Bridge Barriers and Parapets

This paper aims to review and compare the information on the function, level of service, design and specific types of bridge barriers and parapets used under the two most widely recognized and accepted crash-testing standards viz American NCHRP Report 350 and European Union EN 1317. This will help to promote discussion and clarify issues on the use of traffic barriers associated with bridges within India or similar areas around the world. Reference to research documents and codes is made within the text.

Basically two types of barriers exist on bridges, these being:

• traffic barriers and
• pedestrian barriers

Pedestrian barriers are typically the post/baluster type. Traffic barriers however come in avariety of configurations to suit particular circumstances, including flexible, semi-rigid, rigid beam and post, and rigid concrete barriers. Concrete parapets are preferred as they present a cleaner fascia and have improved noise attenuation characteristics.

Parapet panels are usually located along the outside edges of a bridge and some times extend along abutment embankment wingwalls.

Pedestrian barriers on road bridges and footbridges are required to safeguard pedestrians and/or cyclists.

The AASHTO LRFD section 13 specifies geometric requirements in clause 8.1 and design loads in clause 8.2. The minimum height for pedestrian railing is 1070 mm from top of the walkway. Where cyclists may use the walkway, minimum railing height shall be 1070 mm in accordance with the AASHTO Guide for the development of bicycle facilities. However locations where high-speed high-angle impact with the railing are more likely to occur or in locations with site specific safety concerns, railing above minimum should be considered.

The minimum height of the pedestrian parapet in accordance with Table 1 of BS 7818 are specified below. On cycleway bridges or accommodation bridges frequently used by equestrians, the height above the adjoining paved surface must be increased to 1800mm.

Traffic barriers are normally installed on bridges to prevent errant vehicles from running off the edge of the bridge or to protect pedestrians or other motorists in adjacent areas.

Previous bridge design codes required bridge barriers to be designed for a given static load, at key locations on the railings and satisfy certain geometric requirements. Latest revisions of Bridge Design Code still provides static design loads, however these loads are only provided to allow the design of proto type barriers for crash testing, for the design of deck cantilevers supporting the barriers and designing the bolted anchorage connections while emphasis is given to the use of barriers that have been crash tested to a particular standard.

The aim of traffic barriers is to improve site safety. This aim equates to minimal damage and injury to impacting vehicles, vehicle occupants, others on the bridge and traffic, property, roadways, railroads or waterways below the bridge, consistent with the assessed level of risk at the bridge site.

Assessment of the level of risk is the critical component in determining the type of traffic barriers that are suitable for a particular bridge site. Some factors which contribute to the assessment of risk include:

• total traffic volume
• types and proportions of vehicles in the traffic population
• identifying the type of vehicle to be contained
• road alignment
• bridge width
• general site conditions and
• the consequences of not containing the identified vehicle within the roadway.

A good example of the selection procedures used to determine the performance level required of a railing at a particular bridge site is the AASHTO Guide Specification for Bridge Railings 1989.

The American NCHRP Report 350 and the European Union EN 1317 are currently the two most widely recognized and accepted crash-testing standards. The American Federal Highway Administration (FHWA) issues letters of approval for bridge rails that comply with NCHRP 350 and maintains a website of all approved bridge rails. At this stage, a similar central repository of Comité European de-Normalisation (CEN) approved bridge rails does not appear to exist. It is understood that CEN stipulates that compliant systems have to be crash tested to the requirements of EN 1317 but does not issue certificates of compliance. Without such a system, it would be difficult for a road authority to verify CEN compliance of any proposed barrier system.

Australia as an example which does not have their indigenous testing board have developed traffic barrier specifications which are based on crash testing requirements developed by the United States Transportation Research Board (National Cooperative Highway Research Programme (NCHRP) Report 350). Table"A" provided below document a comparison between Australian Standard and TRB Report 350 and the nearest equivalent to a database of compliant EN 1317 bridge barriers as a "Study of Overseas tested Bridge Barriers.’

Table A – Comparison of Overseas tested Bridge Barriers in terms of Impact Severity Values
NCHRP 350							EN1317												AS5100
TL	Mass (kg)	Type	Speed (km/h)	Angle (Degrees)		IS (kJ)	Level	Mass (kg)		Type			Speed (km/h)		Angle (Degrees)			IS (kJ)	PL	Mass (kg)	Type	Speed (km/h)	Angle (Degrees)	IS (kJ)
1	820	C	50	20		9.3
	2000	P	50	25		34.5
							N1	1500		C			80		20			43.3
2	820	C	70	20		18.1													Low	820	C	70	20	18.1
	2000	P	70	25		67.5														2000	P	70	25	67.5
							N2	900		C			100		20			40.6
								1500		C			110		20			81.9
							H1	900		C			100		20			40.6
								10000		R			70		15			126.6
3	820	C	100	20		37.0
	2000	P	100	25		137.8
4	820	C	100	20		37.0													Regular	820	C	100	20	37.0
	8000	S	80	15		132.3														8000	S	80	15	132.3
							H2	900		C			100		20			40.6
								13000		B			70		20			287.5
							H3	900		C			100		20			40.6
								16000		R			80		20			462.1
							H4a	900		C			100		20			40.6
								30000		A			65		20			572.0
5	820	C	100	20		37.0													Medium	820	C	100	20	37.0
	36000	V	80	15		595.4														36000	V	80	15	595.4
6	820	C	100	20		37.0													Special Cat.1	820	C	100	20	37.0
	36000	T	80	15		595.4														36000	T	80	15	595.4
							H4b		900		C			100		20	40.6		Special Cat.2	820	C	To be specified by road authority
									38000		A			65		20	724.6			44000	V/T
																			Special Cat.3	820	C	To be specified by road authority
																				To be specified
Ledged of “Vehicle Type”
A=Articulated Heavy Good Vehicle					R=Rigid Heavy Good Vehicle
B=Bus					S=Single Unit Van Truck
C=Small car					T=Tanker Type semi-Trailer
P=Four Wheel Drive, Utility					V=Van Type Semi Trailer
The data is arranged in the order of increasing Test Levels within a standard and increasing “Impact Severity” across the standards

The "Impact Severity" index is used in both NCHRP 350 and in the European Union code EN1317 (although it is described as "Containment Level") for comparisons between individual test within a testing regime to account for tolerances in the various variables i.e. the mass of the test vehicles, its speed and angle of impact. The Impact severity formula measures the available kinetic energy just prior to vehicle impact.

The Impact Severity is defined by the following formula:

IS = ½ M (V sinθ)² (joules)

where:
M = Mass of vehicle (kg)
V = Velocity (m/s)
θ = Angle of Impact

The IS is a valuable tool as it can function as an equivalence measure and can be used to evaluate tested barriers against other crash test standards. This approach is valid where the nature of the test vehicles (for the relevant categories) and other acceptance criteria (i.e. occupant risk and post impact vehicular behavior) are similar. With all other parameters being equal, the resulting kinetic energy or IS value of the test vehicle can be used to rank or derive the equivalency of a tested barrier under an alternate crash standard. The IS cannot be used in-lieu of a crash test and therefore cannot be used to increase the rating of a tested barrier. The IS does not examine the performance of a bridge rail directly, rather the performance of a bridge rail is inferred from the highest test level that was passed. Hence, while a particular barrier may be able to satisfy the requirements of a higher test level, the IS cannot be used to demonstrate this potential nor can it be used to predict the post impact behavior of the vehicle. This point is particularly relevant to the containment level of a tested barrier and its ability to prevent a colliding vehicle from tripping over the barrier.

Each bridge needs to meet a particular performance level in terms of traffic barrier and a traffic barrier suitable to that performance level should be chosen and specified. In evaluating the type of barrier to be used, it is critical to note that the Road Authority must carefully investigate the degree of risk, clearly identify the types and mix of vehicles that are to be contained and determine the Performance Level required at the bridge site conditions with proper road restrain risk assessment. The selection should be based on road type, grade, curvature, deck height and under-structure.

As per Australian standards:

NCHRP Report 350 Test Level 2 barriers are to be used in situations of low risk and thus defined as "Low performance level" traffic barrier which are required to contain a level of impact typical of a 2.0 tonne utility vehicle (light vehicles) at 70 km/h and an impact angle of 25 degrees.

NCHRP Report 350 Test Level 4 barriers are defined as "Regular performance level" which is required to contain a level of impact typical of an 8.0 tonne truck at 80 km/h and an impact angle of 15 degrees. This performance level barrieris provided for the effective containment of general traffic on all roads; which means that the regular barrier will be applicable and appropriate to the majority of bridge sites.

NCHRP Report 350 Test Level 5 barriers are defined as "Medium performance level" which is required to contain a level of impact typical of a 36.0 tonne articulated van at 80 km/h and an impact angle of 15 degrees. This performance level barrier is provided for site-specific, medium to high risk situations for the effective containment of medium to high mass vehicles and buses on all roads." The emphasis is on successful containment of the vehicle on the bridge to mitigate against injury to third parties at risk off or below the bridge. It is important to distinguish the requirement of identification of the type of vehicle to be contained as a critical factor. The cost of crash-testing 36 tonne trucks is large and subsequently there are fewer crash tested barrier systems available for selection. Concrete barriers have proved capable of containing substantial forces. The "F" Type concrete barriers are based on the AASHTO profiles and are considered to be compliant with NCHRP 350. The 810 mm tall barrier is rated as TL-4, while the taller1060mm high barrier has a TL-5 rating. The New Jersey Concrete Barriers are also NCHRP 350 compliant, with the 810 mm (32")tall barrier rated at TL-4 while the taller 1060 mm (42") barrier is TL-5 compliant.

NCHRP Report 350 Test Level 6 barriers is defined as one of the "Special performance levels" which is required to contain a level of impact typical of a 36.0 tonne tanker as compared to the articulated truck which has higher center of gravity and impart higher energy at impact compared to an articulated vehicle of the same weight. The second Special category identifies a 44 t articulated van at a test speed of 100 km/h which is almost double the energy or severity of impact of the first Special category. For the American NCHRP 350 compliant barriers, there is currently no barrier rated higher than TL-6. Potentially, a compliant EN 1317 H4b barrier may be suitable as a Special category 2 barrier because its IS value is about 30% higher than the TL-6. The suitability of the H4b barrier will depend on the actual requirements of the Authority and these will include the height and location of the centre of gravity of the test vehicle, the collision speed and the angle of impact. The German Maxi-rail is a proprietary barrier system, which is a compliant H4b barrier. This barrier may be suitable as a Special category 2 barrier where the centre of gravity of the T44 vehicle is less than 1.9 m and the impact speed is limited to 80 km/h at a maximum impact angle of 15 degrees. The third Special category is to be specified by the Authority.

For bridge traffic barriers to be acceptable, it must be crash tested and comply with the acceptance criteria as set out in National Cooperative Highway search programme (NCHRP) Report 350 or to other appropriate Standards as determined by the relevant Road Authority. However, it is not practical or economically feasible to test every barrier and components. Barriers should also be acceptable if they can be evaluated as being both geometrically and structurally equal to a compliant crash-tested system. Alternatively, approval may be given based on the evaluation of performance of an existing barrier to the Authority’s requirements.

The author greatfully acknowledges the support from Design Department of "ASHGHAL - Qatar Public Works Authority" and the colleagues from "Aurecon" in particular Eric De Fleuriotwho who is Technical Director and Head of Bridges for Aurecon (Middle East).