Assuming that continuous fillet welds are used, the welds may be minimum size per the AISC Specification Table J2.4 (fillet welds) if the member is subjected only to axial compression or tension. If the member is subjected to flexure, the shear flow (kips/in.) can be calculated from the beam shear Vu as VuQ/I and the weld sized to provide for this required strength; Q is the first moment about the neutral axis of the flange area and I is the moment of inertia of the entire cross-section.

Yes. This common practice results in a member that is known as a hybrid girder.

Tension-field action is the post-buckling development of diagonal tensile stresses in slender plate-girder web panels and compressive forces in the transverse stiffeners that border those panels. When tension-field action is considered in design, the AISC Specification G2.2 provisions apply; otherwise the AISC Specification Section G2.1 applies.