Thickness of a plate?? 1

[owainsx]

Hi All

My name is Oren and this is my first questions in here (I learn a lot from this forum - thanks)
I have a very simple question which unable to handle and sorry that the units are in SI
I have a box that need to hold 150 kg weight the box dimensions are 400x400x50 mm, for this discussion the box is not relevant only the area which is 400x400 mm
I need to calculate the thickness of the box, let say I'm using 1020 metal plate.

Thank you
Oren

 

[MintJulep]

The most direct path is to find the case in Roark's formulas for stress and strain that matches your load distribution and edge supports, then use the appropriate formulas to calculate stress for various commercially available thicknesses until you find the thinnest available plate that has acceptable stress.

Make sure that you include factors for any uncertainty in loading, and any additional forces caused by handling.

 

[rb1957]

For a problem like this (and not a real world situation) I'd assume uniformly distributed load (ie pressure = 150*9.81/(400*400) N/mm^2) = 0.0091 N/mm^2 (a tiny pressure ... 1 atm = 14.7psi = 101325N/m^2) = 1.33psi

"Hoffen wir mal, dass alles gut geht !"
General Paulus, Nov 1942, outside Stalingrad after the launch of Operation Uranus.

 

[3DDave]

Holding the weight is rarely the main problem; how much deflection is acceptable is the thing people see. The result is that the box is strong enough, but flexes into a different shape that isn't if the external supports don't confine it.

As such, the problem as stated is missing a large amount of critical information such as:

Is the item a rigid 400mm X 400mm X 50mm plate or is it tungsten powder that can shift CG and has no stiffness?
Is the box to sit on a rigid shelf or will a couple of people be trying to coordinate shifting grip to carry it?
Does it have an integral lid that can carry shear loads to keep it from twisting or is it an open bucket?

400 mm X 4 is a 1600 mm perimeter; about .1 kg (1 N) being lifted per 1 mm, which is in the range of many fabrics.

So, why the need for 1020 steel specifically? What characteristic is required that requires the use of metal?

 

[Stress_Eng]

Another thing that could have a significant input to how you analyse the plate is what form of loading could it be subjected to during its lifetime? Is it going to be subjected to some form of testing? One comes to mind, a drop test, be it on a flat or a corner, and to what height. Will it see items being dropped onto it. Once you have the loading sorted, you can then determine the appropriate form of analysis. For hand analysis, and as said, Roark is a good start. How is the plate to be assembled, welded (HAZ), bolt / rivet? Each giving a different edge condition and will require analysis, both the plate and the form of joint influencing the analysis.

 

[owainsx]

Hi All

Many thanks but none of you gave me the answer :-( how to calculate but thanks
I found a formula
t=3WL² /2b*sigma
where W is the force
L the length and b the width
Do you familiar with this formula?
it is supposed to be very simple, no
just to calculate the thickness of a 400x400 plate with weight of 150 KG and I chose for this example the 1020 it can also be ST37.2 or any other material.
Please help

 

[MintJulep]

I found a formula

And it's a very nice formula.

Where did you found it?

Why did you pick that one?

 

[owainsx]

From structure engineering and chat GPT

 

[dvd]

A good approach is presented in "Design of Weldments" by Omer Blodgett, section 4.6. You can buy this book for $17.50 from the Lincoln Welding Foundation, or maybe find it somewhere in digital form.

 

[Stress_Eng]

In Roark, for a simply supported plate, you have simple equations for max stress, deflection and distributed edge reaction force. Are you sizing the thickness based on stress (yield or ultimate), deflection or some other criteria (maybe a fatigue stress level)?

 

[zeusfaber]

"How to calculate" and whether the formula that ChatGPT pulled out of thin air is useful or misleading depends on all the questions 3DDave and the others are trying to get you to address.

If the 150 kg is a solid lump sitting on the bottom of the box, which you never intend to move and which is itself sitting on top of a solid, flat lump of concrete, then your equation is, for all practical purposes, t=0.

If you want to pick the box up by the edges, that's a different requirement - the join between the walls and the floor needs to be strong (thick) enough to carry the load. It's different again if the load is liquid or leaning on the wall and again if you want the box to remain roughly box-shaped.

90ish% of engineering is understanding the problem - plugging numbers into a formula is the last and easiest stage (and also easiest to teach, hence the disconnect between engineering education and engineering practice). If the engineers here aren't answering your question, it may be because you aren't asking the right questions yet.