Local axes and the roll angle
In step 1, we built the simply supported beam in the 3D canvas and read exactly the numbers the 2D canvas gave us. In 2D, though, we never had to think about the orientation of the section, because a beam drawn in a plane always has its web in that plane. A beam drawn in space can be turned about its own axis to any angle, so the model has to record the intended angle. In this step, we'll put two copies of step 1's beam side by side under the same load, one with its web vertical and one rolled through 90 degrees onto its side, and see what that one change does to the moment and the deflection. Along the way, we'll meet the local axes of a member and the roll angle that turns them.
1.0 The idea
Every frame member in the 3D canvas has its own set of local axes. Local x runs along the member from its start node to its end node. Local y points in the direction of the section's depth (down the web of an I-section, from flange to flange), and local z completes the right-handed set, running across the flanges. When you select a frame member, Structure Bench draws these axes as a small red, green and blue triad at its midspan (red for x, green for y and blue for z), so you can see which way the section is facing.
The section's properties are defined about these local axes. For our UB 305 x 165 x 40, the catalogue gives a second moment of area of 8,500 cm⁴ about local z, the strong (or major) axis value , and 764 cm⁴ about local y, the value . These two values are properties of the section, so they never change, whatever you do to the member. What change is where local y and local z point in space, and that determines which of the two values resists the load.