By crossbars, do you mean wires running from post to post (the length of the row) or two sets of wires in parallel at 18 and 30 inches above the ground, connected by a bar?

If you are using double wires in parallel, 12 to 18 inches is wide enough to contain the canes. The ones at 18 inches above ground will not be useful, if you want two parallel sets, 30 and 48 inches above ground would be better. Healthy raspberries can go 6 to 8 feet tall.

Without any crossbars, you can simply loosely tie any canes to length-of-row single wires. With crossbars, you need to tuck wayward canes in between the wire sets as the canes get taller. Raspberry canes are flexible and tucking them in is easy. The canes need to be about a foot taller than the wires, or they will not stay tucked.