Beam span

[Neil_B]

I don't check in on this topic too often but thought it would be the best place to ask. I searched around for a span calculator but couldn't find one I needed.
 I am putting up a saw shed and cut a maple into a 6 x 8" x 20' beam for the log entry door. Wondered though, if a 4 x 8 x 20 would work instead as it would square up the beam a little better.
I'm in Canada so snow is a factor but I planned on putting in bracing at least 3 to 4 feet from the corner of the post/beam, at 45deg of course. This would allow me to still get long logs in through the bottom half of the doorway and still support the beam.

Thanks for any advice
Neil

[Jim_Rogers]

In order to do a beam analysis an engineer would need more info, like what is the snow load per square foot for your area?
What other timbers will be attached to this timber?
Is this a shed style roof?
And/or this is just the header over the door?
If it's a header over the door what else will be attached to it, including spacing of these timbers?
Is that hard maple or soft maple?
Are the braces at 3' or 4'? There is a difference and that spacing is important to know.
There is a lot to consider when you're trying to figure out what size beam will be OK.
I have a beam analysis program and if you can tell me the answers to these questions maybe
 I can figure it out for you.
 :P
Jim

[Jeff]

Well looky here. Don P did this one using our base forum calcs.
https://forestryforum.com/members/donp/beamclc4.htm

Pretty cool eh?

[Jim_Rogers]

Very nice.

Is there a link to that some where else for others to find it?
I couldn't find it in the tool box.
Jim

[Jeff]

Jim, at some point, I am going to reorganize the calculators for this website. They are on the TImber Buyers Network right now (we are the offspring so to speak of that website). I will add a dropdown list for them somewhere including the great one that Don has been creating.

tado list stuff...

[Jim_Rogers]

Ok

[Neil_B]

Thanks Jeff, I tried the calc but really didn't know what it was telling me :-[

Maybe I'll go through it again and transfer the info back here but for now I'll answer Jim's questions.

There will be no other timbers on this beam, just the trusses.
A frame roof, maybe just rafters, haven't decided yet. Pitch undecided at this point
Hard maple,
Will put the braces at 4',
So when the beam is on the posts, ash 6x6, there will be 19' span. Corner braces on 4' down the post and 4' into the beam. Final span between braces should be 11' :
Snow load for my area is 33.28 lb/sqft or 1.6 kPa
Neil

[Jim_Rogers]

Neil:
I'm not sure if you understand that in order to figure out how strong you need to make your beam you need to know all the facts and information.
You'll need to decide the size of the roof and where and what will be bearing on this beam.
You'll need to make a diagram like this sample below:



If you can see it you need to know how many square feet at 33.28 will be bearing on this timber. Do you have any idea?
I understand your comment: "There will be no other timbers on this beam, just the trusses." But what shape are the trusses and how many will be resting on the timber?
What's the spacing?
More information is needed to get a correct answer.
Jim

[Neil_B]

Ok, thanks Jim,
Figured it might be easier than that, but if you have a copy of that diagram to email to me I could get some specific numbers for you. All I know for sure right now, is that the shed will be 16' wide so looking at at least 10 feet + of roof on that side of the building with trusses spaced 2' apart.

Looks like I may have to shovel off the roof everyday ???

[beenthere]

New_sawyer
Maybe consider putting a 'gable' end over the wide door entry and a truss above (like a dormer on a house). That way you will not have to deal with the snow load, snow melt, rain gutters, etc. and can get the span strength you will need. Just the weight of the maple beam you are talking about may not be enough to keep it straight (sagging over time) let alone the load from the roof being supported from that side. Just a more expensive thought.

[IndyIan]

Just to add my two cents.
Every timber frame book I've read says not to count the braces to shorten your span in your calculations.  This is to add a safety factor.  Also if your building has high winds hitting it one of your braces may actually be in tension, stressing the beam more instead of helping it.

One thing you do have going for you is that the amount of deflection in the beam can be greater in your saw shed than would be acceptable in a house.  You don't want it look too droopy with a pile of snow on it but your not worrying about cracking drywall on the ceiling either.

Maybe you'll need to use two maple beams or 14"x10" piece of white pine or a steel I-beam.   Wood would be better for a saw shed though ;D

If you play around with the calculator you'll find that the depth of the beam can be more important than the strength of wood.

Ian  

[Jim_Rogers]

Neil:
 If you increase the pitch of the roof to something like a 12/12, a 45° slope and use something like a metal roof the snow won't stay on the roof very long.
I'd be happy to make up a rough drawing of the saw shed for you with a nice little floor plan program I have to show you what it would look like.
Send me a floor plan sketch and some wall details and an idea about what you'd like for the roof and I'll make something up for you.
Jim

[Jim_Rogers]

Neil:
Here is an example of a drawing:



Here is a run-in shed for horses:



These are some drawings that have helped the builders to see what they will look like.

Jim

[Neil_B]

That's cool Jim, do you just have to punch in numbers and the program creates the image? I'll try and get some more done on the plans this weekend and copy them to you.

I kinda like beenthere's idea with the gable as it would match my house. Only thing is it would change the whole layout of the shed. Something to consider though.

IndyIan, I had considered the steel beam but the 24 footer I put in my garage was almost $500. Will have to see if that would fit my "budget"

[Minnesota_boy]

You can seriously increase the capacity of a wood beam by cutting a groove in the top and inserting a piece of plate steel, then drill holes and run bolts through it.  It will keep the visual appearance of the wood beam but have the strength of steel.  The wood beam will keep the steel from twisting under load.  Make sure the steel is a tight fit in the groove.

[Jim_Rogers]

The red barn drawing was created in a floor plan program that cost around $49. You have to tell it how tall the walls are and then just draw the shape in plan view. Then tell it what shape the roof is and what shingles you want on it, and it inserts it for you. Then you have to place each door and window in the location where you want them.
The run-in shed was drawn in another timber framing CAD program, one stick is place in at a time, but sometimes you can do many if the spacing is the same.
here is what the first program made the run-in shed look like:

[Don P]

Here is my read of the building;
!9' clearspan - 228"
10' of building width bearing on the beam.
190 square feet of surface...I rounded to 200 for a quick check. I rounded up the snow load to 50 PSF to add for dead load.
50PSFx200SF=10,000 lb load on the beam
I punched that in and came up with 6.42" deflection and a section size of 64 vs the 247 necessary...the 6x8 is WAY undersize.  I came up with an 8x14 in soft maple as being close.
I specced in a Parallam (on a version of the calc I'm working on) which is stiffer and stronger and came up with a 6x14.

Jim, does that jibe with your program?

[Neil_B]

I think maybe I'd better look for another log . It was the only 20 footer I had around so thought I would try it. Oh well, didn't pay anything for it so maybe I can use it elsewhere in the building.

Thanks a bunch

[Don P]

Hold on though I want Jim to check in, I'm double checking a number that felt wrong, not sure the moment is calculating right... :-[

That's why I'm trying to get us a calc to spec this stuff...not sure this is it yet, its clunky and tough to use yet, but you see the need.

What caught my eye was the 6x8, that is a size often specced for 8' porch carry beams on the log cabins I've done, posts are typically about 8' apart so the span looked big. Span kills a timber fast,  try not to go any farther than necessary. If I remember right, fiberstress squares with span, deflection cubes...you get into trouble faster than you anticipate.

Here's the index to the calcs I've put on the webspace Jeff is kind enough to let us play on.
https://forestryforum.com/members/donp/CalculatorIndex.htm

[Jim_Rogers]

Don P:
I haven't tried to figure anything out because I didn't have any dimension on the size of New_Sawyer's saw shed.

I can understand your clear span dimension, but my drawing of the barn was my drawing from an old customer not the dimensions of New_Sawyer's shed.

If he was to make it that size your figures could be correct in regard to the number of square feet.

Now I took his 33.28 lbs per sq-ft and and rounded it off to 34, and then added 10 lbs per sq-ft for dead load, figure that to be the average for roof decking and shingles. Totaling 44 lbs per sq-ft.
But not knowing how much of the roof was bearing on the timber, and/or the shape of the roof, it's hard to figure.
If it's a hip roof like the above drawing then the amount on the header beam could be less, maybe?
That's why I wanted to know the floor plan dimensions and some idea about the roof design before I tried to figure the actual load on the timber.
Quite honestly, I thought a 6x8 was too small at a 20' span, but I didn't want to say so, until I had some facts to back it up.
I'm kinda glad you said it first.

New_Sawyer if you have some ideas in your head about the size of your saw shed, make up a drawing and email it to me and I'll look at it, as we have discussed.
Adding steel to your current beam might work. If you added some angle iron to the bottom two corners it would help make that 6x8 stronger, or you could make a cut and put in a piece of flat stock as Minnesota_Boy has suggested. That isn't a bad way of making the current beam work.
Putting a gable over the door pushes the weight of the roof over the opening to the sides at the valley rafter connection to the posts. This could help reduce the load for the header beam and make this current timber work.
But you'll have to design the post and valley rafters big enough for the load and make the connection correctly.

It can be done many different ways.
Jim

[Don P]

I went back to the books and checked the calculator, I believe it is putting out good numbers. I would like to walk you all through what it is doing to see if I can give an explanation of how to use it. Kind of working on an instruction page I guess, please give me feedback if this is of interest and how to make it more understandable or useable.

Checking a uniformly loaded simple beam in bending. This is a rectangular wood beam supported on both ends and loaded evenly along its length. A rafter, joist, girder or header is often a beam of this type.

 We need to know what the load is on the beam. Use as an example a door header with a 19' clearspan on a 16 foot wide building that has 2 foot overhangs.



The combined Live Load and Dead Load on the building's roof is 50 pounds per square foot. The area in gray shows the portion of the roof bearing on the beam, 19' X 10'...190 square feet. Multiply that by 50 pounds of weight on every square foot = 9500 pounds bearing on the beam.

In order to find the section modulus required we need to find the maximum bending moment and divide it by the allowable fiberstress for the species and grade we would like to use.

The formula for finding the maximum bending moment is:
 Mmax = WL / 8
 Mmax = (9500 X 19) / 8
 Mmax = 22562.5 ft/lbs

To find the section modulus required we need to divide the maximum bending moment in inch pounds by the allowable fiberstress:
Sreq = Mmax X 12 / Fb
        = (22562.5 X 12) / 1150
        = 270,744 in/lbs / 1150 lbs/sq in
        = 235.4 inches² required

We are looking for a beam with a section modulus of 235.4 in²
The formula for determining section modulus for a rectangular beam is
S = bd² / 6
Lets try a 6 X 8...
S = 6 X ( 8 X 8) / 6
   = 6 X 64 / 6
   = 64
we needed a value of 235.4 or greater, we have 64...not even close. Try again with an 8X14 it comes to 261.3...that works as does a 10 X 12 at 240 in²

That concludes the first check, for extreme fiberstress in bending.
I'll stop there for now and see what you all say...

[Neil_B]

I understand now, Thanks Don. Always helps to have a short explanation to go with the numbers. The other calculators are great to play around with also.

Still trying to decide how I want to lay this out but will try and get some dimensions in soon.

[Jim_Rogers]

Don P:

Very nice instruction about how to figure the beam size.

I did seem to see on error though. Your example is a 14' wide building with a 2' over hang, each side. That's 18', half of 18' is 9'. 9x19=171,171x50=8550.

Mmax (8550x19)/8 =20,306.25

(20306.25 x 12)/1150=211.89 in²

A 9x12 just makes it but I would want more for insurance.

[Don P]

I do appreciate the feedback. I'll keep plugging away at an explanation of the other checks.

19' clearspan on a 16 foot wide building that has 2 foot overhangs

I shrank the picture too much I think, after I read your response I looked at the picture and the 16 optimized to look alot like 14'. I'll clean up the pic for the instruction page and use a bigger font.
Does your program list design values by species? I've had a hard time finding horizontal shear values.

[Jim_Rogers]

Don:
My program doesn't list design values by species, you have to look them up if you use the "custom value" button.
I've got to get a set of values from the big book, or buy a book that has these, like the National Design Specifications book.
I do have some charts from different timber framing books, but not all of them.

Sorry about mis-reading your dimension figure and not re-reading your text instructions.

Jim