Drag the section handles or use the sliders — same force through a smaller section means higher stress.
Stress isn't a material property — it's a condition. The same 100-kip load through a 10"×10" column puts every square inch of steel to relatively gentle work. Squeeze that column down to 2"×2" and the same load is packed into 4 square inches instead of 100. The material doesn't know the load changed. It just knows how hard it's being pushed per square inch. That's stress. When that number exceeds what the material can hold, it yields.
Stress is force per unit area: σ = F / A. It's what the material actually "feels." A 100-kip force on a 10 in² section creates 10 ksi of stress. The same force on a 1 in² section creates 100 ksi — enough to fracture steel. This is why member sizing is fundamentally about controlling stress. Bigger sections spread the same force over more area, keeping stress below the material's capacity.
Normal stress (σ) acts perpendicular to the cross-section — tension pulls fibers apart, compression pushes them together. Shear stress (τ) acts parallel to the cross-section — it tries to slide one face past the other. In bending, both exist simultaneously: normal stress from the bending moment (maximum at extreme fibers) and shear stress from the transverse force (maximum at the neutral axis). Design must check both.
Axial stress is uniform — every square inch of the section carries exactly the same load. That uniformity is what makes σ = P/A so clean. But when loads act differently, the distribution stops being uniform, and a new equation takes over.