on the second chapter or dead air you will need to alert the horde by lowering a crate using 
the crane you can skip this without teleporting or any hard to do glitches when facing the crane 
walk right and where the crate will be lowered. On the left side of the builing that you have to 
get across to there will be a telephone/power box jump on to it and you should hang then get a 
team mate to pull you up and you will be on the building without using the crane and alerting the 
horde (valve has taken the power box out of verses mode)