Plastic Moment Example - Singly Symmetric I-shape

In this example the plastic moment of a singly symmetric I-shaped steel beam section is calculated. The section is constructed by welding a 6”x1” top flange and a 10”x1” bottom flange to a 20”x1/2” web using steel with a yield stress of 50ksi. This video provides a step-by-step example for calculating the plastic moment of a built-up steel I-beam. The lesson begins by identifying the plastic neutral axis, which for this singly symmetric and homogenous section is found by using either equilibrium on the fully plastic section or the equal area axis theorem. The example then explains that the plastic moment is derived by summing the moments of the plastic force resultants - the product of the yield stress and the area of each component - about the plastic neutral axis. Then, the plastic section modulus is determined, demonstrating that it can be calculated using either element areas or force resultants depending on whether the steel is homogenous or hybrid. This example builds off of an earlier example where the elastic bending properties of the sections were investigated. That example is available here: https://youtu.be/fE5ckATxDSs This example also complements an earlier example where the plastic moment of a doubly symmetric beam section was calculated. That example is available here: https://youtu.be/Zdop3epU8PI 0:00 – Introduction 1:05 – Definition of Plastic Neutral Axis 2:30 – Plastic Stress Distribution 3:20 – Plastic Force Resultants 4:45 – Solving for the Plastic Neutral Axis 6:22 – Plastic Moment 11:08 – Plastic Section Modulus 11:50 – Mathematical Shortcut for Finding the PNA 15:25 – Equal Area Axis Theorem 17:20 – Plastic Section Modulus from Element Areas 18:38 – Summary and Discussion