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Metal Gate posts strengths & stresses?

  1. Oct 7, 2015 #1
    Hi folks,

    Just wondering if anyone can help with a few questions I have.

    I'm working on a design of a gate which, instead of using a traditional hinge system, will consist of an outer cylindrical post with the gate leaf welded directly onto it, and an inner cylindrical post which will bear the load and will be fixed into the ground. The Outer post will pivot around the inner post, using the same axis, and will be equally spaced using bearings.

    My question is, how do I stop any 'sagging' of the gate? Are there any equations I can use e.g. something which incorporates the height of the post, the length of the gate, the weight of the gate etc. OR is it just a simple matter of making sure the gate post is heavier than the gate to centre the weight at the post?

    Like with everything, I plan to keep the cost down so I don't necessarily want to buy heavy material with a sole purpose of, err... being heavy.

    Maybe a thinner walled post and filled with postcrete?

    I appreciate any help on this matter. (Apologies if I've used the wrong terminology)


  2. jcsd
  3. Oct 7, 2015 #2
    I also meant to say that as the slampost (where the gate closes to) will be free standing, would I need some sort of cross member linking the two post under ground? also would this help with the squareness and stability of the two posts?

  4. Oct 9, 2015 #3
    I hope I'm not too late to this party, but if I understand your issue, there's only two realistic ways of preventing sagging unless you have some other means of supporting the gate.
    Referring to this picture, you can either make the internal pipe thicker, but that doesn't really increase your interior pipe's rotational resistance (i.e. you could triple the thickness and maybe get 20% less sag out of gate. If you could counterweight the sag, though, you could have a pretty stout gate. With the set up in the picture, you would need to have a cable/eye bolt with a working capacity of (Weight of Gate * Width of Gate)/(2*Height of Gate*cos(angle that cable makes with ground)). For example, if your gate weighs 500 lb, is 8' wide 12' tall, and the cable is 60 degrees to the ground, then you will have:

    Tension = 500lb*8ft/(2*12ft*cos(60)) = 400lb.

    Note**** make sure your concrete counterweight is at least as heavy as your estimated tension, with a few hundred extra pounds to be safe (one foot of concrete = 150 lb).
  5. Oct 11, 2015 #4
    There are two forms of "sag" one encounters in a gate. One is deformation of the gate itself. In order to prevent this the structure needs to have an appropriate internal guy or brace. It is the truss like structure that is of importance. Old farm gates had a single heavy diagonal from upper post end to bottom latch end. More artistic gates are often a true truss structure as required by visual presentation. In either case the load can be estimated by assuming the total weight applied at a lever arm 1/2 the length from post to latch.

    The other source of sag is from the post deforming from vertical. Infinitybyzero has a good idea on this. A back tie of some type is critical. The load (as torque applied to the post) can be approximated as the weight of the gate at a length 1/2 of the distance from post to latch. Remember that this calculation is not true at the top of the post. It is the torque applied at the middle. Thus the load that needs to be supported at the top of the post is approximately 2x the calculation. The anchor that he shows for the post is also important The torque on the post will be essentially a rotation about the center of the post. This torque is also always perpendicular to the gate. When you open it or move it off axis the orientation of the force moves also.
  6. Oct 12, 2015 #5
    Cheers for your reply guys. I think I will elaborate a bit more on the dimensions of my current design.

    Below is my current design. In order to avoid any sagging I wanted to make the gate leaf as light as possible, so thin walled material was used. And to counteract this weight I wanted to put more weight on the centre of the turning axes. (bear in mind, the gate leaf - blue section, and outer post - orange section) are welded together as one section.)

    I Think I have worked out the approximate centre of gravity using online help. (CG in the diagram).

    - This is a double gate leaf design.
    - A cross member will be fixed between each post just below ground level.

    My question will be:
    1) Is my general thinking that as long as the two post are heavier and focussing the overall weight of the gate on the axes, the gate should not sag?
    2) If I use thinner walled material for the inner post (its currently 10mm thick) could I fill the post up with concrete to increase its mass, acting in the same way as above?
    3) If the above IS correct, how will I know what my weight limit is for the gate leaf (Blue) so I can incorporate new leaf designs?

  7. Oct 12, 2015 #6

    The weight for the Inner post for above ground is 38.72 kg!

  8. Oct 12, 2015 #7


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    Do you have an explanation why the below ground cross member is needed. What function does it do.?

    The CG that you have indicated is where both the gate and hinge balance.
    You could have the gate rotate about this point, ( with a change of your design so that it rotates about this point ). And the inner post will stay vertical. The outer post then becomes just the counterweight for the gate,

    The CG of the gate, what you are interested in is midway from hinge to end of gate. See Ketch22 above.

    1. heavier posts - generally mean structural stronger posts with thicker walls,
    2. concrete fill.- will aid somewhat in that the circular post will keep its circular shape throughout the opening closing,
    too thin and the post will bend and fail at ground level
    3. Depth of post in the ground and soil conditions in your area. see Infinityby zero , concrete fill around post

    You could aso, and this works in any position of the gate
    Put a wheel , or runner, on the gate end
    Counterbalance the gate with weight, so that net force is downwards on the inner post.
  9. Oct 12, 2015 #8
    1) The pipes are essentially infinitely stiff compared to the gate leaf (if you are using light, thin tubed sections). You could have a 10" solid steel round bar for a gate post and your gate would still sag if it were made of very light metal.

    2) Concrete fill would not increase the bending stiffness of your support by a significant amount, since you're just adding material to the center of gravity of the pipe itself.
  10. Oct 12, 2015 #9
    I am led to believe that the cross member will act as a brace? It was a question I asked in post #2 so its not something I have decided on yet.

    I think the real question I'm trying to ask is regarding the bending (deflection I think is the technical wording) of the inner gate post. At what weight or force will the post start to deflect? And will my current setup help to limit the deflection?

    Sorry if I'm not clear, as you can tell this kind of stuff is not what I do.
  11. Oct 14, 2015 #10


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    Perhaps look up field gates. There are rules of thumb for how deep the fence post should go. Have worked for farmers for years.
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