🔄 Rotated Neutral Axis: A Smarter Way to Generate the PMM Surface

When checking seismic design for shear walls, engineers often rely on pre-defined PMM interaction curves. But with today’s tools, we can generate the full 3D PMM surface directly — efficiently and transparently.

At the core of this lies one powerful idea:

📐 Rotate the neutral axis — not the structure.

🧠 How It Works

Instead of rotating the wall or moments, we rotate the strain field applied to the wall section:

Y = -x * sin(theta) + y *cos(theta)

epsilon = -0.003 + phi (Y - Y_min)

Where:

• theta is the angle of the neutral axis (sweep direction)

• phi is the curvature magnitude

• Y defines the rotated coordinate along the strain gradient

By sweeping through theta and phi, we compute:

• Concrete and steel stresses (fiber by fiber)

• Internal forces: axial (P), moments (M2, M3)

• And generate the entire 3D PMM surface — from pure flexure to axial-compression-dominant interaction

⚡ Why It’s Powerful

• ✅ Geometrically flexible — works with any wall shape

• ✅ Mechanically transparent — no black-box assumptions

• ✅ Computationally efficient — no need for nonlinear solvers

• ✅ Seismic-ready — aligns naturally with demand rotations from ETABS

💡 The Result

Using this method, PMM generation becomes:

• Clean

• Intuitive

• Precise

And it reveals how real walls behave under combined axial and biaxial bending — in every rotated direction.