What is the Allowable Load on a Tie-Down with a Bent Bar and Fiberglass Support?

AI Thread Summary
The discussion focuses on calculating the allowable load on a tie-down involving a bent bar and fiberglass support. The allowable shear stress in the fiberglass is 300 osi, and the allowable bearing pressure between the washer and fiberglass is 550 psi. The calculations yield allowable loads of approximately 607 lb and 619 lb, derived from the areas of the washer and the shear stress. The participants seek clarification on the shear stress calculations, particularly how the relevant area affects the load. Understanding these calculations is crucial for ensuring safety and compliance in the tie-down design.
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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?
 
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550~psi = \frac {L}{A_{washer}}

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

Allowable load = 2L = 607.46 lbs

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

A = \pi D_{out} \cdot t

This will give you load = 618.50 lbs

Do you understand why this is so ?
 
wow... you rock. Yeah I understand why. Thanks so much!
 
Well, actually I don't quite understand the shear stress part. Can you explain that?
 
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