Arahan Teknik (Jalan) 1-85 - Manual on Design Guidelines of Longitudinal Traffic Barrier

8/3/2019 Arahan Teknik (Jalan) 1-85 - Manual on Design Guidelines of Longitudinal Traffic Barrier

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Roads BranchPublic Works Department Malaysia

Jalan Sultan Salahuddin50582 Kuala Lumpur

Arahan Teknik (Jalan) 1/85

5.0m0m

7.0m0m

Manual On Design

Guideline Of Longitudinal

Traffic Barrier


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ARAHAN TEKNIK (JALAN) 1/85

MANUAL ON DESIGN GUIDELINES Of

LONGITUDINAL TRAFFIC BARRIER

PREFACE

This manual updates the Arahan Teknik (Jalan) 1/85 published in 1985 with few

minor changes.This issue omits the Chapter on Specification and Bill of Quantities which

was included in the earlier publication and the guardrail specification is now incorporated

into the Standard Specifications for Road Works. Departmental policy on guardrail

installation has been included in this issue.

This Arahan Teknik will be continually updated from time to time and in this respect

any feedback from users will be most welcomed. Any comments should be sent to

Cawangan Jalan,Ibu pejabat JKR, Kuala Lumpur.


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Cawangan Jalan, Ibu Pejabat JKR, K.L

PART I - GENERAL

1.1 INTRODUCTION

There is a need to provide highway designengineers with a choice of safe and effective

guardrail and medium barrier systems. The

application of traffic barriers should result in

safer highways. However, guardrails have

sometimes been misused, perhaps due to a

misconception of their function. Guardrails are

not installed to protect roadside objects or

prevent accident occurance, but to protect

vehicle occupants from possible serious injury.

Therefore, if a guardrail is to be installed, the

engineer must be satisfied that the severity of

hitting the barrier must be less than the severity

of hitting the hazard or leaving the highway.

It is recognised that traffic barriers are

hazards in themselves, and therefore their

application should be examined carefully.

Emphasis is placed on reducing the number of

such installations to only those that can be

firmly justified.

The purpose of this document is to give some

introduction on the several types of trafficbarrier systems available, some design

guidelines on corrugated steel beam highway

guardrail mounted on timber/steel posts and

New Jersey barrier.

The considerations outlined are concerned

primarily with the selection and installation of

new traffic barriers, but may also be used for

review of existing installations. Sound

application of these considerations will facili-

tate the provision of appropriate traffic barriers

to ensure roadsides incorporate a consistent andeconomic degree of safety.

Types of barrier not described may be

developed, or be found in use in other

countries. However, they should not be adopted

within Malaysia without careful identification

of critical design features, investigation of

performance records or preliminary testing.

1.2 DEFINITION

i. Traffic barriers are highway

appurtenances that provide a relative

degree of protection to vehicle occupantsfrom hazardous roadside features and from

errant vehicles encroaching across a

median.

ii.Traffic barriers are classified into two

basic groups according to functions:

(a) Longitudinal

Longitudinal traffic barriers perform

by redirecting errant vehicles away

from the roadside hazard.

(b) Crash Cushion

Crash cushion barriers function

primarily by decelerating errant

vehicles to a stop, thus greatly

reducing severity of a head-on impact

with fixed objects that exist in

off-ramp gore areas.

(This document only covers

Longitudinal traffic barriers.)

1.3 FUNCTION OF HIGHWAYTRAFFIC BARRIER

The primary function of highway traffic

barrier is to safely redirect errant vehicles and

thus reduce the severity of run-off road

accidents and number of highway fatalities, and

to minimise personal injuries. Highway traffic

barrier installations on shoulders prevent

vehicles access to steep embankments or fixed

objects, whereas median barriers are used

between the roadways of divided highways toprevent "across the median" collisions with

opposing traffic.

Barrier installations are therefore warranted

(or justified) only at highway locations where

the consequence of an errant vehicle leaving

the roadway is judged to be more hazardous

than the impact with the barrier installation.

This relative accident severity determination is

FOR INTERNAL USE ONLY


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valid regardless of whether one or one

thousand vehicles leave the highway at a point.

Hence accident frequency is not a principal

factor in determining barrier warrants.However,accident frequency factors do assist in

establishing a preferred order of construction of

two or more warranted installations.

However, it is noted that the installation of

highway barrier itself forms a road hazard as

the system is usually an elongated target which

is located closer to the roadway than the object

itself. For this reason,the highway designer

should make every effort to design without

guardrail. This can be done by

a) providing wide shoulders, verges and

medians;

b) providing adequate clearances to

structures;

c) flattening embankment slopes with firm

even surfaces;

d) clearing the roadside of fixed objects.

It may be necessary to consider the

provision of safety barriers where the above

measures cannot be applied or areconsidered impracticable.

1.4 TYPES OF HIGHWAY TRAFFICBARRIER

The traffic barrier systems, generally

tailored for specific highway requirements at a

given site, are commonly classified according

to lateral stiffness into the

following three categories.

i. Rigid Barriers which are normally used

where lateral deflections are not

permitted,such as locations at narrow

medians.As hese systems must be

essentially unyielding, they are almost

exclusively constructed of massive

sections of concrete.

ii. Semi-rigid Barriers which are used where

small to moderate lateral deflection is

acceptable. It can be classified into two

groups:

(a) strong beam/strong post and(b) strong beam/weak post.

The strong beam/weak post concept,is that

the posts near the point of impact are

purposely designed to break away so that

the force of impact is distributed by beam

action to a relatively larger number of posts.

Attributes of this system are (1) barrier

performance is independent of impact point

at or between posts and of soil properties,

and (2) vehicle snagging on a post is

virtually eliminated.

iii. Flexible Barriers which relies on large

dynamic deflections to redirect errant

vehicles gradually. This system either

weak beam/strong post or weak beam/weak

post types generally consists of posts

connected by steel cables.

The flexible barrier system is the multiple

wire rope beam mounted via

offset brackets to post. Tests have shownthat vehicles become pocketed or

snagged.

Some examples of each type of barrier

system are as shown in Figures 1.1, 1.2 and

1.3. The most common barrier system in used

is the semi-rigid barriers of strong beam/strong

post, a balanced design, consisting of a

corrugated steel beam mounted on various

types of posts.



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PART 2 . - DESIGN GUIDELINES

2.1 DESIGN PROCEDURE

For any new barrier installation, therecommended design procedure is as

follows:

i. Establish "point of need" or "length of

need" by warranting consideration as in

sub-section 2.2.

ii. Based on the unobstructed space available

for system deflection, select a barrier

system as in sub-section 2.3. For bridge

rail selection, the system must be

structurally compatible with the bridge.

iii.Determine design particulars for the

selected system, such as terminal

treatments and adjustments for highway

curvature.

iv.Make installation layout drawings. Note

that for guardrails and median barriers,

installations should be extended to a

reasonable distance upstream beyond the

warranted area to prevent vehicle access toa warranting feature. A method for

establishing this necessary extension is

presented in sub-section 2.4.

v.Make a field review, near the completion

of highway construction, before setting the

final installation limits. Short gaps

between installations should be avoided.

2.2 WARRANTS CONSIDERATION

Traffic barrier warrants are decision criteria

that identify sites along highway needing traffic

barrier installations. These warrants are delin-

eated in terms of geometry and location of

roadside features; and for the case of median

barriers traffic volume is also a decision factor.

Warranting criteria presented below have been

developed from analysis of run-off-the-road

accident statistics and are applicable to

highways in general.

2.2.1 Determination of need

Traffic barriers should be considered under the

following conditions:

i. Roadways on high embankment and

embankment with steep side slopes,

ii. On highways with roadside obstacles and

hazards such as structures and

appurtenances,

iii. Divided highways with narrow medians,

carrying large volume of traffic,

iv. Other conditions such as sharp horizontal

curves, pedestrian protection and severe

accident experience.

The three principal features are (A) slopes

embankment, (B) roadside obstacles, and (C)

opposing traffic which are discussed in more

detail in the following. Other factors are

dependent on site conditions, traffic character-

istics and accident experiences

which are considered individually.

(A) Slopes Embankment

Height and slope of roadway embankments

are basic factors in determining traffic

barrier needs for embankment. For low, flat

embankments, out of control vehicles can "ride

out" a slope with a hazard less than that

associated with striking a barrier. For high,

steep embankments, the hazard of being

redirected by a guardrail is less than that of the

vehicle being permitted access to the slope.A

dividing line between these extremes is asshown in Figure 2.1. This curve is independent

of accident frequency and embankment slope

material.



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(B) Roadside Obstacles

For warranting purposes, a 10m zone

adjacent to the travelled way is

recommended as the minimum for being clear

of roadside obstacles. If the 10m zone cannot

be cleared of roadside obstacles such as bridge

piers or permanent buildings, due to practical

or economic reasons, a traffic barrier may be

warranted.

Examples of roadside hazards that warrant

traffic barriers are:

(a) rough rock cuts,

(b) large boulders,

(c) permanent bodies of water with

depth > 0.6 m (2 ft),

(d) line of large trees ( butt dia > 150

mm (6 in)),

(e) bridge piers and abutment at underpasses,

(f) retaining walls and culvert headwalls,

(g) culvert end or wingwalls forming

abrupt drops greater than about 1.0 m in

height.

(h) gap between twin bridges,

(i) narrowing of roadway (loss of shoulder)

over structure,

(j) street lighting poles,

(k) railway tracks.

As most of these hazards extend a considerable length along the roadway, the probability

of errant vehicles striking the hazards is higher.

Where feasible, these roadside obstacles that

warrant the traffic barrier should be moved

from the 10 m wide zone adjacent to the

roadway. If this is not possible,traffic barriers

are to be installed.When guardrails are used on

embankments along shoulders they should be

placed at a minimum distance of 1.5 metres

beyond the edge of the road pavement.



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(C) Opposing Traffic

A longitudinal traffic barrier is used in

narrow medians to prevent across the median,

head-on collisions between vehicles in

opposing traffic. Warrants for these barriers are

determined by median width and the averaged

daily traffic volume.The median barrier need

can be determined as shown in Figure 2.2. It is

suggested that this daily traffic volume be

based on a 2-year projection. Median barriersare not warranted if median width exceeds 15

metres except on the basis of adverse accident

experience. It is noted that although accident

frequency generally increases after a traffic

barrier has been installed in a median, this is

attributed to the decrease in manouvering space

for run-off-the-road vehicles

2.3

\

2.3 SELECTION CRITERIA

Principally, the factors considered in

selecting an appropriate longitudinal barrier

system are:

i. The obstructed space available for lateral

deflection or maximum desired deflection

for a guardrail,

ii. The roadway or bridge structurecross-section, and

iii. The installation and maintenance costs.



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2.3.1 Deflection

The major factor in selecting a traffic barrier

system is matching dynamic lateral deflection

characteristics of a system to the space

available at the highway site. For the systems

to perform in a similar manner in actual

service, minimum unobstructed distances

behind guardrails and median barriers must beequal to or greater than this deflection.

Summaries of basic characteristics of

guardrails, bridge rails and median barriers

systems are presented in Figures 2.3, 2.4 and

2.5 respectively. Deflection, an important

system characteristic, is the maximum lateral

deflection that a system experiences during

impact and redirection of a selected vehicle.

Deflections of systems vary from 0 to 3.6

metres (12 ft) for guardrails and medianbarriers and from 0 to 0.6 metre (2 ft) for

bridge rails.



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Rigid barriers requires the least amount of

lateral space, because of their minimal

deflection under impact, whereas semi-rigid

barriers, which suffer greater deflections under

impact require more lateral space.

The most economical barrier, corrugated sheet

steel guardrail, requires the greatest lateral

space of the types commonly used.



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2.3.2 Roadway and Bridge Cross-section

Roadway and bridge cross-section can

significantly affect traffic barrier performance.

Kerbs, sloped shoulders and stepped medianscan cause errant vehicles to vault a barrier or to

strike it so that the vehicle overturns. Optimum

barrier system performance is provided by a

level surface in front of the barrier. Preferably,

face of barrier should be aligned above the face

of kerb; if however kerbs must be in front of

the barrier, they should be of the low,

mountable type to avoid dynamic jump by

vehicles.

2.3.3 Installation and Maintenance Costs

Although cost of installation generally

increases as system rigidity increases, cost ofrepair and maintenance generally decreases.

However, cost of vehicle damage is higher for

more rigid system. If two or more guardrail

systems satisfy lateral deflection requirements,

final system selection can be made on the basis

of:

Figure 2.6 shows the clearance required to

minimise any potential for vehicles to vault the

guardrail after hitting the kerb. Where barriersare installed on superelevated sections of

highway, the vertical axis of the barrier should

be inclined in order to remain perpendicular to

the pavement surface. This is particularly

important for slope-face.This is particularly

important for slope-face concrete barriers.

(1) local preference,

(2) availability and cost of materials,

(3) ease of installation,

(4) interaction of the barrier or supports

with any subsurface rock, services,

or drainage structures or with surface

drainage paths, and

(5) ease and frequency of maintenance

and repair including effects on traffic

operations.



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2.4 INSTALLATION LENGTH

Installation should be extended upstream from

the warranted limits to prevent vehicle access

behind the protective system. It is not necessaryto extend the installation downstream past the

hazard on highways with one way traffic. For

highways with two way undivided traffic, the

installation should extend downstream.

A method to establish the length-of-need of the

installation is based on a 100 metres

encroachment distance. The length-of-need iscalculated by: L=(1-A)x100m

where

L=Lenght-of-need

A=Distance of barrier from the edge of

pavament

B=Distance of objet from the edge

pavament



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The length of need of a median barrier will

include the full length of road along which the

barrier is required to prevent cross-median

vehicle movement, as well as any length

required to shield other hazards.

Short sections of guardrail' should be avoided

as they are ineffective and often introduce new

hazards instead. An isolated length of guardrail

on an embankment should not be less than 30

metres. For high speed facilities, a minimum of

75 metres is desirable. Short length of guardrail

is only useful as a warning of the presence of

obstruction or hazard but is inadequate as a

physical protection.

2.5 TERMINAL SECTIONS

Regardless of the type of barrier system

employed, a typical installation is composed of

three components:

(a) upstream terminal section,

(b) center section of "length-of-need",and

(c) downstream terminal section.

To prevent an errant vehicle from striking the

warranting feature, the installation must beextended a considerable distance upstream.

Furthermore, terminal sections must be added

to both ends to anchor the system in order that

redirecting force can develop in the rail.

There are three general types of guardrail

terminal treatments:

i. flares,

ii. ramps, and

iii. straight extensions. 2.5.1 Flared

2.51 Flared Terminals

Flared terminals swing away from the

pavement edge either in a straight or parabolic

manner as shown in Figure 2.8. Height of railwith respect to local grade is held constant. A

minimum offset of 1.2 metres (4 ft) should be

provided but where space permits 2.5 metres to

3 metres should be used. The flare should be

gradual to flatten the angle of impact by

vehicles leaving the road. As a guide, the

length of flare, should not be less than ten

times the offset

2.5.2 Ramped Terminals

Ramped terminals provide a guardrail slope to

the beam from effective rail height to grade

level as shown in Figure 2.9.The beam may be

twisted 90 within the ramp section and is

generally anchored at-grade to a concrete

footing.



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2.5.3 Straight Extensions

Straight extensions are additional lengths of

the typical guardrail system, generally with a

standard end-wing added to the beam used, asshown in Figure 2.10.

2.5.4 Terminals Treatment

Guardrail end treatment is an important safety

consideration and an improper designed end

treatment present a hazard to traffic. Guardrail

ends must be strengthened to prevent excessive

deflection and the possibility of the rail end

penetrating the vehicle occupant compartment.

It should be noted that the ramps tends to

launch an errant vehicle and the flare increases

the angle of impingement. To remove this

danger, the approach ends must be anchored to

the ground to give the needed stability to

adjoining sections and should be flared well

away from the travelled way to prevent

vehicles from striking the anchored ends with a

resulting over-ride or roll-over. If the approach

ends are not flared back, then they should be

blended into the approach environment.

On approaches to structures, the guardrailmust be securely attached to the structure in

order to give maximum protection and to

develop full strength of the rail in tension and

provide a relatively smooth configuration on

the traffic side.

2.6 STANDARD GUARDRAILSYSTEMS ADOPTED

The above design guidelines give a variety of

choices of different types of traffic barrier.However, for economic consideration and ease

of maintenance, standard systems are adopted

as follows:

i. Rigid guardrail adopts the New

Jersey design as shown in Figure 2.11.

ii. Semirigid guardrail follows the existing

practice of corrugated sheet steel beam

guardrail mounted on blocked-out steel

posts. In rural areas, timber posts of

either Chengal or Berlian can be used.

iii. For terminal sections, flared terminal

treatment is used. In case where space is

not available, ramped terminal can be

used. Straight extension terminal is used

only where terminal is pointing away

from one directional traffic flow. Return

section terminal is used at narrow

median installation.



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2.7DEPARTMENTAL POLICY

i. For all new semi-rigid guardrail

installations,corrugated sheet steel beam

guardrails mounted on steel posts are to beused. Timber posts may be used on rural

roads of R3 standard and below. The timber

posts shall be of heavy hardwood only of

either Chengal or Bilian.

ii. In the replacement of damaged existing

corrugated sheet steel beam guardrails that

are mounted on timber posts the following

guidelines shall be used to ensure

uniformity and consistency:

a) where the damaged timber posts are more

than five years old, then all the timber posts

in the section shall be replaced with stee

posts.

b) Where the damaged timber posts are less

han 5 years old, then the damaged timber

posts can be replaced with new timber posts

but of either Chengal or Bilian only.

iii. If any deviation from the above guidelines

is considered necessary for specific reasons,Pengarah Cawangan Jalan, Ibu pejabat JKR

should be consulted for installation on

Federal roads and the State Pengarah JKR

for installations on State roads.

PART 3 - COMMON TYPES OF

TRAFFIC BARRIER

3.1 CORRUGATED SHEET STEEL

BEAM GUARDRAIL

The corrugated sheet steel beam guardrail

commonly used consists of sheet steel beam of

W-shape cross-section attached to block-outs or

spacers supported on posts. Generally, the

block-outs and posts are constructed of steel or

timber.

The corrugated sheet steel beam guardrail is

classified as being 'semi-rigid' because it

deflects substantially but not excessively under

the U.S. standard structural adequacy crash

test; i.e. it undergoes a dynamic deflection of

0.8 - 0.9 m and a permanent deflection of

0.5 - 0.6 m when hit at an angle of 25 degrees

by a 2 tonne vehicle travelling at up to 100

km/hr. It follows that this type of guardrail can

require extensive repair after a severe impact,

and this may have safety, cost and road

capacity implications.

Under substantial impact the guardrail has

been designed to behave as follows:

a) The W-beam first bends and then flattens

out forming a wide tension band to contain

the impacting vehicle.

b) The posts are initially restrained by passive

pressure in the soil, resulting in local failure

of the soil at the ground line and for a short

distance below.

c) Wooden posts rotate, with their point of

rotation some distance below the ground.Steel posts partially rotate, but also bend

near the ground line.

d) Deflection of the posts and block-outs

causes the line of action of the restraining

force, acting on the side of the vehicle,

initially to rise, before ultimately dropping,

thus minimising the risk of vehicle vaulting

or rollover; the block-outs also lessen the

risk of vehiclewheels snagging on the posts.



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e) The posts eventually yield and the rail

tears away from the bolt heads and restrains

the vehicle by tension.

The guardrail deflection lessens the rate ofchange of momentum of the impacting vehicle

and its occupants which can significantly

reduce vehicle damage and personal injury.

Sometimes, however, a stiffer barrier is

required, capable of giving more restraint to

heavy vehicles, or of limiting deflections on

impact; narrow medians on roads with

restricted cross-section. Corrugated sheet steel

beam guardrail may not be appropriate in such

situations.

3.2 NEW JERSEY CONCRETEBARRIER

The New Jersey barrier is considered to be a

'rigid' barrier as it is designed not to deflect

significantly under impact. A vehicle, hitting

the barrier at a low angle, first strikes the lower

sloping face of the barrier, rides up the slope,

and then is redirected along the travelled way

by the upper, nearly vertical, face. Energy is

absorbed on lifting the vehicle and bydeformation of the vehicle's suspension and

body.

Generally, it is desirable that this type of

barrier be located within about 1.0 to 3.0

meters of the adjacent edge of the traffic lane to

minimise the potential for large angle impacts,

while maintaining adequate lateral clearance

for normal traffic movements.

This barrier is narrower than a double-sidedcorrugated sheet steel beam guardrail, and has

the ability to withstand more severe impacts,

making it better suited for use in confined situ-

ations. Sometimes this barrier can be made

wider to support lighting posts, or other

engineering services.

This barrier can be constructed either in-situ or

of 6 metres long precast units and sits on the

base course material and restrained by a

minimum of 50 mm thick layer of wearing

course on both sides.

GLOSSARY

Barrier -

See 'Traffic barrier'

Crash cushion.

A safety barrier or terminal designed primarily

to resist and-on impacts,and consisting

generally of a partly confined bundle of

expendable,crushable,elements to absorb the

energy of an impacting vehicle.

Errant vehicle.

A vehicle that enters a roadside during a

generally unplanned manoeuvre, e.g. because

the driver lost control or swerved to avoid

another vehicle or obstacle.

Guardrail.A semi-rigid safety barrier, generally

consistingof steel rails supported on steel or

timber posts, designed primarily to resist lateral

impacts.

Hazard.

See Roadside hazard.

Length of need.

The total length of a longitudinal barrier

needed to prevent errant vehicles colliding"

with roadside hazards. The length is measured

'parallel' to the road and should allow for both

directions of travel.

Rigid barrier.

A safety barrier, generally constructed of con-

crete, which undergoes no perceptible

deflection or deformation under normal

vehicular impact.


Documents

Arahan Teknik (Jalan) 1-85 - Manual on Design Guidelines of Longitudinal Traffic Barrier