U.S. Department of Transportation
Federal Highway Administration
TECHNICAL ADVISORY
T 5040.32
April 13, 1992
Par.
(1) Figure 1: Curved W-Beam Guardrail Installation for an 8½-foot radius.
(2) Figure 2: Curved W-Beam Guardrail Installation for a 35-foot radius.
(3) Figure 3a: Special Anchor Details.
(4) Figure 3b: Special Anchor Details.
(5) Figure 4: Yuma County, AZ, Curved W-Beam Guardrail Installation for an 8½-foot radius.
(1) Breakaway CRT posts are used within the curved "nose" of the guardrail installation. Wood blockouts are not used on the CRT posts. The W-beam rail in the curved area is attached directly to the CRT post with a button-head bolt which has no washer. This is done to have the posts break away in the curved nose area and thus separate from the rail. This minimizes rotation of the rail during impact and minimizes the likelihood that a vehicle will vault over the guardrail upon impact.
(2) For the 8½-foot radius layout (Figure 1), the guardrail is not bolted to the one CRT post at the center of the curved nose area. This allows the center post to easily separate from the guardrail upon impact, and facilitates guardrail deflection without having this bolt ripping or snagging the W-beam rail section.
(3) A flat approach to the curved guardrail installation is necessary in order to ensure proper performance of the system. The slope in front of the installation should not exceed 15:1. If the installation is on a superelevated section, analysis should be performed in order to evaluate the potential for vaulting of an errant vehicle.
(4) The embankment slope should break at least 2 feet behind the post (so that the post will have adequate bearing strength when hit). It is desirable that the embankment slopes behind the guardrail not be steeper than 2:1. Successful crash tests were done on installations with 2:1 slopes behind the guardrail.
(5) Considerable deflection of the W-beam guardrail can be expected with higher speed impacts on the curved guardrail portion of the installation. Therefore, the area behind the curved portion of the guardrail, shown as the cross-hatched areas on Figures 1, 2 and 4, must be kept free of fixedobjects.
(1) When used in close proximity to a bridge with a rigid bridge rail, these design layouts require an adequate space between the curved guardrail installation and the bridge end (approximately 25 feet) to place a crashworthy W-beam transition from the W-beam guardrail to a rigid bridge rail.
(2) Since the special end anchor shown in Figures 3A and 3B has not been crash tested as a guardrail terminal, its use should be limited to low-speed, low-volume facilities with a stop condition such as intersecting driveways or service-type roadways. For most intersecting public highways, the curved guardrail installation should be either terminated along the intersecting roadway with an acceptable terminal system, or connected to an existing guardrail system.
(3) The special end anchor system was developed for use when it is necessary to end the guardrail system immediately after the curved section. This end anchor uses many components from the breakaway cable terminal design. It also includes another cable to connect the steel foundation of the next-to-last post to the end post anchoring cable system. One novel feature incorporated is a pivoting pipe section which isplaced over the end post and improves rail performance by allowing it to swivel as it is deflected by a car. This special end anchor is not a crashworthy terminal for high speed highways. Therefore, as stated previously, its use should be limited to driveways or service roadways.
(4) In the high speed crash tests, some heavy debris was observed flying about in the area behind the impact. Judgment must be used when installing these sections where people are likely to be present in the area behind the curved section.
(1) It is important to note that the Yuma County design shown in Figure 4 was successfully crash tested at 50 mph. Radii larger than 8½ feet should not be used without further testing.
(2) All of the attached designs are based on an intersection angle of 90 degrees. If field conditions vary excessively from 90 degrees, it will be necessary to specially detail a curved guardrail section so that the curved rails will fit the intersection geometry, and that only full sections of W-beam rail will be shop bent for installation.
/s/
Thomas O. Willett, Director
Office of Engineering
/s/
R. Clarke Bennett, Director
Office of Highway Safety
Attachments
[SEE PRINTED COPY OF TA FOR ATTACHMENTS - FIGURES 1, 2, 3a, 3b AND 4]