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

Arahan Teknik (Jalan) 1/85

Manual On Design Guideline Of Longitudinal Traffic Barrier

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Roads Branch Public Works Department Malaysia Jalan Sultan Salahuddin 50582 Kuala Lumpur

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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PART I - GENERAL

1.2 DEFINITION

1.1 INTRODUCTION

i. Traffic barriers are highway appurtenances that provide a relative degree of protection to vehicle occupants from hazardous roadside features and from errant vehicles encroaching across a median.

There is a need to provide highway design engineers 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.

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.)

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 traffic barrier 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 facilitate the provision of appropriate traffic barriers to ensure roadsides incorporate a consistent and economic 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.

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1.3

FUNCTION OF HIGHWAY TRAFFIC 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 to prevent "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

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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 are considered impracticable. 1.4 TYPES OF HIGHWAY TRAFFIC BARRIER 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.

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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 shown that 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.

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.

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PART 2 . - DESIGN GUIDELINES

2.2.1 Determination of need

2.1 DESIGN PROCEDURE

Traffic barriers should be considered under the following conditions:

For any new barrier installation, the recommended design procedure is as follows:

i. Roadways on high embankment and embankment with steep side slopes,

i. Establish "point of need" or "length of need" by warranting consideration as in sub-section 2.2.

ii. On highways with roadside obstacles and hazards such as structures and appurtenances,

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. Divided highways with narrow medians, carrying large volume of traffic,

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 to a 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 delineated 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. Cawangan Jalan, Ibu Pejabat JKR, K.L

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 characteristics 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 as shown 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),

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(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 consider able 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.

(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. Cawangan Jalan, Ibu Pejabat JKR, K.L

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2.3 \

(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 barriers are 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

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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 structure cross-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 be equal to or greater than this deflection. Summaries of basic characteristics of guardrails, bridge rails and median barriers

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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 median barriers 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.

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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 medians can 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.

Figure 2.6 shows the clearance required to minimise any potential for vehicles to vault the guardrail after hitting the kerb. Where barriers are 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, 2.3.3 Installation and Maintenance Costs (2) availability and cost of materials, Although cost of installation generally increases as system rigidity increases, cost of repair 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:

(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

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INSTALLATION LENGTH

Installation should be extended upstream from the warranted limits to prevent vehicle access behind the protective system. It is not necessary to 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.

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A method to establish the length-of-need of the installation is based on a 100 metres encroachment distance. The length-of-need is calculated 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:

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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 rail with 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.

(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 be extended 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

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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, as shown 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.

2.6

STANDARD GUARDRAIL SYSTEMS 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.

On approaches to structures, the guardrail must 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.

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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 be used. 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.

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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 of change 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 CONCRETE BARRIER

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 by deformation 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-sided corrugated sheet steel beam guardrail, and has the ability to withstand more severe impacts, making it better suited for use in confined situations. Sometimes this barrier can be made wider to support lighting posts, or other engineering services.

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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 concrete, which undergoes no perceptible deflection or deformation under normal vehicular impact.

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