bridge barriers, parapets, guidelines, selection

BRIDGING THE MILLENNIA

DESIGN GUIDELINES FOR BRIDGE BARRIERS

Vincent Colosimo, B.Eng, G.Dip.CE.,G.Dip.ME., MICE, MI.Aust. Engineer-Bridge Loads, PBE Section, VicRoads

ABSTRACT

The (Austroads et al. 1996) code outlines four levels of service for barrier performance. For level two, three and four, the code gives both the criteria for containment and rough design guidelines. However, for high performance level 1 barriers, it does not give specific design criteria. The form and height of such barriers must be obtained from studies, tests or specialist literature. For level two barriers, it gives design forces appropriate only for containing light vehicles.

The existing barriers and concrete parapets used by VicRoads have been designed to safely contain a standard two tonne car. Investigations have indicated that the concrete parapet has only marginal reserve capacity beyond this requirement.

The aim of this paper is, therefore, to fill this void and assist designers by providing design guidelines and concept proposals for barriers of various levels of performance. It also identifies inconsistencies in the Austroads' code sections on barriers and proposes modifications. The performance characteristics of both current and proposed barriers, are addressed.

This paper also aims to rationalise and provide uniform practices for the selection and design of barriers and improve road safety.

Keywords: bridge barriers, parapets, guidelines, selection, containment, road safety.

ACKNOWLEDGMENT

The author wishes to thank the Chief Executive of VicRoads Mr. Colin Jordan for his permission to publish this paper. The views expressed in this paper are those of the author and do not necessarily reflect the views of VicRoads.

BIOGRAPHY

Vince Colosimo is an Engineer in the Assets Group of the Bridge Section, VicRoads. He joined the organisation in 1966 and has extensive experience in the design of bridges and other road structures. Other experience includes road design and bridge construction. He has served as the Bridge Loads Engineer, responsible for heavy load permit and load rating assessments. He has been involved with research, testing, and developmental work associated with standardisation of components for bridge and road structures and provides specialist support to the Department, other areas of VicRoads and external organisations.

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INTRODUCTION

1. The (Austroads et al. 1996) Bridge Code outlines four levels of service for barrier performance. For level two, three and four, the Code gives both, the criteria for containment and rough design guidelines.. However for level 1 bathers, while it details identified vehicles to be contained, which include high centre of gravity vehicles and heavy trucks up to the design speed, it does not give specific design criteria. The form and height of such barriers must be obtained from studies, tests or specialist literature. For level 2 barriers it gives design forces appropriate only for containing light vehicles.

2. The aim of this paper is therefore to fill this void and assist designers by providing design guidelines for selecting barriers of various levels of performance. It also details inconsistencies in the (Austroads et al. 1996) Code sections on barriers and recommends modifications. The performance of both the present barriers and future proposals are addressed. Examples of conceptual barrier shapes for the proposed performance levels are given.

BACKGROUND

3 Bridge barriers have been the subject of extensive world wide investigations in recent years as emphasis on road safety has increased.

4. Hirsch (1986) states that most current longitudinal barriers (guardrails, bridge rails, and median bathers) are designed to only restrain and redirect passenger automobiles with the recommended strength test being a 2 tonne passenger vehicle, to be redirected at 100 km/hr and a 25 angle of impact.

5. Ivey (1981) indicates that the vehicle fleets are changing in character. There is a major shift toward smaller vehicles and also towards a greater percentage of trucks. Barrier systems must therefore be designed to cater for both ends of the spectrum, from the mini car to heavy trucks.

6. Local Trucks are generally heavier than their counterparts in the USA. Whilst the USA has retained the 1940's 33 tonne semi-trailer design vehicle with corresponding legal vehicles, Australia moved to the 44 tonne semi-trailer design vehicle in 1976. In addition, Australia's current legal load of 42.5 tome is under review and an early increase to 45 tome is likely.

For Victoria's major freeways and highways, commercial traffic represents approximately 10 to 25 percent of the total volume. CULWAY data indicates that 40 to 50 percent of this traffic is six axle semi-trailers with a mean mass of close to 33 tomes and a 95 th percentile value between 40 to 45 tomes. Hence the mass of Australia's fleet of semi-trailers is significantly higher than their US counterparts and this has been considered in developing the proposed performance levels.

7. This paper extends information provided in a previous paper titled 'Design Proposals For Bridge Barriers', (Colosimo 1996). The paper provides guidance for the selection of the appropriate barrier performance level relevant to specific site conditions, as well as more comprehensive design and geometric requirements for barriers.

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SCOPE

8. These Guidelines are proposed for new or replacement barriers on bridges. Performance Levels and the associated design criteria are provided, relevant to local vehicles, traffic conditions and the road environment. This information can be used for selecting a suitable barrier, taking into account the level of risk at a particular location, aesthetics, performance level and cost/benefit considerations.

The objectives are to reduce the severity of accidents on bridges, and to achieve rational uniformity of standards.

9. Barriers for bicycles, pedestrians and other purposes are not addressed and reference should be made to the requirements of the (Austroads et al. 1996) Bridge Code for this purpose and other related issues.

ROAD SAFETY AND ACCIDENT INFORMATION

10. Bridge Barriers in themselves may be considered as potential hazards to traffic, therefore they need to be detailed and positioned to minimise the severity of vehicle impacts.

11. The limited number of recorded incidents involving bridge barriers makes reliance on Australian statistics impractical. The lack of reliable and accessible Australian data makes the undertaking of statistically based risk analysis difficult and unreliable.

These guidelines are therefore based essentially on risk-benefit analysis incorporated in the AASHTO (1989) Guidelines, with some minor modifications for local traffic and conditions, in part, recommended by Ove Arup & Partners (1991).

TEST RESULTS

12. The design of the existing parapet is based on the same criteria as the California Type 20 bridge barrier. (Nordlin et al. 1971) reported results of five full-scale vehicle impacts on the Californian type 20 rigid barrier. Results indicated that this system will retain and redirect a 2.2 tonne passenger vehicle impacting at speeds up to 105 km/h and at angles from 7 to 25 degrees with the barrier.

Simulation test results were reported by (Bloom, Rudd and Labbra 1974). The type 20 barrier was proven effective in redirecting a standard 2.2 tonne sedan through simulations. However, neither the railing nor the concrete parapet could withstand an 18 tonne single unit truck impacting at 97 km/h and 7° impact angle.

13. Four ways of improving the barrier were identified as follows:-

(a) Increase the parapets resistance to torsion. (b) Increase its horizontal beam stiffness. (c) Provide a barrier to slab connection that will permit some rotation and/or deflection

of the barrier. (d) Strengthen the bridge deck and increase the vertical stiffness of the barrier.

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CODE REQUIREMENTS

Australia

14. The (Austroads et al. 1992) Code classifies railings into three levels of service as follows:-

Level 1 Barriers shall be used in situations of high risk to provide appropriate containment of identified vehicles eg. cars, buses and heavy trucks under severe impact conditions at speeds up to the design speed of the roadway and impact angles of up to 15°. The form and height of such barriers shall be based on suitable studies and/or tests.

Level 2 Barriers shall contain vehicles at intermediate levels of impact. Note that the 1991 previous draft to this Code identified the appropriate vehicles to include both cars and buses. The ultimate design loading being 90 kN (transversely) amended for height of railing and divided by the number of rails. This requirement simulates the traditional standard roadway barrier specification to direct a 2 tonne car at 100 km/h and 25 incidence angle. There is therefore an obvious design load inconsistency in this Code, ie. the 90 kN load relates more closely to the standard roadway barrier Level 3 rather than the Level 2.

Level 3 Barriers shall retain cars at the design speed and impact angles up to 15°'. The design performance requirements correspond to those for standard roadway barriers.

Level 4 Barriers Continuous traffic barriers need not be provided on flood prone, low risk and low level bridges with low traffic volumes less than 150 vehicles/day. The form of such barriers may simply consist of structure delineation by means such as sign posting, guide posts and or castellated kerbs to maximise waterway.

International Standards

15. International practice includes both theoretical design and crash testing for the development of bridge barriers. A review has been made of the current codes and standards available from the USA, the UK, and Europe with particular emphasis on rationalisation of barrier performance levels, including the vehicles to be contained, design forces and barrier heights. Generally these standards include two or three levels of performance for light, medium and heavy vehicles.

United States of America

16. The AASHTO (1989) Guide has served as the basis for the selection, design and testing of bridge barriers by the Federal Highway Administration (FHWA) members. This Guide has considered traffic encroachment rate statistics covering many years and has included a cost/benefit analysis in developing recommendations for bridge barrier performance levels. It specifies a matrix of test vehicles, speed and angle of impact for three barrier performance levels, PL-1, PL-2 and PL-3, including two optional higher performance levels PL-4 and PL-4T. It also provides guidelines for the selection of the appropriate performance level and the design of corresponding barrier(s).

17. In addition AASHTO issued a limit state version of its Bridge Specification (AASHTO 1994), which again specifies the three basic performance levels in the AASHTO (1989) Guide. It specifies minimum heights for barriers as well as (ultimate) design forces and methods of application.

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0

TABLE 1 PERFORMANCE LEVEL DETAILS

Barrier Performance

Level

Vehicles Contained

Design Speed

Km/Hr

Impact Angle

Degrees

Energy Level

Severity Index kNm

Ultimate Lateral Design Force FL kN

Vehicle Dislrib'n Length

Transverse, Long'l or Inward

L, and LL in

Ultimate Loner!

or Inward Force

FL kN

Ultimate Vertical

Downward Force F, kN

Vehicle Distrib'n Length Vertical L, In

Minimum Effective Height

H, nun

T.R.B. Report 350

Test Level

No.

LOW 1.61 CAR 80 20 80 125 1.2 40 20 5.5 500 ' TL-2

2 i CAR 80 20 80

21 CAR 90 15 90

2.4 t *UTILITY 70 20 70

REGULAR 21 CAR 110 20 90 250 I I 80 80 5.5 800 -13,-4

2.41 UTILITY 100 20 110

8 I * RIGID TRUCK 80 15 130

91 LOCAL-BUS 75 15 150

MEDIUM 151 METRO-BUS 100 15 390 500 2.4 170 220 12 1100 TL-5

221 INTER-CITY BUS 90 15 460

33t ART-VAN 85 15 600

361* ART-VAN 80 15 600

HIGH 22 t INTER-CITY BUS 120 15 820 1000 2.5 330 300 15 1400 —TL-6

33 l ART-VAN 110 15 1030

36 1 ART-VAN 100 15 940

44 t * ART-VAN 100 15 1150

Notes: i. * Controlling Strength test Vehicles. ii Table data is based on a lateral combined barrier-vehicle deformation of 0.3 m for the Low and Regular levels,

and 0.5 m for the Medium and High levels.

td

tr

0

t-


18. Current practice in the USA varies, the AASHTO (1989) Guide and States of California and Texas recommend the use of a concrete PL-2 performance barrier, which is only marginally more expensive than a PL-1 barrier and is capable of containing a 2.4 tonne utility (pickup truck) at 100 k/ph and an 8 tonne rigid truck at 80 k/ph, as the basic level barrier provision on all highways. Note that the lower PL-1 performance level barrier is only capable of containing a 2.4 tonne utility at 72 k/ph and 20°.

19. The PL-3 performance level barrier, which is capable of containing a 22 tonne van truck at 80 k/ph and a 15 tonne metro bus at 100 km/h, is used for highways with high traffic volumes with a significant proportion of trucks, and where site specific conditions control. The optional PL-4 performance level barrier is capable of containing a 36 tonne van type semi-trailer at 90 k/ph whilst the PL-4T level is aimed at 36 tonne high centre of gravity tankers . These levels are only used at sites specifically identified as requiring containment of such vehicles.

BRIDGE BARRIER PERFORMANCE LEVELS General

20 It is proposed that the following performance level barriers be provided for new and replacement barriers on bridges. These performance Levels are attuned and correspond to the AASHTO (1994) - Load and Resistance Factor Design (LRFD) specification levels. Their names differ from the Level 4,3,2 and 1 in the (Austroads et al. 1996) code in order to overcome the current code inconsistencies in design criteria between levels. It is recommended that the Australian Bridge Design Code be modified to also conform with the following performance Levels.

* Low Level provision for short low level structures; areas where a small number of heavy vehicles are expected and low speed environments.

* Regular Level provision for the containment of cars, utilities and light trucks. Generally used on high speed roads and highways with a mixture of vehicles.

* Medium Level provision for the containment of buses and medium mass vehicles. Generally used on freeways, arterial roads and major highways with site specific considerations.

* High Level provision for high risk situations and the containment of heavy vehicles, generally on routes with a higher volume of mixed heavy vehicles.

21 Recommended design requirements for barriers that comply with the above performance levels are contained in Table 1.

22. Beyond the performance levels detailed in this guideline, a Special Level non-penetrable performance may be required for site specific, unusual conditions and critical locations where penetration or vaulting by very high centre of gravity and/or heavy vehicles under varying impact conditions, must be prevented.

These barriers need to be designed for site specific cost benefit considerations. Reference may be made to specialist literature such as Hirsch (1986) and Troutbeck (1994) for design guidance.

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Low Level

23. This level corresponds to the AASHTO (1994) LRFD Specifications, PL-1 performance level and is proposed for use on roads with low traffic volumes, low posted speed environments and predominantly light vehicles. The controlling strength test vehicle in the AASHTO (1994) is a 2.4 tonne pickup utility at 70 km/h and 20 degree impact angle. This barrier might, for example, comprise a tested roadside barrier such as a guard fence, modified in stiffness for bridge use. Note that this Level should also contain a medium 1.6 tonne car, at 80 km/h and 20 degrees or at 100 km/h and 15 degrees.

Regular Level

24 This level corresponds to the VicRoads current steel and concrete barrier systems which were designed to contain 2 tonne cars at 100 km/h and an impact angle of 25 degrees. The equivalent test vehicle for this level from AASHTO (1994), is a 2.4 tonne utility 'pickup' at 100 km/h and 20 degrees. It is worth noting that this vehicle also represents the typical four wheel drive vehicle on our roads. However this Level takes into account the reserve strength of current concrete barriers and the controlling test vehicle for strength is an 8 tonne rigid truck at 80 km/h and 15 degrees. This performance level is equivalent to the US PL-2 level, which is consistent with the test level 4 vehicle in the matrix of test vehicles of the TRB (1993) Report 350.

Note that Appendix B of the AASHTO (1989) Guide Specifications states that barrier railings meeting the requirements of the AASHTO - 1989 Standard Specifications for Highway Bridges and of previous codes, were suitable for most rural arterial highways, and thus there was a conscious effort made to match the Performance Level (PL-2) to the performance limit of the current concrete safety parapet. Since our current barriers were designed to similar if not identical code requirements the same performance match is applicable and recommended for our systems.

In addition, correspondence received from Mr. Ralph Bill Bishop of CALTRAN (ie. California Department of Transportation), Division of Structures, dated October 26 1995, included the following information. 'Firstly their standard bridge barrier since 1978 had been the 813 mm high concrete safety shape. Secondly, it was understood that the AASHTO (1994) Specification , would become the dominating Specification by 1998. Thirdly, California felt that all their State Highways should have their new bridge barriers at a Performance Level of Two, the difference in cost between a PL-1 and PL-2 barrier not being that great.'

The existing VicRoads standard concrete parapet, with improved longitudinal continuity provisions, should generally comply with the requirements for the Regular performance level.

Medium Level

25 This level has been set to contain a 15 tonne metro bus at 100 km/hr and a 22 tonne intercity bus at 90 km/h, each at 15 degrees. This level is therefore attuned to the Level 2 requirements in section 1 of (Austroads et al.1996) i.e. to contain hazards intermediate between Level 1 and Level 3, at intermediate levels of impact.

The relevant vehicle of particular interest, contained by this level is a 33 tonne articulated-van (semi-trailer) at 85 km/h and 15 degree angle. This vehicle choice for design, simultaneously ensures that the above buses are contained and addresses the high

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percentage of six axle semi-trailers with a mean mass of over 30 tonnes using the freeways and major highways. For theoretical dynamic modelling analysis the 33 tonne articulated semi-trailer is converted into two independent components; namely an 18 tonne prime mover and a 26 tonne trailer which helps explain the relevance to a 22 tonne bus.

The controlling strength test vehicle in the TRB (1993) Report 350, is a 36 tonne Articulated Van at 80 km/hr and 15 degrees, of Test level 5, relevant to the USA PL-3 performance barriers. Note that the correlation between the 22 tonne test vehicle criteria in AASHTO (1994) and Test Level 5, is that the 22 tonne vehicle may be considered to be the Prime Mover component of the 36 tonne TRB (1993) test vehicle. It is considered that redirecting the prime mover should generally ensure that the trailer would follow and also be redirected , subject to proper fixity of the trailer load.

This performance level can be provided at minor additional cost over the Regular level.

High level

26. This performance level is proposed for heavier high centre of gravity vehicles in high risk situations, such as a freeway, high speed main road, or highway bridge over a freeway, a rail line, occupied houses or factories and other similar situations to protect vehicle occupants and in particular, people and property beneath or adjacent to the bridge. This Level is attuned to Level One of (Austroads et al. 1996), ie. for heavy trucks, buses etc. under severe impact conditions and up to the roadway design speed and 15 degrees. The default controlling vehicle has been selected as a 44 tonne van type semi-trailer at a speed of 100 kmph and a 15 degree angle. Choice of this Level should depend upon the specific situation, the traffic environment and level of risk.

BARRIER PERFORMANCE LEVEL SELECTION METHOD

General

27. The selection method is based on the proven AASHTO (1989) Specification procedure, in part as modified by OVE ARUP & Partners (1991), for Local Australian data and traffic conditions.

The procedure is based on the application of benefit-cost analysis taking into account, site conditions, estimates of roadside encroachments (the default value was 0.0003 encroachments/km/year/vpd on one side of road), and barrier performance levels. The procedure has been further simplified by leaving out the extra three traffic multiplication factors introduced by Ove ARUP (1991), namely, Bridge Road Width Ratio, Bridge Width and Speed Continuity. These additional factors very rarely affect the level of provision chosen and if relevant, may be considered by reference to specialist literature. Refer to Ove ARUP (1991) or Colosimo (1996) for guidance.

It should be noted that the selection procedure outlined below is still under review. However it is proposed to recommend it for adoption by VicRoads shortly after the review process.

The selection method is a four level system which in addition allows the possibility of a 'barrier not required' solution, or the possibility of a barrier of very high performance 'one-of special' design, for situations of extreme risk.

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Adjust AADT [ =(RT)(GC)(CU)(US)AADT ]

Medium Level Barrier Re• uired

Regular Level Barrier Required

V Low Level Barrier Required

Determine Additional Design Information Barrier offset, Width between kerbs, Risk factors - clause 29

Yes

Is Barrier Required at all?

Barrier not Required

Determine Additional Design Information Design speed, Road type, Approach & Departure Grade, Land Use Risk-clause3l

Compare Adjusted AADT with Threshold Limits - clause 32 I


BARRIER SELECTION FLOW CHART

Determine Initial Design Information AADT, %Cv, Land use, Deck height, Depth of water - clause 28

Yes Is bridge

at a High Risk Location?

High Level Barrier Required

No

Fig. 1 - Barrier selection flow chart

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The Flow Chart of the selection process is shown as Figure 1.

High Level Barrier Criteria

28. A high level barrier selection decision reflects the situation where, if a barrier is penetrated by a vehicle, then there is a high probability of loss of life or serious injury to the vehicle occupant(s) and or other persons. A high level barrier should be considered in the following situations. Each of these situations is referred to in (Austroads et al. 1996).

(i) Bridges over major roadways with an AADT of 20,000 or more vehicles per day. (ii) Bridges over electrified railways or goods lines carrying large quantities of either

noxious or flammable substances. (iii) Bridges over water greater than 3 metres deep. (iv) Bridges over high occupancy land use such as houses, factories etc. (v) Bridges more than 10 metres high. (vi) Bridges on horizontal curves with a radius of 600 metres or less.

High level barriers should be specified if any of the above conditions apply, and the commercial traffic levels at the bridge site are as follows :

• Greater than 2,000 commercial vehicles per day in rural locations or • Greater than 4,000 commercial vehicles per day in urban locations.

29. For a designer to consider a 'Special level' of protection greater than the High level, it is proposed that at least two of the site conditions and commercial talc count stated above need to apply in combination.

Situations Where a Barrier is not Required

30. For certain bridge or culvert sites, conditions may be such that the barrier may constitute a greater hazard than if no barrier is provided at all.Situations where a barrier is not required are based upon the requirements outlined in the (Austroads et al. 1996) Code for a level four barrier, but with more stringent conditions attached i.e. if all the following nine conditions are satisfied or excluded:

traffic volumes are less than 150 vehicles/day. (ii) radius of curvature of the bridge is greater than 1500 metres and where the

approaches have sight distance greater than stopping distance. (iii) width between kerbs is not less than 6.6 metres for a two lane bridge or 4.2 metres

for a single lane bridge. (iv) barrier offset is at least 1.2 metres from the edge of the traffic lanes. (v) rural location without anticipated pedestrian traffic. (vi) under structure land is low risk i.e. the conditions under and about the bridge must

not increase the level of risk to the occupants of a vehicle leaving the bridge, eg. dangerous obstacles in near proximity.

(vii) low level bridges subject to frequent flooding where the bridge is over water normally less than 1.2 metres deep, or is less than 1.5 metres above the ground or water course invert.

(viii) for low level flood prone sites where the provision of barriers prevents the passage of debris or the barriers are frequently damaged by heavy debris or both.

(ix) For flood prone sites, sign posting is erected, warning that the bridge is subject to frequent flooding, clearly indicating the position of the crossing and continuous kerbs not less than 250 nun high provided on the bridge deck. Castellated kerbs may be used where afflux is critical, where permitted by the Road Authority.

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Class C

Class B

v— Class A

1 1 1


ROAD TYPE FACTOR

60 65 70 75 80 85 90 95 100 105 110 Design Speed (km/h)

Fig. 2 - Road Type Factor

GRADE CONTINUITY FACTOR

c; 1.5

1.4 E

1.3

1.2

1.1

1

0 1 2 3 4 5 6 7 8 9 10 Grade Continuity (%)

Fig. 3 - Grade Continuity Factor

45

2.5

2

6- 1.5

1

0.5


CURVATURE FACTOR

3

2.5

B 2

1.5

1

1 t I

600 700 800 900 1000 1100 1200 1300 1400 Radius of Curvature

Fig. 4 - Curvature Factor

DECK HEIGHT AND UNDER STRUCTURE USE FACTOR

High Risk Land Use .

-4... ,

Medium Risk Land . '

. . . , r

. Low

1 Risk Land

,

0 5 10 15

20 Height above Under-Structure Land Use (m)

Fig. 5 - Deck height and under structure use Factor

46

3

2.5

2

1.5

1

1000

O 0

2 100

qF

10 4.0

'0

1

Bather Type Low Regular Medium

Range of AADT 0 - 2,600 2.600 to 27,000 > 27,000


THRESHOLD LIMITS 60 Km/h

1000

C>' 100

E-

1 0

1

— - -- - -- Rail Offset .= > 3.7m

m — m —

Medium Level 1.2

..... -- -. ... - - ......

0.3

...

-... - j allerr.' LW .... _ .......

..... . AirAi viii - Regular --- 1

Low Level

5 10 15 20 25 30 35 40 % Commercial Vehicles

Fig. 6 - Threshold Limits 60 Km/h

THRESHOLD LIMITS 100 Km/h

— Rail Offset -

- Medium

,

>3.7m - m m m

Level -- --

er 2.4 1.2

z 03

114t7

i ArMi WI

Regular Level acriffitirgliw 66

jar I:w :1..

z ... ......awa _

.

fti

mir

-.91c Iferli,

5 10 15 20 25 30 35 40 % Commercial Vehicles

Fig. 7 - Threshold Limits 100 Km/h

Example for 100 Km/h design speed, 16% commercial vehicle and 1.2m bather offset

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Adjustment to AADT

3 I The adjusted AADT Method is as used in AASHTO (1989) but the weighting of the factors has, in some instances, been marginally modified. There are four adjustment factors to be considered, to modify the estimated current traffic volume (AATD) - at the time of construction.The projected traffic growth assumed in this method is 2 % per annum for 30 years.

(i) Road Type (RT) : Class A: Divided, or undivided with 5 or more lanes Class B: Undivided with 4 or fewer lanes Class C: One way.

(ii) Grade Continuity (GC) = Average grade + change in Grade' GA + I G1 - G2

Note that GA denotes the average grade as a % through the bridge zone. The expression I GI - G2 J is an absolute value for the algebraic difference in approach grade GI and departure grade G2.

(iii) Curvature (CU): The factors chosen reflect the increasing importance of the radius of curvature as it decreases towards the 600 metre mark.

(iv) Deck Height and Under Structure Land Use (US): The high risk land use category is included here as there are some instances where a lower volume road may require a High level barrier. The modification graph in Fig. 5 plots three risk levels relevant to land use underneath the bridge structure, defined as follows :- High Risk Land Use : Refers to Land used in such a way that there is a significant risk to persons or property below the structure eg. Over major roadways; railways; houses; factories; deep water etc. Medium Risk Land Use : Refers to Land used in such a way that there is an occasional risk to persons or property below the structure eg. Over roads with AADT < 10,000 vpd; country rail lines with occasional services; walking trails or areas with occasional human populations. Low Risk Land Use : Refers to Land used in such a way that there is a minimal or insignificant risk to persons or property below the structure eg. Over open fields; bushland; water less than 1 metre deep etc.

Adjusted AADT = (RT) (GC) (CU) (US) * AADT

The graphical models with adjusting factors are shown in Fig.'s 2 to 5.

Final Barrier Level Performance Selection

32. The final selection process involves comparing the modified AADT to the threshold limits in Figures 6 and 7, relevant to specific design speeds and performance Levels. These threshold graphs also include an additional variable, namely the bridge rail offset to the traffic lane.An example of the use of the threshold limits charts is shown on Figure 7.

PERFORMANCE TESTING

33. In the USA, a compulsory part of the design of any new type of barrier is performance testing in accordance with TRB (1993) Report 350. Similarly, in the UK the BSi (1992) BS6779 allows new concrete parapets to be designed in accordance with its requirements but all new steel barriers must be crash tested.

48


34. It may not be economically realistic to set a similar requirement for Australia, but attention should be given to international testing and outcomes in continuing to review and improve our standards.

35. An official Memorandum (FHWA 1993) from the U.S Department of Transportation to Regional Highway Administrators, in its attachment titled 'Background and Procedures for gaining Federal Highway Administration Acceptance for Highway Features', states that 'the AASHTO (1989) Specification includes three Performance Levels that, in their effect, approximately match three test levels in the TRB (1993) Report 350. Any new bridge railing testing (post May 16, 1994) should be in accordance with Report 350. Test Levels 2, 4, and 5 in Report 350 should be substituted for performance Levels PL-1, PL-2, and PL-3, respectively in AASHTO (1989) '.

The AASHTO (1994), has adopted essentially the same three basic Levels of Performance PL-1, PL-2, and PL-3. These Guidelines include corresponding performance levels named: Low, Regular, and Medium respectively, which are matched to the same ultimate design forces (Refer Table 1), and performance tests ie. Test Levels: 2, 4, and 5 of the TRB (1993) Report 350 .

36. The controlling strength vehicle for the High Level Performance refers to a heavy 44 tonne semitrailer and is not tied directly to the TRB (1993) Test level 6, which includes a 36 tonne medium mass tanker. However barriers that have satisfied the Test Level 6 requirements, with minor strength improvements could fulfill requirements in this guideline, for the high level.

37. Barrier systems tested successfully for all the requirements of the equivalent performance level tests in the TRB (1993) Report 350, are considered acceptable and can form the basis for some of our own standardised developments. In addition the State Road Authority may approve Barriers designed in accordance with the requirements of these Guidelines.

DESIGN CRITERIA

Barrier Design Forces

38. Table 1 specifies the (ultimate) design forces, distribution length, effective height, energy level severity index (ie. the kinetic energy of the impact) and the test level, relevant to each barrier performance level vehicles to be contained, for specific impact criteria.

39. The Design Force is defined as an equivalent static force which represents the dynamic force imparted to a railing system by a specified vehicle, impacting a railing at a designated speed and angle.

40. The design force(s) have been purposefully chosen to align with values in AASHTO (1994), for three reasons, firstly because research and development in the USA is generally ahead of the international effort in this field, secondly because we have traditionally followed the AASHTO recommendations in the past, and thirdly to ensure that with minor improvements we can adopt systems that have been developed and successfully tested to fulfill the requirements of the relevant levels in the internationally accepted testing document, the TRB (1993) Report 350. The design force values have been marginally rounded off, resulting in values which subject to consistent design, should double the containment strength capacity between successive Levels of barrier performance.

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41. The mathematical dynamic vehicle-barrier impact model quoted by Hirsch (1986) has been calibrated and used to determine the design strength and height requirements for barriers. The average force by this theory has been factored, as suggested by both Hirsch (1986) and the BSi (1994) C.E.N. code , by a factor of 2.5 to give good correlation with 50 millisecond test forces (or impact forces) on stiff to rigid rails. These are the design forces quoted in both AASHTO (1994) and Table 1.

42. Note that the (ultimate) design force is applied to the total barrier system and refers to the containment capacity of the whole barrier panel(s) that deform and absorb the energy of impact. This design force is considerably higher than the traditional design load quoted in our Codes, which is applied and distributed to a section or localised area. Results of analysis of the current barriers indicate containment capacities in the order of two to three times the original design load.

Therefore the equivalent 'traditional design load' that could be applied in accordance with past Codes, or the (Austroads et al 1996) code methodology and distribution, is approximately equal to the average force. For the Regular Level it would be 0.4 times 250 kN = 100 kN, which agrees closely with the 90 kN nominal design load in the (Austroads et al 1996) code or 99 kN in the NAASRA - 1976 'Bridge Design Specification' . However, such equivalence depends upon the continuity of the barrier and the form of connection to the supporting structure, and thus the equivalent traditional design load(s) must only be considered to be approximate. They are therefore recommended only to be used for preliminary designs and comparing with existing or past practices.

43. The design of a barrier system using the ultimate design forces provided in Table 1, requires a detailed analysis, such as a yield line analysis for a concrete parapet or an inelastic plastic moment analysis for a steel post and rail barrier. All forces should be applied to the longitudinal rail elements. The distribution of longitudinal loads to posts should be consistent with the continuity of rail elements. Distribution of transverse loads should be consistent with the assumed failure mechanism of the railing system.

44. Load factors for the Permanent effects are recommended as per the (Austroads et al 1996) code, and a load factor of 1.0 , as per the (AASHTO 1994) is recommended for the transverse, longitudinal and vertical loads on the barrier as given in Table 1.

Geometry

45. Bridge barriers should generally be positioned vertically on bridge decks. However for low height barriers positioned on bridge grades less than 2 %, cost-benefit consideration may be made for positioning normally to their base surface.

46. The main references for the following geometric requirements concerning Separation of Rail Elements have been the AASHTO (1994) and the (Austroads et al 1996).

47. For traffic railings, the recommended criteria for maximum clear opening below the bottom rail, the set-back distance, and maximum opening between rails, is as follows as:-

i. the total depth of the rail(s) in contact with the vehicle (EA) should not be less than 25% of the height of the barrier (H)

ii. for post and rail barriers, the lowest rail should be centred between 380 mm and 500 mm above the reference surface. The reference surface is the roadway pavement level, or the footway surface level

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iii. for post and rail barriers, the vertical clear opening between rails should be, not more than 380 mm, but preferably less than 350 mm. The vertical clear opening between rails is defined as the distance between square or rectangular shapes and the extreme traffic side ridge of curved or circular shapes.

iv. for post and rail barriers, the minimum post set-back, should be:-

A • 100 mm for > 0.5 and 200 mm for --E --

A < 0.3

E Linear interpolation shall be used for A between 0.3 and 0.5 H

The post set-back is defined as the horizontal distance between the traffic faces of the railing and the traffic face of plane faced posts or the centre of circular posts

v. For post and rail barriers, the traffic faces of all traffic rails shall be within 25 mm of a vertical plane, through the face of the rail closest to the traffic.

Rails which are not positioned in accordance with these provisions i. - v., should not be considered traffic rails for the purpose of resisting the design loading given in Table 1.

Barrier Heights

48. Table 1 specifies the minimum effective height of the vehicle rollover force (H,) for each Performance Level. These values have been based upon international practice, performance during testing and in-service conditions as well as theoretical Dynamic Model Analysis detailed in Hirsch (1986). The effective height of a barrier may be considered to be the height of the resultant reactive force provided by the lateral resistance of the individual components of the barrier. Traffic barriers should have an effective height greater than or equal to the required minimum effective height (H„). The methodology for computing such values is provided in AASHTO (1994). The equivalent actual heights for rigid concrete parapets may be approximately equal to or marginally higher than the required effective height (H,).

49. For low performance level traffic barriers, the effective height (H,) is 500 mm. However for a metal, concrete or a combined metal and concrete barrier with a vertical face, the actual height should not be less than 700 mm unless prototype testing indicates that a lower system fulfills the requirements of the PL - 1 performance in the TRB (1993) Report 350 .

50. The regular performance level effective height of 800 mm is in part based on satisfactory performance in preventing light vehicles rolling over such barriers, under the specified impact criteria ( AASHTO 1994 ).

51. The medium performance effective height of 1100 mm is also based on the tyre height of a heavy vehicle and the successful record in the US of 1067 mm high concrete parapets, in redirecting commercial vehicles.

52. The high level effective height of 1400 mm is also aimed at the tray height of semi-trailers to prevent such vehicles from overturning.

53 In order to prevent a special tanker type semi-trailer from overturning, a greater height, in the order of 1700 to 2000 mm may be required for a concrete parapet to prevent

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overturning of such a vehicle. Hence, in the case of a very high risk situation, an individual assessment is still required.

54. Bridge traffic barriers over railways should satisfy the effective height requirements but in addition, and as per the (BSi 1992) BS6779, it is recommended that their minimum height be 1250 mm.

SPECIFIC FEATURES

Design

55. Anchorages: The yield strength of steel anchor bolts for the barrier shall be fully developed by bond, hooks, attachment to embedded plates or any combination thereof. Reinforcing steel for concrete barriers shall have sufficient embedment length to develop the yield strength, (AASHTO 1994) . To minimise damage to bridge decks it is necessary to design progressive strength systems in which barriers and their connections fail prior to the supporting structural elements. In lieu of a detailed structural analysis to ensure this behaviour, it is recommended that the barrier anchorages be designed to have a structural strength 20 % higher then the connecting barrier elements at the base of the barrier.

56. Bridge Deck Cantilever: The forces transmitted to the bridge deck may be determined from an ultimate strength analysis of the barrier system using the design forces given in Table 1.

57. Continuity: International practice and codes specify the importance of structural longitudinal continuity of barriers in containing vehicles. Continuity and full lateral strength are therefore recommended to be provided throughout the barrier length. In the case of steel railing, splices may be provided by bolted sleeve joints or full penetration butt welds. Full rail continuity should be provided for bending and shear, and 75 % tensile continuity in the splice section to allow for the reduction in strength due to bolt holes. Appropriate consideration must be given to expansion and contraction movements of the structure when designing a continuous barrier. For joint movements greater than 50 mm consideration may be given to providing expansion joints which fully bridge the gap. However additional or stiffer posts shall be provided adjacent to the joints, to ensure full lateral strength and containment capacity of the barrier, (BSi 1992).

58. Failure Mechanisms: Attention needs to be given in barrier design to the potential failure mechanisms to ensure that individual components do not become dangerous air borne projectiles after vehicle impact. The barrier should not rotate more than 40 degrees under test vehicle conditions to avoid ramping and vehicle climbing. In addition the Type 'F' concrete safety shape in (Austroads et al . 1996) is recommended over the traditional more pronounced 'New Jersey' profile to minimise reverse roll of light vehicles.

End Treatment And Transitions

59. An approach barrier and transition system should be capable of resisting predominant vehicle impacts, based on cost benefit and risk considerations. The lateral strength and stiffness of the barrier, within the transition length should vary and transition between the flexible approach barrier system and the rigid or semi rigid bridge barrier value. The approach flexible barrier should also have a crash worthy end terminal.

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BARRIER CONCEPTS DESIGN - V.G.DRAWN T.B.

LOW LEVEL

LOW LEVEL

LOW LEVEL

REGULAR LEVEL

REGULAR LEVEL

REGULAR LEVEL

MEDIUM LEVEL

MEDIUM LEVEL

HIGH LEVEL

Fig. B - Barrier Concepts

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A smooth face and tensile continuity needs to be maintained throughout. The exposed rail ends, posts and sharp changes in the geometry of the barrier components, kerbs etc. should be avoided or transitioned out or down with a proposed minimum splay or slope of 1 in 10 for the barrier components, and 1 in 20 for the kerb discontinuities.

Visibility and Aesthetics

60. In developing standard bridge barriers and the use of barriers at intersections where sight distance is important, consideration should be given to the placement of barriers and their 'See-through aspects'. Solid concrete (or steel) parapets may present sight distance problems or restrict general lateral visibility. Consideration may be given to the use of a steel post and rail or composite steel and concrete barrier for such situations, and also to improve the aesthetics.

COSTS

61. An estimate has been made of the cost of supply and erection of concrete barriers that would comply with the performance levels nominated above. These costs are $220, $300, $370 and $500 per linear metre for low, regular, medium and high performance level concrete parapet type barriers. The estimated cost for the regular level barrier is also applicable to VicRoads existing standard steel RHS barriers. Note that the additional cost of providing a higher performance level barrier is small when compared to the overall bridge costs and the cost to the Australian community of approximately $900,000 per death and $150,000 per injury.

STANDARD BRIDGE BARRIERS

62. Considerable progress has already been made in developing a recommended range of standard barriers which can be introduced as interim standards. Some promising concept proposal for the various Performance Levels are shown in Figure 8. In addition an internal VicRoads Technical Note titled Design Requirements for Bridge Traffic Barriers' has been prepared generally in accord with these guidelines and its publication should rationalise design practices in Victoria, (Colosimo 1997).

FUTURE DIRECTIONS

Implement the proposals in these guidelines for new bridges, by finalising a Technical Note on selection procedures for bridge barriers for VicRoads.

Develop and implement Guidelines for the assessment of Special High risk location barriers.

Review the proposed performance levels and associated design criteria with other state road authorities to seek agreement at the AUSTROADS level .

Develop a limited number of standard barriers based on these guidelines and approved tested systems.

Prepare an appropriate technical note covering: structural requirements, performance levels and warrants for bridge approach barriers to conform with performance levels in these guidelines.

Provide expert advise on design of appropriate bridge barriers on a one off basis during this development period.

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NUDGING THE MILLENNIA

REFERENCES:

I. AASHTO (1989). Guide Specification for Bridge Railings, American Association of State Highway and Transportation Officials, Washington, D.C.20001.

2.. AASHTO (1994). LRFD Bridge Design Specifications, First Edition, American Association of State Highway and Transportation Officials, Washington, D.C.

3. AUSTROADS, A.A.RA. and S.A. (1992) . Australian Bridge Design Code, Austroads Incorporatet, Sydney.

4. BLOOM, J.A, RUDD, T.J, LABBRA, J.J. (1974). Establishment of Interim Guidelines for bridge rails required to contain heavy vehicles Volumes 1, 2, 3. FHWA - RD - 75 - 45, 46, 47,.

5. B.S.i. (1992). British Standard 6779, Highway Parapets for bridges and other structures, Part 1. Specifications for vehicle containment parapets of metal construction. Part 2. Specifications for vehicle containment parapets of concrete construction, Department of Transport, T.RRL. London

6. B.S.i. (1994). prEN 1317-1&2, (C.E.N. - European Code) Road Restraint Systems, Department of Transport, London.

7. COLOSIMO_ V. (1996), Design Proposals For Bridge Barriers, Asia-Pacific Symposium on Bridge Loading and Fatigue, 17-18 December.

8. COLOSIMO V. (1997). Design Requirements for Bridge Traffic Barriers. VicRoads -Principal Bridge Engineer's Section, internal report No 971001, (Approved by PBE 7 th. October 1997).

9. FHWA (1993). Memorandum to Regional Highway Administrators, U.S. Department of Transportation, Federal Highway Administration.

10. HIRSCH, T.J. (1986). Longitudinal Barriers for buses and trucks. Texas transportation Institute. Reported in Transportation Research Record 1052, pp 95-102, TRB, NRC..

11. IVEY. D.L. (1981). Smaller cars and highway safety. Texas transportation researcher, April PP5-8.

12. NORDLIN. E.F. (1971). Woodstrom, J.H. Hackett, RP, and Folson, J.J. Dynamic tests of California Type 20 bridge barrier rail California Division of Highways, January.

13. OVE ARUP & PARTNERS (1991), Study of Rational Standards for bridge bathers and bridge approach treatments Technical Report No. 2- Review of current standards and formulation of new guidelines, Volumes 1 & 2 , July 1991. Prepared for VICROADS.

14. T.RB. (1993). Report 350 Recommended Procedures for the Safety Performance Evaluation of Highway Features. Transportation Research Board, National Research Council, Washington, D.C..

15. T.R.B. (1986). Symposium on Geometric Design for Large Trucks. Transport Research Board, T.R.R 1052 , National Research Council, Washington DC.

16. TROUTBECK (Prof.) R.T. and BUNKER J.B.(1994), Rational Standards for Bridge Bathers, procedure for Estimating Collision Frequency and Severity for Bridge Barriers, Queensland University of Technology.

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bridge barriers, parapets, guidelines, selection

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