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Destroying a building by exploding petrol underneath it.
reynardo wrote in little_details
A friend of mine has a query for her Urban Commando type story. Modern, current day, this world, Australian city, this universe, no magical or other odd powers.

She's got the zombies *and* the bad guys trapped in a building. The heroine has had a change of heart and doesn't want the bad guys to become zombies, so she wants to instead destroy the building by blowing up some cars in the basement carpark, or alternately just siphoning the petrol onto the carpark floor and exploding that. How many litres/Gallons of petrol detonated 2 floors below might give her the destruction of at least 50 square meters of real estate, as in a small block with shops on the ground floor and offices above. The bad guys and zombies are trapped on the upper floor.

I've read of a couple of instances - This one in Prahran, Melbourne in 1968 had "several gallons" of petrol, and would have destroyed the office and set the building alight. This one in Sydney last year was only 10 litres of petrol, which doesn't seem like very much. Whereas this idea of the Fuel Air bomb didn't need much more than a fuel can's worth - or so he said. Yet I feel that wouldn't be enough.

Searches done on "blowing up buildings with petrol", "blowing up buildings with gasoline" and the old favourite "why am I doing this for my friend?"

Ideas? Scientific studies? Anonymous confessions of similar instances?

the biggest issue with petrol(gasoline) is that the fuel/air mix needs to be "right", otherwise you just get a stinky mess. Your friend could always handwave it with a take from "The Walking Dead" - the facility dealt with unpleasant materials and already had a self-destruct mechanism built in?

Ohhh tempting. Thank you!

it was quite spectacular - a CDC facility rigged with a fuel-air bomb.

"Big badda boom" indeed!

Here are some quick links on how much explosive power petrol/gasoline has, and its behavior. The internet consensus seems to be that 8 US fluid ounces are equivalent to 5 sticks of dynamite, so you might look up how much dynamite it takes to demolish your size of building.

There's also the point that smaller explosions in strategic places could be a better bet for leveling the whole building, like an actual, professional demolition.

The other thing to note is that flash fires from petrol are more common than explosions. Petrol fumes often behave like this: That said, there's also this:

My boss's brother is famous in the neighborhood for trying to do the same thing the guys in the videos did, except in a pit. He poured the fuel on the logs and branches, then went to take a phone call or something. Came back a few minutes later, threw something into the pit to light it, and I'm told logs went flying 50 feet in the air. So I think you can reasonably handwave the proper air/gas mixture so long as it's in a confined space. I think the bigger question is, how is the MC going to set off this explosion without killing herself in the process?

Damn - not killing herself is fairly important. Of course, the classic scene with the protagonist walking away with a smile and not even a glance back, while, behind her, the building explodes with pieces flying everywhere...

I'm doing civil engineering at uni so I should hopefully have some things to add.

I would perhaps look away from "exploding" the building from a few parked cars and more towards collapsing it. Any building made of concrete will have steel reinforcing bars embedded in it. All engineers will design that steel to have an adequate cover for fire, enough concrete so that the heat won't weaken the steel. However, if you take away that cover, it will only be about 5cm or so, then the reinforced concrete column/slab is much more likely to fail. If enough columns fail then you are definitely going to have a catastrophic failure of all the various structural members that hang off those columns. Think chain reaction.

The columns most likely to cause this will be in the middle of the building, where the highest loads are.

Perhaps there is a petrol station located near the car park and your heroine grabs a petrol tanker or something, crashes it into the car park, she then sets the leaking petrol on fire and runs. The petrol ignites, weakens the steel and the building collapses. The collapse then causes the huge petrol tanks under the station to rupture and ignite from the still burning carpark, cue the huge, Hollywood explosion.

Hope this helps somewhat...

Oh that all sounds lovely... or rather, it'll be one hell of a bang - thank you!