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Precomposite Stresses in Reinforced Steel Sections

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When strengthening a steel section (an open-web joist, beam, or girder in my most recent case), I always did the following (ASD):
1. assume a more or less permanent load (mainly the DL taken by the member), and compute the stresses and deflection due to this load on the original section.
2. compute the additional stresses and deflection based on the reinforced section (due mainly to LL and any additional DL - like permanent mechanical equipment)
3. add the two together (at appropriate points on the section) to determine the total stress, and compare with allowable.  If the section is overstressed, then require some load removal prior to reinforcing (e.g. jacking to remove some DL).
Recently, an engineer in my office told me he only checks the 1st condition.  In step 2, he puts the entire DL + LL on the new reinforced (i.e. increased) section, and doesn't consider the locked in stresses due to the permanent load on the original section.
While I know AISC ASD allows this for composite beams, it seems this is based on certain approximations and simplifications.  I just think that it cannot be correct in an elastic approach to the problem, especially when considering things like coverplating, etc.   (I know using a plastic section approach would make a big difference, but I only talking elastic in this case)
Any thoughts?