Plastic Moment Example - Doubly Symmetric I-shape

In this example, the plastic moment of a doubly symmetric I-shaped steel beam section is calculated. The section is constructed by welding 8”x1” flanges 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 doubly symmetric and homogenous section is located at the geometric center of the section (mid-height). 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, demonstrating that it can be calculated using either element areas or force resultants depending on whether the steel is homogenous or hybrid. The example concludes by comparing the results of this built-up section to standard rolled shapes, emphasizing that while area-based calculations work for homogenous steel sections, hybrid or composite sections require a force-based approach. 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/WPNEFKzHoR8 0:00 – Introduction 0:58 – Plastic Neutral Axis 2:00 – Plastic Stress Distribution 4:45 – Plastic Force Resultants 6:07 – Plastic Moment 8:37 – Plastic Section Modulus 9:20 – Homogenous, Hybrid, and Composite Sections Defined 10:26 – Equal Area Axis Theorem 11:50 – Plastic Section Modulus from Element Areas 14:05 – Summary and Discussion