Triangular Section:

A triangular section along base b and height h is illustrated in Figure . The location of neutral axis is at a distance h/3 from the base.

Moment of inertia around the neutral axis, I = bh ³/36

The topmost layer is at distance of 2h /3 from the neutral axis & the bottommost layer is at a distance of h/3 from the neutral axis.

y_top =(2/3)h and y_bottom =(1/3)    h

                  Beam Section                                          Bending Stress Distribution

Figure

∴  Section modulus, Z = I / y_max

= ( bh ³ /36) ×   (3/2h )  = bh ² /24

Consider σ_max be the maximum bending stress at the topmost layer.

∴  Moment of resistance, M = σ_max  Z = σ_max ( bh ² /24)