Sawhorse Capacity 3

[dozer]

I've been asked to determine the capacity of some homemade sawhorses we have in the shop. These are made of carbon steel HSS sections. I was wondering if anyone knows what manufacturers of commercial sawhorses do for the following:

1) Safety factor
2) Impact factor (if any)
3) How load applied? Concentrated, uniform?
4) Where load applied? Even though a concentrated load at the center would seem to be the worst case, I can imagine a case where a concentrated load at the very end would control because of web failure doing a check per AISC 14th ed., section J10.
5) Deflection of supported load. Imagine two sawhorses supporting a bundle of rebar. The rebar sags significantly. In my case, the top horizontal member is a tube (HSS). Because of this sag, instead of each vertical wall of the tube carrying the load evenly, the inside wall (wall closest to cg of rebar bundle) would carry most or even all of the load.

 

[skeletron]

With shop devices, I would start conservative and then slim the design down as needed. Impact factor applies if a crane is involved, otherwise it depends on your safety margin.

I found this type of work interesting because of the marriage of prior knowledge of LSD but acknowledgement that working stress design might actually be a better fit. It could be tedious because the wide variability in loads and applications. One strategy I used was to include usage/safety notes on the drawings. That helped protect against someone using the device incorrectly and then blaming the engineer. So in your case, I would include a maximum spacing between sawhorses for the rebar.

 

[BridgeSmith]

1) Since it's internal, I would say report nominal capacity (yield capacity). Whatever you use be sure to clearly indicate it in the findings/report.
2) Probably not, since any factor would be a WAG. Again, clearly indicate what you used. Perhaps include a calculated increase factor for an assumed common drop distance, i.e. effective load increased by x% if x material is dropped on the sawhorse from x inches. I can't help you with where to find reference materials for that.
3 & 4) Depends on what will be supported. If there's a variety, including single, heavy sections, point load at the center should be the design loading case.
5) Unlikely to control the capacity, but check local yielding if you're concerned.

 

[dhengr]

Dozer:
1 & 2. Well…, you’ve got a pretty good start at the list of things you should check. And, as you get into your analysis, you will add to that list. It seems to me that sawhorses in a fab. shop are not unlike many reusable components in a lifting system which are covered by the ASME BTH - 1 Std. and have some FoS as high as 5. They are used pretty indiscriminately, not well maintained or cared for, sit out in the rain to rust until needed again, sometimes cut while cutting the material they are supporting, etc. etc. The shop assumes that if material will fit on them, they must be able to support it, come what may. You pretty much have to imagine all the ways the shop may misuse them and be sure that situation is not a killer or maimer. So, a high FoS is a reasonable assumption to start with, since the shop will find ways to use that up. Certainly, they will drop loads on them, so impact is worth considering. All of these are up to your engineering judgement and experience out in the shop.
3, 4 & 5. All the worst possible loads in the worst possible ways, positions, orientations, etc. are fair game. And, you have already listed a few of them, add lateral loads on the top HSS. Yes, one web of the top HSS could carry much of the load and thus the legs on that side too. The two legs on one end can be overloaded too, actually lifting the other two legs off the floor. The feet of the legs should splay out beyond the end of the top HSS, so they can’t tip the horse over endwise. Same reason the legs splay side to side, as relates to lateral loads and your sagging rebar bundle.
6. Check all welds for cap’y. and good load path. Check top HSS for bending and shear cap’y. and legs for col. cap’y, as a beam/column. What cross connection is there btwn. the two legs, tying them together, and immediately under the top HSS to carry it? In effect, determine all these various member and joint cap’ys., then work back to all your imagined loadings.

Edit: If you are doing this for your own shop (or a client for that matter), and to satisfy your insurer, you might suggest to your boss that you could give a safety talk on sawhorse usage to your shop people. Of course, your report should show all of your assumptions, FoS’ and allowable cap’ys. for various loading conditions. Don’t get too complicated with the allowable loads, this just get confusing for the shop; center point load and uniform load on top HSS, and max. load per sawhorse, then your safety instructions. You certainly should have inspected the sawhorses which are out there, so you know what exists, and maybe what needs fixin. Your report might also comment on inspection and maintenance requirements for your shop sawhorses.

 

[BridgeSmith]

skeletron & dhengr, I would agree with you for designing the sawhorses, but unless I misunderstood the OP, this is an evaluation of existing sawhorses, isn't it? dozer is tasked with reporting what the sawhorses can carry, right? Shouldn't that be nominal loading?

 

[skeletron]

@HotRod10: yes you are correct. The OP is asking determine the capacity of existing sawhorses.

I still approach these things as I would a new design except that the geometry is determined.
Dhengr has a pretty thorough write-up above.

I think your rated capacity (which should be stenciled on the sawhorse) is based on your load and safety factor. Whatever the load that the unit is rated for, the shop workers will drop that load on the sawhorse.

 

[racookpe1978]

If the analysis is on an EXISTING sawhorse, then the EAXACT weld details (which will be different for every leg) and the actual leg material and backbone material MUST be measured for each different sawhorse.

One is likely from 3x3x1/8 angle iron, the next 3x3x1/4 on three legs, 3x3x1/8 on the 4th leg - but its bent and cut up too. The third, fourth and fifth were made later, and use two back-to-back C4's for the backbone, but the first and second use WF's. Expect them to be made from scrap and mis-matched parts with unmatched welds under (or over) every different leg. And none of the welds will be NDE'd, and most were made by the trainee, apprentice, or most useless welder at the time. The good welders were working on paid jobs, not shop junk.

 

[BridgeSmith]

Yeah skeletron, if the load capacity is to be posted on the sawhorses, I'd agree it should be some sort of 'working load', which for other items I have seen, is 1/3 to 1/4 of the ultimate (nominal) strength.

 

[WinelandV]

How about a LOAD TEST....? Might be quicker and easier. Set failure as an excessive deformation (1/4"? 3/8"?). Then determine the rated capacity by dividing by 1.25 of 1.5.

Easy, peasy, lemon-squeezy.

 

[CANPRO]

I’m given these types of requests often from the shop, and honestly I find this type of work refreshing - I think you’re far enough removed from the codes that you can use your own judgment and first principals to put a reasonable load rating on the shop equipment. Of course references like AISC manuals/design guides are a great help. But be aware that the sawhorse might be used for something that is governed by some written code (if they put planks on it and use it for access you’ve got yourself a scaffold component).

When I’m asked “how much can this hold?”, my first question back is always “how much do you need it to hold?”. It gives you a target to check, which is much easier than trying to find the weakest link and maximum possible capacity. You can run the numbers and determine what you have for a FoS based on what the shop wants. You may find for example they only need a 5,000 lbs rating and maybe you would have been comfortable with 7,000 lbs...you can tell them it’s good for 6,000 lbs, you look like a hero and you can sleep easy.

 

[Ron247]

I have not seen your saw horses, but if the beam portion overhangs the legs, you have a serious stability issue to check. I made guys cut the overhangs off since I have seen someone set something on the overhang and then start hitting it sideways. I assume the legs are not fastened down to the floor.

Crank all the numbers you want, in the end do a load test to some degree.

As far as Factor of Safety, to me, it should be 3 (Failure/3) or more since there is so many ways someone can use and abuse the saw horses. For example, Make your mind also think of round pipe rolling off the edge etc. We also know someone will be right there underfoot when it fails.

 

[Ron]

Since this is internal and you are in the US, you should check the OSHA requirements for such devices. That should give you the safety factor you need.

Agree with others that a load test would be more useful.

 

[dozer]

Thanks for all the replies. I didn't expect to get so much feedback. I would love to test these but these but one I checked will handle a 35 kip concentrated load using AISC code and assuming no side load and applied so as not to cause twisting. I'm pretty sure our shop is not going to want to do testing of that magnitude.

Lots of good ideas. I'll use them as a check list of sorts as I proceed. Thanks again.

 

[Compositepro]

I suggest that for a case like this that the only purpose calculations would be to know what load to use for proof testing. The calculation itself will do nothing to prove safety.

 

[charliealphabravo]

I have been asked to do a similar load rating of some Racks Of Unknown Source (R.O.U.S.) in our shop and I find myself thinking about the same questions.

I think load testing alone is also not adequate in the absence of calculations. Consider that most structures are put into service based solely on calculation. Without strain or deflection measurements you will not know the stress levels. Without calculations you will not know the controlling failure mode or where to take your strain/deflection measurements to observe maximum effect. I suppose load testing presumes careful observation for distortion and buckling which may effectively alleviate the need for much calculation, especially if you conservatively de-rate the structure.

Some risks cannot be resolved with either calculations or proof testing. I find myself wondering about the quality of welds and the quality of the steel. Calculations and load testing only has meaning when you have some confidence in these qualities.

 

[kingnero]

I did something similar once (automatic loading table but for really heavy loads), and based my impact calculations on the faster speed of the two speeds, of the cranes.
It's hard to drop something on sawhorses, as they're always (?) loaded using the overhead crane or with a bobcat/forklift. If you know the velocity of the downward speed, I'd say that's a fairly good starting point.