From diagram in $\Delta ABD$

$F_c=R cos(\beta-\alpha)$----------------1

Also from$\Delta ACD$

$F_S=R cos[\varphi+\beta-\alpha]$

therfore R=$\frac{F_s}{cos[\Phi+\beta-\alpha]}$-----------------------2

Shear stress;$f_s=\frac{F_s}{A_s}$

therefore $F_S=f_s \times \frac{t_1\times b}{sin\Phi}$-----------------------3

Putting the value of $F_s$ from eqn.3 in eqn. 2 and then the value of R in eqn.1

$F_c=f_s \times \frac{t_1 \times b}{sin \Phi} \times \frac{1}{cos[\Phi+\beta-\alpha]} \times cos(\beta-\alpha) $ …