In case of timber structures, the simple bending formula M = fz may be applied for A. Rectangular beams up to 300 mm depth B. All rectangular beams C. Solid circular beams only D. All square cross-section beams

The correct answer is: A. Rectangular beams up to 300 mm depth.

The simple bending formula M = fz is a simplified version of the more general bending formula M = ∫ (σ(x) y) dx, where σ(x) is the stress at a distance x from the neutral axis, y is the distance from the neutral axis to the point of interest, and M is the bending moment. The simple bending formula is only valid for beams with a rectangular cross-section and a depth of up to 300 mm. For beams with a larger depth, the more general bending formula must be used.

Option B is incorrect because the simple bending formula is not valid for all rectangular beams. It is only valid for beams with a depth of up to 300 mm.

Option C is incorrect because the simple bending formula is not valid for solid circular beams. It is only valid for rectangular beams.

Option D is incorrect because the simple bending formula is not valid for all square cross-section beams. It is only valid for rectangular beams with a depth of up to 300 mm.