December 12, 2019, 06:48:33 AM
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Topic: structure load  (Read 250 times)

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Offline RomanKatz

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structure load
« on: November 25, 2019, 04:31:32 AM »
Hello everyone,

I have a question regarding applied load on a structure.

See attached PDF file of a IBC platform. It is made of 5*5 cm (2mm thickness) steel. I don't know the type of steel, but I can say that It is used for blocking the doors of shipped Flexy-tanks. That's how I have it.
 So an IBC is to be placed on this structure and I wonder if it will hold. I calculated the Inertia moment but what does it mean? its just a number for me.

Anyway, can someone show how to approach this and what to do, I will be very thankful.
Best Regards,
Roman Katz

Offline Enthalpy

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Re: structure load
« Reply #1 on: November 25, 2019, 12:17:05 PM »
Hi RomanKatz,

You can reasonably assume that the steel is S235 or a local equivalent. It's ferritic, with about 0.2%C, plus the Mn amount that guarantees 235MPa yield stress, and little more. There are very few steel compositions used to make profiles like your square tube, very few that are easily welded, none that is cheaper, and the other ones are stronger.

What is an IBC and a flexy-tank? You want to load this construction with a heavy item?

The construction can fail through many ways. The first one you could check is the bending of the beams. The others involve side buckling of the feet and are more complicated, here still  accessible to hand computation.

Can you predict how the weight is distributed, especially if the construction is used abnormally? Can it load a single beam?

Then the worst case would be: all weigh at the centre of a single beam (and possibly a shock if the mass is deposited brutally, difficult to compute). You compute a bending moment using half the force and half the beam length, and compare with the bending moment acceptable by this beam. You need a safety factor between both.

The construction wouldn't be strong against side loads, if any is expected.

Offline RomanKatz

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Re: structure load
« Reply #2 on: November 26, 2019, 03:22:10 AM »
Enthalpy, thank you for your reply.

IBC is a 1000L (250Gallons) plastic cube (1m*1*1) that can be transported via forklift. So it is expected to be placed on top of the construction. Because of that, the weight is distributed on all 4 top side bars(maybe even across all 6). Assuming that the operator will place the IBC calmly, what calculation should I do? static even load across the beam that is fixed on both sides?
Best Regards,
Roman Katz

Offline Enthalpy

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Re: structure load
« Reply #3 on: November 26, 2019, 06:05:28 PM »
That's clearer.

At least the 2mm walls of the tubes won't buckle before attaining the proof stress.

I wonder about the shock if loading a tank full of water on the construction. At 0.1m/s, the steel tubes can't absorb the kinetic energy within their elastic limit. The plastic tank probably neither. I'm ready to neglect that because the S235 can absorb a huge energy before breaking. But over many movements, either the steel frame or the plastic tank will get a permanent deformation, not quite elegant.

I have not checked whether the feet buckle to the side. This can be evaluated but is more difficult. Side forces, say if transporting the full tank, or if earthquakes are expected, may be dimensioning too.

Here are already names to discuss the static loads. By the way, mechanical design uses mm, not cm.

Offline Enthalpy

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Re: structure load
« Reply #4 on: November 26, 2019, 06:58:08 PM »
The full tank shall weigh 10kN. 995kg×9.806m/s2 plus some plastic.

To determine a load distribution on the beams, we must make some assumption, because a perfectly stiff tank would have just three contact points with the metal, unrealistic.

A uniform load on beams ABCD is reasonable from a deformable tank bottom, without justifying further. But I suppose the construction must withstand a slightly eccentric tank that relies on ABC only. Then if the tank survives, C feels 10kN/3 + 10kN/3/2 = 5kN, not the 3.3kN on C and B and 1.7kN on A and D if the tank is centred.

You can compute the bending moment on C: it's FX/2 if F+F is the length and X+X the distributed force.

You can compute what bending moment the beam withstands: it's σ(H4-h4)/H where H=50mm and h=50-2-2=46mm. S235 steel guarantees σ=235MPa proof stress.

I recommend to compute all in SI units. People using mm, daN and hbar get very annoyed when computing less usual behaviours, for vibrations, shocks, forces by fluids and so on.

Then you can compare both. I'd want a safety factor >=2 if the only possible loss is money, but here with shocks, I'd prefer much more.

You must also check the beam E (F is the same). It feels 2.5kN at 1/3 length from beam C and 1.7kN at 2/3 length from beam B. I expect a stronger bending moment on E than C. Or if the tank is round enough that it relies only on B and C, the E feels 2.5kN at 1/3 an 2/3 length.

Offline RomanKatz

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Re: structure load
« Reply #5 on: December 04, 2019, 05:27:02 AM »
Thanks very much. It was very helpful.
Best Regards,
Roman Katz

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