NSR - Mechanical Engineering problem - materials and bending moments

I'm building a really wide gate that will be swinging off a single post planted in the ground and I need to know what to make the post out of.

It will be constructed of 2x4 11ga rectangular tubing (4.75lb/ft), total width will be 25ft, and let's say the 4 uprights are 5ft long each, so that's like 330 lbs total, plus I will be hanging pickets on it to make it look like the rest of my fence. It will be supported on the free end most of the time, just needs to be able to swing open for short periods of time without bending the post over at the ground. Assume I can plant the post so it absolutely won't move...I'll probably bury it 5' in concrete. What do I need to use for the post? Thanks in advance!

Quote:

---

Originally Posted by khakis

I'm building a really wide gate that will be swinging off a single post planted in the ground and I need to know what to make the post out of.

It will be constructed of 2x4 11ga rectangular tubing (4.75lb/ft), total width will be 25ft, and let's say the 4 uprights are 5ft long each, so that's like 330 lbs total, plus I will be hanging pickets on it to make it look like the rest of my fence. It will be supported on the free end most of the time, just needs to be able to swing open for short periods of time without bending the post over at the ground. Assume I can plant the post so it absolutely won't move...I'll probably bury it 5' in concrete. What do I need to use for the post? Thanks in advance!

---

4x4 tube with a 1/4" wall would suffice. Pretty conservative assuming the pickets weigh a hundred or so pounds and you may have a person hanging off the end for some reason...also, assuming steel at a yield strength of 46 ksi.

Don't have an answer for you, but this post brought back sweet memories of the LRFD.

cool, thanks! I was thinking 4" sch 80 pipe...that should do the trick, right?

Assuming the 25 foot long gate weighs 13#/ft and you add a 200# person at the end, you get a bending force of 9063 #-ft. The allowable bending capacity of a 4" dia schedule 80 pipe is 9650 #-ft so it's strong enough. However, my concern would be the deflection in the whole system. The connection of the post to the ground will have a tendency to rotate, the pole will bend, and the gate will sag. 25' is a long gate and without something to stiffen it up it has the potential to deflect a lot.

Cool thanks...I had originally thought that the gate as drawn would be stiff enough, but the more I look at it the more I think I will probably put a cable from upper left to lower right to help with the sagging. Does that seem like it would work?

You could run diagonal stiffeners/gussets to a couple of other anchors further back from the road. Or you could used larger sized square pipe set in concrete, and hope the soil is solid enough to keep the whole thing from slowly tipping over the years.

Oh your concern is with the post tipping? I was hoping that a 5' deep hole 8-10" in diameter filled with concrete, combined with our hard-ass high clay soil would distribute the force enough to keep it from moving. Also, the free end of the gate will be supported 99% of the time, so the post won't have constant torque on it. Maybe with a 25' span that's still a lot to ask? I could also shorten it some, like maybe down to 20' if that would help drastically. Extending the post another 5' and putting a cable from there to the top right corner could also be a possibility

Really appreciate the help, by the way....

use 2 12.5' gates?

If you decrease the length of the gate to 20': M = [13plf*(20'^2)/2] + [200*20'] = 6600 #-ft (wow, I'm a nerd...). Realize that 4000#-' comes from the assumed weight of a person hanging on the end of the gate.

So, decreasing the length to 20' will decrease the bending force by almost 30%. That will help a lot. My biggest concern would be the gate deflecting. The best way to fix that would be to put diagonals in each bay or to sheath the gate with a metal plate.

As far as predicting the required depth of the post, the building code has some equations for embedded posts but there are a lot of factors that go into it and it is generally pretty conservative because it's aimed at pole foundations for buildings where failure is a big deal. Using some rough estimates the equation recommends that your 18" concrete hole be 18' deep. That seems a pretty extreme...

Probably not much help with specific post material recomedations but calculating the load is fairly straightforward. For example, you have a cantilevered beam that will be free at one end and fixed at the other so the max shear force on the fixed end: Vmax = -(q*L) /2, with q being the total load (mass times gravity) and L being the length of the beam. The max bending moment: Mmax = -(q*L^2) /2 . Note that The max bending moment increases proportional to the square of the length.

The other consideration is the distance between your top hinge and your bottom hinge (A-B) and how well they are aligned because the stress is tensile at the top hinge and compressive at the bottom hinge with misalignment adding a z component.

The values of the main variables: q (load), L (length), A (top hinge), B (bottom hinge) will all determine the mechanics of the vertical post. In other words, increasing the length as Buke points out has a big impact on the forces acting on the post whereas increasing the distance between the hinges helps distribute the load.

triangles man, triangles

Counterweights, life is all about balance.