Strength (mostly axial)of square steel tubing

[slashragnarok]

I am designing a Power Rack like this. Would I benefit anyhow using 3"x3" tubing instead of 2"x2" tubing? The most weight that thing is going to be subjected to is around 800 lbs (400 kg approx). Also will bolting the pieces be dangerous or should I just weld them? Finally I also want to omit the slightly elevated cross member on the sides. Will that be OK?

Thanks.

 

[Mech_Engineer]

Generally I would say use the same size the off-the-shelf ones use. The new 3" will be proportionally stronger in tension/compression due to the increased cross-section, but more importantly it will be more than 3x stronger in bending due to the increase in size.

 

[slashragnarok]

Thanks. The off the shelf ones use both 2x2 and 3x3. The question is how much do I need. The thickness of the tubing is around 3 mm (11 gauge steel). There are also racks with 7 gauge steel (4.5mm). 7 gauge is overkill. Since there is very little dynamic loading involved I don't want to keep more than 1.2-1.5 factor of safety. So finally the question boils down to whether 2x2 11 gauge steel tubing racks can be rated for 800 lbs or not.

 

[Mech_Engineer]

It will depend on your geometry, calculating a safety factor requires that you can calculate stress in the part and how its supporting the load. Are you able to duplicate an existing design?

 

[AlephZero]

Since there is very little dynamic loading involved I don't want to keep more than 1.2-1.5 factor of safety.

I don't know anything specifically about weight training equipment, but that sounds a risky way to go. Calculating the loads assuming all the joints are perfectly rigid etc is straightforward, but the joints and adjustable parts are most likely where the failures will start from.

Don't forget to consider what the dynamic loads would be if something goes horribly wrong - e.g. half of the 800 pound weight falls off because somebody forgot to fix it properly, or whatever.

IMO safety factors of 5 or 10 would probably be more sensible than 1.2 or 1.5.

As mech-engineer said, copying an existing design is a good starting point.

 

[slashragnarok]

I don't know anything specifically about weight training equipment, but that sounds a risky way to go. Calculating the loads assuming all the joints are perfectly rigid etc is straightforward, but the joints and adjustable parts are most likely where the failures will start from.

Don't forget to consider what the dynamic loads would be if something goes horribly wrong - e.g. half of the 800 pound weight falls off because somebody forgot to fix it properly, or whatever.

IMO safety factors of 5 or 10 would probably be more sensible than 1.2 or 1.5.

As mech-engineer said, copying an existing design is a good starting point.

Sorry I didn't mention it but when I wrote 800 lbs I already factored in some safety. There's no way I'll ever lift more than 500 lbs.

@mech-engineer - I am trying to build off an existing design. The link I posted (Pic).

 

[Mech_Engineer]

This sounds like a safety-significant application, what's your motivation for designing and building this yourself instead of just purchasing one?

 

[slashragnarok]

This sounds like a safety-significant application, what's your motivation for designing and building this yourself instead of just purchasing one?

Firstly I'm in India and all the good manufacturers are in the US. Second, the parts I need are cheap here and labour is cheaper still. We have good workmen here and it is easy to copy designs. If you look at the pic I posted, it's ridiculously simple to replicate it. But, I am confused by the sheer magnitude of designs available. They also vary in terms of safety and complexity. We have bolted ones and welded ones. Ones with 2x2 steel and 3x3 steel. 11 gauge thickness vs 7 gauge. I don't want to deliberately overdesign the thing. This isn't an airplane. So I want your valuable opinions on how to get a safe yet optimally designed structure.

Now let me explain a bit about how the thing works. Please refer to the attached photo and if possible, please pardon my abysmal attempt at labelling the picture.

The thick black line labelled Approximate bar is the actual weight which would be resting on the Hooks for racking. The hooks have cylindrical projections at the back which are slightly smaller than the holes through which they are secured. An athlete would stand inside the structure and position the hooks as per her convenience. The hooks give the barbell a starting position from which the bar may be loaded onto one's back or other suitable positions. After she is done, the weights are put back on the hooks. The pins labelled safety are for emergency situations wherein a lifter having failed to lift the weight could rest the weight on the pins and escape unharmed.

So the points of load are the two hooks secured into the holes and in some cases the two safeties. Another possible loading point would be the member labelled chin up bar from which a person would hang and pull herself up. Here are some more pics to show how the thing works (note these are different designs).

Power rack

http://www.menshealth.co.uk/cm/menshealthuk/images/9W/power-rack-build-muscle-05082011-medium_new.jpg

And here's an alternate and more secure version of the hooks for resting the weights on

J hook

You can clearly see how the hooks are secure.

Hope this clarifies my situation. Thanks.

 

[Mech_Engineer]

I don't think there's any other feedback I can give you, your best bet is to err on the side of caution. Good luck.

 

[Monkey]

I would use the 3x3 without doubt, especially as you said parts are cheap there. I´d like to know how you intend to make the j-hooks? (Im a weightlifter too, so I know the equipment you are speaking of). You couldn't go far wrong duplicating the ROGUE equipment (infinity or monster lite ranges), it is the best in the market (IMO) - They do make a 2x2" design (Echo), but the 2"x3" is the minimum professional rack which they rate to 1000LBS. More is better in the case of Lbs rating :-)

Jhooks are another matter, nightmare trying to make those, I've tried…

 

[Monkey]

PS. Also bear in mind the pin sizes you are going to be using! The box section size isn't as important really (as long as using a minimal 2" 11 gauge) as the pin sizes you are using - You will want 1" tube for safeties in reality, and you would then be better off with larger than 2" for the box IMO. 3" 11 gauge is what I would go with in your situation. Also if ou are going to buy the Jcups (recommended) then you will want box to fit the majority of Jcups, ROGUE do a 2x3 and a 3x3 jcup, the 2x2 jcup is not good as it does have protective plastic inserts. Off the shelf Jcups will of course cost more than the cost of all your material though, so I guess you will be making them somehow?