Thread by @zahed

The Kerch Strait bridge has been struck by an explosion.

Leaving aside the military implications to other experts (and plausible deniability for Ukraine), let’s look again at the structural engineering implications for this bridge and outcomes for the rest of the war (1/x)

https://twitter.com/Ukraine/status/1578634825417650176

When I reviewed the damage for the Antonovski bridge, I remarked that the Kerch Strait bridge would constitute a more difficult challenge. That challenge has been accepted (by… someone) and the result is as good as we could expect, even though the bridge is repairable IMO (2/x)

https://twitter.com/zahed/status/1552221470373421056

First, let’s look at the structure itself. The road section consists of simply supported prestressed girders supported by concrete piers. Unlike the box girders of the Antonovski bridge, these girders aren’t interconnected (i.e., loads aren’t transmitted from span to span) (3/x)

This highlights a key structural weakness… once a girder is dislodged from the supports, adjacent girders follow, leaving a clean break at the pier.

An attack from above, with energy dispersed by the road deck and piers absorbing compression would be insufficient (4/x)

An explosion from below, however, with the road deck absorbing the blast wave, would be sufficient to dislodge these connections. I believe this is what happened... if so, a brilliant strategic decision combining military and engineering expertise. (5/x)

Russia is claiming that the explosion was caused by a lorry explosion. Again, I believe a blast from a truck would be dissipated by the road deck, resulting in localised damage.

Look at how many HIMARS attacks it took to disable the Antonovski bridge. (6/x)

As for the multiple road deck spans that have collapsed, this is likely due to the primary span collapse pulling the adjacent spans down via the road deck. I believe the piers are still intact, which means the decks are repairable, but would take many months (7/x)

On to the rail bridge. This bridge is constructed from simply supported steel box girders on tall concrete piers.

Let’s talk about the differences between steel and concrete and why the rail bridge may have been designed this way (8/x)

Steel is ductile, which means that it can absorb energy through deformation without fracture.

Concrete (without steel reinforcement) is brittle, which means that when stressed, it can fail catastrophically and unpredictably.

The Antonovski bridge is now in this state. (9/x)

The explosion that likely disabled the road deck would have been unlikely to disable the rail bridge, due to the high placement. I believe this was by design.

Similarly, an explosion from above could be absorbed more easily by the steel.

That leaves one option… fire (10/x)

As we saw with the WTC attack on September 11th, it was fire that brought down the towers, not the impact of planes themselves. This required a long period of intense heat to degrade the steel to the point where collapse was inevitable (11/x)

The only way this could be replicated at Kerch was to have a fuel source to provide this long period of heat exposure. A rail car full of fuel tanks provided this opportunity, and much of what we saw on fire was fuel draping the bridge structure.

However… (12/x)

https://twitter.com/olliecarroll/status/1578605748942053376

The fire appears out and the bridge still stands. If there is any deflection in the bridge, the steel will have lost a considerable amount of strength.

If it hasn’t reached that point (see graph above), it may be salvageable, but not for many months. (13/x)

Overall, this is as much as any military strategist or engineer could have hoped for if the intent was to disable the bridge for resupplying troops.

A road deck is still intact and the rail bridge could be operable in a few months.

Well played, whoever is responsible. (14/14)

ADDENDUM: It’s hard to calculate how steel on the rail bridge was impacted by fire, looking at photos from afar.

But there’s a clue… look at the steel railings and walkways that have wilted under the heat. A good sign the box girders are possibly damaged beyond repair. 🔥

https://twitter.com/JimmySecUK/status/1578737471323779072