Complex sentence: When designing very tall buildings supplying enough elevators is a problem--travellers wanting to reach a specific higher floor may conceivably have to stop at a very large number of other floors on the way up to let other passengers off and on.
Split into simpler sentences: When designing very tall buildings supplying enough elevators is a problem. Travellers wanting to reach a specific higher floor may conceivably have to stop at a very large number of other floors on the way up to let other passengers off and on.