$(10$ marks$)$ Fig. $1-2$ shows the arrangement of shear walls of a $25-$storey $RC$ shear wall structure subjected to wind load of $w=50k\frac{N}{m}$ along $y-$direction. The total height of building is $H=75m.$ The arrangement of shear walls is symmetrical. Take elastic modulus is $E=45$GPa. Ignore the effects due to across$-$wind and torsion. Assume all walls have the same thickness $t=500mm$ and the floor diaphragm is rigid.

a$)$ Determine the shear forces and overturning moments acting on walls $1$ and $2$ at the base level using approximate method of analysis.

$(5$ marks$)$

b$)$ Estimate the top drift of the structure due to wind along y$-$direction.

$(3$ marks$)$

c$)$ Comment on the torsional stiffness of the structure. Suggest one method to increase the torsional stiffness without altering the wall thickness and elastic modulus.

$(2$ marks$)$

Fig. $1-2$ Shear wall structure