IMHO not more or less unlikely, simply impossible.

Lightning Strike Energy == 1,000,000,000 Joules

1 Joule heats 1g water 0.24 C

1 L water == 100g

1 cu.meter water = 1000L

Room Temp == 23 C

Thus:

(77/0.24) = 320.8 Joules raises 1g water from 23 C to 100 C (boiling).

(1E9 Joules / 320.8 Joules) lightning strike with no losses along the way heats 3,116,883g of water.

3,116,883g of water == 31,168 L of water == 31 cu.meters of water. This is a block of water 1m x 3m x 10m.

Steam volume is 1600x liquid water volume. So even if we say that 90% of the energy is lost in the transfer, that's still 3 cubic meters of water that flash-boils into steam, quite possibly causing damage to the bridge.

So: perhaps unlikely, but quite possible.

Edit: 3,116,883g of water == 3,117 L of water == 3 cu.meters of water. Damn math errors, always creeping in. Still unlikely, still possible.

3,116,883g water is 3,117 liters and not 31,168 liters: http://m.wolframalpha.com/input/?i=3%2C116%2C883g+water+in+l...

1L of water is 1000g, then you need additional 2261 joules per gram to evaporate. So 1E9j will evaporate 384 liters of water, so 0.384 cubic meters (unless I lost a degree of magnitude here or there myself :)).

Indeed. Lightning does this to trees all the time, there's a lot of crazy videos on YouTube of lightning causing trees to explode.

https://en.wikipedia.org/wiki/Exploding_tree#Lightning

Trees have relatively high resistance so they absorb a lot of energy. The lower the resistance, the lower the impact.

Yes but the steel cable ought to conduct the electricity through the water column into the structure.

If there is sufficient resistor in the structure for massive steam explosion, then it's very unlikely that the lightning would ever strike there.

I am no expert in this field, so: why are you so sure this is impossible? If I understand it correctly, a lightning strike to the (already slightly damaged) cable stays will seek its way through the steel cables, heating them up. If there is water between the cables and the concrete, a small steam explosion will occur. If the cables are already corroded (which they were, according to the article) and thus brittle, this may cause heavy damage to them.

As the mentioned corrosion is indicative of water present, and since the cable stays are a single point of failure (also explained in the article), I personally find this a quite likely chain of events.

Check a representation of one of the stays, here:

https://static01.nyt.com/newsgraphics/2018/08/27/italy-bridg...

Basically the concrete around the steel cables is nothing but a "protection box".

From the little I know about explosions, the idea is that there is a sudden expansion from the inside to the outside. For having any damage to parts near the "core" there must be sufficient "containment", that the concrete simply cannot represent IMHO.

The actual steel used for cables is high tensile and - believe me - extremely resistant besides rather hard while concrete (particularly concrete so ammalorated to let the water creep in) has very little tensile strength.

So, IF actually there was a steam explosion, the result/effect would have been much more likely (besides "nothing") unscathed cables and ruined concrete around them (with no structural failure of the bridge)

As an alternative to explosives it is common to use expanding cement grout in actual concrete demolition, which essentially is explosive on slow motion. When dealing with reinforced concrete you normally need to cut the rebars with a torch (or other means) after the concrete has cracked.

Check (just an example this kind of cement originated in Japan in the '70's and now there are more similar products than stars in the sky):

https://www.youtube.com/watch?v=7cvcnkzK8Jk

Presumably the bridge had a lightning protection system installed.