Sorry for necro.
I got some ideas with mechanic.
First of all, isn't it odd that single crunlde stepped on plate can force stone bridge 10x10 tiles to raise? Same for other mechanizabe objects.
So, first of my suggestions is - every mechanical stuff requires power to operate.
[1] If finely balanced, a moderate amount of power might be needed equally in either direction, of course. Perhaps the given Architect's skill could be applied to how well balanced (or favoured toward the required state, such that it responds effortlessly/immediately to the being sent in that direction) such constructions are. That's a completely new suggestion (from me at least), so you might want to ignore that for the time-being.
I get the impression that the mechanics of a bridge are such that some sort of counterbalancing weight is ready to pull the bridge up, and only a single brake stuck in a gear, or a tether on a rope is keeping that from happening, and that pressure plate's small motion releases the rope or brake, letting the weight-based mechanics work.
Of course, the fact that you never have to set these things up, or apply power, or anything else, and they are perfectly resettable on their own all imply some sort of dwarfy magic taking place.
It would be cool if bridges worked sort of like an elevator where you needed to set up counterbalances, though.
Supposedly, some real-life drawbridges are counterbalanced so well that a 30-horsepower engine lifts a bridge that weighs several tons. You can basically just lock it in place once it is in the air, and the counterbalancing means that some steel pegs sliding into their slots can keep the bridge in place in the raised position without using power at all.