Steel truss
The most common bridge type encountered on documented routes. Primary structural elements are the truss members (top chord, bottom chord, diagonals, and verticals). Inspect all visible members for corrosion, section loss, buckling, and connection integrity. Connections (gusset plates, bolts, rivets) are the critical inspection points. A single failed connection can compromise the entire truss. Section loss from corrosion is most common at the bottom chord (water accumulation) and at connections (moisture traps). If any truss member shows section loss exceeding 20% or any connection shows visible failure (missing bolts, cracked gusset plates, separated members), the bridge should be considered compromised.
Deck on steel truss bridges is typically concrete, steel grating, or timber. Assess the deck independently of the truss structure. A sound truss with a degraded deck requires deck repair or reinforcement before crossing.
Concrete beam
Concrete beam bridges carry load through reinforced concrete beams (girders) supporting a concrete deck. Inspect the beams from the underside. Key indicators: spalling on the beam soffit, exposed rebar, cracking patterns (longitudinal cracks along the bottom face indicate flexural overload or rebar corrosion; diagonal cracks near the supports indicate shear stress). If rebar is exposed and corroded, the beam has lost reinforcing capacity in that region. Concrete beams with corrosion-related spalling over more than 25% of the beam length should be considered compromised.
Concrete beam bridges are generally more durable than steel truss bridges in the absence of maintenance, because the concrete provides passive protection to the reinforcement. However, once the concrete cover fails and water reaches the rebar, degradation accelerates rapidly.
Timber
Timber bridges are less common on documented routes and are typically shorter span (under 20 m). The primary structural members are timber beams or stringers supporting a timber deck. Probe all accessible timber members with a pointed tool. Sound timber resists penetration; degraded timber yields. Pay particular attention to the bearing points where the beams rest on the abutments—this is where moisture accumulation and fungal degradation are most advanced. Timber bridges with beam degradation at the bearing points should not be crossed. Deck planks should be probed individually; replace or reinforce any plank that shows probe penetration exceeding 10 mm.
Suspension
Suspension bridges are uncommon on documented routes. Where encountered, the primary inspection points are the cables, the towers, and the anchorages. Cable inspection: look for broken wires (visible as protruding wire ends), corrosion (surface rust is cosmetic; deep rust with visible section loss is structural), and excessive sag. Tower inspection: assess for lean, cracking, and connection integrity at the saddle points where cables pass over the towers. Anchorage inspection: ensure the anchorage is stable and the cable attachment is intact. Suspension bridges are complex structures; conservative assessment is warranted. If any cable shows more than 5% wire breakage by visual count, the bridge should be considered compromised.