How is Section Modulus Used in Bending Steel?


To determine if a steel section can be curved you need to first determine its section modulus and then see if you have the correct bending equipment. Section modulus is the direct measure of the strength of the steel. Bending a steel section that has a larger section modulus than another will be stronger and harder to bend.  Section modulus is a geometric property for a given cross-section used in the design of flexural members. There are two types of section moduli, the elastic section modulus (S) and the plastic section modulus (Z). In the case for bending a steel section it is important to calculate ‘S’ by taking the moment of inertia of the area of the cross section of a structural member – divided by the distance from the neutral axis to the furthest point of the steel section. This is where the steel will bend first.  The bending moment that it takes to yield that section equals the section modulus times the yield strength.

In simple terms, the section modulus is the ratio of bending moment to bending stress for steel. If your steel has a high section modulus it will be harder to bend and can withstand a high moment without having high bending stress. You need to divide the maximum bending moment by the section modulus to get the bending stress and then compare the bending stress to the allowable tensile stress to see if the steel can take that much moment. All bending equipment have section modulus ratings. A three-roll section bender can be designed to bend steel with section modulus between 0.4 to 500 in3.  So, to determine if the equipment can bend your steel section, you need to calculate the section modulus mathematically or by referring to engineering tables. Then, if that amount is equal or less than that of the calculated strength of the bending machine, your machine should be able to curve the section.

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