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.
In this example, the wood grain direction is not your primary concern. In any situation, the load on the shelves will be too small to stress the supporting wood.
This is because the screws and the fixing in the wall are under the most strain, not the support piece (in this example, the piece of wood).
When the shelf is overloaded, the screws will bend or tear from the wall plug, there will be little damage to the supporting piece of wood. It is more important to pay attention to how the shelves are attached to the wall if they will hold much weight.
In other contexts, grain orientation is indeed important, and the wood will be stronger when the grain is parallel to the exerted force.
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.