Lessons · Engineering · Choosing a section from its properties
Choosing a section from the numbers in the table
A steel section is chosen by its properties, not its look. The section modulus relates bending moment to stress, so the required modulus is the moment divided by the allowable stress. Pick the lightest section in the table whose modulus is at least that.
Hone is a place to practise a career, one idea a day. This is one of its lessons, written out in full and free to read without an account.
In beta. This lesson was written for Hone and has not yet been checked by a licensed engineer. Practice material, not professional advice. What that means.
What it is for
It is the one place in a structural calculation where the answer really is looked up, and the skill is knowing which number to look up and which way round the division goes.
How to think about it
Work out the required section modulus as moment over allowable stress, keeping units consistent. Then read down the table for the first section that meets it, preferring the lightest, and check deflection separately afterwards.
Worked example
Required modulus = moment / allowable stress.Both from the problem.
Watch units: kN·m and N/mm squared need converting.This is where most errors live.
Read the table for the first section at or above it.Lightest that qualifies.
Then check deflection, which may force a deeper one.Strength is not the end.
Your turn
Moment 60 kN·m, allowable stress 150 N/mm². Write the required section modulus in cm³. (60 kN·m = 60x10⁶ N·mm)
60x10^6 / 150 = 400000 mm^3 = cm^3
Solve one, graded on the server
The trap
Mixing units. kN·m over N/mm squared gives nonsense unless the moment is converted first, and the nonsense looks like a plausible number.