This is a problem which will be increasingly prevalent all over the network in the not too distant future. Many of these structures cost a million pounds or more to design, fabricate and install and it's becoming clear that their lifespan will be no more than about 30 years.
The concept is completely wrong, box section steel is just not suitable for structures such as these, as the evidence shows, it will rust away in no time at all.
There doesn't seem to be any problem with the cast iron structures which most of the pre-grouping companies built well over a century ago!