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

• To: seaint(--nospam--at)seaint.org
• Subject: RE: Was embedded timber post (80 Tons trailer impact on Structure)
• From: "Casey K. Hemmatyar" <khemmatyar(--nospam--at)gmail.com>
• Date: Fri, 23 May 2008 12:03:10 -0700

Chris:

You're right but the is a 1/2 factor in the basic equation.

I need to go for my lunch!

Regards

Casey K. Hemmatyar, SE

-----Original Message-----
From: Christopher Wright [mailto:chrisw(--nospam--at)skypoint.com]
Sent: Friday, May 23, 2008 11:54 AM
To: seaint(--nospam--at)seaint.org
Subject: Re: Was embedded timber post (80 Tons trailer impact on Structure)

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)skypoint.com   | this distance" (last words of Gen.

.......................................| John Sedgwick, Spotsylvania

1864)

http://www.skypoint.com/~chrisw/

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