June 8, 1993
U.S. Department of Transportation
Federal Highway Administration
- Related Technical Information
PURPOSE To transmit information on W-beam and thrie beam guardrail transition designs for attachment to modified and unmodified concrete safety shape bridge rails, wingwalls or parapets.
Almost all existing W-beam guardrail systems that connect directly to a bridge rail without adequate blockouts or a rub rail near the bridge connection should be considered unsatisfactory because they can result in vehicle snagging, which in turn can contribute to a catastrophic accident. Not only are these hazardous transitions between guardrail and bridge rail common, but indeed many State standards still detail the transition without adequate blockouts, rub rail or other important features.
Crash tests have demonstrated that a stiffer guardrail transition to the bridge rail is necessary. Stiffer transitions can be accomplished through reduced post spacing, larger posts, nested W-beam elements, and other specialized features.
On January 28, 1988, the Federal Highway Administration (FHWA) issued Technical Advisory T 5040.26, Guardrail Transitions. Its purpose was to transmit information onnew and retrofitted transitions from W-beam guardrail to modified concrete safety shape barrier which had been successfully tested. Subsequently, additional transition designs for use with unmodified safety shape barriers have been crash tested and have performed successfully.
This Technical Advisory transmits additional transition designs which have been successfully crash tested with a 4500 lb. car at 60 mph and 25 degrees. Any of these designs can be used either as a retrofit design or for new construction.
These transitions are direct transitions from the New Jersey concrete safety shape to the W-beam guardrail. Except for the design shown in Figure 8A, this can be done without altering the New Jersey shape profile. These transition designs can also be used on concrete barriers with F-shape profiles.
The Precast Curb Transition (Figures 6A and 6B) uses a precast tapered curb, whereas the significant detail(s) of the Wood Post Direct Transition and the Steel Post Direct Transition (Figures 7A and 7B) are a specially fabricated rub rail and steel pipe blockout. The Thrie Beam Transition to Modified Safety Shape (Figures 8A and 8B) features a modified receiver cast into a concrete safety shape barrier, along with modification to the standard safety shape configuration in order to accept the larger thrie beam end shoe. It has a bevel cut into the bottom of the end of the safety shape to minimize wheel snagging. When used with a New Jersey profile, it also has a groove cut into the concrete to receive the lower flange of the thrie beam end connector.
It should be noted that the standard safety shape concrete barrier section does not contain sufficientsteel reinforcement to resist the forces transmitted into it by the transition section during a collision. Use of these retrofit transitions will require one of two adjustments: either the removal and replacement of the last 12-foot section of barrier with a new section containing sufficient reinforcing steel (see Figure 3C of Technical Advisory T 5040.26 for typical bar sizing and layout), or the installation of a suitably reinforced section of barrier at the end of the existing barrier, to which the attachment may be made.
Crash testing of additional guardrail transitions is planned over the next several years. It is anticipated that additional transition designs will become available throughout this period.
Standard drawings and plan sheets should be reviewed for adequacy of designs, and these sheets should be replaced or upgraded as needed. This is one of the best ways to prevent the continued installation of transitions which are known to be inadequate.
Existing transitions should be analyzed, and upgraded if appropriate, as a part of an ongoing construction or rehabilitation project. The decision to upgrade should be related to the quality of the existing transition, traffic speed and volume, and the potential for large-angle impacts.
As with most bridge rail transitions, the crashworthy transitions discussed in this Technical Advisory require that special attention be given to the design of associated drainage features. These features must be compatible with the transition selected for use. Coordination will be required among those responsible for the design of bridge rail, guardrail and drainage. For example, the use of more closely spaced posts in a transition may require that special attention be given to the location of drainage inlets and related pipes.
Untested designs currently in use should be verified as crashworthy by testing in accordance with National Cooperative Highway Research Program (NCHRP) Report 350, Recommended Procedures for the Safe Performance Evaluation of Highway Features, as should any new designs or modifications to existing crashworthy designs.
RELATED TECHNICAL INFORMATION The Wood Post transitions (Figures 6 and 7) allow W-beam guardrail to be directly connected to either the F-shape or New Jersey shape concrete barrier. These transitions may be used with permanent or temporary concrete barriers under the following conditions:
Particular care should be taken in checking the adequacy of existing concrete barriers. The concrete barrier must be strong enough to accommodate the loads which can build up at the end anchor connection. Sufficient strength is required in order to ensure that the concrete section does not fail when impacted, and that the end anchor connection to the concrete is strongenough to accept the tensile forces resulting from vehicle impacts on the W-beam transition section. Concrete and reinforcement similar to that shown on Figure 3C of Technical Advisory T 5040.26 is considered satisfactory.
These transitions have been designed to meet all established performance criteria with a 4500 lb. car impacting at 60 mph and 25 degrees. For those test conditions, the reinforcing must be designed so that the bridge end (or independent end block) will be able to resist a lateral ultimate load of 60 kips. It must also be able to resist an ultimate longitudinal load of 120 kips from the W-beam and rub rail. This information is provided for the use of designers who may wish to produce their own reinforcing details, or have to check the existing bridge ends.
The end of the concrete barrier must not extend vertically above the W-beam connection. Previous crash tests have demonstrated that vertical concrete extensions above 33.5 inches can snag the vehicle hood, forcing it through the windshield and into the passenger compartment. New concrete barriers should therefore betransitioned down from 32 to 27 inches; this will provide a smooth and inclined surface similar to that shown in Figures 6A and 6B.
The W-beam guardrail transitions detailed in this supplement require a rub rail or concrete curb to prevent the wheels of an impacting vehicle from under-riding the barrier and snagging on the toe of the concrete barrier. Past experience with other rub rails and curbs indicates that modifications can result in unsatisfactory results; therefore, only the rub rail and curb detailed in this supplement should be used. The curb shown in Figures 7A and 7B is not freestanding, and requires both a connection to the concrete barrier as well as to the ground.
Transitions may be made to either permanent or temporary concrete safety shapes, provided that the concrete barrier is part of a continuous system which provides sufficient strength and stability in order to prevent rotation (overturning) or significant deflection. Strength and rigidity at the end of questionable concrete barriers may be improved by pinning the barrier to the ground or supporting system, or supporting the back of the barrier (such as backfilling behind the barrier).
The additional drawings, in a format suitable for use on the Intergraph CAD system, are available from the Federal Highway Administration, Office of Highway Safety, HHS-11, 400 Seventh Street, S.W., Washington, D.C. 20590.
Thomas O. Willett, Director
Office of Engineering
R. Clarke Bennett, Director
Office of Highway Safety
Links to the 7 attachments are provided below.
Each thumbnail graphic is a link to the larger technical drawing on a separate page.
(The attachment number is provided above each thumbnail graphic.)
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