While calculating single member headers in outside walls, the bearing length or moment will sometimes fall short of the design criteria my firm specifies. The detail we use from the IRC places a flat wood plate the full length of the span below the structural member the full thickness of the exterior wall, i.e. 2x6 typically. The header is often directly below the wall double top plate, but also may be lower in the wall height, in which case we will specify a single top plate above the structural member. We know this adds to the strength and stability of the structural member, but there is not an option or accessory to show the additional reinforcement for the header.
Guest
It sounds like you're trying to put numbers to how much additional strength adjacent members give to your main header, and this is all code-qualified. I don't think wood has % composite like steel does, so if you want to members to work together, you need to design the inter-member connections for the full shear flow (VQ/I) of each section of built-up members.