1. ) Bottom bars must resist both positive and negative moments due to continuity
2. Bottom bars require additional capacity to resist tensile forces from shrinkage
3. Top bars are governed by temperature effects rather than structural demands
4. Bottom bars act as primary reinforcement for positive moment between girders

Explanation

Explanation:
Larger diameter bars are specified in the bottom mat because they serve as the primary reinforcement for positive moment between girders—the controlling condition for most bridge decks under AASHTO LRFD design. The bottom reinforcement typically resists moments from the AASHTO design truck wheel loads positioned to maximize positive moment between girders (creating tension in the bottom fibers). Top reinforcement primarily resists negative moment over girders and temperature/shrinkage effects. While both mats experience both positive and negative moments (A), the bottom mat demands govern; shrinkage effects (B) affect both mats; and temperature considerations (C) apply to both mats but aren't the primary design driver.

Learning Resources:
• Review AASHTO LRFD Bridge Design Specifications, 9th Edition (2020), Section 9.7.3.2 for deck design requirements.