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light failure

light failure - winters high winds and salt air When...

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Mary Pat Tumelty CIVE 375 3/14/2008 Failure of a Plastic Wall Mounted Light Fixture This light fixture (Figure 1) developed several cracks as a result of an initial failure of bracket (Figure 2) that held the top onto the housing. The failure that I am focusing on is the large on the top piece that runs along the corner (Figure 3). The top is mad of a plastic about 1/16” thick. (As a side note, this is the second light fixture that has failed in the same manner in a similar location) The light fixture was located on the Easterly outside wall of house. The house is located on the Northeast end of Ocean City, New Jersey, which is about a house away from the beach and about two blocks away from the most Northern part of the island. The fixture has faced about 15 to 25 years of bright morning sunlight, harsh shore
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Unformatted text preview: winters, high winds, and salt air. When plastic is exposed to sunlight it becomes brittle. The high wind must have gotten into the lamp and after repeated wind loads the bracket holding the top piece on broke. This allowed the top to fly off creating a trajectory and then hit the concrete patio. At this point the force of the impact with the ground cause the large failure in top, which quickly sheared because of the brittle plastic. The load that caused the crack is equal to 755.6 pounds. This happened when the shear was at its maximum, a value of 5.58 ksi. Since it failed at τ max the angle of failure is 45 ° . This also means that it failed in shear, which clearly the angle of the crack on the top piece of the lamp....
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