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Re: Stopping That Truck !!
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- Subject: Re: Stopping That Truck !!
- From: "John MacLean" <john_maclean(--nospam--at)pomeroy.ca>
- Date: Thu, 24 Jan 2002 13:23:29 -0800
Stan Caldwell wrote: <<Question: What is the design force on the bollard? on the wire rope?>> Interesting problem. It seems very similar to the "fall arrest force" problem where you have to design a fall restraint system for a guy working with a safety line. Here's my basic take on it which may be similar to the thought process you have already gone through. Your 7500 lb truck travelling at 20 mph has a kinetic energy (1/2 mv^2) of about 100 kip-ft. So you basically need a system that will absorb 100 kip-ft of energy. Some of the energy will be absorbed in totalling the truck. Then again it could be one of the Ford trucks built up here at the Oshawa truck plant in which case it will remain elastic and all the energy will go into your cables and bollards. I guess you could figure out some kind of spring constant "k" for the system. In that case the deflection of the system, d, would be Fmax/k. Fmax is the maximum force applied to the truck. If the "spring" is elastic, the work done in stopping the truck will be Fmaxd/2 because the force will ramp up linearly from 0 to Fmax. Since d = Fmax/k then the work done = Fmax^2/2k. So Fmax = sqrt(100kip-ft/2k) = 7 x sqrt(1/k) kip or Fmax = 100kip-ft/2d = 50 kip-ft/d So if you can stop the truck in 1 ft, Fmax = 50 kip; 2ft, Fmax = 25 kip; 6", Fmax = 100 kip; etc. If the deflection is 12" midway between the bollards (spaced at 12') then Fmax = 50kip. The tension in the cable would be 50 kip x sqrt(12^2+72^2)/12 = 304 kip. If the deflection includes movement of the bollards then the force would be even greater since the cable angle would be smaller. The hard thing is to figure out what k or d is. There's going to be some non-linearity in the system. Primarily, I would think, in your clay foundations. I've also got a feeling that the cable system will be non-linear due just to geometry. I would be inclined to do a rough design of a system based on a 50 kip load then work out a spring constant for the system and cycle back through the design til I was happy. I would assume very little if any energy was absorbed by crunching sheet metal. I would tend to assume the system was stiff rather than flexible. I would tend not to design the bollards to yield if possible. There is a certain deterrence in the simple appearance of strength (as Stalin used to say "quantity has a certain quality all it's own"). I'm not totally clear on how the bollards work. Is the cable continuous across several bollards and anchored intermittently or is it anchored to each bollard. If the cable is anchored to the bollards then the bollards would have to be designed for the cable force. If the cable slides through an eye at the bollard then it would have to be designed for the impact of the truck hitting the bollard itself which is a whole nother problem (but similar). There's my take for what it's worth. Cheers, John MacLean Vancouver, BC ******* ****** ******* ******** ******* ******* ******* *** * Read list FAQ at: http://www.seaint.org/list_FAQ.asp * * This email was sent to you via Structural Engineers * Association of Southern California (SEAOSC) server. To * subscribe (no fee) or UnSubscribe, please go to: * * http://www.seaint.org/sealist1.asp * * Questions to seaint-ad(--nospam--at)seaint.org. Remember, any email you * send to the list is public domain and may be re-posted * without your permission. Make sure you visit our web * site at: http://www.seaint.org ******* ****** ****** ****** ******* ****** ****** ********
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