Post by thread:[EE]: Best place to weaken a beam

Simply placing reinforcing OSB over the slot is not likely to replace the functionality of the top fibers of the beam. You might consider replacing the board with a steel beam that is not tall, and then placing wood between the beam and floor for fastening and support, except where you want the 'slot': _____________________________________ |wwwwwwwwwwwwwww| |wwwwwwwwwwww| SSSSSSSSSSSSSSSSSSSSSSSSSSSSS Underscore = floor w = wood S = steel beam Alternately replace the hypothetical beam with wood - you'll need a wider beam of wood, though. I don't know what the loading factor is, but for instance you might find that 2 2x8's equal the capacity of one 2x10, though there will be greater flexing. We could give you more detailed answers (or alternatives) if you tell us what the slot is for. -Adam On 10/10/07, Cedric Chang <spam_OUTccTakeThisOuTnope9.com> wrote: {Quote hidden}> I am considering cutting some slots into some floor beams and > reinforcing above the slots with OSB. My question is ....... where > is the safest place to cut a slot ???? near the supporting wall or > out in the center. I am thinking it would be better to cut the slot > near the supporting wall ; I don't have my college texts on static > systems anymore and I can't really support my thoughts. Maybe it > does not matter where the slot is cut ! > > Best > > Cedric! > -

Peter Todd <.....peteKILLspam@spam@petertodd.ca> wrote: > On Thu, Oct 11, 2007 at 10:00:22AM -0600, Cedric Chang wrote: > > I really appreciate the replies and will let you all know what I do > > and what happens. > > One further comment: I have a lot of faith in OSB since I have seen > > many I-beams built out of them. > > I have a lot of respect for engineers because they manage to build I-beams > out of OSB. :) The engineered I-beams I've seen use OSB for the "webbing" between the top and bottom rails. The real strength of the beam is in the rails, which are usually solid wood. The webbing just keeps them parallel, and resists the compressive load across the height of the beam, which requires relatively little strength. As everyone else has said, the answer is generally "within the center 1/3 of the height, and not near any other weakenings". -- Dave Tweed

Since OSB is a relatively new product, I haven't needed to become familiar with it's properties, and therefore can't say anything about it. But, since there are less parallel fibers in a particular direct, I think we can say that it's allowable tensile strength is going to be less than that of the parent wood specie. Note that plywood has a lower allowable bending when the span is perpendicular to the face grain direction. And yes, if the depth of the beam is less the deflection (springy) will be significantly more since the factor is now the 4th power of the depth. If you can throw enough width at it, you might be able to have the strength and stiffness, but somewhere the actual weight of the beam will become great and not even carry it's own weight. And steel is flexible and has limitations too. The answer to this problem is beyond the scope of this list, and should (must) be left to professionals that can assure that safety will prevail with no loss of life, injury or damage to property. :) This not like trying some electronics, where it is no big thing if a 2n2222 transistor was undersized amperage, blows, and just solder in another or replace with a beefier part. :( I like the definition of Engineering: "The economic application of scientific principles." :) M. Adam Davis wrote: {Quote hidden}> Simply placing reinforcing OSB over the slot is not likely to replace > the functionality of the top fibers of the beam. > > You might consider replacing the board with a steel beam that is not > tall, and then placing wood between the beam and floor for fastening > and support, except where you want the 'slot': > _____________________________________ > |wwwwwwwwwwwwwww| |wwwwwwwwwwww| > SSSSSSSSSSSSSSSSSSSSSSSSSSSSS > > Underscore = floor > w = wood > S = steel beam > > Alternately replace the hypothetical beam with wood - you'll need a > wider beam of wood, though. I don't know what the loading factor is, > but for instance you might find that 2 2x8's equal the capacity of > one 2x10, though there will be greater flexing. > > We could give you more detailed answers (or alternatives) if you tell > us what the slot is for. > > -Adam > > > On 10/10/07, Cedric Chang <ccKILLspamnope9.com> wrote: > >> I am considering cutting some slots into some floor beams and >> reinforcing above the slots with OSB. My question is ....... where >> is the safest place to cut a slot ???? near the supporting wall or >> out in the center. I am thinking it would be better to cut the slot >> near the supporting wall ; I don't have my college texts on static >> systems anymore and I can't really support my thoughts. Maybe it >> does not matter where the slot is cut ! >> >> Best >> >> Cedric! >> --

More to it than that, take a thick magazine or book, and bend it, while allowing the pages to slide against each other. Then bend again, squeezing the ends of the pages together sufficient to prevent sliding, and notice how stiff the book is. This is called shear flow, and the web of the beam must resist this also, and the glued joint is designed for this force. The maximum force of this shear flow at the end of the beam is at the mid height. These subjects are covered in 1st semester of mechanics of materials class, just like Ohms law is covered in 1st EE class. Dave Tweed wrote: {Quote hidden}> Peter Todd <.....peteKILLspam.....petertodd.ca> wrote: > >> On Thu, Oct 11, 2007 at 10:00:22AM -0600, Cedric Chang wrote: >> >>> I really appreciate the replies and will let you all know what I do >>> and what happens. >>> One further comment: I have a lot of faith in OSB since I have seen >>> many I-beams built out of them. >>> >> I have a lot of respect for engineers because they manage to build I-beams >> out of OSB. :) >> > > The engineered I-beams I've seen use OSB for the "webbing" between the top > and bottom rails. The real strength of the beam is in the rails, which are > usually solid wood. The webbing just keeps them parallel, and resists the > compressive load across the height of the beam, which requires relatively > little strength. > > As everyone else has said, the answer is generally "within the center 1/3 > of the height, and not near any other weakenings". > > -- Dave Tweed >