Span → Girder depth

Source: AASHTO LRFD Bridge Design Specifications, Table 2.5.2.6.3-1 — traditional minimum depths for constant-depth superstructures, expressed as depth = coefficient × span (L). These are minimum depths for first sizing. The shaded band shows a typical depth range, from the AASHTO minimum (dashed line) up to roughly 1.3× that value — depths often adopted in practice for stiffness and economy. Final sections must satisfy a full strength, service and fatigue check. Each type also has a practical maximum span; beyond it the tool shows Not recommended rather than a depth, since another superstructure type would normally be chosen. Coefficients are dimensionless, so the result follows your span unit (shown here in metres).