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  1. #11
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    as to the other part of your question, i think it was a question. reinforcing bar joist: I've had to run 5/8 round stock to the four corners of the angel iron this keeps the angle from twisting and letting the bar roll over or bending.
    I've also had to run angle iron top to bottom next to the the center supports to make it handle more load. did i mention 15+years of structural welding.
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  2. #12
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    Quote Originally Posted by enlpck View Post
    Ok. See if I can be clear here without getting into math or calculations...


    There are several factors to consider: The strength of the beam against deflection is primarily from the material farthest from the center, which is usually the flange, so adding material to the flange will increase resistance to deflection (as show in the pic attached above) The flange material is the main element in load carrying until one of the other failure modes is met.

    There are limits to this: shear and buckling.

    The shear strength comes from the web. The loading at most points in the beam is such that the top flange is in compression, the bottom in tension, and near the ends of a beam (point loads near away from the ends, uniformly distributed load, or a combination) this means that the bottom of the web is pulled toward the center of the span and the top is pushed away, putting the web in shear (you can see this by taking book and bending it. Watch where the sheets of paper go) The web resists this shear load, and must be increased in thickness eventually, or it will fail, even if the flanges are sufficient for the tension and compression loads they are under.

    Buckling can occurs several ways, the two most important are when the flange buckles (it begins to wrinkle like cloth and loses its ability to carry shear) and when the beam starts to twist and deflects sideways.

    Which failure mode is more important depends on the loading, other bracing, and the dimensions of the beam. Normally, for a beam that is otherwise unbraced and is long compared to its height, it will buckle to the side. If there is bracing, or the flanges have been significantly thickened, buckling or shear of the web may occur first.

    There are other way for a beam to fail as well (web buckling under a point load, flange deflection from a load applied to the edges of the flange without a path to the web, load applied off the center line of the beam-- which will really tend toward buckling to the side-- and so on), but the two above are the most common.

    If the beam isn't real long without bracing, adding to the flanges as shown above is indicated. As the span gets longer without bracing, other considerations come in. The same general rules apply to box tube, but both sides of box tube act as web, and the sides (being off the centerline) act to stiffen against buckling to the side, so box tube with wall thickness the same as the flange thickness of a W beam will be stronger, a little against bending in the vertical plane, and a good bit against buckling sideways, and a moderate bit against shear.

    As a side note, indiscriminate addition of material to the flange may actually do no good, or weaken the beam, if done incorrectly, as the added material may be strained more than the original material and yield or fail, leading to failure of the original material. I will restrain myself from going into detail unless asked.
    long version of what i said.
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  3. #13
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    There are several factors to consider: The strength of the beam against deflection is primarily from the material farthest from the center, which is usually the flange, so adding material to the flange will increase resistance to deflection (as show in the pic attached above) The flange material is the main element in load carrying until one of the other failure modes is met.

    There are limits to this: shear and buckling.
    Good, good and simple and describes the sequence for lack of better words.

  4. #14
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    OK from what it sounds like judging by what Enlpck, S Berry and vin man have said, and correct me if I'm wrong.

    Its stronger to add metal to the top and bottom flange of a beam instead of putting a plate in the webb area.

    Next question, Does a H or I-beam work the same way as a Bar joist.
    Meanining, Is the top flange under compression and the bottom flange under tension.
    And how would you reinforce this, would you add material to the flanges or would you run a plate standing on end betwen the upper & lower flange.

    Keep in mind there would already be proper X bracing installed along with the typical 1" angle running across from joist to joist and the beams are being reinforced to carry the load of a bigger RTU- Roof top unit.

    So what do you do in this situation to reinforce this beam.

  5. #15
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    Heres another example,

    Some one wants to put a pool table in their basement but theres a column in the way so it needs to be moved 4' down to make room for the pool table.

    The engineer says put a new footing in for the column and this is how you reinforce the beam.

    How have engineers had you guys reinforce the beams.

  6. #16
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    Take your pick, which one is stronger?
    I have no idea!
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  7. #17
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    Forgot this one!
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  8. #18
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    Quote Originally Posted by vin-man welding View Post
    as to the other part of your question, i think it was a question. reinforcing bar joist: I've had to run 5/8 round stock to the four corners of the angel iron this keeps the angle from twisting and letting the bar roll over or bending.
    I've also had to run angle iron top to bottom next to the the center supports to make it handle more load. did i mention 15+years of structural welding.
    Portable Welder: did you miss this one, i gave the answer befor the question.
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  9. #19
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    Quote Originally Posted by Portable Welder View Post
    OK from what it sounds like judging by what Enlpck, S Berry and vin man have said, and correct me if I'm wrong.

    Its stronger to add metal to the top and bottom flange of a beam instead of putting a plate in the webb area.
    Next question, Does a H or I-beam work the same way as a Bar joist.
    Meanining, Is the top flange under compression and the bottom flange under tension.
    And how would you reinforce this, would you add material to the flanges or would you run a plate standing on end betwen the upper & lower flange.

    Keep in mind there would already be proper X bracing installed along with the typical 1" angle running across from joist to joist and the beams are being reinforced to carry the load of a bigger RTU- Roof top unit.

    So what do you do in this situation to reinforce this beam.
    no it's the other way, load is webb and side load is flange. but plating the top or bottom will increase the vertical load at the point of inertia.
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  10. #20
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    Quote Originally Posted by vin-man welding View Post
    but plating the top or bottom will increase the vertical load at the point of inertia.
    Huh?


    Quote Originally Posted by vin-man welding View Post
    no it's the other way, load is webb and side load is flange

    v-m, for the vast majority of situations where a horizontal beam carries a vertical load, the web is NOT the limiting factor. Yes, the flange is what provides resistance to bending under side loads, but this is because the flange is the part that has material farthest from the neutral plane in side load (neutral plane: the plane through the material where no stretching or compressing takes place in bending). The material farthest from the neutral plane (the flange for vertical load in the original example) undergoes the most stretch/compression, and is therefore under the most stress, and is most important in carrying load in cases where shear isn't dominant and there is sufficient bracing to prevent buckling.

    Portable-Welder: the load distribution is generally the same with a bar joist and a beam-- the load top is under compression and the bottom under tension in both-- but the bar joist is much less stiff against side load and buckling to the side. I would STRONGLY suggest getting and engineer to evaluate a situation like this and provide details. That is what he is paid for and why he carries insurance. Each situation is different.

    In the first case you described, I would guess that the solution would be a) replace the joist with a heavier one, b) reinforce the joist by adding material at the top and at the bottom, or c) (most likely is the capacity and space are there, as it is easiest and cheapest) run W-beams ACROSS the joists to spread the load over several of them, rather than one or two. Buckling is likely not an issue for any of these solutions if the cross bracing is sufficient in the first case, unless the load is quite large. In the case of an S-section (I-beam), not much is gained by reinforcing between the flanges in most cases, as they are fairly narrow and thick. In a W-section (H-beam; wide flange) the reinforcement greatly reduces the risk of buckling in longer beams, but is only one of several solutions. If the beam isn't too long relative to its size, it is of little use.

    In the second case, lots of options. A simple case: most likely would be a pair of columns, one to each side of the original, and a beam to catch load where the original was, if the column needs to move sideways. If the column needs to move along the beam, it can get quite complicated, in particular because small increases in span (say from 12' to 16', from moving 4') can require a much larger beam. The size of the beam needed goes up much faster then span for a given load, and can change the limiting factor from allowable deflection to load carrying capacity based on yield, shear, or buckling.

    For more information: See (among many others) _Structures_: Schoedek; _Strength_of_materials_: Den Hartog; _Procedure_book_of_arc_welding_: James F. Lincoln foundation; _Design_of_welded_Structures_: Blodgett; AISC_manual_of_Steel_construction_:AISC;

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