I was just by that bridge a few weeks ago. I was told that the missing span was removed because it was unstable.

http://maps.google.com/maps?hl=en&ie=UTF8&ll=43.265363,-89.727659&spn=0,359.966547&t=h&z=16&layer=c&cbll=43.265439,-89.727602&panoid=1KJGuy-tDjVd6nSX-t8Y8w&cbp=12,134.57,,0,25.44

The memory is a bit hazy but I remember reports that one of the piers had significant undercutting from the current and the bridge deck appeared to slope down river. A also seem to remeber that there were four or five different types of spans in the structure? Maybe some kind of record but not unsusal for the Milwaukee.

Here's some details about the bridge demolition. Pretty interesting pictures.

http://www.implosionworld.com/emergency3.htm