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Which beam is stronger?

  1. Aug 9, 2012 #1
    This is for a deck:

    Which beam is stronger?

    A) a beam with cross-section dimensions 4"x10" and 8' long
    B) a beam with cross-section dimensions 4"x12" and 12' long

    Both beams are same material and are both simply supported at the ends. Same load type (either concentrated or distributed doesn't really matter) and same load location.

    Here are my thoughts:
    Moment of inertia for beams: A: 333.33 in^4; B: 576 in^4
    Lengths of beams: A: 96 in; B: 144 in

    Using the equation for a concentrated load at the center: P = (deflection*48*E*I)/(L^3)
    The deflection of beam A is about half the deflection of B. Does this mean A is stronger with the given loads?
  2. jcsd
  3. Aug 10, 2012 #2
    A good way to compare is to use what is known as the section modulus (usually given the symbol Z or S)

    Values of Z are often tabulated in structural tables, but can easily be computed as the ratio of the moment of intertia to the distance from the neutral axis to the edge of the beam (c).

    [tex]Z = \frac{I}{c}[/tex]

    In the case of a rectangular beam c=h/2 where h is the height of the beam.

    If M is the maximum moment on the beam ( computed directly from the loads and geometry of the beam) and f is the allowable maximum stress in the material, a particularly simple equation then results.

    [tex]Z = \frac{M}{f}[/tex]

    does this help?
  4. Aug 12, 2012 #3


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    walt88: Yes, that is correct. If both beams have exactly the same material strength, beam A is 1.953 times stiffer than beam B with respect to (w.r.t.) deflection, and 1.042 times stronger than beam B w.r.t. stress.
  5. Aug 26, 2012 #4
    Don't confuse member strength and stiffness with performance under load. The larger section is both stronger and stiffer, but because it is on a longer span, it may be more highly stressed and may deflect more under the same load.
  6. Aug 27, 2012 #5


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    It's true, with some quick calcs it looks like the stress under the same load will be 4.2% higher in the longer beam. In other words, the shorter beam is stronger under the same load.

    It takes more than just the section modulus to compare beam strengths when the length changes too...

    Attached Files:

  7. Aug 27, 2012 #6
    Yes indeed, but this is taken care of in the comparison to the section modulus to the actual applied moment.
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