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Old 03-08-2012, 11:45 AM   #1
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Help calculating beam load


Hi,
I have an existing enclosed patio and would like to turn the roof into a deck/balcony.

The patio is 14'x22' with a 6x14 beam held up by 6x6 posts 14' away from the house. The beam is running parallel with the house with 2x10 joists supported by the house itself and the beam.

Assume the live load and dead load on the building's roof is 50 PSF, may be 60 PSF (I live in California but I need to double check this).

The weight bearing on the beam should be: 14*22*50/2 = 7,700 pounds, since the beam only holds half the roof's weight.

Assume allowable fiberstress of beam is 8.
Maximum bending moment: Mmax = WL/8
Mmax = 7700 * 22 / 8 = 21,175 ft/lbs
(Should this length be 14 ft not 22 ft?)

Section Required = Mmax X 12 / Fb
21175 ft/lbs * 12 in/ft / 1000 PSI = 254.1 cu in

Section modulus of my beam is: S = bd2 / 6
S = 6 * (14*14) / 6 = 168 cu in

Either my math is wrong or this big, beefy 6x14 beam isn't nearly as strong as I would suspect.

Deflection and sheering are more than adequate with the 2x10 joists.
b = 1.5
d = 9.25
Deflection: D =(5 * W * L3) / (384 * E * (b * d3/ 12))
Shear =(( 3 / 2 )* V ) / ( bf * df )

Thanks in advance.

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Old 03-08-2012, 12:25 PM   #2
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Help calculating beam load


Please this is a DYI forum most pro's here are going to tell you open the yellow pages consult a engineer get your plans stamped with there seal bring it to the building department and enjoy life. No one is going to go out on limb here and say yep that will work.

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Old 03-08-2012, 01:19 PM   #3
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Help calculating beam load


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Originally Posted by Nailbags View Post
Please this is a DYI forum most pro's here are going to tell you open the yellow pages consult a engineer get your plans stamped with there seal bring it to the building department and enjoy life. No one is going to go out on limb here and say yep that will work.
I plan on getting permits for it but I like to do my homework first before bringing my plans to an architect/engineer.
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Old 03-08-2012, 01:29 PM   #4
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Help calculating beam load


Your math is a bit oddly presented, but the beam is undersized based on the loads you presented. However, there are a few unusual figures in your calculations you need to check. First off, is the beam really full sized 6x14, or is it three 2x14 nailed together, which would mean it is less than 6x14? Or maybe it is a glulam or parallam? And it seems you are using 1000 psi as the allowable fiber stress on the beam, even though in line 7 if your post you list the allowable stress as 8, which I assume is a typo?

Regardless, I cannot get into the business of reviewing static computations for mission critical applications such as a structural beam. I do suggest you verify all dimensions and review the allowable stress on the beam.
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Old 03-08-2012, 01:52 PM   #5
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Help calculating beam load


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Originally Posted by OniKoroshi View Post
I like to do my homework first before bringing my plans to an architect/engineer.
Good for you but in the US you might be accused of being anti-business. The best customers are the ones who are dumb, uninformed and don't want to be informed.
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Old 03-08-2012, 02:58 PM   #6
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Help calculating beam load


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Originally Posted by Daniel Holzman View Post
Your math is a bit oddly presented, but the beam is undersized based on the loads you presented. However, there are a few unusual figures in your calculations you need to check. First off, is the beam really full sized 6x14, or is it three 2x14 nailed together, which would mean it is less than 6x14? Or maybe it is a glulam or parallam? And it seems you are using 1000 psi as the allowable fiber stress on the beam, even though in line 7 if your post you list the allowable stress as 8, which I assume is a typo?

Regardless, I cannot get into the business of reviewing static computations for mission critical applications such as a structural beam. I do suggest you verify all dimensions and review the allowable stress on the beam.
It's a solid 6x14 beam.

My original calculations had 1150 psi but I figured 1000 psi to be safe. I forgot to change the allowable stress in line 8 to accommodate for that.

I got my calculations from the forestyforums.
http://www.forestryforum.com/members...beamsizing.htm
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Old 03-08-2012, 03:55 PM   #7
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Help calculating beam load


You actually have a full size 6x14 dimensioned lumber beam. Quite large, but as you discovered, inadequate. Dimensional lumber of that size has other issues sometimes, like twisting and splitting, which should be checked before considering its use. Most folks use glulams or parallams when they get that large due to superior stability and higher strength, but you already have the beam in place.

The figure of 50 psf for roof load is pretty high, although totally reasonable if you live in high snow country. Where I live, snow load is about 32 psf, plus about 10 psf for dead load from framing, makes a total of about 42 psf design. Course I am in MA, we may not get as much snow as you, but two years ago when we had up to 4 feet of snow on roofs, there were numerous collapses due to inadequate beam size.
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Old 03-08-2012, 04:34 PM   #8
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Originally Posted by Daniel Holzman View Post
You actually have a full size 6x14 dimensioned lumber beam. Quite large, but as you discovered, inadequate. Dimensional lumber of that size has other issues sometimes, like twisting and splitting, which should be checked before considering its use. Most folks use glulams or parallams when they get that large due to superior stability and higher strength, but you already have the beam in place.

The figure of 50 psf for roof load is pretty high, although totally reasonable if you live in high snow country. Where I live, snow load is about 32 psf, plus about 10 psf for dead load from framing, makes a total of about 42 psf design. Course I am in MA, we may not get as much snow as you, but two years ago when we had up to 4 feet of snow on roofs, there were numerous collapses due to inadequate beam size.
I live in southern California so the chances of snow are a little low
I thought 50 psf for roof load was something standard for a walkable roof that needs to hold the weight of people and minimal furniture. If the psf of live and dead load is much less than maybe this beam will suffice as long as it's in good condition. The patio is getting torn down to the main structure so I'm just trying to get a general idea of what needs to be done. I'll have some real numbers after I visit the permit office next week but I'd like to come prepared with as much information as possible. Thanks again.
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Old 03-08-2012, 06:27 PM   #9
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Help calculating beam load


What is your location (city)? If you are in a D (sub 0,1 or 2) seismic zone, California requires this to be designed by a licensed engineer since this on the second floor.

And I don't believe there are any parts of So. Cal. that are in anything but a D zone.

You or a designer can do the architectural stuff though.

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Old 03-09-2012, 12:54 AM   #10
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Help calculating beam load


I'll find out more information about what needs to be done since I'm in a D zone when I visit the permit office next week.

As for my calculations, I definitely was doing the wrong calculations. After briefly speaking with an architect, I now know that 6x14 beam is grossly overkill for my application. The 16" o.c. wall underneath the beam would create many 16" wide tributaries. I was calculating a load for a beam only supported by posts.

He did mention that even though the 2x10 joists are structurally adequate to support the load of being a deck, the current code for decks may be overkill at around 100 PSF and might require me to switch to 2x12 joists or sister some more 2x10's to the joists.
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Old 03-09-2012, 06:46 PM   #11
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Help calculating beam load


Quote:
Originally Posted by Yoyizit View Post
Good for you but in the US you might be accused of being anti-business. The best customers are the ones who are dumb, uninformed and don't want to be informed.

i totally agree with this statement.. i dont know how many times ive dealt with clients that watched a few too many episodes of holmes on homes and felt they knew everything they needed to know.. it creates nothing but problems for the builders.. these guys are the pro's and if they truly are, they consult the people that can give them the correct info.

regarding usage of conventional lumber.. for anything relating large spans its next to non existant nowadays.. the lumber available is very low quality and like someone else stated. the larger the peice of wood the less stable it will be. .engineered lumber is the standard practice for a very large beam which will be under a great deal of stress.

locally nothing less than 3 ply 2x10 with posts no more than 7' apart will pass an inspection.. for joists , anything spanning more than 6' must be 2x8 , 12" o.c with blocking in between. as for where your located i dont know.. i highly suggest you dont even mention your calculations for your project when you do consult a engineer. just do up sketch of what is there now and what you would like the new design to be with no mention of what you beam sizes and joist sizes will be. it will make the meeting go much smoother
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Old 03-22-2012, 04:11 PM   #12
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Help calculating beam load


Quote:
Originally Posted by OniKoroshi View Post
Hi,
I have an existing enclosed patio and would like to turn the roof into a deck/balcony.

The patio is 14'x22' with a 6x14 beam held up by 6x6 posts 14' away from the house. The beam is running parallel with the house with 2x10 joists supported by the house itself and the beam.

Assume the live load and dead load on the building's roof is 50 PSF, may be 60 PSF (I live in California but I need to double check this).

The weight bearing on the beam should be: 14*22*50/2 = 7,700 pounds, since the beam only holds half the roof's weight.

Assume allowable fiberstress of beam is 8.
Maximum bending moment: Mmax = WL^2/8
Mmax = 7700 (pounds/foot) * 22 ft / 8 = 21,175 ft/lbs (ft-lbs)
(Should this length be 14 ft not 22 ft?)

Section Required = Mmax X 12 / Fb
21175 ft/lbs * 12 in/ft / 1000 PSI = 254.1 cu in

Section modulus of my beam is: S = bd2 / 6
S = 6 * (14*14) / 6 = 168 cu in

Either my math is wrong or this big, beefy 6x14 beam isn't nearly as strong as I would suspect.

Deflection and sheering are more than adequate with the 2x10 joists.
b = 1.5
d = 9.25
Deflection: D =(5 * W * L3) / (384 * E * (b * d3/ 12))
Shear =(( 3 / 2 )* V ) / ( bf * df )

Thanks in advance.
More than a few problems in there....


Consult a licensed professional. I admire you for looking to learn, always a good thing, but its best to learn from someone who is capable of doing the work and is able to see the project.
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Old 03-22-2012, 05:13 PM   #13
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Help calculating beam load


I missed the little detail that you are planning to build a deck, I thought it was a roof. Deck loads are much higher than roof loads, as you determined, although 100 psf is NOT universally used, in my town the live load is 50 psf per the building inspector. One thing I don't quite follow is why you have a full size 6x14 inch beam supported on a conventional stud wall, or did I miss something?
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Old 03-22-2012, 09:52 PM   #14
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Help calculating beam load


Quote:
Originally Posted by Daniel Holzman View Post
I missed the little detail that you are planning to build a deck, I thought it was a roof. Deck loads are much higher than roof loads, as you determined, although 100 psf is NOT universally used, in my town the live load is 50 psf per the building inspector. One thing I don't quite follow is why you have a full size 6x14 inch beam supported on a conventional stud wall, or did I miss something?
As I understood it, this was a question of the roof, which would correspond to the 50 psf loading (though I have no knowledge of his local codes).

I too was confused about the stud wall, as it seemed in the original post that the beam was to be supported on each end by posts. Perhaps he was referring to the portion of the roof at the house? A bearing wall supporting the exterior side of this roof would certainly drop the member size of the exterior "beam" by a large factor.

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