What does this calculation tell us? For board (3) to remain attached to the other two boards, 198.53 lb of shear force in the x direction must be transmitted to board (3) from boards (1) and (2) along every inch of beam length in the region left of support B.

Each pair of nails can transmit 600 lb of shear force. The nail spacing along the length of the beam must be established such that the required shear strength of 198.53 lb/in. is provided.