When would I use symmetrical screw crosses over mutually parallel inclined screws?

Inclined screws can be arranged mutually parallel to one another or in symmetrical screw crosses.
Inclined screws can be arranged mutually parallel to one another when the design load is one-directional only. In this case, the screws will carry load primarily along the axis in tension. When designing for reverse-loading scenarios, symmetrical screw crosses can be used to create equal resistance in both directions. In this case, one screw per cross will carry load primarily in tension, while the other screw will carry load primarily in compression along the axis. Since the shear-tension screw and shear-compression screw within each cross only need to be offset 1.5D, greater overall capacity can be achieved in some cases by virtue of the fact that a greater number of inclined screws can be fit into a given area. Shear compression screws, however, cancel out the beneficial effects of friction, however, so the design resistance per screw is lower when using symmetrical screw crosses.
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Relevant Technical Resources

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