Toolbox for Work Zone Barriers

 

Barriers

The purposes of this section are to provide quick access to basic information about barriers used by Caltrans and to allow quick, rough comparisons of different kinds of barriers in support of Work Zone planning. To access available information and make comparisons among various devices, please use the Overview, Dimensions, Guidelines, Notes, All Information, Compare, and Sort links in the navigation area to the left of this page.

The primary sources of this information are NCHRP 350 approval letters.

The next steps in developing these pages are to improve the information presented and to review the presentation format. This will require practitioner input.

Following is a statement of basic assumptions about barriers used in developing these pages. The source of this information has been derived from other states. This information is believed to be appropriate for use in California, but it will almost certainly need modifications and/or clarifications both for accuracy and to reflect practitioner perspectives.

Basic Definition of a Barrier

Barrier / Traffic Barrier — A device which provides a physical barrier through which a vehicle would not normally pass. Permanant barriers are designed to minimize harm to vehicle occupants; temporary barriers need also to protect construction and maintenance workers. Geometric and operational restrictions in work zones frequently preclude the use of the same design standards for barriers and terminals that apply to permanent systems.

Traffic barriers are designed so that a vehicle hitting the barrier is steered back onto the road. Concrete barriers redirect errant vehicles due to their shape; other barriers often redirect by designing supports so that they break off on impact, allowing the barrier to deform and push the vehicle back on track. All barriers either move on impact, as with concrete barriers, or deform, as with metal barriers. Accordingly, if used to protect work zones, all require a buffer zone between their placement and the area they are intended to protect.

Basic Types of Barriers

Rigid Traffic Barrier — A longitudinal barrier which does not deflect upon impact and dissipates a negligible amount of the vehicle's impact energy. An example is the FShape concrete barrier.

SemiRigid Traffic Barrier — A longitudinal barrier, ranging from practically rigid to quite flexible, which will dissipate some of the impact energy through yielding of the rail and post elements and in some cases the soil. An example is the Wbeam barrier.

Flexible Traffic Barrier — A longitudinal barrier that offers relatively large lateral deflection, with supports which serve primarily to hold the barrier in place. Examples are cable and weak post traffic barrier.

Median Traffic Barrier, Roadside Traffic Barrier — A "roadside barrier" should be crashworth for hits from one side. A "median barrier" should be crashworthy for hits from either side. Either a "roadside barrier" or a "median barrier" may be used in a median.

Public Barrier — Most barriers are intended to provide protection from errant vehicles. Some barriers may protect people not in a vehicle, such as near a park or playground.

Hazard Zone Barrier — Most barriers are intended to provide protection from errant vehicles. Some barriers may protect hazardous facilities, such as an oil tank or chemical plant, that could cause a disaster if struck.

Multiple Hits Barrier — Most barriers provide protection from unusual incidents. In some cases, there may be a reasonable chance of multiple hits in a short period of time. For example, a cable barrier may not be appropriate on a mountain curve in icy weather, for a first vehicle might down the cable leaving next cars vunerable to harm. In general, the frequency of impacts is roughly related to the amount of traffic, so more durable barriers are needed as the frequency of impacts increase. This means that cable barriers are only appropriate for low traffic volumes; metal barriers are appropriate only for medium volumes; and at high volumes concrete barriers are preferred to minimize hazards both to vehicles and repair crews that would otherwise be needed to maintain metal or cable barriers.

K-Rail — A temporary, concrete barrier designed to protect by raising the front of an errant vehicle up onto a lower sloped face and then redirecting the vehicle. For low-impact angles, the redirection minimizes sheet metal damage. For high-speed and high-angle impacts, there is a danger of launching or rolling over the errant vehicle.

It is only necessary to lift the vehicle enough to reduce the friction between the tires and the paved surface. This aides in banking and redirecting the vehicle. If the vehicle is lifted too high into the air, it may yaw, pitch, or roll, which can cause the vehicle to roll over when the wheels come in contact with the ground again. Preferably, concrete safety shape barriers should be adjacent to a paved surface so that the wheels cannot dig into the soil and cause the vehicle to overturn.

For high impact angles, the K-Rail is actually a multistage barrier. The front bumper impacts the upper sloped face and slides upwards. This interaction initiates lifting of the vehicle. If the bumper is relatively weak, the front end starts to crush before any uplift occurs. Then, as the vehicle becomes more nearly parallel with the barrier, the wheel contacts the lower sloped face. Most of the additional lift of the vehicle is caused by the lower sloped face compressing the front suspension. However, wheel side-scrubbing forces provide some additional lift, particularly if the barrier face is rough. Therefore, exposed aggregate and other rough surface finishes should be avoided. Modern vehicles have relatively short distances between the bumper and the wheel; as a result, bumper contact is followed almost immediately by wheel contact.

Concrete barriers require a flush median and are generally used on high volume roadways with narrow width medians. They require a crash cushion or sloped end terminal. They also require channelizing devices and pavement marking in front of barrier tapers, a lateral buffer space between the edge of the driving lane and the barrier, and a longitudinal buffer space between the channelizer taper and the barrier taper. They may require a glare screen to prevent headlight glare.

Other states call this type of barrier a "Jersey" barrier, because they were developed in New Jersey.

W-Beam, F-Shape, Vertical Parapets, Constant-Slope — These are variants on a K-Rail. Vertical parapets absorb energy by crushing a vehicle rather than lifting it. Constant slope barriers in California have an angle of 9.1 degrees and do not need a thick asphalt overlay and are used on curved ramps and for resurfacing operations.

Portable Concrete Barriers — Portable concrete barriers are often used in construction work zones. To mimimize lateral deflection, longer segments are used along with pins or loops with high bending moments to join the segments.

Low-Profile Concrete Barriers — Low-profile barriers have a negative slope that holds a vehicle down by pushing on the bumper. They are used in work zones and intersections where driver sight distance would be blocked by a higher parrier.

Metal Barriers — Newer versions of metal barriers are highly portable and quick to deploy. They are less expensive than portable concrete barriers while offering lift-deflection and low exit angles. However, although they meet NCHRP level 4 tests, deployment has been relatively slow, in part due to the costs of replacing damaged units. They are not suitable for sloped terrain.

Cable Barriers — Many other states are rapidly deploying modified cable barriers due to their low cost, suitability for sloped terrain, and "forgiveness" to errant motorists. They are often used to retrofit highways originally build without a median barrier. In California, early designs and installations when combined with median vegetation, such as oleanders, are being replaced as quickly as possible due to their propensity to launch errant vehicles into the air and over onto on-coming traffic. Cable barriers have high maintenance costs and are not suitable for high-traffic or high-hit frequency applications.

Balsi Beam — The Balsi Beam is a highly mobile, temporary barrier developed by Caltrans to provide protection for short-term, relatively small work zones. A Balsi Beam is a specially designed trailor that can be driven to a work site at normal speeds without the need of special permits. Once at the site, it can be deployed from within the tractor truck in about 10 minutes. The Balsi Beam consists of high-strength steel box sections that are able to extend to a total of about 30 feet. Future versions will also include power and tools for commonly performed tasks. Currently, the Balsi Beam is believed to be suitable for bridge maintenance, irrigation repair, culvert repairs, median repairs, and rolling zone tasks. Where longer spans of protection are required, additional attenuator trucks are used to provide a "shadow" area.

 

 

 

 

 

 

The purpose of this site is to help plan Work Zones.

The basic idea is to present basic information about barriers and attentuators in a format that will allow quick comparisons in support of Work Zone planning. The primary sources of the information presented here are NCHRP 350 approval letters. The next steps in developing these pages are to improve the information presented and to review the presentation format. This will require practitioner input.

This site is hosted by AHMCT at UC Davis. The content of these pages reflects work in progress. Please send comments to ahmctWeb (at) ucdavis.edu.