The pencil supports the load as a beam, the roof hangs from the beam. This time stick a pencil under the ridge and support it at each end on a stack of books or anything solid. Take the thread bottom chord off the card and set it up as a roof again. Since a piece of string worked we know it is a tension tie.īefore leaving that experiment there is another way to avoid thrusting the walls. Push down on the ridge and the thread keeps the card from being able to spread. Set the model back up but this time run a piece of thread from side to side across the bottom and tape it to each side. If the roof had been on top of walls it would have pushed the top of the walls out. The card's feet slide out, that's the horizontal thrust. Push down on the ridge (the fold in the card). Take a greeting card or piece of cardboard folded like a card and set it on a slick tabletop like a tent or roof. If there is a rigid triangle formed it is locked into shape, triangles do not change shape.Īnother way to see rafter thrust is a tabletop experiment. The tie is usually a ceiling or second floor joist or the bottom chord of a truss. A tie across from wall to wall in the diagram would arrest the horizontal component of the vertical thrust. This picture should help describe what the vertical load on a roof is trying to do.Īs the roof's weight pushes down vertically, the rafters try to push the walls out horizontally so that they can "snap through" and drop the ridge to the ground. I would be happy to answer questions here or in PM. This seemed like it might be the most unobtrusive place to put this. In constructing a deck, I think it would be highly unlikely that going to 4x lumber for floor joists would make any sense at all rather than following conventional construction practice that meets applicable codes and industry standard construction practices.I saw a post that led me to think a ramble on rafter thrust might not be a bad idea. 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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