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 DIY Chatroom Home Improvement Forum beam extends beyond post

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05-19-2010, 01:42 AM   #1
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## beam extends beyond post

In a different thread, DH discussed having two simple beams vs one continuous one, and another individual asked an intriguing question. If a beam is supported by two posts, and the beam extends past the posts, how does that affect load calcs? Too weird for a simple response? I can see how a uniform load would be very different (maybe my sight is bad, though) from a simple beam, and I can see how having weight only on the "handing out" end of the beam might get screwy, as that would/could cause some lifting between the two posts. So; are load calcs for a beam that is (just) hanging out too complex for discussion here? If not, how does one figure what the beam will carry uniformly loaded and loaded with all the weight on the unsupported part? Thanks. john

 05-19-2010, 08:41 AM #2 Member   Join Date: Jul 2008 Location: NW of D.C. Posts: 5,990 Rewards Points: 2,000 You have particulars or you just want a general formula? Your posting a diagram would be good.

05-19-2010, 03:23 PM   #3
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## general

Yoy: Nothing specific. Say a beam, parallel to a house, is supported by 3 posts. The posts are all X apart, and the beam extends past the posts Y on each end. One post is at the center of the beam. The beam supports dimensional lumber joists that are attached to the house on the other end, as in a fairly typical car port. It would seem that the larger Y is, the more radical an impact it will have on the beam between the posts. Again, this may just be too complex for here, but I thought there might be some general rules-of-thumb, like "Don't have Y more than 1/3 of X" for a uniform load"; or "Don't put more weight on the end of the beam that Z% of the uniform load capacity of the beam". Probably are not any rules of thumb, but I figured I'd ask. Thanks. john

 05-19-2010, 03:38 PM #4 Member   Join Date: Jul 2008 Location: NW of D.C. Posts: 5,990 Rewards Points: 2,000 It'll be a while, unless DH beats me to it. I'd say there are values for Y and w [uniform loading in pounds per inch] where the center post does nothing. Plus, it's symmetrical about the center post. This should be easy.
 05-19-2010, 05:21 PM #5 Civil Engineer   Join Date: Mar 2009 Location: Boston Posts: 5,832 Rewards Points: 5,246 The particular situation you describe is known as a cantilevered beam. The part of the beam that extends beyond a support is the cantilever part. The analysis of continuous beams over multiple supports that are cantilevered is relatively complex, because the beam is not statically determinate. You are correct in believing that the position of the loads has a major impact on the moment diagram and the deflection. Actual design of such a beam is complicated due to the need to determine the most severe loading condition, the fact that negative moment typically develops over the supports, and the fact that negative moment can develop over an entire span during installation. There are charts and diagrams for the simplest conditions, some of which are available on line. The good news is that for typical house construction, the minimum required size shown in the building code for continuous beam situations such as headers over multiple supports are conservative. The bad news is that cantilevered beams are typically not discussed in the code, and need to be individually designed using formulas too complex for me to discuss in this forum.
05-19-2010, 06:49 PM   #6
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## got it

Dan: thanks. i figured it was more than pi(r squared). done deal. j

 05-19-2010, 07:13 PM #7 Member   Join Date: Jul 2008 Location: NW of D.C. Posts: 5,990 Rewards Points: 2,000 This link is close to what you want. http://www.fpl.fs.fed.us/documnts/fplrn/fplrn060.pdf I guess if I can get their answers I'm on the right track.
05-19-2010, 10:05 PM   #8
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## roger that

Yoy: thanks. will read when i get some time. j

06-11-2010, 07:47 PM   #9
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Quote:
 Originally Posted by jklingel Yoy: Nothing specific. Say a beam, parallel to a house, is supported by 3 posts. The posts are all X apart, and the beam extends past the posts Y on each end. One post is at the center of the beam. The beam supports dimensional lumber joists that are attached to the house on the other end, as in a fairly typical car port. It would seem that the larger Y is, the more radical an impact it will have on the beam between the posts. Again, this may just be too complex for here, but I thought there might be some general rules-of-thumb, like "Don't have Y more than 1/3 of X" for a uniform load"; or "Don't put more weight on the end of the beam that Z% of the uniform load capacity of the beam". Probably are not any rules of thumb, but I figured I'd ask. Thanks. john

Dude its really no complicated at all. What is the dimensions of the beam u should be asking yourself. and u really don't want it to overhang more than 1ft past the post if it were 4x10 beam

06-11-2010, 08:42 PM   #10
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Quote:
 Originally Posted by jerecri Dude its really no complicated at all. What is the dimensions of the beam u should be asking yourself. and u really don't want it to overhang more than 1ft past the post if it were 4x10 beam
And exactly how did you come up with this 1' measurement ?

 06-14-2010, 10:20 PM #11 Member   Join Date: Feb 2010 Location: MI's Western UP Posts: 603 Rewards Points: 506 I did all tat crap back in college. it can get complicated to do the long way, but I sort of remember some cheater formula for a simple case like you are describing. the general rule of thumb for construction is have it overhanging no more than 1/3 of the overall length of the board. so if you want a 4' overhang, make sure you have another 8' going into the structure.

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