What is strance about this is that traffic had been reduced in both directions if reports I heard are true. One would think the load was probably 1/2 what it normally was. Why would it fail now if these plates were the issue?
Good question, but conspiracy theories (about terrorism) will only get you flamed from some quarters.
One Freeper tied it to the obesity crisis.
A fatigue crack can move through material very slowly, even over a period of years. The final failure through the overloaded remaining material can happen at any load.
I've seen fatigue cracks that leave only 5% of the original material remaining before a rapid final break.
Due to Bush's terrible economy and Bush's high gas prices, only 1/2 of the normal traffic was necessary for the bridge to fail, because Bush's Global Warming had already weakened the plates, and they failed, even before Al Queda could blow them up because Bush didn't catch them due to Bush's quagmire in Iraq.