This brief provides general guidelines regarding the selection, installation, and maintenance of W-beam guardrail terminals. In addition, common issues of concern are identified for these elements with generally accepted practices to address these issues. The American Association of State Highway and Transportation Officials (AASHTO) Roadside Design Guide 4th Edition Chapter 8.3 provides additional guidance on terminal design concepts.
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There a re three primary W-beam guardrail end terminal designs in use at present: Burke-in-backslope, non-energy-absorbing, and energy-absorbing. Figure 1 shows th e relative trajectories of a vehicle impacting non-energy-absorbing and energy-absorbing terminals head-on and at high speed (62 mph). The decision to use either an energy-absorbing terminal or a non-energy-absorbing terminal should be based on the likelihood of a near end-on impact and th e nature of the recovery area immediately behind and beyond the terminal.
Figure 1: Vehicle Trajectories by Terminal Type
Non Energy-absorbing – A terminal that does not dissipate a significant amount of kinetic energy in a head-on crash and is a gating system that allows the vehicle to traverse the area behind and parallel to the guardrail. Some key characteristics include:
Energy-absorbing - A terminal that dissipates a significant amount of kinetic energy in a head on crash. Some key characteristics include:
Buried-in-Backslope – A terminal that terminates a W-beam guardrail installation by burying the end in the backslope. Grading is critical for a buried-in-backslope terminal because the terrain leading up to the buried-in-backslope must be traversable and contain no fixed object hazards. If the backs lope is relatively flat, a vehicle can ride up the slope and bypass the terminal. When th is condition exists at a site, the designer must ensure that the hazard remains shielded by assessing the available clear run out distance behind the rail and the barrier length-of-need. Also, there are other "grading" design considerations to follow:
Figure 2 is a suggested flowchart that can be used by a designer to select the most appropriate terminal for a specific location. It's important to note that the starting point is to verify that a barrier is actually needed. If so, t hen the correct length of need should be confirmed. If a total length of barrier is less than about 150 feet, an energy-absorbing terminal should be selected for the reason previously stated. When an appropriate backslope exists near the end of the barrier. the buried-in-backslope terminal should be considered. When no suitable backslope exists, either a non-energy-absorbing or energy-absorbing may be appropriate.
Figure 2: Terminal Selection Flow Chart
Terminal Selection: Common Issues of Concern and Current Generally Accepted Practices.
Common Issue of Concern Current General Accepted PracticeSide-by-side Terminals:
If the ends of two barriers are within seven feet of
each other, they should be combined and
terminated as a median barrier or a bullnose
design should be considered. This should
reduce the potential for
the vehicle to reach the
hazard or obstruction.
Bullnose Guardrail System for Median Applications
Common Issue of Concern Current General Accepted PracticeCurbs
The presence of a curb
can introduce instability
as the vehicle hits the
terminal and should be
avoided or minimized if
possible. In addition,
added rub rails or other
items not part of the
original design might
affect the performance of
the terminal and should
not be added.
Curb-Guardrail
Combinations
Refer to NCHRP Report 537 Recommended Guidelines avoided or minimized if for Curb and Curb-Barrier Installations. There are tested curb and guardrail and end terminal combinations covered.
Common Issue of Concern Current General Accepted Practice
Guardrail insufficient length to shield
the hazard.
Inadequate Length of Need (LON)
Extending the barrier or a buried-in-backslope to appropriately shield the hazard.
Common Issue of Concern Current General Accepted PracticeTerminal flare rate can be excessive on a flared terminal.
A gating terminal may be considered here because of existing run out area.
Terminal Installation:
When installing the terminal, the manufacturer's installation manual should be followed closely. Additionally, grading in the area of the terminal is important because terminals are tested for crashworthiness on flat and unobstructed terrain. As shown in Figure 3, there are three grading locations of concern around barrier terminals:
All of these areas should be carefully considered during the design phase or a project. Engineered earthwork and specification of a platform* should also be considered to achieve successful terminal performance. In addition, necessary earthwork should be completed prior to the installation of the safety feature.
*A platform is the required grading for both adjacent & advance areas to acceptable criteria per the Roadside Design Guide.
Figure 3: Terminal Grading Areas
The "grading platform" in the photo has a drop-off that creates a significantly greater hazard than previously existed.
Advance Area:
The "advance area" consists of the
space traversed by an errant
motorist before the terminal is
struck. If a terminal "platform" is
constructed, it must be smoothly
blended into the existing roadside
embankment so a motorist has an
opportunity to return to the
roadway without striking the
terminal or losing control of the
vehicle by dropping off the edge of
a steep platform before impact.
Before selecting a grading platform, the designer should first consider the following:
Terminal Installation: Common Issues of Concern and Current Generally Accepted Practice
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Common Issue of Concern Current General Accepted PracticeAdjacent Area:When the area immediately behind a terminal (i.e., the "adjacent area") is steep or non-traversable, at vehicle can overturn after breaking through the terminal. A minimum traversable area behind the terminal is an essential part good barrier design.
A field check should be made to determine if a run-out area exists. A run-out area requires the following:Common Issue of Concern Current General Accepted Practice
Adjacent Area
Although the terminal shown here
is an energy-absorbing design, any
impact into th e end will most likely
end with a vehicle striking the
utility pole.
Common Issue of Concern Current General Accepted Practice
Advance & Adjacent Area:
The adjacent grading is the area
around the 1st post and is critical to
help develop the anchor strength
and ensure that the post stubs and
strut do not stick out more than 4"
above the ground. The terminal in
the photo is neither crashworthy
nor a good anchor. When a ground strut anchor is used, it is normally
at 'ground level'.
To achieve successful terminal performance, the designer should consider including engineered earthwork as a key component on the final construction plan for the terminal installation.
Common Issue of Concern Current General Accepted Practice
The installation shown here is an energy-absorbing design, so vehicle impacting head-on would likely be stopped safely before reaching the concrete barrier. However, any angled hits at the end would result in significant intrusion behind the rail and into the rock outcropping.
The guardrail should have been extended to shield the secondary hazard (i.e., the rock wall). A good field check to determine if shielding secondary hazards may be worthwhile is to note whether or not the area immediately upstream from the terminal would warrant shielding in the absence of a primary hazard (i.e., end of the bridge barrier).
The Roadside Design Guide identifies maintenance factors grouped into three categories: (1) routine maintenance, (2) Crash Maintenance, and (3) material and storage requirements. Common examples of routine maintenance and material and storage requirements are listed below. Routine maintenance includes inspecting roadside devices at regular intervals to determine the condition of the device and required repair needed for the device. Proper materials and storage of them ensures routine maintenance is carried out appropriately using proper components when completing repairs.
Extruder Heads
Routine Maintenance: Care is needed when installing and
repairing extruder head type terminals to ensure that the head is
properly attached to the rail. This photo shows a case where the
terminal of the head is not properly attached to the rail and will,
therefore, not perform properly, and should be repaired
immediately. This situation can also occur if the barrier is
impacted upstream with sufficient force and deflection that the
rail pulls out of the head.
Cable attachments
Routine Maintenance: The cable is critical in providing tensile
strength in the rail. For some designs, the cable must be able to
detach from the rail during an impact. The photo shows a location
where the cable is not attached and where the bolts holding the
cable were installed backwards.
Cable anchorage
Routine Maintenance: The photo shows a location where the
shoulder bolts holding the cable were installed backwards and a
metallic butterfly reflector was placed within the end treatment
performance area, which may adversely affect the separation of
rail from the post Attachments to the guardrail within this
performance area should not be made.
Mismatched Parts
Material and Storage Requirements: The photo shows an in-service
installation using components from two different
systems. This is likely due to improper maintenance
decisions being made after an impact. Parts from one
system to another system are not interchangeable unless
specified by the manufacturer.
In efforts to effectively address the highlighted concerns, the following existing resources and noteworthy practices are provided for consideration by State Departments of Transportation and other highway agencies.
Inspector /Maintenance & Designer Mentoring Training
State and local agencies should conduct training at regular intervals for DOT personnel, consultants,
and contractors to ensure the optimal barrier design and installation of new roadside safety devices,
and the inspection and maintenance of existing devices. This noteworthy practice would serve to
eliminate common installation and maintenance errors that adversely affect the intended
performance of the roadside safety device.
Installer Certification
Installer Certification is training for the roadside safety system installers that may be offered at
regular intervals to maintain a specific knowledge base of both existing and new systems. Agencies
that offer this training may also make this a requirement for installation of roadside safety systems in
their jurisdiction. This noteworthy practice also may serve to eliminate common installation errors
that may adversely affect the intended performance of the roadside safety device.
Engineered earthwork design in construction plans
Crash testing for end treatments is performed on flat or near flat terrain. In real world applications,
this type of terrain is fairly rare and some grading is likely needed. Therefore, end treatments may
require individual construction details and cross sections with regard to earthwork analysis. If this
information is not included in the plan, the end treatment may not fit or function as intended when
installed in the field. In some cases, improperly installed end treatments can degrade the strength or
performance of an entire barrier system.
When roadside safety systems such as traffic barriers and terminals are installed exactly as shown on project plans or replaced in-kind after a crash, the end result can be an installation that may not effectively shield the primary hazard, may be too short or too long, may not shield obvious "secondary" hazards in its immediate vicinity, or may not be needed at all. A per-installation review checklist can be used to recognize field adjustments to a design that are needed to ensure an optimal installation.
BACKGROUND
Crashworthy end terminals are installed at the ends of W-beam guardrail to develop the strength of the barrier system and to provide crash protection to occupants of vehicles that impact the end of barrier installations. Full-scale crash tests for end terminals are based on impact performance criteria as provided in the American Association of State Highway and Transportation Officials (AASHTO) Manual for Assessing Safety Hardware (MASH). The Federal Highway Administration (FHWA) procedures require that end terminals installed on federally funded projects must meet MASH or the National Cooperative Highway Research Program (NCHRP) Report 350 crash test criteria for eligible use on the National Highway System (NHS). The AASHTO Roadside Design Guide (RDG) provides guidance on the selection and installation of W-beam guardrail end terminals that are based on systems that comply with MASH or with the earlier NCHRP Report 350 criteria. According to MASH, an “In-Service Performance Evaluation” (ISPE) remains a follow-up to the crash test experiments, which partially assess the efficacy of an end terminal. More thorough and in-depth knowledge of end terminals are essential to the validation of their implementation. NCHRP Report 490: In-Service Performance Evaluation of Traffic Barriers, published in 2003, provides insight to procedures and guidelines for performing in-service evaluations that include the in-service performance evaluation of end terminals. Although the roadside safety community has agreed about the importance of in-service evaluations and procedures in existence, few in-service performance evaluations have been completed. Simply relying on full-scale crash tests to assess the long-term performance of end terminals may overlook important aspects of crash safety as well as other aspects of device performance. Crash tests that are conducted under specific standardized and idealized conditions do not consider the full range of vehicle impacts that are expected to occur in the real world—including such factors as vehicle weight, type, and configuration, impact speed and angle, and vehicle orientation. They do not consider the wide range of environmental conditions to which devices are exposed, such as snow and ice, saturated soil conditions, frozen ground, and extreme temperature ranges. The effects of these conditions are generally unknown, although it is not unreasonable to expect that they may affect device performance. Other factors, such as end terminal maintenance, repair of minor impacts, and variations in device installations can affect their true real world performance. End terminals that perform acceptably during full-scale crash tests may not provide the level of protection expected over the range of these conditions that may be encountered over the life of the end terminal. Evaluating end terminal performance in the field over the life of the device is the most effective way to judge the long-term effectiveness of the hardware. Currently, the majority of states are raising their guardrail heights to the new 31-inch guardrail height requirements while using W-beam end terminals to protect the ends of new installations. Relatively small numbers of newer devices have been successfully crash tested to MASH crash test criteria. While many more end terminals have been successfully crash tested to NCHRP Report 350 crash test criteria, it is expected that not all of these earlier devices will meet the newer MASH criteria. The limited highway resources of many states make it imperative that safety features installed on new construction projects or upgraded on existing highway projects, must be accomplished in the most cost-effective manner possible. Projects funded to replace devices that perform acceptably, while leaving poor-performing devices in service at other locations, is not appropriate. Research is needed to more thoroughly understand the real world performance of W-beam guardrail end terminals through the evaluation of the in-service safety performance of the most common designs currently in service on the nation’s highway system.
OBJECTIVE
The objective of this research is to evaluate the in-service crash performance of a select number of common W-beam guardrail end terminals currently installed throughout the United States. For each common end terminal, the evaluation should include:
STATUS: Terminated; transferred under TRB Studies and Special Programs Division; completed as TRB Special Report 323.
Terminated; transferred under TRB Studies and Special Programs Division; completed as TRB Special Report 323.
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