# What thickness of steel sheet do I need to achieve a given strength?

I have a customer that originally bought 0.0351" thick sheet steel, CSA grade, G90 zinc coating. They moved to 0.0236" Grade80, G90 to cut costs. The thinner, harder steel satisfied their customer's strength requirements. Their customer would now like a heavier zinc coating (G110) for corrosion protection but our customer wants to keep the total metal thickness the same meaning the base metal thickness will need to decrease. Strength is determined by the base metal thickness, not the zinc coating! What is the formula to determine how thin I can go and stay within the strength requirements of my customer's customer?

Now- Here is where you are going to beat me up. I do not know the strength requirements yet of my customer's customer. If I assume that requirement to be 'x', I could still calculate strength.

• Without knowing the requirements, there really isn't any way to answer this. @wasabi tried to give you some general considerations, but it still isn't an answer. The items mentioned might not even be relevant to the way that this part is loaded. It would be much better if you could wait until you know more of your requirements.
– hazzey
Jun 23 '15 at 18:06

If the sheet suffers only compression, then you need to check how the sheet will collapse: by stress (unlikely, only if a very short sheet) or by buckling. The allowable compression force by stress is obtained in the same way as in tension: allowable stress times cross-sectional area (assuming distributed load). By buckling the absolute maximum value is given by Euler: $$F = \dfrac{\pi^2EI}{(kL)^2}$$ where $EI$ is the sheet's stiffness and $kL$ is the effective length (a function of the boundary conditions). This is the absolute maximum, but no code will ever allow you to get close to this value. Since this is a function of the sheet's moment of inertia, the strength is a function of the orientation of the sheet. The lower value is obviously given for the sheet's "weak axis", so here the strength is also linearly proportional to thickness (double the thickness, double the strength).