I s'pose it depends on the door design. If it closes flush there may be no purchase. Still, a crowbar is a good thing to have in an emergency shelter, if not for opening the door, for other things after.

That said, decades back when still on the farm we used a come-along to keep the wire taut when running barbed wire fencing.

After years of reading prepping channels: When the Loo Roll Apocalypse struck, did I have stockpiles of any of the things that were actually in short supply? Loo rolls, flour, pasta, bread, sanitiser.....
NO! :-)
qjs