Riddles and Puzzles

[Zenrax]

That one is old. Here's a riddle:

A truck is delivering steel ingots to a foundry. It comes to a bridge, with a sign that says 'This bridge's weight limit is 14,000 pounds. It will immediately collapse if even one milligram more that that amount is put on the bridge.'

Edit: The truck weighs EXACTLY 14,000 pounds.

The truck drives halfway across, but then a butterfly lands on the truck. Does the bridge collapse, and why or why not?