life extension of column base

huangyhg

life extension of column base
i am going over many of the older columns in a thermal generating plant, which have been exposed to a corrosive environment, mainly in the lower 6 to 8 inches of the column base. the steel is delaminating severly on some of them, and is deteriorating quite significantly with regards to its thickness.
i have seen two solutions done within the plant before: temporarily shoring the column, removing the lower 3', and welding in a new column section followed by proper coating. the other solution was welding a 3/4" thick steel plate on the web of the w-section column about 12" up from the bottom. this appears to have helped greatly in extending the life of the column, but i am not sure if this is good practice or just a guessed-patch job by a fabricator.
is there any way to determine practical reinforcing plate thicknesses and dimensions to weld to the web's of these columns to extended the life of the columns without full replacement? i of course want a cost effective way to reinforce the existing web and flange material of these w-section columns and to prevent futher deterioration. any suggestions are greatly appreciated!

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i guess you could measure the column where it's not deteriorated and use the same size plates (or at least the same areas) to reinforce the corroded region.
but i like the first way (shoring and replacing) a lot better. you don't have to worry about any little areas of corrosion being left and continuing to spread into the new fix. and you're assured of getting bearing of the new  entire section on the base plate.  plus it will be easier to coat, without a bunch of nooks and crannies.
with the level of corrosion that you describe, determining how much "good" steel remains is most likely not practical. therefore, you can't tell what you are starting with, so making accurate calculations about what to add won't work. agree with jedclampett, replace the section.
don't be too concerned about a cost effective solution.  while that is a goal, in an industrial plant keeping the plant operational is far more important. later, if structural failure results in an unplanned plant outage, the fact that the repairs were cost effective will be an absolutely worthless explanation.
keep in mind, the web plate solution is only applicable when there are sound metals (web/flanges)around to weld the plate to. prior to welding, the deteriorated sections shall be thoroughly cleaned (by sand blasting), and coated with rust prevention materials. this type of repair is not ideal (at least from engineers' view), but it is more economical than other methods for columns with modest load.  
we were going to try the weld repair method on some columns corroded by acid. it end up that we had to go the replace method to get anchor point. i don't recall the exact details of job but the cheap repair method turned into a giant fubar.
ps;
the reason i didn't stay close to this job it was our group who inspected and put out a memo about the inspection and requesting that the structural and civil  group take a look at affected columns. they didn't get the memo, they say,  and the maintenance group took it on their own to fix the problem.