Why should archers have all the fun with bonus damage?
A crossbow can be constructed with a Str modifier similar to a composite longbow or shortbow. The cost is the same, +100 gp per point of Str modifier.
If your Str modifier is less than that of the crossbow, it requires 1 additional move action to reload for every point of Str modifier you have below the crossbow. For example, if you have a Str of 18 (+4 modifier) and use a light crossbow that has a +7 Str modifier, it requires 4 move actions to reload.
Reloading devices can be purchased for crossbows to reduce the reloading time.
A hook and stirrup costs 4 gp, adds no weight, and reduces the loading time due to excess Str by 1 move action.
A lever costs 9 gp, weighs 1/2 lb., and reduces the loading time due to excess Str by 2 move actions.
A crank costs 16 gp, weighs 2 lbs., and reduces the loading time due to excess Str by 3 move actions, to a minimum of 1 move action.
A windlass 25 gp, weighs 4 lbs., and reduces the loading time due to excess Str by 4 move actions, to a minimum of 1 move action.
You can purchase masterwork versions of these for +50 gp that reduce the loading time due to excess Str by 1 additional move action.
For example, you have Str 12 (+1 modifier) and are using a light crossbow (Str +8). It would normally take 8 move actions to reload. If you use a lever, it would take only 6 move actions to reload. If you use a masterwork windlass, it would take only 3 move actions to reload.
A person can use a crossbow with a Str bonus no more than +20 above his Str, that would take him a maximum of 1 minute (20 move actions) to reload. (Reductions for reloading devices do not raise this maximum.)
Above that and he can neither reload the crossbow, nor fire it effectively. (While he could pull the trigger, the recoil would knock him prone, and the bolt would fly off in a random direction, with no chance to hit, and doing no damage.)