Seismic retrofit: jacketing, restrainers, seat extensions and isolation

How bridges get built · Finishing, articulation and maintenance · Seismic retrofit: jacketing, restrainers, seat extensions and isolation

Also called: column jacketing, steel jacket retrofit, cable restrainers, seat extensions, isolation retrofit, seismic strengthening of bridges

How it is done

Screen the network and rank bridges by seismic hazard, vulnerability (seat width, column details, skew, curvature, soil) and importance; assess the selected bridges by capacity and demand analysis (pushover, displacement-based methods).
Choose the strategy for each bridge and agree the performance level (no collapse, or usable after the design event): prevent unseating, add ductility, add strength, reduce demand by isolation and damping, or accept damage in non-critical members.
Prevent unseating: install cable or rod restrainers across in-span hinges and at abutments, extend seats with concrete corbels or steel brackets, and add shear keys or stoppers for transverse movement.
Jacket the columns: steel shells (elliptical for rectangular columns) with the gap grouted, reinforced concrete jackets, or wrapped carbon or glass FRP, leaving a clear gap at top and bottom so the jacket confines without adding flexural strength.
Strengthen what the jacketed column would now overload: footing overlays with added piles or anchors, cap-beam bolsters and joint shear reinforcement, working around live traffic and services.
For isolation or damping retrofit, jack the deck, replace the bearings with lead-rubber or friction pendulum isolators, add viscous dampers where needed, and widen joints and gaps for the larger displacements.
Check the retrofitted bridge with the same analysis, inspect welds and grout, and record the new articulation and devices; isolators and dampers need periodic inspection.

Key numbers

Steel column jacketsplate about 10–25 mm thick, grouted gap of 10–25 mm, clear gap of about 50 mm at top and bottom
Minimum seat width (AASHTO LRFD 4.7.4.4)N = (200 + 0.0017L + 0.0067H)(1 + 0.000125S²) mm, with L and H in mm and skew S in degrees, increased by the zone factor
Cable restrainerstypically 19 mm galvanised wire rope units with slack set to allow thermal movement
Isolation retrofitisolated period about 2–3 s; design displacements commonly 150–600 mm
Jacketed columnsdisplacement ductility capacity of about 4 or more is the usual target

Where it fits

Bridges in seismic regions designed before ductile detailing became standard (in California, before the 1970s–90s code changes).
Bridges with short seats, in-span hinges, high skew, non-ductile columns or lap splices in plastic hinge zones.
Lifeline routes that must stay open after an earthquake.

Where it does not

Jackets that add flexural strength and move the damage into the footing or cap.
Isolation on soft soil sites or where joint gaps cannot be widened.
Retrofitting bridges that also need replacement for condition, capacity or width.

Choosing it

Fit restrainers and seat extensions first when unseating is the main risk (short seats, skewed or curved bridges, in-span hinges); they are cheap and prevent collapse but do not protect the columns.
Choose steel jackets for circular columns with lap-splice or shear deficits where appearance is secondary; choose FRP where weight, speed or architecture rule; choose RC jackets where strength and stiffness must also rise and the foundation can take it.
Choose isolation over strengthening when piers and foundations cannot economically take the elastic forces and the deck can be allowed to move; avoid it on soft soils and where large joint movements are impractical.
Prefer replacement when foundations, columns and seats are all deficient and the bridge also has condition or functional problems.

Plant, pace and money

PlantSteel plate rolling and welding, grout pumps, FRP wrapping kits, drilling for restrainers and anchors, jacks for bearing replacement, access and traffic management.

ProductivityIndicative: a few column jackets per crew per week; isolation retrofit of a large bridge takes one to three years.

CostIndicative and highly variable: restrainers and seat extensions are cheap per bridge; column jacketing and foundation work dominate typical retrofits; isolation retrofit of a major crossing runs to tens of millions of USD. Owners usually judge retrofit against the cost of replacement.

Risks and controls

What goes wrong

Moving the weak link instead of removing it.
Hidden lap splices, poor concrete or undocumented reinforcement found during the work.
Restrainers set too stiff or too slack.
Work next to live traffic.
Isolators and dampers neglected in later maintenance.

Quality assurance

Weld and grout inspection of steel jackets, FRP thickness and bond checks, restrainer slack settings, prototype and production tests of isolators (EN 15129 or AASHTO), as-built articulation records.

Origins

Retrofit began in California after the 1971 San Fernando earthquake with cable restrainers at hinges. The collapse of the Cypress Street Viaduct in 1989 and the tests of Priestley and Seible at UC San Diego made steel column jacketing standard, and the 1994 Northridge and 1995 Kobe earthquakes extended retrofit programmes to Japan and worldwide.

Examples

California's state programme after the 1989 and 1994 earthquakes, Japan's nationwide column jacketing after Kobe 1995, and isolation retrofits of long crossings such as the 1962 Benicia–Martinez bridge.

Case studies

Cypress Street Viaduct (I-880), OaklandUSA · 1989The upper deck of the double-deck viaduct collapsed onto the lower deck in the Loma Prieta earthquake, killing 42; it drove the Caltrans column-jacketing programme based on UC San Diego tests.

Hanshin Expressway Route 3, Fukae sectionJapan · 1995About 635 m of single-column viaduct overturned in the Kobe earthquake after column failure from inadequate transverse reinforcement and detailing; Japan then jacketed RC bridge columns on a national scale.

Benicia–Martinez Bridge (1962 crossing)USA · early 2000sSeismic isolation retrofit of the long steel truss crossing with the largest friction pendulum bearings manufactured up to that time.