Overhead cranes run on rails or "down-shop rails". The downshop rails themselves also need supporting, either from the factory walls via "knee joints" or from rails supported by legs.
The legs can be either single and tied basck to the building or double lattice work and totally free standing.
Double legs like those pictured below being lifted off the lorry are used when we cannot tie back to the existing factory walls to give the crane stability.
Single legs are are cheaper, but need to be tied back the the building to give support, I.E. to stop the crain from swaying.
"Knee joints" to support the crane downshop rails
Crane rails on I-beam section usually on 40 or 50mm rails.
The crane end carriages run on the crane rails above.