Last reviewed 30 Sept 2026 · 7 min read
Why steel bridges need rehabilitation
Many steel bridges, especially on railways, are 80–150 years old. They were designed for lighter loads, built with riveted connections and wrought iron or early mild steel, and are still carrying heavier, more frequent traffic. Their typical problems are corrosion, fatigue cracking, loose rivets, section loss, distortion and inadequate capacity for the present loads. Because a steel bridge often has no redundancy (a fracture-critical member may bring down the span), a careful assessment and timely rehabilitation are essential.
Assessment
- Records and drawings — original details, materials (old steel may have inclusions and lower strength and poor weldability), previous repairs, load history.
- Inspection — corrosion, cracks, loose rivets, distortion, condition of the paint, condition of bearings; fatigue-prone details identified.
- Measurements — thickness of corroded members by ultrasonic gauge; dimensions; material tests on coupons (yield, tensile strength, chemistry, toughness, weldability).
- Analysis — with the reduced sections and the actual properties; fatigue life from the stress range and the number of cycles; the load rating.
- Load test or strain monitoring under trains to obtain the real stress ranges.
Strengthening of members
Plate girders
- Flange cover plates — extra plates bolted (HSFG) or welded to the flanges to increase the section modulus.
- Web stiffeners — additional vertical or bearing stiffeners to prevent web buckling and to increase the shear capacity.
- Web plates — added plates over corroded or thin webs.
- Composite action — adding shear connectors and a concrete deck, if the girder supports a deck.
Trusses
- Adding plates or angles to the chords and web members (compression members: increase the area and the radius of gyration; tension members: increase the net area).
- Bracing — additional lateral and portal bracing to prevent buckling and to improve the stability.
- Gusset plate strengthening — doubler plates, or replacement, where corrosion or fatigue has reduced the capacity; the Whitmore section and block-shear checks are part of the assessment.
- Member replacement — a severely corroded member is replaced while the load is carried by temporary supports or jacking.
Stringers and cross-girders
In through-girder railway bridges the stringers and cross-girders carry the track directly and see the most fatigue. They are strengthened with cover plates or replaced, and the connections (riveted angles) are upgraded.
Connections — riveted and bolted
- Loose rivets are found by the hammer test and are replaced.
- Replacement of rivets by HSFG bolts — the rivets are drilled out and replaced with high-strength friction-grip bolts (of the same or larger diameter) tightened to the specified tension. HSFG bolts are stiffer, do not slip at service load, and have better fatigue resistance. Replacement should proceed in a pattern so that the joint keeps its strength (only a small fraction of rivets in a group is replaced at a time, often less than 10–20 %).
- Adding connection plates, splice plates and gussets to improve the capacity of a joint.
Fatigue cracks
Where they occur: at rivet and bolt holes, welds (toes and roots), re-entrant corners, coped ends of stringers, web–flange junctions, stiffener ends and details of low fatigue category. Distortion-induced fatigue occurs where a stiff member is connected to a flexible one that moves.
Repair sequence
- Detect and confirm — visual, dye penetrant or magnetic particle test, ultrasonic test to find the extent.
- Investigate the cause — high stress range, poor detail, out-of-plane distortion, defect in the weld. Measure the stress range if possible.
- Arrest the crack immediately — drill a stop hole (about 20–30 mm diameter) at the tip of the crack, centred on the tip so that the tip is removed; the sharp tip is replaced by a round hole with a much lower stress concentration. Fit a high-strength bolt (tightened) in the hole, to compress the hole and increase its fatigue life.
- Repair permanently — gouge and weld the crack (if the steel is weldable) with a full-penetration weld ground smooth and inspected, often with weld toe treatment (peening or grinding); or bolt a splice plate (with HSFG bolts) across the cracked section; or replace the member.
- Retrofit to reduce the stress range — add a plate or stiffener, soften the connection or stiffen the detail, add a load-path to reduce distortion, or provide a gap to relieve the restraint — so that the crack does not restart.
- Monitor — repeat inspections and strain measurements.
Fracture-critical members (non-redundant tension members) demand extra care and frequent inspection.