U.S. Department of Transportation
Federal Highway Administration
TECHNICAL ADVISORY
GUARDRAIL TRANSITIONS
T 5040.34
June 8, 1993
Par.
- Purpose
- Background
- Summary
- Recommendations
- 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.
- BACKGROUND
- 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.
- SUMMARY
- 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.
- RECOMMENDATIONS
- 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.
signed by
Thomas O. Willett, Director
Office of Engineering
signed by
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.)
Attachment 1, Figure 6A
Attachment 1, Figure 6B
Attachment 2, Figure 7A
Attachment 2, Figure 7B
Attachment 2, Figure 7C
Attachment 3, Figure 8A
Attachment 3, Figure 8B
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