The grain must b vertical or perpendicular to the spar !! It also must be a tight fit between the ribs !! Most homebuilts use an I beam type instead of a full box to save weight. The sheer web is only their to stop the bending moments of the spar. The bottom spar is in tention and the top spar is in compression, while positive G loads are in effect. just the oppisite in negative gs
the tip only transfers the loads from the top spar to the bottom spar, and vise versa !!