design header wall thickness required by para. 403.1.2. For welded pipe, when the branch does not intersect the longitudinal or helical seam weld of the header, the allowable stress value for seamless pipe of comparable grade may be used in determining th for the purpose of reinforcement calculations only. When the branch does intersect the longitudinal or helical seam weld of the header, the allowable stress value, S, of the header shall be used in the calculation. The allowable stress value, S, of the branch shall be used in calculating tb.