There are two possibilities to pass the weir and both have to do with one room you already have located:
You have entered a storage room in the basement through a ducting. Two ways to procede:
1. In this storage room, there is a fuse. You can pick up the fuse and place it on the roof inside one of the cranes...
2. There is an office in the groundfloor, check the working desk. There is a readable and a key. The key is the storage key that allows you to open the storage room from the inside.
I hope you play version 3. If not, I strongly recommend you to switc