Maximum Load for a custom steel sawhorse 3

[erbm]

We have custom made steel channel/angle sawhorses in our shop. They are used basically to hold up heavy steel plate, typically 2 under each plate. I want to conservatively "rate" them for a max. load that each one can hold by assuming a uniform load across it. They are constructed of a horizontal "C" channel at the top with "A" frame angles at each end. Some even have bracing under the "C" channel along its length as pictured.
Can someone help with the proper beam (Roark's stress and strain) formulas needed to analyze this?
I'm thinking it will fall under a BEAM that is simply supported at each end, but I don't know if I should check the design with other formulas.

 

[btrueblood]

You also need to check the legs to be sure they won't buckle, see Roark's chapter on "Elastic Stability".

I'd probably want to test one of them...

 

[redpicker]

Why not just "Proof Load" them to twice your rated load? That way, nobody can question your calculations and you don't have to worry about material or fabrication quality issues.

rp

 

[erbm]

I'd like to calculate a starting point or "rated load" first, and then we can proof test. We can even put the horses in our press brake to test. So I'm just wondering if I'm on the right path to come up with a max. tonage the horse can hold. Is there a link to a spreadsheet that has equations to run stress loads and check buckling, deflection?
Thanks for the comments.

 

[MikeHalloran]

The channel _would_ be loaded as a beam with a concentrated load at the center only when/if you set the plate edges down on the sawhorse in the process of placing the plates on the horses.

Once you have placed the plate horizontally on the horses, the channel on top is not nearly as stiff as a heavy plate, so it's really not loaded at all until the a-frames start to buckle and the diagonals get some tension or compression. Okay, the channel flanges get some direct compression, depending on the corner details.

If everything is placed symmetrically, the plate is basically supported by the four a-frames.

How about farming this out to a college junior or some FEA junkie?




Mike Halloran
Pembroke Pines, FL, USA

 

[boo1]

The small frame would require analysis of beam bending, columns strength and welds. FEA model or 2x what proof load of that you are loading it to now..

 

[berkshire]

erbm
If you have a load meter on your press brake, just set one on the brake, and keep increasing the load until it buckles, then back off 50% for the rest. You will have your answer before the FEA guy has loaded the problem.
B.E

The good engineer does not need to memorize every formula; he just needs to know where he can find them when he needs them. Old professor

 

[Ron]

btb and rp have it...proof load to twice the capacity you need and live with it.

 

[MintJulep]

Don't forget twisting when it's not set on a perfectly level floor.

And loading on the legs to account for getting whacked by something when loaded.

And probably a bunch of other abuse loads.

 

[msquared48]

Man, that's one honkin' sawhorse. Should should support at least two or three Clydesdales...

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[unclesyd]

I agreed with the proof testing to get an idea of a safe working load, but have one question.
Does OSHA or your insurance carrier have any requirements for said sawhorse?

The reason for this that we came under fire for using site built pipe stands and jacks. We were not allowed to test and use we had to get store bought ons. The ones we bought we not nearly as robust or steady as ours but the had a little stamp on the support column with all the vitals.

We did have numerous sawhorses built on the same pattern with angle iron for legs on the larger and pipe legs on the smaller. The only difference is we have shoes on the legs to help in moving them across floors.

 

[nuche1973]

what you have is a continuous span beam. My Roark's book has it under section 8.3 Three moment equation. The center support is your knee braces. You'll then have to check those braces in compression with the resultant load along their axies, as columns. Also, your legs will need to be checked with respect to your resultant loads from the channel. Use Roark's chapter on columns (in my book its chapter 12). Your end conditions will be fixed-free for the legs and fixed-fixed for the braces (if they are welded) pin-pin if bolted.

There are days when I wake up feeling like the dumbest man on the planet, then there are days when I confirm it.

 

[dicer]

Knowing the cross sectional dimensions of the material would help.
As long as the load is applied across the whole top surface, I'd say with a quick look it would safely hold 20,000lbs. This is a been there done that quess too. Are the welds good?
It will be interesting what you all calculate the load to be.

 

[ESPcomposites]

Looking at the structure, the likely failure mode will be instability of the legs. As mentioned, if the load is large enough to cover the legs, the load will ultimately make its way there in the event the top member experiences a bending failure. It would just be localized and the load will redistribute to the legs. But the loading condition has not be specified fully so that is just something to consider.

That said, the standard Euler solution will likely overestimate the buckling capability. That solution is for long columns and stable cross sections. You probably have a short/intermediate column and definitely have an unstable cross-section (angle). As such, you should calculate the crippling strength of the cross section and use that in combination with a Johnson-Euler solution. That is the approach using in the aircraft industry since the cross sections are often unstable (L, C, I, T).

Brian

http://www.espcomposites.com