Parapets, barriers and edge details

How bridges get built · Finishing, articulation and maintenance · Parapets, barriers and edge details

Also called: bridge barriers, crash barriers, parapet construction, slipformed barriers

Parapets, barriers and edge details — concept diagram — Concept diagram (TheBridgeEng)

How it is done

Select the system from the containment level required by road class, speed, traffic mix and what lies below, and use only a crash-tested system; design the deck edge or cantilever for the barrier impact load.
Cast the deck edge with the barrier starter bars, or with cast-in sockets and anchor bolts set by template to line, level and spacing; check cover and positions before the pour.
Concrete barriers: slipform from a stringline in one continuous pass, or cast in travelling forms, with contraction joints at regular spacing and full joints at every deck movement joint; cure and seal.
Steel and aluminium parapets: bolt the posts to the sockets at the specified torque, set rails to line, and provide sliding splices at deck expansion joints.
Finish the edge: drip groove or drip strip under the cantilever, edge drainage and outlets, cable ducts, lighting columns, noise or wind screens, and protective screens over railways where required.
Install the transition from the bridge parapet to the approach barrier and the end terminals exactly as tested, with graded stiffness and no snagging points.
Inspect line and level, test anchor torque or pull-out on a sample, and record the system and its containment level in the bridge file.

Key numbers

Containment classesEN 1317 N1–N2, H1–H4b, L1–L4; AASHTO MASH TL-1 to TL-6, with TL-4 and TL-5 usual on highway bridges
Concrete barrier heightabout 810 mm (TL-4 under NCHRP 350), 915 mm (TL-4 under MASH), 1,070 mm (TL-5)
Design transverse impact force (AASHTO LRFD Table A13.2-1): about 240 kN for TL-4 (raised for MASH) and about 550 kN for TL-5
Concrete barrier self-weightabout 5–8 kN/m
Dynamic deflectionessentially zero for rigid concrete barriers; a few hundred millimetres for steel parapets, which sets clearance to columns and screens
Indicative costUSD 300–800 per m depending on type and containment

Where it fits

All bridges; containment level by road class.

Where it does not

Barrier bases without drainage (water sits against the deck edge); untested transitions.

Choosing it

Choose a concrete barrier over a steel parapet on highway bridges with long runs and heavy trucks: no deflection, little maintenance, fast to slipform; it adds dead load and blocks views and snow clearance.
Choose steel or aluminium parapets on urban, long-span or weight-sensitive bridges, where appearance, drainage through the edge and quick replacement after impact matter.
Choose high containment (H4 or TL-5/TL-6) over railways, busy roads and hazardous sites, and on bridges with heavy goods traffic; the deck edge must then be designed for the larger impact load.

Plant, pace and money

PlantSlipform paver or forms, small crane for steel parapets.

ProductivitySlipformed 300-1,000 m per day.

CostUSD 300-800 per metre depending on type.

Risks and controls

What goes wrong

Bar anchorage at the deck edge, cracking of slipformed barriers, transitions failing in impact.

Quality assurance

Crash test certificates for the system, anchorage inspection, transition details checked.

Origins

Rigid concrete safety shapes began with the New Jersey barrier in the 1950s, followed by the F-shape and the single-slope barrier from crash testing in the 1970s and 1980s. Acceptance by full-scale crash test moved from NCHRP Report 350 (1993) to MASH (2009) in the United States and to EN 1317 in Europe.

Examples

F-shape and single-slope concrete barriers on highways; steel parapets on urban and rail bridges.

Case studies

Great Heck rail crash (Selby)UK · 2001A road vehicle left the M62 on the approach to an overbridge and came to rest on the East Coast Main Line, where it was struck by an express that then hit a freight train; 10 people died. It led to national guidance on vehicle restraint at road-over-rail bridges and their approaches.

Queensferry CrossingUK · 2017Wind shielding along both deck edges lets the bridge stay open in winds that used to restrict traffic on the neighbouring Forth Road Bridge.

Millau ViaductFrance · 2004Wind screens along the deck edges protect traffic on a deck up to about 270 m above the Tarn valley.