$4\mathrm{.}11$

A rectangular high$-$rise building structure with a height of $H=40m$ and a windward face width of $B=30m$ is constructed in the suburbs of a city. The ground roughness index is denoted as a$.=0\mathrm{.}15.$ The basic wind pressure is $W_0=0\mathrm{.}5k\frac{N}{m^2}.$ The ground roughness index for the standard terrain is also a$.=0\mathrm{.}15.$ Assume that the influence of along$-$wind turbulence is not considered. The height is divided into four segments for approximate calculations. Determine the bending moment at the bottom of the building generated by the along$-$wind action.