Really, it depends on which failure mechanism you're looking at. Flexure in an individual plank, flexure in an entire span, shear for a plank or a span, pillars failing in compression or flexure.
I suggest only looking at flexure for each individual span and compression for the pillar, just to keep things simple.
For a flexural failure, that's just when the giant is heavy enough that the span beneath him bends until it breaks due to the moment applied by the giant exceeding the moment capacity for the span. The moment the giant applies is a function of its own weight and the length of the span squared, as well as the location of the giant along the span, but just assume the giant is standing smack dab in the middle, since that's the worst case scenario.
For a compressive failure in a support pillar for a single giant, that's pretty obvious. Calculate the maximum axial force the pillar can take and if the giant exceeds that, then when the giant's standing on top of the pillar, it breaks.
Now, if you have a lot of giants, the point load of a single giant is instead a distributed load across the bridge. That's all. Intensity and span length squared are still the factors in flexure, and now span length becomes a factor in pillar compression, since each pillar is supporting the distributed load across a single span's length.
Moment capacity is a bit more complicated, as shapes and structures become far more relevant. And if you're using wood, it becomes even more complicated due to the odd nature of wood failure, in that it can fail and break, but still carry the weight until the giant crosses. It just damages the bridge badly.