![]() ![]() This is due to the increase in the moment of inertia by an extra 2 inches of nominal width size. If you examine the tables provided by Dupe, you will determine that going up to the next size 2x increases allowable span much more than use the same 2x at the next closer standard spacing. If your solution is to go to 4x lumber to attempt to fix the problem of an inadequate floor joist, then you probably need to step back and reevaluate your design or question your understanding of the concept of moment of inertia as it relates to supporting floor loads.Ģx lumber works well for standard construction techniques, including the ability to fasten it in a structurally sound method as well as being able to handle it and fabricate with it using common tools. The various organizations providing span tables for 2x construction did so for well thought out and time honored reasons.īasically, if a floor joist is determined to be inadequate to support a given load at a given span, then you must either reduce the span to a safe one for the desired floor joist size, reduce the spacing between floor joists to a lesser but still practical standard spacing, or go to a larger 2x lumber size of sufficient width to safely support the load at the required span under the given load. The lumber yard you called asked a very pertinent question. ![]() The following is posted soley in service to public safety.ĭupe posted important information relating to allowable spans for various sizes of standard 2x lumber. ![]() I don't know what code covers post and beam construction, but that's probably where you need to head. I think regular code doesn't list them because it's cover light wood framing, not heavy timber framing (post and beam). Also if these are sawn beams with no stampings on them, now that's a whole other issue. I would direct you to local building department. Engineered beams take this to an extreme and thus why something of the same dimensions has much higher ratings, they know what they got in that beam, so you can't use their numbers and apply to a sawn beam. The conservative nature makes sense both from simplicity but also when laminating multiple members you have a lower statistical chance of a bad flaw being in the same spot of the beam (one board could have a crack, the other doesn't vs one crack thru it all). So that is one place that touches on this. Now I would assume a 3圆 to be 2.5" thick vs 2 2byes which would be 3", so maybe some conservative nature. It list the same span values for the solid verses the build up. It has a few rows that call out 3圆 or 2 -2圆, 3x10 or 2 - 2x10, etc. There is an interesting table for deck beam spans. So in IRC 2015, most tables for headers and such have a note below them saying to can interpolate for values between those listed. ![]()
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