They wanted to simulate the look of the Word Trade Center towers collapsed on their stage. They knew using real I-beams wasn’t an option, so they used styrofoam to create the I-beams, large chunks of concrete, and the cross.
They used wet painting techniques and monster mud to give the pieces dimension. Then they painted everything to give it a rustic look.
Fire - Beams - Nail
To make it look like fire ate away at the beams they used nail...