for the people who are going on about bikes being used outside – that is all very well but there is this thing called ventilation which cellars don’t normally have much of. The spores and bio activity will also change the pH of the moisture on the bike and any ferrous materials will rust very deeply, aluminium will oxidise into white powder, rubber parts will get pitted from bacteria and mould growth.
I’ve lived in a flat with damp and seen what damage it can do and I’ve had equipment in a damp coal store that rusted very quickly.
I can understand why you want to use the space but I would run a dehumidifier down there or rig up some fans to get forced ventilation if there is a vent. Then it will be a much nicer environment for your bikes and you going down there. A wooden A frame would be fine, just get some pressure treated stuff and ideally further waterproof the feet with some bitumen paint. You could get quite creative and hook up some waterproof lights and make a nice frame for working on the bikes.
edit: at very least make sure all the bikes are taken out regularly and wash them even if they are not dirty from being used.