London Bridge Structure: Box Girder Frame Bridge

The current London Bridge (opened 1973) is a box girder bridge—a type of frame structure, not a shell.

Structural Classification

• Frame structure: Resists loads through bending in rigidly connected members (beams, columns, slabs)
• Box girder: Hollow rectangular/trapezoidal cross-section; top flange = deck, bottom flange = soffit, webs = sides
• Behavior: Acts as a beam; torsionally stiff; efficient for curvature

Design & Construction

• Architect: Lord Holford
• Engineers: Mott, Hay & Anderson (Peter Henderson, lead)
• Method: Balanced cantilever construction from piers
• Materials: Prestressed concrete (post-tensioned)
• Spans: 67 m + 104 m + 67 m (three spans)

Why Box Girder?

• Spans 104 m over navigation channel
• Shallow depth (2.7 m) = low profile, minimal approach gradients
• Torsional rigidity for curved alignment
• Aesthetics: clean lines, hidden services in cells
• Durability: protected tendons, inspectable interior

Not a Shell Structure

Shell structures (e.g., Sydney Opera House roofs, cooling towers) derive strength from curved surface geometry carrying membrane stresses. Box girders are beam-like—strength from section depth and material distribution away from neutral axis.

Fifth Bridge on Site

This is the fifth London Bridge. The previous (Rennie's) was sold to Robert McCulloch and rebuilt at Lake Havasu City, Arizona (1971).