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Thread: Pickup Truck Flatbed Design

  1. #1
    Associate Engineer
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    Bang Head Pickup Truck Flatbed Design

    Hello Everyone. Thank you for allowing me to post my questions here.

    I have an old one-ton dually pickup truck with a fifth-wheel hitch. It has two steel places mounted in the existing bed which are attached to brackets off the main frame, and which hold the fifth-wheel hitch plate mechanism. It also has a 100 gallon auxiliary aluminum diesel tank in the bed just behind the cab.

    The bed is to be replaced with a flatbed design. The framework will lay directly on the existing frame rails or supported just above them to the needed tire clearance height. The two main rails are approximately three foot (3') on center.

    In any case, the frame will be created with cross pieces, with similar material boxing it in at the two sides. The spacing between the cross members can be whatever is necessary for proper weight distribution. The bed will be no more than eight feet (8') wide and nine feet (9') long. The framework will be covered by tongue and groove pine boards, laying front to back. The boards will be 1.5" thick. Some additional metal edge trim and details will be added which do not effect the weight or strength.

    The idea is to make the truck still carry the 2000 pounds of load rating, but mostly used for the fifth-wheel hitch and toolboxes placed over and under the bed at the sides. The full auxiliary tank adds eight hundred pounds, and tool boxes may add no more than five hundred.

    With the current high cost of low carbon steel (mild steel) (A500, A36, ???), it seems wise not to over-build. However, I am at a loss as to which type and configuration to use. Square tube, rectangular tube, C-channel, or whatever. I am hoping to weld it together with my little flux core arc welder, and thought that eighth inch thickness material should be enough and make it easier to weld. And that is about all I know.

    Watching videos on YouTube from others who are doing this sort of build leaves me questioning the strength-to-cost benefits of their endeavors.

    Does anyone have any ideas to help me calculate the necessary size, shape and thickness trade-offs for my project?

    Thank you.

  2. #2
    Associate Engineer
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    I just noticed something. If the distance between the main beams or support rails of the vehicle are only about 36 inches, then the real issue is the cantilever of the overhung frame members outside those rails. That is where most of the load is and that is where the real support strength needs to be measured.

    It seems simple to reinforce the fifth-wheel hitch area because it was always designed to attach directly to the main vehicle frame rails anyway with simple angle iron pieces. That stands alone and does not have anything to do with the bed. Even the fuel tank spans the main frame rails on its own.

    Therefore, doesn't the weight of the toolboxes and other attached items to the overhung area become the real issue here?

  3. #3
    Associate Engineer
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    I guess all I need is a way to calculate the strength of the overhung members and their load-bearing ability. Is that correct?

  4. #4
    Associate Engineer
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    I'm still trying to figure out what I can on my own, but all I found was "Bending, Deflection and Stress Equations Calculator for Cantilevered Beam with One Load Applied at End". Sadly, I could make neither heads nor tails of it. Can anyone make some suggestions?

  5. #5
    Associate Engineer
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    Am I looking for "end-loaded cantilever beams" calculations?

    Almost any response is appreciated.

  6. #6
    Administrator Kelly Bramble's Avatar
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    You're not getting many replies because of the complexity and amount of work involved in your question.

    If you're more concerned with price than function this will complicate the design.

    Here's some quick math:

    Working (maximum, capacity) load = Rated load - (total mass removed) + (total mass added)

    I would likely use rectangular structural shape with the long section perpendicular to the load. Without a free body diagram with dimensional data, it can too hard to estimate loading.

    I suspect that you need to estimate the maximum loading at the highest load location and analyze the worst-case scenario assuming a factor of safety of about 1.5.
    What you’re calculating is the resultant stress due to the loading and comparing that to the known yield rating of the structural shape used.

    For example, if you use carbon steel rated at 56,000 psi yield you would want the applied loading to not exceed 56,000 psi / 1.5 (FOS) or 37,333 psi applied stress. The deflection may matter.

    Most likely you actually need one of the following beam stress and deflection calculators.
    Area moment of inertia here:
    https://www.engineersedge.com/sectio...ies_menu.shtml
    Beam stress and deflection
    https://www.engineersedge.com/beam_b..._bending2e.htm
    https://www.engineersedge.com/beam_b..._bending2e.htm

    https://www.engineersedge.com/beam_calc_menu.shtml

    Be aware that one would need FEA software to do a full analysis of the loading.
    Tell me and I forget. Teach me and I remember. Involve me and I learn.

  7. #7
    Principle Engineer
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    The problem gets further complicated when you consider chassis flex and jounce. Over building might not be that expensive.

  8. #8
    Associate Engineer
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    Thank you all for your good comments. That's what I need; many people coming together to give ideas.

    This relatively simple project has created (or shed light on) a void in my understanding. So many people are doing this sort of project and displaying their methods on videos. Simple rectangular tubing or channel is used without any comment. When I look at the design of the stock bed, I can't help but wonder if everyone is over-building even with their simple selections. That thin bed material can be easily bent by overloading and yet no one seems to be concerned with that. Even 1x1x1/8 tubing seems stronger then the stock bed support members. What might I be missing?

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