In particular stress analysis programs are amazingly complex, with the ability to calculate a square foot of structure in any given environment - tension, torsion, compression etc...
I remember a certain gentleman from WS Atkins coming to see me when I was building 486's (oh those were the days! - NOT). He travelled 300 miles in order to run their customised piece of software, which analysed a 4m square piece of bridge under heavy wind conditions - He booked into a local hotel and came back the next day after starting the program running.
It took 24 hours to complete the calculation back then!!!! Even with only 256 colours, the resultant displays were pretty amazing to look at. Like something out of the Star Trek engineering displays!!
The software professionals use is generally proprietary, in other words custom built for them. After all, would you trust a bridge that had been built by Microsoft and not patched before crossing it? - LOL
Regds
Bob
Take a look here if you are interested in finding out about some of the 'off the shelf' software which is for the bridge building industry.
http://www.bridgesite.com/softwarelinks.htm
Regards
Bob