Max Span Length On 4-2x10 Fir Beam? - Building & Construction - DIY Chatroom Home Improvement Forum

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08-03-2009, 05:20 PM   #1
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## Max span length on 4-2x10 fir beam?

Hey everyone, does anybody know what the maximum span length is on a basement beam that is made of 4-2x10 fir laminated? The spans there now are 9 (which i would like to increase) 10 and 11ft, although, due to markings on the boards, it looks like the 11ft one has been moved sometime in the past. I don't know where to find the span tables for these and I need to get my plan made up to give it to the city. I would like to move it to 11ft if possible, it not I will put another post a ft from the wall so that I can move this one 1 more foot over and out of the TV viewing area.

G

***Sorry that should have read "Max span length on 4-2x10 fir beam?" ,not 'joist' ***

Last edited by gadget463; 08-03-2009 at 05:22 PM. Reason: messed up the title

08-03-2009, 05:57 PM   #2
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You are quoting the "maximum span". That is the maximum and it can be reduced by the load on it.

 08-03-2009, 06:48 PM #3 Civil Engineer   Join Date: Mar 2009 Location: Boston Posts: 5,467 Rewards Points: 4,514 The maximum allowable span on a beam is determined by the following factors: 1. The moment of inertia of the beam, which is determined by the width and depth for a rectangular beam. 2. The maximum allowable fiber stress on the wood, which is determined by the species and grade of the wood. 3. The load on the beam, which is determined by the geometry of the beam, and the amount of weight it is carrying (this includes both live and dead load). 4. For a laminated beam, the maximum span may be limited by the strength of the connections between the individual boards (how strong is the glue, if they are glued, or how many nails are there per foot). 5. The end conditions on the beam, in other words how is it connected at the ends. If there are intermediate supports, those generally need to be examined as well. 6. Occasionally there are issues associated with cross bracing, which is used to limit lateral buckling. So unfortunately, it is not as simple as looking up in a table. You may need to have the beam looked at by an engineer, or at least someone familiar with structural analysis, to determine the allowable span.
 08-03-2009, 06:49 PM #4 contractor   Join Date: Jul 2009 Location: Boise, ID Posts: 20 Rewards Points: 10 max span is determined by the load it is holding up. what's up from that point, how much weight is it holding? and what are the posts sitting on? if you put a post on thin concrete you may crack it and get some sag. If it's over a wooden floor, you may need to increase the framing under it to hold it up. sounds like you may be in over your head on this one. (nothing personal, just don't want you to drop your house on your head.)
 08-03-2009, 06:51 PM #5 Newbie   Join Date: Aug 2009 Location: Saskatchewan Posts: 2 Rewards Points: 10 I appreciate that, I was just trying to prepare my drawing for the city to get a permit, but i didn't want to get an engineer involved this early but I guess I may have to. Thanks for all the advice. G

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