W-Beam guardrail has two "humps" and one "valley". Humps are rises that face towards the traffic (convex surface), while the valley (concave surface) points away from the traffic lane.  The other type is Thrie-beam, or T-Beam.  This has three humps and two valleys.  (Click Here to View Guardrail Specifications)

There are two basic thicknesses of the metal used in guardrail, as well. 12 guage is the most common, roughly 1/8" thick. 10 guage guardrail, on the other hand, is about 3/16" thick. This added thickness creates a stronger guardrail, but also greatly increases it's weight.

Guardrail can also come shop-bent, or curved for certain roadside applications. The two curves are concave and convex, which are shown in the diagram below:

A fourth characteristic of guardrail is the length of each piece, or panel.  This size ranges from 25' 0" down to 6' 3".  This length is not the overall length of the panel, however, it represents the distance between the outermost post holes. Thus a 12' 6" x 6' 3" panel is actually 13' 6 1/2" long, with post holes centered at 6' 3" apart. These hole distances can vary, as well, from being punched only at the ends, every 6' 3" or 3' 1 1/2", even at custom dimensions. This brings us into the next topic...

There are a number of fabrication techniques, but we present here a very generic overview of the forming and galvanization of guardrail items. First a large roll, or coil, of flat metal sheet is slit into the necessary width. W-Beam is cut into 18" wide pieces, while T-Beam is 25" wide.  This is done by unrolling the coil and sending the flat sheet through a slitter.  This flat rectangular piece is punched by a large press with bits placed at the appropriate dimensions.  The punched sheet is slid down the line into the roll-former, where the "humps" and "valleys" are made. The rail shape is then cut at the appropriate length. This rolled panel of black guardrail, as it is called due to its color, is then sent to be galvanized.  The galvanizing procedure includes dipping the panels into prewash solutions of water, acid, and finally into zinc, which is kept molten at a temperature of over 800 degrees C.  This turns the panel into a shiny silver color, as it is now coated in zinc.   Below you can see the standard hot-dip galvanizing procedure (Image source American Galvanizers Association)

Galvanization helps prevents rusting from occuring. Rust occurs when a metal is oxidized (loses electrons) to the environment.  Zinc is more willing to lose its electrons than steel, so it oxidizes first, leaving "healthy" steel underneath for a much longer period of time. Guardrail beams may be galvanized before or after fabrication. Uncoated edges resulting from shearing or punching will not be considered objectionable. It is important that any item intended to perform as tested throughout its lifespan be properly protected against environmental corrosion. Thus, we recommend specifying "AASHTO M-180 Type II" coating for all guardrail applications. Click here to read about different coating thicknesses and application methods. (pdf)

Guardrail can be put along any roadside - bridges, medians, embankments - any area where there is a risk of a vehicle going off the roadway
 
Typical installation consists of a panel of guardrail attached to posts that have been pounded into the ground.  These posts can be of varying lengths depending upon the needs of the installation.  The posts are spaced at intervals corresponding to the number of punches in the guardrail panel.  Therefore, a 12' 6" piece of guardrail at 6' 3" post spacing requires 3 posts.

Between the post and rail are offset blocks.  These are 6" x 8" x 14" (typical) blocks used to bring the center line of the guardrail-to-post unit away from the post.  By offsetting this line, an impact against the rail at that point is buffered more effectively.

Posts and blocks can be either made out of wood or steel. Steel is strong and consistent, however the federal government decided not to allow steel blocks after October 1998. Wood blocks are effective blockouts, however there are some environmental issues that degrade performance over the serviceable lifespan. Recycled plastic blocks are relatively new to the safety industry and provide an environmentally friendly, durable, and consistent product. We have developed a plastic block, made from post consumer and post industrial plastics.  The fabrication of this block is controlled, unlike wood, providing better quality, as well as the plastic composition, which provides for a block that will last for around 30 years.  To see more about this product, CLICK HERE.  

Posts, as stated above, can also be made of wood or metal.  Metal posts are used in the length, or stretches of guardrail.  At the ends however, wood posts are usually used.  End treatments are an important part of guardrail, and will be covered next.   

Guardrail is an effective barrier, as it can contain and redirect a vehicle. However, it is an extremely stiff object to impact directly at the end. Wherever guardrail stops, there needs to be something that not only provides sufficient anchorage for the rest of the "run" - but is also safe for vehicles impacting end-on. 

This anchor can range from a simple buffer when the guardrail is installed in a low-speed application, such as a parking facility or warehouse, or can be a full scale terminal system for high-speed roadways. The purpose of the terminal is to anchor the barrier, yet also protect anyone that happens to hit the guardrail at this point.  If no end treatment were used, the stiff panel of guardrail could either enter the vehicle injuring the occupants and/or cause the vehicle to roll-over also injuring anyone inside.  Therefore, a large focus of designers and researchers relates to providing more effective and safer end terminals.  There are two basic types of terminals:First is the "flared" system, where the guardrail is offset away from the roadway. This is like the FLEAT and REGENT systems. In a "Flared Gating" terminal, the momentum of the vehicle is decreased as the posts are broken and the rail is bent. The "gating" term means that vehicles impacting the end will "gate" through and into a designed clear zone behind and downstream of the barrier. In a "Flared Energy Absorbing" terminal, such as the FLEAT, an impact head kinks the rail in addition to the post breakage to slow the vehicle down. Energy absorbing terminals will typically stop a vehicle before the clear zone.

Secondly, there is a "tangential" end terminal. These systems lay parallel to the road, and are all considered "Energy Absorbing" in that they bring the vehicle to a controlled stop by absorbing the vehicles momentum. This can be done in several ways.  The BEST, for example, slices the guardrail into strips then curls them out to the side. The SKT, like the FLEAT, kinks the rail instead of cutting it. Another tangential terminal for median applications, is the FLEAT-MT.

For all systems, any impact that is after the the critical impact point (CIP), typically post 3, will redirect the vehicle back into the traffic lane, safely avoiding the hazard. However, NEARLY ALL TERMINALS, if impacted at an angle on the nose, will "gate" the vehicle - regardless of the presence of an impact head. This should be kept in mind when placing a guardrail terminal, such that adequate area is provided for a vehicle to enter after impacting the end of any terminal.