S=I/c; larger values mean a beam resists more moment for a given stress.
S=I/c relates maximum bending stress to moment: σ=M/S. Rectangular beam bh²/6, circle πd³/32. Standard tables list S for rolled sections. Design chooses S≥M/σ_allow.
A 200 mm square section has S=1.33×10⁶ mm³.
The section modulus combines geometry into one number for bending design, so it is a quick way to compare beam sections.
S=I/c; larger values mean a beam resists more moment for a given stress.
The section modulus combines geometry into one number for bending design, so it is a quick way to compare beam sections.
I is the second moment of area and describes stiffness. S is I divided by the distance to the outer fibre and describes strength in bending.
It allows a designer to check bending capacity directly by dividing the moment by S and comparing with the allowable stress.
Typical applications include selecting a steel section from a catalogue for a required moment, comparing two designs of a box beam for the same bending capacity and checking a timber beam section for a given bending stress limit.
Study "Bending stress" first (topic 55 of 106 in Engineering Mechanics). It covers the groundwork this topic builds on.
Units m³. Larger S carries more moment. Plastic section modulus Z is higher.
A 200 mm square section has S=1.33×10⁶ mm³.
The next topic is "Beam deflection", which builds directly on this one.
It is classed as Intermediate in Engineering Mechanics (topic 56 of 106). It is an intermediate topic. Work through the beginner topics in Engineering Mechanics first.
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