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Need help with a statics problem

  1. May 27, 2014 #1
    Thanks for checking out my post-

    I work at a launch facility, and we have light fixtures around the site that have proven to be poorly designed by the manufacturer. Multiple fixtures joints have failed over time due to lack of proper structural support - My goal is to add sufficient supports to these fixtures without over-complicating things, and then backing up my corrective action with calculations.

    The largest force acting on these fixtures is from the rocket blast, which I assumed to act as a blast wave at a 45-degree angle to the top surface of the fixture (based on the fixture location). I also used wind speed and fixture weight as additional forces, and then I calculated the resultant moment acting on the joint. From that, I simply calculated the reaction moment on the other side of the joint, and then determined the force acting on the new bolt I plan to add for support.

    See attached for my calculation. As you can see, my final shear stress on the bolt was found to be 311 ksi, which seems way too large to be correct.. I was pretty conservative in this approach, but I still would not expect an answer on this order of magnitude.

    Any help would be extremely appreciated.

    Attached Files:

  2. jcsd
  3. May 27, 2014 #2


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    I think your loading due to wind pressure is a tad low.

    For a wind speed of 120 mph, P = 0.00256*V^2 approximately, according to the ASCE.

    For V = 120 mph, P = 37 lb/ft^2 = 0.256 psi, instead of 0.10 psi.

    http://www-classes.usc.edu/architecture/structures/wind/ASCE7 Wind Load.pdf

    Using a 1/4" bolt to support a load of 450 lbs at 24" is much too small. You've either got to mitigate the load or find a different way to support this fixture.
  4. May 29, 2014 #3
    Thanks for the input, you're right about the wind pressure. But my confusion here is that 311 ksi seems extremely large. And now apparently this is actually too low... I just want to make sure this solution makes sense or if I'm way off base here. Thanks again for responding.
  5. Jun 1, 2014 #4


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    steves1080: Although you perhaps could adjust your wind pressure, your calculations are correct, and your answer is correct, for the given assumptions.

    However, the applied forces actually act at the lamp centerline, which is a moment arm of 382 mm, instead of 613 mm. Therefore, this reduces your current stress; but your bolt is still overstressed.

    Is your bolt in single shear, or double shear? I.e., does it have one shear plane, or two shear planes? Know what I mean?

    Use a moment arm of 382 mm. And if your bolt has one shear plane, then perhaps try an M16 bolt or a 0.6250-11 UNC-2A bolt.
    Last edited: Jun 1, 2014
  6. Jun 2, 2014 #5
    Thanks so much for the response. Yes I suppose using a moment at the edge of the plate may have just been overly conservative.. it makes sense to place the forces in the middle of the plate, which is more realistic.

    I assumed one shear plane, because you basically have one bolt through two flush surfaces, each trying to move in opposite directions perpendicular to the bolt.

    Thanks again!
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