I would be very surprised if what is supporting the bridge is the iron tubes.
Some one said if its like the Wagga Bridge, the concrete support structures will be end of life, if this is the case then the bridge is in its last years and must go sooner or later. If the concrete support structures have another 25 years in them, then replace the deck with something more suitable and use the spans elsewhere.RTT, a small correction which does not substantially alter the meaning of your post.
The support structures (caissson piles) are iron, not concrete. Being iron they are old and brittle, and in the Wagga case, literally cracking.
What seems more likely is that they were used as caissons in which case the tubes were sunk into the river bed until they reached firm support (rock??) then emptied and filled with concrete which then becomes the pier. The iron tubes then become irrelevant and can quietly rust away below water level.
That was what happened with the Hawkesbury River bridges and the lack of concrete or the poor quality of what was used was a major factor in what ultimately doomed the original structure.
What does make me wonder however is that in the case of the Whitton Lattice Truss bridges the iron tubes continue right up to bridge level and are braced to one another. That bracing is not there for the fun of it.
Continuing the tubes to bridge level may just have been a simpler way of constructing the above water piers in that it provided formwork for the concrete within and bracing for it.
I will now sit back and wait for someone to tell me the piers for the Hawkesbury bridges were stone and concrete respectively and there were no iron or steel tubes involved. Think before you do.
Of course its just sandstone and concrete
No seriously thanks for that info, I would love to know how they built these massive piers back in the day and what they actually look like under the water line and below the bottom of the river.
Also how deep is the river there?
Some years ago the ARHS ran a tour which involved a boat trip around the two bridges and commentary on the issues involved with each.
There were some quite comprehensive tour notes handed out on the day and while I cannot find mine they are here somewhere.
If you can get hold of a copy they are very well worth reading.
As to what the piers of the original bridge look like below water level, and the river bottom, that would be the billion dollar question.
I am working from memory of the tour notes from here on so while I write in good faith I cannot swear to its unquestionable accuracy but in the construction it was found that one of the piers was not up to standard and the contractor was made to remove the top of a caisson and rebuild it. A comment was made that, while that had fixed the problem they could see, "what was below that was anybody's guess."
Some years later when the iron caissons had rusted away a check was made of one of the actual piers and it was found that a rod could be pushed into what was supposedly solid concrete. It was described as a blue gelatinous mass of no structural value.
The iron caissons for the original bridge were arranged in a single row of three tubes which apparently makes it more difficult to sink them straight while the new bridge had four tubes arranged in a square which makes it easier.
As a consequence one of the piers for the original was out of place and in one the stone pier had to be cantilevered out from the three caissons to get it where it was supposed to be.
If I can find my copy of the tour notes (meaning if Mrs 7334 has an idea where they are) I will check what I have written but if this is to go any further it really should be in a new thread.