Why destroy Laketown's bridge?

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To segue back to the topic of the thread, while thinking about the Washer under the Bridge, I still think that Smaug wanted mostly to attack Laketown via the bridge. My argument turns on one item: Does Smaug know that he has a hole in his armoured belly?

What if he did know? Wouldn't he want to protect himself from attack from below? If he could march, or stomp, across the bridge, attack from below would be highly unlikely as there was no place for a Turin to stand as there is no understructure to the bridge. By collapsing the bridge, the Master forced Smaug to the air, which presented his Achilles heel, or belly, to attack (still however unlikely).

If he didn't know, well, I'm not sure that my argument falls down like so much bridgework, but it doesn't help it either.