what is the load direction?

Load direction? Sorry, this was not a question specifc to any application just in general, is it better to have rivets (single shear or double shear) lined up in a "square"pattern (upper) or in a "triangular"(or "stitched") pattern (lower). Not necessarily the pattern shown.
The load direction would then be horizontal or vertical, it doesn't matter. (I think. That is why I am asking).

For reference, AC43-13B recommends the upper version.

Timbo

The stress distribution is definitely affected by load direction.  In the staggard pattern shown if the load is vertical, the shadow effect of the rivet cross section subtracts more area from the nominal cross section of the lapped members and therefore higher net stress.  If loaded horizontally the net stress is more nearly the same as the nonstaggard pattern but the first row of rivets must sustain more strain than subsequent rivets before they begin to share the load in this direction of loading.  the equations for determining the optimum spacing are not unduly complex but the analysis is hardly rudimentary.  Spacing between rivets and edge distance must be
considered.  Generally the nonstaggard spacing is preferred but there are exceptions.  It also depends on whether their is pressure involved causing hoop stress along with simple tension or shear stress superimposed.

Jesus is THE life,
Leonard

The staggered pattern gives better results in a fluid tight application as the clamp up regions around each fastener overlap more efficiently, minimizing a leak path. I've seen this pattern in shear webs of integral fuselage and wing fuel tanks.