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Maximum Load of a Beam

  1. Oct 8, 2013 #1
    1. The problem statement, all variables and given/known data

    The bar 1 in the figure has a cross-sectional area of 0.75 in^2. If the stress in this bar must be
    limited to 30 ksi (30,000 psi) determine the maximum load that P that can be supported by
    the structure.

    2. Relevant equations

    σ = F/A
    τ = Fd

    3. The attempt at a solution

    σ = F/A
    F = (30,000 psi)(0.75 in^2)

    τ = Fd
    τ = [(30,000psi)(0.75in^2)](6ft) --- torque/moment about B.

    Then equate to second torque as change distance and force, but still same torque. To clarify, I mean τ = F1d1= F2d2
    aka τ = (force in beam 1)(6ft) = (P)(10ft)

    τ2 = F2d2=(30,000psi)(0.75in^2)(6ft)
    F2(10ft) = (30,000psi)(0.75in^2)(6ft)
    F2= (30,000psi*0.75in^2*6ft)/10ft
    F2= 13500 lb ft

    That should be reasonable as the original force (@ beam 1) would have been 30000*0.75 = 22500 lb ft. Since the distance to P is larger, the force should be smaller (and it is) to keep the torque the same.

    Is there anything wrong with this answer or my method? We are studying considerably more difficult material, so I question the above easy solution.

    Attached Files:

    Last edited: Oct 8, 2013
  2. jcsd
  3. Oct 9, 2013 #2
    Yep your work looks right to me. This is a pretty basic problem so if you know what your doing it will seem easy.
    Don't worry though you'll get more difficult questions eventually.
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