Selection, Installation and Maintenance of W-Beam Guardrail End Terminals

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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Terminal Selection:

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

Characteristics of the Different Terminal Types:

1. 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:

   • Does not significantly reduce vehicle speed in a near head-on hit
   • Run out distance can exceed 150 feet
   • Best specified when t here is a long, clear, traversable area behind and parallel to the guard rail installation, such as often found in a flat freeway median

2. Energy-absorbing - A terminal that dissipates a significant amount of kinetic energy in a head on crash. Some key characteristics include:

   • Barrier installations less than 150 feet in advance of any shielded object must be energy absorbing
   • Energy-absorbing ' terminals have been shown to stop an impacting pick-up truck in about 50 feet when struck head-on
   • Best suited to locations where traversable area behind barrier is limited; or, contains fixed object hazards.

3. 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:

   • The backslope itself must be sufficiently steep to prevent a vehicle from climbing over the rail
   • The barrier flare rate must be appropriate for the roadway design speed and traffic volume
   • The height of the rail must remain constant in relation to the roadway edge at least until the guardrail crosses the ditch flow-line
   • W-bearn rub rail must be added if the distance from the bottom of the primary rail and the ground exceeds about 17 inches.

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 Practice

Side-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 Practice

Curbs
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 Practice

Terminal 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:

1. Advance area
2. Adjacent area
3. Run-out area

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

Common Issue of Concern Current General Accepted Practice
The "grading platform" in the photo has a drop-off that creates a significantly greater hazard than previously existed.

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:

1. extending the barrier a short distance to a flatter location.
2. specifying a non-flared end treatment.

Terminal Installation: Common Issues of Concern and Current Generally Accepted Practice

Common Issue of Concern Current General Accepted Practice

Adjacent 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:

1. Minimum traversable area of 20-feet wide & 75-fect long. This distance is based on the final resting position typically found for a small car during terminal is an essential part of crash testing.
2. A heavier vehicle at a higher speed will typically travel a greater distance behind and beyond the terminal.

 

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.

In many situations, it simply may not be practical to shield every hazard. This barrier was installed primarily to shield the slope along the curve and is effective for that purpose, but it should have been lengthened if practical,to shield the pole also. An energy-absorbing terminal can slow a vehicle in line with the rail and is preferable here rather than a non-energy absorbing design.

 

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).

Terminal Maintenance:

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.

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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.

• Use of Roadside Safety Systems Pre-Installation Field Review Checklist
  

  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.

The Complete Guide to Guardrail

Guardrail, also known as highway guardrail or sometimes guide rail is, as the name suggests, a type of railing that is installed on roads and highways. While it's not technically a fence, it is often installed by fence companies.

Let's take a closer look at guardrail, including what it is, how it's made, how it's installed and more.

What Is Guardrail?

Guardrail is a type of barrier that is intended to keep vehicles on highways or roads. It's often installed on sharp corners or along elevated roadways, where vehicles leaving the road would be very dangerous for the occupants.

Guardrail is designed to slow vehicles down and to redirect them to a safer trajectory whenever possible. So, while a vehicle that does run into highway guardrail will definitely show signs of damage, if the guardrail does its job, it won't leave the road but rather be safely redirected until the driver can stop.

Guardrail is also used to protect buildings and other structures on roads, highways and even on industrial sites. In this case, the sturdy guardrail structure can be used to prevent vehicle access as well as provide roadway safety.

Types of Guardrail

Different parts of the world may have slightly different types of guardrails or guide rail, but there are three main types of guardrails in common use, especially in North America. These are:

• W beam guardrail
• Thrie beam guardrail
• Cable rail

W beam guardrail and thrie beam guardrail are built using roll-formed steel railing sections, while (as the name suggests), cable rail does a similar thing but using high tensile cables instead of solid steel sections.

What Is Guardrail Made From?

W beam and thrie beam guardrail are manufactured from roll-formed, hot dip galvanized mild steel.

Cable rail systems are made from high-tensile steel cables. This is usually also galvanized to protect it against corrosion.

How Is Guardrail Installed?

W beam and thrie beam guardrail systems are designed to be installed on either steel or wooden posts, depending on the site and the specification required. Some also have spacer blocks, which keep the rail section slightly away from the post itself.

Whether you choose wood or steel posts for your guardrail, the posts themselves are very strong and are carefully installed in engineer-designed footings to ensure that the guardrail is as strong and durable as possible.

Cable rail, on the other hand, is usually installed with specially made steel posts, which include a variety of fittings to keep the cables perfectly spaced and taut.

How Are Guardrail Sections Attached to Posts?

Steel guardrail sections are overlapped slightly to match up predrilled and punched holes in each piece. Then each piece is bolted in place using high-quality fasteners. Both adjacent guardrail sections are caught by this hardware, so it creates a single continuous section of guardrail.

Can Guardrail Be Installed on Curves?

Steel w beam and thrie beam guardrail can be custom manufactured to follow curves, but the metal sections themselves don't bend ' so you can't bend them on site!

Cable rail systems are usually more forgiving and can usually be installed to follow curves on-site without too much trouble.

How Is Guardrail Ended?

W beam and thrie beam guardrail systems have purpose-manufactured end sections that are used to finish off the ends of each run of guardrail. These are usually rolled back on themselves to prevent sharp edges that could potentially be dangerous.

Other end designs for guardrail and cable rail are available, though, so the best choice is often to provide the manufacturer with a site plan and have them recommend the best and safest end options for your guardrail project.

Is Guardrail Easy to Install?

Guardrail is a fairly simple type of barrier product, and once you get the hang of the installation method, it's not very difficult to figure out. A bit like building with blocks!

However, guardrail is very heavy, which does mean you will need a strong fence crew to lift and position the posts and pieces. It also requires fairly hefty footings, which means you will need digging equipment, and because everything is predrilled for assembly, it does also require some precision with post placement.

Can You Repair Guardrail?

It is possible to replace some damaged sections of guardrail after a vehicle has collided with it; however, you will usually need to have the guardrail inspected by a specialist to determine which parts need to be replaced.

Vehicular damage to guardrail is not always easily visible, but even invisible cracks in posts and other damage could make the system fail the next time it needs to protect a vehicle.

Who Installs Guardrail?

Some fence companies do install highway guardrail and cable rail systems, but usually commercial fence and industrial fence contractors.

Some roadbuilders and civil engineering contractors might also do their own guardrail installation.

If you're looking for a company to install guardrail for you, be sure to look for one who has experience and references. This is a very important safety feature, and it's best left to the pros!

For more information, please visit Galvanized Highway W Beam Guardrail.