I-Girder vs Box Girder: Key Differences

Both are longitudinal bridge girders, but they differ in cross-section, span range, and application.

I-Girder (PSC I-Girder / T-Girder)

• Cross-section resembles an "I" or "T"
• Simpler to precast and transport
• Economical for spans of 20–35 m
• Used extensively in road bridges, ROBs, metro viaducts
• Lower torsional stiffness

Box Girder

• Hollow rectangular or trapezoidal cross-section
• High torsional stiffness — suited for curved alignments
• Efficient for spans of 30–80 m (segmental) or longer (in-situ)
• Used in HSR viaducts, major highway bridges, cable-stayed bridge approaches
• More complex to cast (requires internal formwork)

Rule of thumb: For straight, shorter spans with simple loading → I-girder. For curved alignments, longer spans, or where torsion is significant → box girder.