# Strength of square steel tube

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1. May 12, 2017

### pHartless

I have a problem to do with my project in college. I'm creating a hanging fire pit, basically made of 3 lengths of square steel bar connected to a plate at the top and on hinges. The 3 legs of the frame move in slots and when in the open position would be at 45 degrees. The actual fire pit is hanging from the plate via a chain. My problem is that my tutor argues the weight of the chain/fire pit would cause the square tubes to bend when in the open position, but me and another tutor argue it would not- and none of know how to go about calculating it.

So does anybody know the maximum weight 3 20x20mm square steel tubes when arranged at 45 degrees like I described would be able to take before deforming?

2. May 13, 2017

### Nidum

Please post a clear diagram of the assembly .

3. May 13, 2017

### pHartless

4. May 13, 2017

### RogueOne

Is this designed as using square bar, or square tube? If it is tube, what is the wall thickness of the tube?

5. May 13, 2017

### pHartless

It is using square tube, wall thickness being 2mm

6. May 13, 2017

### Nidum

(1) Intuitively 20 mm square tubes which are 1850 mm long would seem to be a bit feeble to support any significant load unless they were additionally braced somehow .

(2) Even if the actual legs were adequate for your purpose I would still be concerned about the stability of the tripod because there does not seem to be any very reliable means of stopping the legs spreading and causing the tripod to collapse as a mechanism .

(3) Try modifying the design and post a new sketch when you have finished .

Last edited: May 13, 2017
7. May 13, 2017

### pHartless

the legs move in slots in a plate just below the plate they are attached to by the hinges, which prevents them from spreading. Can you not scrutinize my design I'm not asking you for design advice, my question was whats the maximum weight they could take before deforming and if you can help me with that great.

8. May 15, 2017

### RogueOne

Actually, his question clarified for me that this structure can be thought of as a space frame, which makes it much more simple to calculate.

The bending forces put on the tubes will be negligible. Most of the laod will be longitudinal to the tube's lengths. So this becomes a question, mostly, of their buckling/compressive strength.

The amount of load that can be applied downward by a chain connected to the top, theoretically, will be 3x the buckling strength of one of those square tubes. Do you know any of the ansi standards for the tube that you want to use? If not, I could make assumptions and tell you an approximate strength.

9. May 15, 2017

### RogueOne

For a 73" long square tube with a .065" (16 gauge) wall thickness made of steel, The maximum column load is 374lbs.

If you make your frame precisely enough to distribute that load perfectly, then your frame will accept somewhere around 1122lbs net weight hanging from the top of them.

However, if you actually light a fire in that hanging grate, then the steel will heat up. If it gets hot enough from the fire, it will lose strength.

10. May 16, 2017

### gary350

Look it up in the Machinist Hand Book formulas are there. Compression strength is many times stronger than bending strength for 3 legs at a 45 degree angle. 20mm will be very strong.

I built 1 of those for camping from 3/8" round steel rods it holds up a 15 lb pot of stew easy over the fire. 3/8" hollow rods would have been better much lighter weight for hauling around. My 3 legs were about 5 ft tall at 30 degree angle.
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11. May 16, 2017

### RogueOne

Keep in mind that the tubes are hinge-jointed. "pin jointed" describes it as well. That is important in defining this structure.

12. May 16, 2017

### Shane Kennedy

In that arrangement, assuming the legs aren't restrained i.e. pin jointed, at the top from moving wider, the legs are purely in compression. At 45degrees, the load on each is the weight*(root2)/3. That does make for an slightly unstable tripod though. Unless the tubes are bent to start with, they are unlikely to buckle. A larger, thinner wall tube will be even stiffer.

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