Gusset Connection Axial

AISC Steel Construction Manual - Part 13 in the 14th edition. It's the black tab that says "BRACE"

It doesn't have your exact situation, but has enough that you should be able to work with it.

Our cross frames for bridge girders are very similar, except that instead of a W-beam, the bottom horizontal is an angle. Outside of the standard checks for shear on the bolts and bearing at the bolt holes, we don't really check anything specific to the gusset plate. The bridge design spec has a minimum thickness for the gusset plates (5/16") and limits on the distance between the angles (the unsupported width of the gusset plate) so buckling of the plate is precluded. I suppose without those limits, you'd need to consider plate buckling as a possible failure mode, although if your drawing is to scale, it seems unlikely there'd be any reduction in capacity for that.

Rod Smith, P.E., The artist formerly known as HotRod10

AISC Design Guide 29. Look for chevron bracing connections.

There are several critical sections identified for the plate flexure and shear.