بسم الله الرحمن الرحيم
1- How to model
it depends on how many spans and span length
usually we used FEM software such as Adapt-ABI or tower-6
2- manual calculation of this type stresses and section properyies
it is almost the same way that you calculate prestressed bridge with one more extra calculation for the stressing block
3- vedio about this type of bridges
use the following links
Prestressed Concrete Bridges: Design and Construction - Nigel R. Hewson - Google Books
https://www.google.ae/search?q=externally+prestressed+concrete&ie=utf-8&oe=utf-8&aq=t&rls=org.mozilla:en-USfficial&client=firefox-a#hl=en&client=firefox-a&hs=lio&tbo=d&rls=org.mozilla:en-US%3Aofficial&sclient=psy-ab&q=externally+prestressed+concrete+bridge&oq=externally+prestressed+concrete+bridge&gs_l=serp.3..33i29l4.5749.11940.0.12450.7.7.0.0.0.0.141.883.0j7.7.0.les%3B..0.0...1c.1.jnSKpcY1bEQ&pbx=1&bav=on.2,or.r_gc.r_pw.r_qf.&bvm=bv.1355534169,d.bmk&fp=2cb25063a44ebac2&bpcl=40096503&biw=1429&bih=637
https://www.google.ae/search?q=exte...b25063a44ebac2&bpcl=40096503&biw=1429&bih=637