square steel tubing strength

[fuel-depot]

Hi there.

I realise this is a "how long is a peice of string" question but im hoping someone can point me in the direction of working out what size steel I would need to use for a project.

Im wanting to build a wheelchair ramp that will be about 2200m long with a width of about 700mm, I want to hinge the centre it so i can easily store/lift it when not in use.

So really what im trying to work out (im far from an engineer) is what thickness/size/type of steel would i need to use, how much bracing is required and what is the best configuration for strength. I was going to either screw in ply on top or something similar for the ramp surface. (I am trying to keep it light)

thanks for any help

dave

[Master Splinter]

If you are trying to keep it light, you might be better served by using some of the "C" or "Z" profiles (purlins) that are sold for roofing/decking - they are a more efficient shape for taking a load, plus they come with a set of span tables so you can work out what sort of size you need. See here. (http://www.stramit.com.au/products/structural/purlins/selection.html)

A quick browse of that suggests that their smallest member should be up to it, and should weigh around 8 kg for the steel. But you might like to reconsider the idea of folding them in the middle, as you would need pretty hefty hinges and a locking mechanism. (it would be the area under the most load - remember, thats a person in a wheelchair, and possibly someone pushing them, so thats at least 160 kilos of dead weight, so I would be wanting something that could take 5-600 kilos...I am not an engineer, but thats the sort of safety margin that sounds good to me!)

An easier solution may be to buy two or three of the ready made scaffold planks in steel or aluminium (http://www.plumberschoice.com.au/page/shop/browse/a/category/e/Scaffold_planks) or timber (http://www.tilling.com.au/index.php?option=com_content&task=section&id=6&Itemid=38) or even plastic (http://www.gratingco.com.au/scaffolding.shtml) and just fabricating a clip to hold the planks in the right place. Strong, easy to carry, and no fabrication fiddles.

[echnidna]

50mm x 2mm rhs would handle that

[Chris Parks]

I built one as a torsion box. Two sheets of 3 ply and some 20mm pieces as the seperation timber. It was very light and very strong.

[fuel-depot]

back to the drawingborad

cheers

[GDW]

G'day - I was chasing some formulas to calculate safe working load on some stands I was making to level up my new motorhome on my sloping driveway, even asked some practicing engineers who no longer did those sort of calculations.
The end result for me was a Mechanical engineering reference text in my local library (can't remember the title) which had a table of safe working load for various beam profiles for mild steel (table 5.2.4).

For hollow rectangles the equation for greatest safe load in lbs (it was an american text) was:

890(AD-ad)/L for load in the center and,
1780(AD-ad)/L for load distributed across beam

Where L = distance between supports (ft), A = cross sectional area of the beam (sq in), D = depth of beam (in), a = Interior cross sectional area (sq in), d = interior depth (in).

I built a series of equations in excel to convert the measurements to metric and calculate the safe working loads for various sized steel profiles and played until I had a design that would carry at least 4 times the load. To date the stands have held up OK.

GDW

[soundman]

The biggest problem is going to be the hinge?

The joint will be at the point of maximum stress and the forces will be enormous.

If you can get away without a a break or hinge it would be soooo much easier.

cheers