Beam AB is simply supported (pin at A, roller at B) and made of three planks glued together into an I section: a top flange 100 mm wide and 20 mm thick, a web 20 mm thick and 80 mm deep, and a bottom flange 60 mm wide and 20 mm thick (total depth 120 mm). The centroid C is 68.3 mm above the bottom face and the centroidal second moment of area about the neutral axis is $8.63\times10^{-6}\ \mathrm{m^4}$. Two downward loads $P_1 = P_2 = 3.0$ kN act at distances $a = 0.6$ m and $a + b = 1.0$ m from A, and the span is $2a + b = 1.6$ m. Each glued joint is 20 mm wide.

Determine the shear stress in each glued joint (joint u, between the top flange and the web, and joint l, between the web and the bottom flange) at section n-n, located between A and $P_1$.