by the numbers I got over 50% greater

Don P · July 23, 2020, 06:03:24 PM

Sorry I was slowly editing the post above while y'all were ciphering, reread it then go again.

SpaceBus · July 24, 2020, 07:44:32 AM

I'm saving the links to the calcs, but where did you find them?

btulloh · July 24, 2020, 07:54:34 AM

Click on the red tool box on the left, at the bottom of the sponsors list. Takes you to these calcs and a bunch more. The Forestry Forum Toolbox.

Sixacresand · July 24, 2020, 08:27:02 AM

if you put the ends of a board on cement blocks and it can support your weight in the middle, then it is strong enough. Even after that test, hopefully, no one would try to stand in the middle of any one rafter twelve feet up.

Don P · July 24, 2020, 09:05:56 AM

Proof loading is a good real world way of checking things but a man standing on a board may or may not be a realistic proof load. We have one member here with a 300 pound per square foot snow load. If the rafters are on 2' centers then there is a 600 lb load per foot of length on that rafter. Even here in the mountains of VA the snow load can exceed my weight on a rafter. Typically in wood the proof load is 2x design load to provide a factor of safety. A rule of thumb to remember is that a center point load produces 2x the bending moment compared to a uniformly distributed load. The 200lb man standing in the middle of the board is the same as a 400 lb snow load spread out over the length of the rafter.

During construction I've looked up to see 2 guys in the middle of a rafter sitting on a couple of squares of shingles, a quick half ton.

I was in a barn yesterday with at least a half dozen broken rafters, several ties pulled out of their nailing. First order of business was get this equipment out of here and don't bump anything, the angels are bound to be very tired.

Tom King · July 25, 2020, 03:24:55 PM

I got called to look at a sagging roof, on an 1850 house. The sag was noticeably severe, but the house had been standing for 165 years.

When I went up in the attic, and turned on my utility light, the hair on the back of my neck stood up, and chills ran up, and down my spine. The rafters were 3x3's, 20 feet long. Fortunately, they were all clear Heart Pine, so the tensile strength was keeping the roof up, but some had split, as you can see in the picture.

We built massive trusses in place, using 2x12's, and 2x8's. The trusses tied to the ceiling joists in the middle of their span, which was also the middle of the rafter span. We pulled those boards up through a 14x18" original access door into the attic. Fortunately, the little hole was in line with the back steps, but we could just barely get 16 footers up there.

With the trusses in place, I eased the roof back up to flat with opposing wedges on top of the trusses, and tied them to the trusses. The middles of the hips had to be raised with hydraulic jacks, and had sagged over 5 inches.

This picture is when the trusses were in process. You can see one of the split rafters to the right.
They kept us there working for another 2-1/2 years, on that house, after we fixed the roof.

Don P · July 25, 2020, 04:42:10 PM

"They don't build them like they used to, we have laws against that now :D"

Durf700 · July 25, 2020, 09:25:39 PM

I've tried to figure out the calculator for beam size but didn't have any luck. I need to build a beam for a 20' overhead door for the mill. I am using eastern hemlock for the barn. I have a log already picked out to mill and should be able to get a 6x12x20 beam out of it. I also plan on installing 6x6 angle bracing at a 45 degree angle which will be mounted 6 feet in from each end of the 20 foot span going down and secured to the 6x6s on each side. so the center area of the opening with no bracing under it will be 8 feet.

am I going to be ok? my local snow load for stick building is 65lbs , however manufactured homes can have a 30# snow load according to my state snow load map at work.

my barn will have a 5/12 pitch roof with colored steel so snow typically comes off quite quick. I have the same barn at my home currently and the snow is almost aways off it within a day of a storm.

thanks

Don P · July 25, 2020, 10:19:47 PM

Hoping the building will last long term you're designing for the bad day, the day snow sticks. I use the full design snow load and then add 10-15 psf for the weight of the materials. From the sounds of it I'd use 75psf.

It sounds like half of the truss is bearing on the rear wall and half of each truss is bearing on the beam. Half the truss width x the 20' beam span = the area supported by the beam.

Multiply that tributary area in square feet x 75 pounds of load per square foot = the load on the beam.

That should get you going, holler back with the truss width if you get stuck.

Durf700 · July 26, 2020, 09:34:26 AM

Quote from: Don P on July 25, 2020, 10:19:47 PM
Hoping the building will last long term you're designing for the bad day, the day snow sticks. I use the full design snow load and then add 10-15 psf for the weight of the materials. From the sounds of it I'd use 75psf.

It sounds like half of the truss is bearing on the rear wall and half of each truss is bearing on the beam. Half the truss width x the 20' beam span = the area supported by the beam.

Multiply that tributary area in square feet x 75 pounds of load per square foot = the load on the beam.

That should get you going, holler back with the truss width if you get stuck.

the width is 28' wide and length is 48'

Don P · July 26, 2020, 06:13:03 PM

Quote from: Don P on July 25, 2020, 10:19:47 PM
It sounds like half of the truss is bearing on the rear wall and half of each truss is bearing on the beam. Half the truss width x the 20' beam span = the area supported by the beam.

Quotethe width is 28' wide

14'x20'=280 square feet supported by the beam

QuoteMultiply that tributary area in square feet x 75 pounds of load per square foot = the load on the beam.

280 square feet x 75 pounds per square foot load=21,000 lbs supported by the beam.

280sf x 10psf =2800lb dead load

plug it all in here
https://forestryforum.com/members/donp/beamclc06b.htm

Section modulus required 840"³, a 6x12 has a section modulus of 144"³

You are not in the realm of wood of any reasonable dimension. You can play with dimensions but I don't think that's gonna work.

A 4 ply 14" deep LVL should work. Its close, let the supplier check that. A quadruple 16" is plenty.
A 20" or larger midspan diameter log with a light flattening on the top would work
A W12x26 I beam would work.

thecfarm · July 26, 2020, 09:19:49 PM

Could always put a 4x4 in the middle of the span that could be removed for the snow load.

Don P · July 26, 2020, 09:57:14 PM

I did that on the shed roof of the barn here. The double 14" LVL spanning 24' supporting an 8' wide shed with a 1' overhang is ok in bending strength but fails in deflection under full load. I said the same thing, I'll prop it in the winter or when we're going to have a snow or if I pull an engine from the beam. I've banged a post in with the snow already there before (2 local gas station canopies and 5 high tunnels that I know of collapsed that day), I've also had a small block engine hanging on it and then thought Ruh-Roh :-[. A removable post will likely not be there when you need it.

Putting a wall behind the saw and breaking the 28' truss span down would change things. Those reactions would need to come from the truss shop. They may be able to design one that would bear on that wall and not require any posts in front.

Turning the trusses the other way would give a 28' opening that doesn't need a beam.

Durf700 · July 26, 2020, 09:59:48 PM

what about the 6x6 angle bracing I am installing ? doesnt that reduce the span to 8 feet ? the angle bracing will come in 6 feet on each side and reduce the opening to 8 feet. I thought that would help out alot. worse case scenario is I install a beam in the middle like mentioned for when I am not there working.

Don P · July 26, 2020, 10:16:35 PM

There's been discussion of that here. Load goes to stiffness. The beam is on 2 vertical posts at the ends, that is pretty stiff. As the beam deflects it will bear on the angled braces until they hook up and become stiff enough to attract some or all of the load. By that point is the beam deflected to the point that it is in failure or did you get it all tight enough to actually do the work. That isn't lags or nails, we are talking tonnage. and it needs to take load as well as the posts.

The posts have an axial load, the beam sitting on top of them has a load pushing straight down their length. With a brace there is now a side bending load on the post. With a combined load it needs to be stiff enough to resist bending or buckling under that load. I've asked an engineer about this recently, his reply was that he would design for the full span and load.

I'm not saying you can't or that it doesn't work in theory, but that is on your own.

SpaceBus · July 27, 2020, 07:43:24 AM

Quote from: Don P on July 26, 2020, 09:57:14 PM
I did that on the shed roof of the barn here. The double 14" LVL spanning 24' supporting an 8' wide shed with a 1' overhang is ok in bending strength but fails in deflection under full load. I said the same thing, I'll prop it in the winter or when we're going to have a snow or if I pull an engine from the beam. I've banged a post in with the snow already there before (2 local gas station canopies and 5 high tunnels that I know of collapsed that day), I've also had a small block engine hanging on it and then thought Ruh-Roh :-[. A removable post will likely not be there when you need it.

Putting a wall behind the saw and breaking the 28' truss span down would change things. Those reactions would need to come from the truss shop. They may be able to design one that would bear on that wall and not require any posts in front.

Turning the trusses the other way would give a 28' opening that doesn't need a beam.

This thread made me consider trusses for my mill shed instead of beams. Especially since trusses work well with splices.

Durf700 · July 27, 2020, 07:25:27 PM

I have a call in on pricing for a steel I beam 20' long 12" high. if the price is reasonable I will just go with that and be done with it I hope.

Don P · July 27, 2020, 08:57:13 PM

The lightest 12" I beam I got a pass with was a W12x26.
W- wide flange
12- 12" tall
26- 26 lbs per foot, this relates to the thickness of the flanges and web and so the bending strength and stiffness of the beam. So make sure it is at least a 26 lb/ft or heavier. That also makes it easy to know what you need to lift, 520 lbs.

At that point I'd be tempted to go all steel on that opening. It's easy to weld smaller but stronger braces between a steel beam and post to create a very rigid frame that beefs up the whole structure. If it's not inspected I'd also be looking for good used.

Durf700 · July 28, 2020, 08:32:58 PM

thanks for the info. I hope to get a price tomorrow on the beam. if the price is good I can get bracing for each side and weld it together to secure a more solid beam into place in the opening.

thanks

Don P · July 28, 2020, 09:40:05 PM

I was curious, the steel manual lists 29 different weights for a W12 beam, from W12x14 (3/16" web and 1/4" flanges that are 4" wide) all the way up to a W12x335 (1-3/4" thick web and 3" thick flanges that are 13" wide)

The W12x26 has 1/4" thick web with 3/8" thick flanges that are 6-1/2" wide.

mike_belben · July 29, 2020, 11:50:33 AM

Quote from: Don P on July 28, 2020, 09:40:05 PM
all the way up to a W12x335 (1-3/4" thick web and 3" thick flanges that are 13" wide)

So its basically an anvil, extruded!

Durf700 · August 04, 2020, 09:08:41 PM

I called my local engineer that I do alot of work with and am going to install a triple 18" LVL beam thats 20' long. its a little more money vs a steel beam but I figure it will be a little easier to fasten the rafters to it and to secure it to the 6x6s.

Don P · August 04, 2020, 10:16:14 PM

Good deal. I was curious,

QuoteA 4 ply 14" deep LVL should work. Its close, let the supplier check that. A quadruple 16" is plenty.

The section modulus of a 4 ply 14" lvl which was very tight is 229"³, the 4 ply 16" is 284"³, and his 3 ply 18" is 299"³.

Durf700 · August 05, 2020, 08:01:00 PM

planning on stopping down and paying for the 3 ply 18" LVL tomorrow morning for the barn. hoping to get it delivered Friday while I am building my trusses so I can install it. will post a few pics once i get it up and the rafters on.

Durf700 · August 11, 2020, 09:35:24 PM

the 3 ply 20' LVL's are up! they got a bit of weight to them! if all goes well I will post some pics this weekend.. hoping to get my trusses and perlings up saturday. ordered my steel today so now I have to keep up!! can't wait to get the barn done so I can leave the saw at the property. will be alot easier to just run over and saw for an hour or 2 after work if I have an order to do.