corroded cross section

huangyhg

corroded cross section
i am looking for opinions on the evaluation of corroded steel w-sections.
when you have a new section, it is easy enough to calculate your sectional modulus, and your maximum bending and shear stresses.  regarding your allowables, you can easily determine if you are compact, non-compact, or slender and come up with your allowable stresses.
when you start to look at a beam that has been in place for awhile, and exhibits heavy pitting along the bottom of the web and bottom flange, how do you determine your allowable stress, and your actual stress?
i have seen people estimate thickness loss and come up with a sectional modulus based on this.  would you also use this assumed thickness loss as your complete thickness for your b/t and h/t ratios to determine your allowable stresses?  if the web started to get very thin, would horizontal shear flow be an issue?
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i forgot to ask, does anyone have any good references to read regarding the evaluation of deteriorated existing   
i can also use a good reading on deteriorated steel structures, would also appreciate a good reference.