1. To ensure the cap remains functional after an extreme event that damages columns
2. To force plastic hinge formation in the columns rather than in the cap during seismic events
3. To accommodate higher live load moments in the cap compared to the columns
4. To allow for future widening of the bridge without replacing the bent cap

Explanation

Explanation: Bridge bent caps are designed with higher moment capacity than columns to ensure "strong cap-weak column" behavior during seismic events—forcing plastic hinges to form in columns where they're more ductile and visible for inspection. This capacity design approach (per AASHTO Guide Specifications for LRFD Seismic Bridge Design) typically requires bent caps to resist 1.2-1.5 times the maximum moment that columns can develop at overstrength. This controls damage location and prevents brittle cap failures that could cause superstructure collapse. Post-event functionality (A), live load distribution (C), and future widening (D) are secondary considerations not driving this specific design requirement.

Learning Resources:

• Review AASHTO Guide Specifications for LRFD Seismic Bridge Design, 2nd Edition (2011), Section 4.11.2 for capacity protection requirements.
• Study the Northridge Earthquake (1994) bridge damage patterns showing column hinging.
• Digital Resource: Watch Caltrans' "Capacity Design of Bridge Substructures" webinar (https://dot.ca.gov/programs/research-innovation-system-information/webinars)
• Tip: When designing bent caps, provide adequate development length for column longitudinal reinforcement to ensure moment transfer at the joint region.