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Re: Was embedded timber post (80 Tons trailer impact on Structure)

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On May 23, 2008, at 12:41 PM, Casey K. Hemmatyar wrote:

Just to refresh your memory, it has a 28,340 k-ft of Force but not Energy.
Not to nag, but you're both wrong.

David figured the energy correctly but misinterpreted the work as a moment. If you're looking at the and came up with the average force that would decelerate the truck in 30 inches. If you ran the truck into a post the post would have to deform 30 inches under the influence of that force--actually the total of the deformation of the post and the deformation of the truck would be 30 inches. What would actually happen is that the initial force would be much greater than the average; the post would snap off and the truck would be damaged but probably keep moving.

Casey also calculated the energy, only in metric units and then put it equal to a force, which it isn't. I'm not sure how the calculation got carried out, but the mass in kg of an object that weighs 80 tons = (80)(2000)/2.2046 = 72575 kg and 70 mph = (70)(88/60) = 102.667 ft/ sec or (0.3048)(102.667) = 31.293 m/sec That makes the kinetic energy (72575)(31.293)^2 =71069 kJ, not 39779 kJ.

The quantity Casey is figuring is an energy not a force, because the relationship he's using is the definition of kinetic energy.

But the actual bottom line is that running a truck into an embedded timber post, will snap the post and not do much to slow the truck, except if the truck jackknifes and rolls after the impact. If you've ever seen a collision of a truck and a telephone pole, it's pretty clear.

Christopher Wright P.E. |"They couldn't hit an elephant at
chrisw(--nospam--at)   | this distance" (last words of Gen.
.......................................| John Sedgwick, Spotsylvania 1864)

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