The whole point of the exercise is that the bridge was demolished prematurely which ruined the French withdrawal to Lindenau.
The withdrawal began at 1100 on 18 October with the trains moving out. Cavalry departed at 1600. Around 0200 on the 19th the French began withdrawing northward towards Leipzig. The allies began their advance/attack at 0700 on the 19th and the French rear guard (Macdonald, Reynier, and Lauriston-XI, VII, V Corps) withdrew into the Leipzig suburbs around 1000 and the allied attack drove them into the city around 1200. The bridge was blown that afternoon during the retreat.
Now when were the foot bridges supposed to be constructed and to what purpose in addition to the main bridge?