Boarding Steps

[Captmike]

Hello all.

I hope nobody here is offended by a non-mechanical engineer seeking advice.
I am an electrical engineer, and tried to use some of the formulas here for stress and deflection, but not sure about specific materials.

Reference drawing below.

I am trying to design boarding steps that will be attached to a dock with the ability to swing up and out of the way - folding back to rest on the dock. When deployed, the steps will swing down such that the lower step is about 1 foot above the water line.

I would like to use 2"X2" 1/8" thick angle steel for the 24" vertical supports, 34" horizontal supports and the step supports. I would like to use 2" 1/8" thick steel plate for the two angle supports.

The steps will be 2X6 pressure treated lumber - two boards for the top step, two boards for the middle step and 4 boards for the lower step.

Two 4 foot pieces of angle steel will be bolted to the deck to anchor the assembly.
The step assembly will be attached to the deck anchors with 1/2" bolts which will allow the entire step assembly to pivot up and back to reset on the dock when not deployed.

I estimate the weight of the assembly will be about 40 lbs for the steel and about 12 pounds for the lumber for a total of 52 pounds.

Does anyone here see a problem with this design?

Thanks in advance,
Mike

[jboggs]

Random comments:
1) Your overall concept seems sound but you need to lay it all out to scale. 2x6 boards will not arrange themselves as you have shown.
2) You show a 4x4 block on the bottom corner. Is it stationary or does it move with the steps? Either way you will want a fixed stop at that point to absorb all the force. Do not plan on some kind of stop arrangement up near the hinge point. The closer the stop is to the hinge, the sooner it will fail.
3) Do not use a bolt as your hinge. You will want some kind of fixed pivot post with a bushing and a washer. Bolts and nuts make lousy pivots.

[JAlberts]

It appears to me that your 4x4 block stops at the bottom are fine as long as they are a part of the step assembly. You would not want to place them as protrusions on the wall.
One thing that does concern me is how you intend to pivot the steps up on to the deck because from what I see it is going to be very difficult for anyone to get the necessary leverage from the top of the dock to achieve that. Also, if youare going to lay the stairs back on the deck, you will require a socket arrangement at the bottom of the rope support post that will allow you to remove it.

[Captmike]

Thanks jboggs and JAlberts.
Jboggs - yes the 4x4 stop block is mounted low on the assembly not the wall. For the pivot, I planned on using stainless shoulder bolt on deck anchor with oilite bushings pressed into holes in the vertical supports.

JAlberts - correct the stop blocks are mounted low on assembly.
Great idea on making the rope support socketed. Thanks!
To pull up the assembly (which I estimate to be approximately 50 pounds), I was thinking of just a rope attached to outside corner of assembly. Maybe attach a small winch to inside of nearby deck rail.

Thanks again guys for taking time to respond.

Mike

[Captmike]

What are your thoughts about a couple of torsion springs at hinge points to counteract a few pounds of gravity?

[jboggs]

Thoughts? Probably more trouble then its worth.
Torsion springs and wet environments? No so much.
The spring centerlines would have to lie on the pivot centerline. That means that they must be outside the frame to not create a tripping hazard.
The details of the application can get complicated pretty quickly.
I think a better weight saving solution might be aluminum construction for both the frame and the steps.

[Captmike]

Would love to use aluminum. I didn't think aluminum had the tensile strength to support 800 lbs (I want it to be capable of 3x or 4x of intended load) concentrated at the pivot holes. Maybe 1/4" aluminum?

[JAlberts]

Captmike: I am still concerned about how you are going to lift the stairs. When you rotate around the current pivots the stair assembly is initially going to swing outward away form the dock face. As a result, even if you place a winch on the dock rail you will still be pulling almost straight back toward the pivot point which will not give you any effective lifting leverage. I think that if you make a flat cardboard model of your arrangement and your planned lifting method you will see the problem.

[jboggs]

800 lbs! You didn't think that little fact was worth mentioning in your original post?

As far as aluminum not having the tensile strength, what does a fully loaded 747 weigh? Its made of aluminum. Whether the material is steel, aluminum, or whatever, you size the members to carry the load. Usually the aluminum required to handle a specific load will weigh less than the steel required to handle the same load. And aluminum works very well around water. In fact, the removable ladders and platforms offered by boat manufacturers are all made of aluminum.

I agree with Jalberts. You could learn a lot by building some prototypes of wood or cardboard to test the geometry yourself before you ever commit to the final construction.

I'm not going to comment any more on the details of your design. Failure could result in human injury, or worse. You might want to get the assistance of a local mechanical engineer, especially if there is any possibility that anyone other your immediate family might ever find themselves on your stand.