Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Homework Help: Mechanics of Materials

  1. Jan 16, 2005 #1
    okay, I've been banging my head against a wall for a few hours. It's a bit difficult to describe but here goes...

    There is a rope being pulled up with force P. The bottom of this rope is tied to a bent bar. The bent bar is shaped like an upside down U with the two ends stuck in a piece of fiberglass that has a thickness of t=(3/8) in. After the plywood on each "prong" is a washer and a screw-head. The screw going into the plywood has a diameter of 1/4 in and the washer has a diameter of 7/8 in. Allowable shear stress in the fiberglass is 300 osi and the allowable bearing pressure between the washer and the fiberglass is 550 psi. What is the allowable load on the tie-down?

    The answer is 607 lb and 619 lb but I can't seem to figure how the book got this. Can anyone help me?
  2. jcsd
  3. Jan 16, 2005 #2


    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    [tex]550~psi = \frac {L}{A_{washer}} [/tex]

    [tex]A_{washer} = \frac{\pi}{4} (D^2_{out} - D^2 _{in}) [/tex]

    Allowable load = 2L = 607.46 lbs

    Similarly for the second part, keeping in mind that the relevant area, A for the shear stress is

    [tex] A = \pi D_{out} \cdot t [/tex]

    This will give you load = 618.50 lbs

    Do you understand why this is so ?
  4. Jan 16, 2005 #3
    wow... you rock. Yeah I understand why. Thanks so much!
  5. Jan 16, 2005 #4
    Well, actually I don't quite understand the shear stress part. Can you explain that?
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook