The safety mechanism for bridge collapses brought on by strong earthquakes, beyond the requirements of seismic design regulations, is:

• A. Bridge Collapse Prevention Device
• B. Guard rail
• C. Unseating Prevention System
• D. Expansion joint

Answer: (C)

When bridges are hit by very strong shaking, a primary collapse risk is the deck sliding or lifting off its supports. An unseating prevention system is designed to keep the deck firmly connected to the bearings and abutments during extreme movements, preventing the catastrophic scenario where the deck becomes unseated and the bridge collapses. This mechanism provides restraints that limit relative displacement between the deck and its supports, effectively preserving continuity of the structure when seismic demands exceed ordinary design margins. It goes beyond standard seismic regulations by targeting this specific failure mode and adding a safeguard that acts only under severe shaking, while allowing normal movements under ordinary conditions.

Guard rails mainly protect vehicles from falling off the bridge and do not address the structural risk of deck unseating during earthquakes. Expansion joints accommodate thermal expansion and contraction and joint movement, but they do not restrain the deck from sliding off supports during extreme shaking. A generic “Bridge Collapse Prevention Device” is not a recognized, specific measure for this failure mode, whereas an unseating prevention system directly targets the issue of deck stability at the supports.