It depends on how it is demolished, aka demoed.
A steel beam might just be removed. A stone column like the ones in Egypt or a masonry chimney might need to be knocked down piece by piece or brick by brick.
Generally for buildings, the word destroyed is reserved for natural disasters or other catastrophes, e.g., destroyed by a fire.
Although correct by definition, most people in the industry wouldn't say chop-off or crop. They would say, "Cut it here, and here. And remove this section." Except, actually for chimneys, which do often get 'cropped'. But I still think remove the top (X feet) is more likely to be said.
Knock-down and tear-down usually refer to the entire subject of the sentence, e.g., tear down this wall.