chrisl

It doesn’t have to be nearly that hot to lose strength.  Even ~200 C you have to derate it by about 10%, and about 50% by 500 C, depending on the steel.  So you can relax out the prestress at temps much lower than where it gets soft.

And one more, this is how hot the train fire was:
https://streamable.com/0tow6w
Melting steel is 'really' hot (I'll leave it to others to google).  Steel rebar loses its strength well below melting point.  Ergo the bridge is damaged, and may not last very long.

Regarding damages suffered by the bridge here is opninn of "Civilian Girkin" ask Nesmyan. Basically, he says the bridge was built with violation of safety code due to lack of required technology in RU for supports/foundation columns. As a result, an explosion could lead to fluctuation of structure and damage we see.
 

I'd say that (maybe) the biggest news right now is not Kerch or the Kyiv attacks.  It's the attacks on infrastructure in NATO countries.  What will these countries do?  What will NATO do?  What will the majority of the public want in these countries? 
Clearly Putin is betting that his nuclear threat will allow him to make these attacks on NATO countries w/o any response, in the hopes that there will then be pressure put on UKR to make a bad deal, soon.  This could have profound affects going forward.
Hopefully people in EU will just get really angry and we'll see more pressure on RU and more support for UKR.  And the first thing to do is to kick RU off security council and call it a terrorist state, with all the implications of that.  I think leaders have resisted officially labeling Putin a terrorist because that's impossible to walk back later, but we past that now.  

Guys were in a proper place in a proper time. 
Kalibr, for example, has march speed 0,8M (and 2-2.5M at the target), so it can be intercepted with some probabbility with MANPAD. 
 

He missed the fact that there is also a train ferry and all the required facilities just next to the bridge. According to wiki article it stopped operation after the bridge was opened, but I think it's a safe bet that it can be re-commissioned again quite quickly.
Still if RU are forced to use the ferry, it will be a huge inconvenience. And the ferry itself and associated facilities would be a relatively easy targets, and rather hard to fix/ replace.

Has anybody verified that Ted Kaczynski is still actually in prison?

If you watch the video from under the rail side a few times and step back and forth through the frames it looks like the truck is on the correct span and also likely over the spot where the hole is.  It's super annoying to do, because the guy didn't hold his phone steady and you can't just put your finger over the truck on your screen and then shuttle forward - the location of the phone moves in that time.  And just to make it more annoying, Russians seem to be just as bad about inappropriate use of portrait mode as the Ukrainians. 

A truck bomb is almost as easy as blowing up a fuel train if there's a war on.  Especially if one of the belligerents is Russia.
They're shipping tons of stuff across the bridge to support the war.  It wouldn't be that hard to either identify a shipment of explosive stuff, like HE shells, and stick a GPS trigger in with it, or just fabricate some paperwork that says there's *supposed* to be a truck bomb in the back.  It wouldn't say "Truck Bomb, Qty 1" but "200 rounds of 152 mm shells" or something like that.  Russia isn't very computerized - the IDs people use are largely typed or handwritten.  Shipping docs are likely all typed and handwritten.  There's probably not some easily accessed databased to check all shipments, or some kind of protocol for calling to check if a truck is full of good bombs or bad bombs (the evil bit isn't set in the data record).  Sometimes the easiest way to hide something is in broad daylight.  When the logistics system is already broken, it's even easier.
It would certainly be easier to blow up an oil tank car.  You could even put charges with GPS triggers on a bunch of tank cars in random places east of the bridge this week and just make sure they have a couple weeks worth of battery. Some of them are likely to end up burning on the bridge. Put them all in different locations on the cars, because once one goes up they'll start searching that spot on all the cars.

And still nobody has posted a picture of the front side of the hole.
If it's a truck bomb we should see the trailer frame imprinted in the asphalt around the hole (if not embedded).
If it's a clean hole it's a missile.

Meteorite
Norse God
Baba Yaga's Hut
I'm pretty sure it was a truck bomb, but I'm willing to accept missile.  I'd like to see the top road surface to see if that big hole is in the righthand lane.  On the way home from getting groceries I came up with my own conspiracy theory: while the truck was parked for 6 hours someone could have put a homing target on it (or maybe they just shipped a homing target as freight) and once the truck was getting onto the bridge for sure, the missile could have been launched in the right direction to pick up the truck as a target at the right time. If the bridge has cell service you could do it with an iPhone and a missile that has the "Find my...[target]" app.

Has anybody verified that Ted Kaczynski is still actually in prison?

I’m in the missile camp for two reasons: 
1. Russia wouldn’t want to admit that Ukraine has a heavy long range missile. 
2. Ukraine wouldn’t want to admit it has a heavy long range missile.
Thus, every other explanation under the sun will be posited, claimed, hinted at and asserted.

I'm open to whatever truth comes out, if any, about the Kerch bomb.  RIght now mostly looks like truck bomb.  but that's not conclusive, so internet is going nuts.  What Steve said does carry some weight -- if RU says it's truck bomb, there's good chance it's not truck bomb.

Meteorite
Norse God
Baba Yaga's Hut
I'm pretty sure it was a truck bomb, but I'm willing to accept missile.  I'd like to see the top road surface to see if that big hole is in the righthand lane.  On the way home from getting groceries I came up with my own conspiracy theory: while the truck was parked for 6 hours someone could have put a homing target on it (or maybe they just shipped a homing target as freight) and once the truck was getting onto the bridge for sure, the missile could have been launched in the right direction to pick up the truck as a target at the right time. If the bridge has cell service you could do it with an iPhone and a missile that has the "Find my...[target]" app.

Apologies to Gary Larson

There is no missile guidance system that works like that I know of, and the timing necessary would be extraordinary if not impossible. 
 
Anyways, here is a whole thread on why the truck we saw explode was probably an exploding truck:
 

Northern Luhansk. RU finally stabilises a line along the boggy Krasna river, but the N-S rail and roads are under UA fire. Ergo, cut.


 
 

I'm still not even convinced there's a boat there at all.  It kind of looks like a bit of wind driven whitecap.  I spent some time stepping through that video when it first showed up and didn't see it continue to the left across the screen (I guess since it was the bomb...), since I still haven't seen even minimally convincing evidence that the explosion came from anywhere but above the bridge deck.
It was a pretty windy night.  The videos of the water show a lot of whitecaps, and a huge amount of the splash got blown back over the bridge, and somehow a lot of people are convinced that the light scattered from the droplets is WP.  A) There was a ginormous splash and a strong wind, and water droplets scatter light really well, and b) WP would probably leave some serious marks on the asphalt road surface, if not set it on fire (which would have been kind of awesome).

I'm still not even convinced there's a boat there at all.  It kind of looks like a bit of wind driven whitecap.  I spent some time stepping through that video when it first showed up and didn't see it continue to the left across the screen (I guess since it was the bomb...), since I still haven't seen even minimally convincing evidence that the explosion came from anywhere but above the bridge deck.
It was a pretty windy night.  The videos of the water show a lot of whitecaps, and a huge amount of the splash got blown back over the bridge, and somehow a lot of people are convinced that the light scattered from the droplets is WP.  A) There was a ginormous splash and a strong wind, and water droplets scatter light really well, and b) WP would probably leave some serious marks on the asphalt road surface, if not set it on fire (which would have been kind of awesome).

Are they saying that the "last two points" that could have taken 30 minutes to traverse but took 6 hours are the ones from Midnight to almost 6 am?  I think there's a very simple explanation for that that doesn't require the driver to be connected to any conspiracy.

The technical origin of that white bar has been explained here in detail, and has nothing to do with the flash being out of the frame of the camera.  Same for the versions that claim a flash in the sky above the truck. My guess is those were a different phone filming a screen with the same recording, and is a result of the phone exposure adjusting to the white band that appears at the bottom of screen (that they have edited out there) or reflecting off something back onto the screen. And again, there is no way the explosion that caused this damage could be out of the frame shown.

Quality pictures of rail bridge damage. Bending of the metal elements was probably caused by explosion itself not heat from fuel fire.
 

The twin towers was a much more complicated structural setup and they survived the collisions just fine.  The heat of the fire weakened the columns and then too many columns started failing to support the floors above them and it collapsed in a domino effect because even full strength floors couldn't support the mass of floors above them piling up.
The Kerch bridge is a much more straightforward structure - it's a lot like a Hot Wheels track, with segments defined by the longitudinal beams and a relatively thin web of steel and asphalt tying them together.  Each segment is resting across multiple piers and mostly held in place by gravity with some flexible constraints to keep it from cracking due to thermal expansion/contraction and wind.  The two decks don't appear to be really tied together - they rest separately on their piers, which are pretty solid (though susceptible to earthquakes and ice), isolated by the bearings that accommodate the expansion/contraction.  So very little of the force that went into the outer deck was transmitted to the inner deck through the structure, and it's just the pressure from the shock wave that did moderate damage to the deck and railing.  
The catch is that there's probably latent damage to at least the nearest bearings on the inner deck.  It got enough force to dent and crease it near the point of explosion, so some of that would have been transmitted.  It's now one of those roads that you take in a small car because you to make a trip and have no choice, but if they drive a bunch of heavy trucks loaded with tanks and ammo over it, it could have cracks that propagate and fail catastrophically.  Especially since it's in a saltwater environment and suffers things like winter and ice.  Water is terrible stuff, especially if it can freeze and thaw (which I'm sure you have more experience than you want with).
The rail bridge is also suspect - it took the pressure wave broadside because it wasn't shadowed, so there could be some hidden cracks started (or expanded if they were already there).  On top of that it had many hours of burning fuel on the steel in multiple spots.  That's going to change the steel properties there, and if they just run trains at full speed over it like nothing happened, they could get a surprise later.

Not quite.
You're correct that you suspend a bomb 3 feet above the roadway and blow it up, the roadway will reflect the 50% of the energy from the bomb that's going downward back upwards.  But all that energy goes downward first, and the roadway experiences a reaction force equal to the force reflecting back up.  And the roadway then deforms inelastically, turning a bunch of that energy into bent metal, concrete, and shredded end connectors, and a big splash.