Common Bridge Truss Types: Engineering the Triangle

Why Trusses?

Triangles don't deform under load. Truss bridges use interconnected triangles to span distances efficiently—material concentrated where forces are highest.

Major Truss Types

1. Pratt Truss (Most Common for Rail)

• Diagonals slope **downward** toward center (tension)
• Verticals in compression
• Efficient for long spans, uniform loads
• Variants: Parker (curved top chord), K-truss (extra verticals)

2. Warren Truss (Economical, Versatile)

• Equilateral triangles, alternating diagonals
• No verticals (basic) or with verticals (modified)
• Equal tension/compression in diagonals
• Simple fabrication, good for medium spans

3. Howe Truss (Historic, Wood-Friendly)

• Diagonals slope **upward** toward center (compression)
• Verticals in tension — ideal for wood (strong in compression)
• Common 19th century covered bridges
• Less efficient for steel

4. Baltimore Truss (Pratt + Warren Hybrid)

• Pratt configuration with subdivided panels (Warren-style)
• Extra verticals reduce panel length, buckling risk
• Heavy railroad use, late 19th/early 20th century
• Pennsylvania Railroad standard

5. Pennsylvania (Petit) Truss

• Parker (curved top) + subdivided panels
• Longest simple-span trusses (up to 150+ m)
• Complex fabrication, high capacity

6. K-Truss (Pratt Variant)

• Two diagonals per panel forming "K" with vertical
• Shorter unbraced lengths — better buckling resistance
• Used for very long spans, heavy loads

Selection Factors

Modern Context

New truss bridges rare—replaced by plate girders, box girders, cable-stayed. But thousands remain in service; understanding their behavior is essential for inspection, rating, and rehabilitation.