Pole barn 32x40 clearspan beam size

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Old 05-04-04, 09:36 AM
profishguide
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Question Pole barn 32x40 clearspan beam size

I'm building a 32'x40' pole barn using 6x6 treated posts on 8' centers and would like to use a glue-lam beam across the 40' length, but am not sure how to calculate the size needed to support the roof or if I need to increase the support post size.
Thanks, Frank
 
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Old 05-04-04, 09:28 PM
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Could you clarify your plan? Is this the ridge beam you are proposing?


What amazes me is how many folks want to create a large structure without (apparently) having set foot in an equivalent structure.

Reverse-engineering is a wonderful thing.
 
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Old 05-05-04, 10:58 AM
profishguide
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Beam size

This will be the ridge beam. I live in a sparsley populated area of alaska, but plan on a trip to town soon to look at other similar sized buildings. There are no building regs out here, but I have looked at some large metal buildings blown down by 90 mph winds we had two years ago and another built out of rough cut spruce logs (ridge beam type) that looked pretty simple to build. I planned to use posts sunk 6' and set in concrete. I always like to err on the safe side, so overkill is OK.
Frank
 
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Old 05-05-04, 07:54 PM
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Hmmm....

In theory the ridge will be supporting half the roof, say 16 by 40, which is 540 square feet, times oh, 40 lbs/foot snow-wind loading, which is coincidentally 540 pounds per lineal foot of beam loading. I haven't really considered the pitch of the roof, since I am a rank amateur.

I suspect that you may have excellent access to extension service literature on things such as panel-lumber beams built of dimensional lumber and plywood.

The reall challenge is that 40 foot freespan in combination with the 540 pounds lineal load. Don't take my word on that 540 figure, since I don't have access to what your local authorities like as wind/snow loadings.

I have tables of panel-lumber beams and 540 over that distance will be a challenge for any material easily transported. I myself have spent little time looking at warehouse ceilings, but ridge beam and rafter constructs have limits. Perhaps best to consider a shed roof with rafters and purlins.

Your rafters would span 32 feet, with the spacing a function of rafter capacity and purlin capacity.

Let's say a rafter every 8 feet, resting upon your posts, so the rafter load would be 320 lbs/ft. This is much easier to achieve. Purlins cross the rafters and support the roof decking.

I suspect your local resources have plenty to offer. Check out the colleges and extension services. Don't forget libraries.

Of course, if you have access to something that would support 540 lbs/ft over 40 feet, then your plan would work.
 
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Old 05-09-04, 04:34 PM
profishguide
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Rafters

Can I make my own rafters? My lumber mill can only make 20' long boards, if so are there any good sites for plans?
 
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Old 05-10-04, 10:34 AM
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according to the info

that I have this would require a 6 3/4" x 24" deep glulam beam. This is based on a 2400 fb beam. That is IF 40 psf is enough load for alaska?. I would suggest buying wood i beams which come in 48' lengths instead of building your own.


BTW this beam weighs about 40 lbs a foot so you might want to consider a crane!
 
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Old 05-10-04, 08:06 PM
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Brainstorming...

I suspect that salvage materials such as bar joists are spoken for before the building is dismantled, but if such materials are available they may be cost-effective.

How valuable is your freespan? Things get easier with columns, even one column. If you insist on freespan, what is the minimum headroom under your joists? The deeper the truss the stronger the truss, and when it's truly deep (say 6-8 foot) a fabricated truss can really hold some weight. Think bridges.

There was a 'golden age' of wooden arch trusses, I suspect some buildings still exist in your area. Neglecting the possibility of obtaining the same quality of wood, reverse-engineering the trusses would be simple. Get permission to climb around in the rafters and make plenty of notes.

As I stated earlier, if you could tolerate one (substantial) column in your building, a nominal 40 by 40 wouldn't be so hard.


Thanks to aUniqueName for his contributions (I suspect his name has an 'alphabet' suffixing).
 
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Old 05-12-04, 09:25 PM
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I have been thinking about this situation and have additional thoughts.


First and foremost, how necessary is a large freespan? Why is it valuable?

I suggest a rethinking of what you need, and when you need it. An incremental approach to this project may produce benefits.


If you plan to work in your own harvested timber that will tend to slow you down. You may not have a place to work. You may not have a place to store your lumber while it dries and ages.

If you were to build a smaller, and taller building, you would have a useful building faster. As the lumber accumulates, attach shed-roof extensions to two opposing sides. This style is found everywhere, from cathedrals to lumber yards. The second story/loft would be a good place to stage materials as you build the extensions.

Your current proposal is for 32 feet by 40 feet of freespan. If you have a bushplane to shelter, I can see the need, MAYBE.

Let's say it's a bushplane 'requiring' the freespan. Are you sure you can't pull it tailfirst and 'spin' the plane past a column and tuck it into the building?

It might be easier to build a turntable than 40 feet of freespan in the wilderness.
 
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Old 05-15-04, 10:43 AM
profishguide
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Thanks for all the info

I'm now going to add a support in the center, which drops my beam length to 20' and makes things alot easier.
 
  #10  
Old 09-14-11, 03:55 PM
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Barn

You said "posts on 8 ft. centers". Is this referring to the side walls along the 40 ft. dimension? You also said "clear span". Does this mean you will use 32 ft. long roof trusses? If so, the roof weight will be supported by the beams running from post to post along the side walls. Ridge beams are not used with roof trusses. Please clarify.

In the calculation above 16 x 40 is 640, not 540.
 
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Old 10-07-11, 02:32 PM
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Let me throw a new wrench into the works. Using a liveload of only 40 psf may not be wise. Unless the location is in some hidden "banana belt" of Alaska. I did a few building retrofit designs in Colorado a few years ago, where the County (LaPlata) design snow load by itself was 110 psf. Might want to use a beefier (than 40) number, just to play it safe. Could be an expensive lesson, and you'd be a very unhappy camper if the thing comes crashing down some day. Be careful out there!
 
 

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