Truss Bridge Span: Definition and Determinants

Definition

The **bridge span** of a truss bridge is the **horizontal distance between the centerlines of its two main support bearings** (piers or abutments). It's the clear opening the truss must bridge without intermediate support.

What Span Includes/Excludes

• Includes: Main truss structure between bearings
• Excludes: Approach spans, approach slabs, wing walls, embankments
• Multi-span truss bridges: Each span measured separately (pier to pier)

Span Ranges by Truss Type

Factors Determining Maximum Span

• Material: Steel > wrought iron > timber
• Depth-to-span ratio: Typically 1:6 to 1:10 (deeper = longer possible)
• Load requirements: Railway (heavier) vs. highway vs. pedestrian
• Fabrication/erection: Transport limits, crane capacity
• Buckling resistance: Compression member slenderness
• Fatigue life: Stress range at details

Span vs. Total Length

Span = single clear distance between supports. Total length = sum of all spans + approaches. A 10-span truss viaduct might have 50 m spans but 600 m total length.

Design Process

1. Site topography → pier locations → required spans
2. Load analysis → member forces per span
3. Truss type selection → depth optimization
4. Member sizing → connection design → camber calculation
5. Constructability check → finalize

Key Insight

Span is the primary structural challenge. Everything—depth, weight, cost, complexity—scales nonlinearly with span. That's why continuous trusses and cable-supported systems dominate long spans today.