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  1. #1
    Join Date
    Aug 2004
    Location
    Milan Michigan
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    1,633

    Default Beam or Bar joist reinforcement.

    I was at another site the other day and we had a discussion as to how to reinforce a beam, Barjoist or a tube that is laying in the horizontal position being used as a beam.

    The load being a verticle load pushing straight down from the top or being pulled straight down from the bottom.

    Wheather the load is a point load or a uniform load.

    My question is on an I shape or box shape what is the best way to reinforce this member.

    On a beam is it stronger to add steel to the top and bottom flange or is it stronger to add a plate of steel to the webb part of the beam.

    Or if the member was a box tube, is it stronger to plate the top and bottom flange or to plate the sides of the tube calling that part the webb.
    Last edited by Portable Welder; 02-22-2009 at 02:01 PM.

  2. #2
    Join Date
    Jul 2006
    Posts
    1,508

    Default

    Not too sure about box tube. Iíve always been told by engineers that an S-shape will fail to one side or the other first, so adding plates to the top and bottom flange will strengthen an S-shape / I-beam.

  3. #3
    Join Date
    Jul 2006
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    Default

    Should have added this in the first post!
    Attached Images Attached Images

  4. #4
    Join Date
    Aug 2004
    Location
    Milan Michigan
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    1,633

    Default

    Thanks for the picture sonora iron.

  5. #5
    Join Date
    Feb 2008
    Location
    North of Phila. PA
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    404

    Default

    You can strengthen a beam in 2 ways.

    1st, you can thicken the flanges as shown above. This increases the strength while not significantly increasing the height.


    2nd You can deepen the beam. Sometimes by building a T on top of the area of the greatest stress. This works for point loads mostly.

    A taller beam will weigh less than a thicker thinner height beam and support more weight. The key is that you separate the flange by a greater distance. I can't lay my hands on my steel table right now, or I'd give you a good example on how much strength gets added just by increasing the web height vs increasing the flange thickness.

  6. #6
    Join Date
    Jul 2006
    Posts
    1,508

    Default

    I donít think so.
    Why will an H-beam / wide flange take more load than an S-shape?
    The width of the flanges.

    Example: (just a quick look, to find two beams close in size and weight)
    24 x 14 x 130# wide flange evenly loaded 15-feet span will take 294,000 pounds.
    24 x 7 7/8 x 120# I-beam evenly loaded 15-foot span will take 223,000 pounds.

  7. #7
    Join Date
    Feb 2008
    Location
    North of Phila. PA
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    Default

    Sonora Iron I'm not sure if your post was in response to mine or not. You are correct in stating that a H-beam is stronger than an S shape of similar size. More steel is in the top and bottom of the H-beam do to it being wider. My statement may have been a bit unclear as far as thickness. I was referring more to if you took the same shape with aprox the same size and jumped up the flange thickness it would be stronger. Widening the flange in effect increases the amount of steel at the maximum load levels and increases the strength over a beam with a narrower flange.

  8. #8
    Join Date
    Oct 2008
    Location
    taxachussetts
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    416

    Default

    I'll try and type this out right. the beam gets it carrying load from the web and the lateral load from the flange. there is some big deferences between a h beam and a I beam or w beam. your best way to make at beam stronger in a renovation is put a thick web c channel in the web of the beam. (example 8" i beam you'd put a 6" channel ) i hope i wrote that so you can understand.
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  9. #9
    Join Date
    Sep 2005
    Location
    16919 Pole Rd. Brethren, MI 49619
    Posts
    4,364

    Default

    Loading from the top or pulling down a plate on flange on top of the beam as in the pic is the easiest.

  10. #10
    Join Date
    Jun 2004
    Location
    nj
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    204

    Default

    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.

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