The forces are not directly additive. Use Force=sqrt(axial^2 + shear^2). Then you could take the resultant force of the shear and axial and interpolate the C_weld value from AISC Tables 8-8. If you need a faster conservative answer, neglect the contribution of the horizontal welds and attribute the force resultant to the single vertical weld, probably without the directional strength increase. AISC eqn 8-1 (page 8-8 in 15th edition).