Building a beam - advice on design

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Old 08-09-13, 06:05 PM
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Building a beam - advice on design

Hi Folks,

After getting sidetracked from my deck rebuild (http://www.doityourself.com/forum/de...-old-deck.html) project, I'm back on track.

Deck was torn down (joists weren't even toenailed into the beam - LOL) and footings are in. So now we're looking at raising the posts and installing the beam and I wouldn't mind feedback on how I think I should build the beam.

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The beam (and deck) will be 23 foot wide, and I have posts at the red markers at 1', 8', 15', 22' points (per the image).

I want to build the beam using 3 2X10s, and the rectangles in the image is the top down view of how I think I should put it together.

Thoughts?

Thanks.
 
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Old 08-10-13, 10:39 AM
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Replacing old deck

Sorry to bump this old thread, http://www.doityourself.com/forum/de...-old-deck.html but I tried posting a new thread to ask about the beam, but it's not showing up.

Tore the old deck down without issue. Joists weren't even toe-nailed in to the beam - LOL.

Looking for advice on beam construction. Planning on using 3 2X10s and just wondering about the best way to attach them.



Red dots are the posts with measurements. The rectangles are the top down view of how I thought I should put the beam together. Outer 2X10s cut at 11.5 feet. Inner 2X10 cut so that there's an 11 foot in center and a 6 foot section on either end.

Thoughts?

Thanks
Bob
 

Last edited by lawrosa; 08-12-13 at 10:47 PM.
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Old 08-12-13, 10:51 PM
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When you post pics or links it needs to get approved if the site thinks its spam...

Welcome to the forums.. Your good to go...

The carpenters should be around soon, if not tomorrow...
 
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Old 08-12-13, 10:54 PM
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Thanks Lawrosa.
Please feel free to delete the "hello moderators thread" I started. Was very strange to me.

Great site you guys have, has been very helpful so far.
 
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Old 08-13-13, 12:45 AM
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Hi Bob,
I'm not a pro on loads, but I try to make sure no two seams line up. The two joints in the center can be shifted, one left and the other right to improve the beam, say 15' and 8'. Then the center beam could be 11.5' and 11.5'.

Just a thought. The pros will be along.

Bud
 
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Old 08-13-13, 03:18 PM
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I think I'd leave things just like they are shown. Shifting the joints either way in the center span actually moves them to locations where bending moments are greatest (closer to the two interior supports). For a continuous, symmetrical beam like yours, the negative bending moments at the interior supports are somewhat higher than the positive bending moments at all of the mid-span locations. The greater the bending moment, the greater the stress in the extreme fibers of the member.
 
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Old 08-13-13, 07:22 PM
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Would it help if I added a metal strap or something at the center point where the outside 2X10s will connect?
 
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Old 08-13-13, 10:37 PM
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I'd vote for the "or something" instead of metal straps. Thin metal added near the neutral axis won't appreciably increase the section modulus of the beam. The gain will be almost imperceptible. However, sistering (on the outsides) a few of the 2 x 10 "drops" that you'll be left with would help considerably. The longer the better.
 
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