Steel Plate Design – Determine a Flat Plate’s Thickness

Steel plate bridge
Steel plate design is rarely covered in industry and is not often available in the simpler design software packages. When it is it will require modelling the plate for a FEA (finite element analysis) which can be unnecessarily time consuming.
Following the 3 steps below you will be impressing clients and sanity checking steel plate sizes in no time.

1. Select the Grade of Steel

In steel plate design, nine times out of ten this will be a very straight forward decision, grade S275. Structural steel plates are easily available in grade S275 and in most cases will be sufficient for what you are after.

BUT, yes there is always a but, there are also times when you should consider higher grade plates. This is almost exclusively when you are dealing with offshore structures and other corrosive environments. That’s a whole other can of worms, so pretend I didn’t say anything!
A quick rule of thumb for steel grades – S275 for when deflection governs sizing, S355 for when strength or durability governs design.

2. Steel Plate Plastic Modulus

Remember how to calculate that one? Most of the time you are not going to find that in standard tables, but it’s so simple to calculate that finding the right table will take longer.

Quick sidestep before I show you the goods, what is the plastic modulus? The plastic modulus is the sum of the first moment of the area above and below the plastic neutral axis.

Which when written looks like:steel plate design - plastic moment structural calculations

Luckily any time we work with steel plates the geometric variables are fairly predictable. We know the plate will be some form of rectangle and can therefore simplify the equation to:

rectangle plastic moment structural calculation

This one looks a bit less daunting!

3. Calculate the Moment Resistance

We now have everything we need to complete the structural calculations; the yield strength of the steel and the plastic modulus. The final step is to multiply the values and divide by a partial material factor. The result will be your plastic moment resistance.

steel plate design - moment resistance structural calculations

Steel Plate Design Example

To illustrate this we’ll calculate the moment resistance of a 10mm thick steel plate over a 100mm wide strip.
1.) Grade – We’ll go straight to grade S275 steel with yield strength of 275N/mm^2.
2) Plastic Modulus – (in mm)

plastic moment structural calculations
3) Design Moment Resistance – Note partial material factor taken as 1, this is determined by each countries design codes.

steel plate design - moment resistance structural calculations

That’s it, the design resistance for a 100mm wide strip of 10mm thick plate is 0.7kNm.

If you would like a more thorough look into structural steel design, our introduction to structural steel design course is currently 80% off on Udemy.

Good luck on your designs!

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