Cantilever beams
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Posted by: wa2qcj ® 05/04/2006, 15:12:53 Author Profile eMail author Edit

I want to construct an extension to my bus. The bus is a 1990 Thomas bus. The frams is at least 1/4 inch thick 10 inch chennel. The engine is in the rear of the bus. The frame extends to the rear of the bus. There is no more than 12 inches of free space from frame end to bumper. I want to use 3 inch, by 1.498 by 0.248 inch channel for this extention. The extention will carry general cargo, a 120 gallon water tank, and be the supporting structure for a trailer hitch. Is the channel strong enough? I am guessing that the channel needs to extend at least 4 feet back along the frame and to be bolted by 1/2 to 3/4 inch #8 strength bolts to the frame in at least 4 places per frame rail. So, people, am I whistling in the wind and to far off for it to be safe, or fairly close to what would be needed. I would really like some good advice on this project.

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Re: Cantilever beams
Re: Cantilever beams -- wa2qcj	Post Reply	Top of thread	Forum

Posted by: Scooter ® 05/05/2006, 14:29:25 Author Profile eMail author Edit

As A follow up on my last, Take the following example based on the info. given. The channel discription matches that of C3 x 5. This has a section modulus (Sx) of 1.24 in.cubed. If you have 12" of availible main frame to attach the extension to, and you want to use a 48" piece, then you have 36" of actual cantilevered member. Now, if we, for safety sake assume the complete load as being applied at the free end, and we are trying to match our above (Sx), we use: M@fixed end= PL, where P=Load, L=Length of cantilever. Now, LOAD ?, OK for example; Water, 120 gal.=1001#, Holding Tank=350#, Hitch=400# (ext.tounge wt.)Misc."stuff"=350#, total=2051# Therefore, PL=2051 x 36=73836 inch lbs. Now, Sr=M/Sa where Sa=SA-36 with an ASME stress allowable of 16,600PSI. Therefore, M/Sa=4.447 in.cubed.< 1.24 therefore it won't work. Actually it would take about 4 ea. of this size channels to do the jobs. Now, rember this is only an "examble" of the approch to finding what is needed. Of cource their are other ways, but this one will work. (Remember the D.O.T. factors also) I hope this will help point you in the right direction. Thanks Scooter PS You will also have to find the shear on the bolts you use to attach the extension to the frame.

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Re: Cantilever beams
Re: Cantilever beams -- wa2qcj	Post Reply	Top of thread	Forum

Posted by: Kelly_Bramble ® 05/05/2006, 08:55:04 Author Profile eMail author Edit

Honestly,120 gallons of water and a trailer hitch could be alot of weight. To say yes or no, one would have to do some calculations and verifiy the supporting structure relative to maximum fatique load. Not a simple forum question. Can you not buy a rated trailer hitch for this particular vehicle? You would sure hate to have a failure driving down the road with your car or boat in tow.

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Re: Re: Cantilever beams
Re: Re: Cantilever beams -- Kelly_Bramble	Post Reply	Top of thread	Forum

Posted by: Scooter ® 05/05/2006, 09:12:30 Author Profile eMail author Edit

I agree with Kelly, this needs to be calculated based on a dynamic load, not static. I wonder wa2qcj if you have included the existing bumper into your planning ? most buses have a stoutly designed & built bumper. Don't forget to bone up on D.O.T. standards for "Rear end protection" devices on Buses. You may have to meet certin dimentional & strength requirnments to keep your Bus legal. Thanks Scooter

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Re: Re: Re: Cantilever beams
Re: Re: Re: Cantilever beams -- Scooter	Post Reply	Top of thread	Forum

Posted by: wa2qcj ® 05/05/2006, 14:54:57 Author Profile eMail author Edit

While I did not mention it, I had planned on making a connection to said bumper on each side. I was also considering (planning) on using about 4 feet of connection length under the bus for support for that 4 feet of cantilever. I would never attempt such a load as I have mentioned with a 1 foot long anchor. I seriously doubt, as you say, that it would work. Minimum distance for the anchoring to the frame would be, to me, at least 4 feet. As for the trailer hitch, I had planned on, although not mentioned, using a commercially made hitch. So, oops for those details, but a 1 foot anchor simply would not work, I was thinking of more like 4 feet minimum. Using a bracket attached to the bumper, and the cantilever beam, would that improve support quite a bit, wouldn't it? I was also considering adding to the support of the channel by using long bolts connecting a cross plate to the top and bottom of the channel and frame for added support. Is this any better? Now, if need be, I can forget, for now, the water tank, but for the move, I would like to have the extension.

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Re: Re: Re: Re: Cantilever beams
Re: Re: Re: Re: Cantilever beams -- wa2qcj	Post Reply	Top of thread	Forum

Posted by: Scooter ® 05/05/2006, 15:34:10 Author Profile eMail author Edit

You have many good views & ideas for your project, However it is difficlut to comment on most of them because we have not had actually seen the Bus, it's undercarriage, Bumper or Bumper attachment brackets. All these items would require evaluation prior to design. You might be better off to get a local Engineering firm to look at your project. After doing so, they could advise you better than we could. The Engineering group you find need not be structrial engineers, any diciplines would be able to help. This would be a good project for an engineering student. Sorry I couldn't give better advice. Thanks, Scooter

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