I think that theory is specifically for Buildings. So, as per the theory for you to clear the exam you will need a minimum of 31/40 in the AM part. That way you will have 31+10+5+5+5 =56/80
Definitely does not work for bridges. I can confirm that. 22/40 in AM, A/IR/A. I should have 22+10+5+20 = 57/80 for bridges. Also someone has posted 23/40 AM, A/A/A on this thread page 1, that is 23+10+10+20 = 63/80 for bridges.