You’re correct.
To replicate the event you're describing, we need to have a multi-million tonne building, close to our steel building and we need to make that first building collapse.
Naturally flying a plane into the first building won't make if fall, so we'll need a few hundred kilos of strategically placed C4 in that building to bring it down.
To hide that C4 explosion we'll use the plane flying into the building as a diversion but at least we're guaranteed to bring down that first building.
Now we have a collapsing multi-million tonne building close to the steel building we are trying to bring down and that event will be creating millions of tons of flying debris.
Naturally, that amount of flying debris won't be enough to collapse the target building, but luckily we also places some C4 in a second building that is also close by.
Once again we are going to need a second plane to hide the C4 explosion in that second building, but that should be easy enough to arrange.
Now we've managed to bring down two massive multi-million tonne buildings and created flying debris with the equivalent energy of thousands of tons of C4 but we always knew that wouldn't be enough energy to bring down our target building.
Luckily, while laying the C4 charges on the other two buildings, we also manage to plant a few hundred kilos of C4 in the third building.
Now to finish the job, all we have to do is detonate those last few kilos of C4 and no one will be the wiser.
What a simple plan. It's fool proof and using the planes to cover up the C4 explosions is a stroke of genius.
The only problem we face is this last explosion makes the building collapse like a demolition.
We should have thought about that!!!