Answer:
The reduction of the shear stiffness is caused by the fact that the fibers perpendicular to each other have a very small stiffness and a low strength. For this reason, the rolling shear strength of CLT panels is also very small.
The shear stresses in yz are compared with the rolling shear strength τyz.
The same applies to the shear stiffnesses. In the direction of the minor axis (yz-plane), the stiffness of the individual boards is considerably smaller than in the direction of the principal axis (xz‑plane).