I'm cutting some shelf supports, and am curious which grain direction is optimal for holding a screw + load.
(Of course this is negligible in the given context, but I'd like to know for reference)
I'm a visual thinker so, here:
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Vertical (quartersawn) grain is the strongest in terms of flexion loads, and consequently one would assume would be slightly stronger against compression loads. The wood is also less likely to split across a grain line vs along one, but I think that's dependant on species. An example here is guitar braces. They use straight grained spruce with vertical grain lines for the optimal strength/stiffness to weight ratio.
Slightly more importantly is wood shift. Wood moves the most parallel to a grain line as humidity changes. One would assume you will have enough compression in the (soft) wood from the screw to tolerate any changes in the wood, however in an application using bolts and hardwood with minimal compression, the change in size of the wood can cause an otherwise torqued-to-spec bolt to come loose. An anecdotal example: I have to tighten the wooden holds on my climbing wall in the winter, because the wood shrinks just enough that the holds will spin under the heavy load of dynamic climbing.
It does not matter here as loads will be too small to stress supports.
With screw passing through wood no noticeable difference A to B. Screws and fixing in wall under most strain, not support piece. If a shelf is overloaded screws will bend or tear from wall plug, little damage to support wood. How shelves are attached to supports is very important if shelves hold much weight.
In other context grain orientation is more important where wood has square section. Stronger when grain is parallel to force exerted.
This is more a question of growth rings than grain. The grain runs lengthwise in the beam, so it is parallel to the wall in either case, as it should be. The growth rings affect the strength of the wood less than the main grain direction, but there is still some difference.
It is unlikely that the load would be high enough to split the wood. Usually wood constructions are limited by the amount of sag (bending due to force) rather than the break strength.
However, seasonal moisture variations may eventually split the wood along the growth ring. The screw direction A would hold the split wood together, while the screw direction B would pull the split wider.
Similarly if the screw is driven too close to the end in direction B, it splits the wood easier than in direction A.