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Thread: Constructing a Deck with Beam supports

  1. #21
    what deflection seems more accurate?....and are you sure I should go bigger than a moment of inertia of 324?? I'm looking at a chart of W I beams and beams with a moment of inertia higher than 200 are beginning to get extremely pricey

  2. #22
    Technical Fellow Kelly_Bramble's Avatar
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    I think 1/2 to 3/4 deflection should be OK if not a little more - others?

    Your design is really simple in that you’re trying to support the loading with two w-flanges. Ultimately strength characteristics, costing of materials and weight are what make truss designs attractive. The difference being that there will be increased manufacturing/fabrication costs associate with a truss type structural bridge design.

    A truss design will distribute the loading such that smaller individual structural components (smaller moments of I) can be used.

  3. #23
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    Ok Kelly, now you have me confused.

    Increasing M of I represents an increase in stiffness, resulting in less deflection.
    If .3" is small, increasing M of I more than 324 will result in even smaller deflection.
    Right?

    If we use M of I of 90, on a 30' span, with a 2,000# load, the deflection is .72".

    srw2104: I am sure you realize we are talking about the capacity of a single beam.
    You will have two beams, effectively doubling the load carrying capacity of the bridge
    over the capacity of a single beam.
    (There are other considerations, but we will get to those later.)

  4. #24
    Technical Fellow Kelly_Bramble's Avatar
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    Yes,

    What I meant and failed to articulate by saying "increasing M of I" is that for FOS the structrual w-flange M of I should be bigger. I should have said you need a FOS (Factor of Safety) of some number (1.5 maybe) and increase the M of I to accomplish this.

    Quote Originally Posted by dalecyr View Post
    Ok Kelly, now you have me confused.

    Increasing M of I represents an increase in stiffness, resulting in less deflection.
    If .3" is small, increasing M of I more than 324 will result in even smaller deflection.
    Right?

    If we use M of I of 90, on a 30' span, with a 2,000# load, the deflection is .72".

    srw2104: I am sure you realize we are talking about the capacity of a single beam.
    You will have two beams, effectively doubling the load carrying capacity of the bridge
    over the capacity of a single beam.
    (There are other considerations, but we will get to those later.)

  5. #25
    yes, i do realize this is a calculation for a single beam. Do you think that since the load is shared between the two supports, I can divide it in half and use a value of W to be equal to that of 1500 instead of 3000?

  6. #26
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    I am probably more conservative than others;
    I would use a load (W) of 2000#, which you state is the expected load.

    This will give a FOS of (approx. (not quite)) 2:1.

    But I would like to see the opinions of others on the forum...

  7. #27
    okay...than you for the help dalecyr and kelly. It is much appreciated

  8. #28
    Lead Engineer RWOLFEJR's Avatar
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    I'd go with 40 foot W14 x 22 twice (I of 199 each) and some rough cut 2" hardwood planks. If the load will indeed be limited to a mower and/or 2,000 lbs. for your own private residence. The 14" beam is lightest you can get with a high enough moment and light = less steel = less dollars. Or as Kelly mentioned you could look into trusses. Beams with all the useless parts cut out. Space the beams maybe a foot wider than your mowers width. and overhang the boards by a foot each side. Sink some big cut stones in the ground a few feet off the bank each side. Drop the beams on the rocks and poke a bunch of holes for stainless carriage bolts. Further you sink the stones the less need for a ramp. Thrwo some rip rap along the edges of the crick and call it a day.

  9. #29
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    +1,
    and Bob has addressed a few more issues that we had not gotten to yet.
    Like making the beam long enough to provide a point of attachment to the footing;
    reducing the need for a ramp by sinking footings into the ground;
    treating the side of the creek to prevent erosion in that area;
    using rough cut lumber to increase M of I on the planks;
    placement of the beams relative to the load distribution and beam supports.

    Do remember tho, this is a generic answer to a somewhat theorectical problem;
    if you change the "problem", the answer no longer applies.

    For example, you have not stated that there could be any lateral load at all.
    Although the I of a W14x22 is 199 vertically, it is only 7 horizontally.
    Run an I of 7 through the equation, and think about what happens when
    some debris gets caught under the bridge and starts to make a dam.

    There are ways to mitigate those forces.
    But then, you did not ask that question.

  10. #30
    Project Engineer CCR5600Design's Avatar
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    It seems it would have been easier to just put larger diameter tires on the mower and lawn cart to increase ground clearance and just drive through the crick...



    Ron

  11. #31
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    Quote Originally Posted by CCR5600Design View Post
    It seems it would have been easier to just put larger diameter tires on the mower and lawn cart
    Or, my personal favorite, save a bunch of money with a couple of yards of concrete, some rebar and corrugated steel drain pipe (if needed) and make a concrete crossing. Have it flow through the pipe for normal flow and over the ford for high flow.

    Much less cost, no cranes, unless of course two men can carry a 40 foot W14 x 22 on their shoulders.

    Dave
    Generally, I will not give you the answer to your question, but I *will* guide you into discovering how to solve this yourself.

  12. #32
    I will absolutely consider that as one of my options...thank you!

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