Structure is a plane frame. Members are of rectangular cross section, all in timber. Beta Angle is 0.0. The analysis results are OK (They show MY to be 0.0). But the timber design output shows values of out-of-plane bending stresses (fby) to be non-zero. What is the reason?

 In timber design per AITC, members designed as a column are to be designed for moments caused by minimum eccentricity of axial load. So even if MY is 0.0, the program during design will compute an MY requirements of the AITC Manual. The minimum eccentricity is assumed to be 1 inch or 1/10th of the depth in that direction, whichever is more. See Chapter 4 of the STAAD.Pro Technical Reference manual for details.