[Rikae]

Originally Posted by STW

The deliberate destruction of the bridge to achieve this goal would only make sense if they first knew about the structural flaw on Smaugs belly.

They would not need to know this for a fact - conventional wisdom to which even Bilbo had access would give them reason to suspect it:

"I have always understood," said Bilbo in a frightened squeak, "that dragons were softer underneath, especially in the region of the -- er -- chest; but doubtless one so fortified has thought of that"

Originally Posted by STW

As long as Smuag had the ability to fly and retreat when tired it is irrelevent as to what he feared.

Since repetition seems to be popular around here anyway, here goes:

Amid shrieks and wailing and the shouts of men he came over them, swept towards the bridges and was foiled!The bridge was gone, and his enemies were on an island in deep water too deep and dark and cool for his liking.

It seems perfectly clear from this quote that Smaug "was foiled" because the bridge was gone. Now, why this is the case may be up for debate, but the fact that it is so is given in the text. Smaug sweeps "towards the bridges" and is foiled, and the next sentence would seem logically to explain how this was so - and it says "the bridge was gone, and his enemies were... (etc.)" What is your explanation for how Smaug was foiled, if it wasn't by the destruction of the bridges?

Originally Posted by STW

Please quote the section of the text which states that Smaug feared the water. He did not go into it because of the vapor that would rise thus blinding him to the escaping people and it would quench him putting out his fires. But it would hardly harm him.

It seems, to me, excessive to quote what Davem already quoted a few posts back. Furthermore, if the water quenched his fire it would seriously impair his attack, which is a perfectly reasonable and obvious explanation for his fear and avoidance of it. This is beside the point at any rate - the point is that the text appears to link the destruction of the bridge with the risk, for Smaug, of ending up in the water or wet, something he wishes to avoid.
Furthermore, the text implies he had some plan involving the bridge, in which he was foiled by its destruction. This alone, even unexplained, shows the destruction of the bridge was of some use against Smaug. The question of the effect of water, because of its placement in the text, strongly suggests Smaug planned to use the bridge in his preferred attack, one which would not be as likely to bring him into contact with the water when the bridge was present. Since an attack by air doesn't carry such a risk in any event, it would appear he would have preferred not to be forced to attack from the air.

Originally Posted by STW

And what supports that belief? A boat on the water was subject to the same aerial attack that those on the ground, or on the bridge, or those who would have reached mainland would have been subject to. There is no difference.

I was referring to alatar's imagined scenario in his post #12, which, as you can see, doesn't involve boats. This is not particularly important, however, if it is understood that Smaug would have preferred to attack by land and would have been more dangerous in such an attack. It's true that the reasons behind this remain open to speculation, but Smaug's preferred attack is clear.

Originally Posted by SRW

Given the "little time" they had for Smaug to close that distance, just how did they manage to quickly get organized, grab the proper tools and supplies, divide into divisions of labor, and take down and destroy that bridge? Was this like the River Kwai where the bridge was wired with explosives? I think not. Just how did they destroy that bridge so quickly in the few moments it took Smaug to arrive and attack the town?

This has already been addressed several times - the bridge was designed to be quickly destroyed. If they had designed it accordingly, a few cuts should have done the trick, and if it was their emergency plan, they likely had the necessary tools readily availiable and people assigned to the task.