U.S. Department of Transportation
Federal Highway Administration
1200 New Jersey Avenue, SE
Washington, DC 20590
202-366-4000
U.S. Department of Transportation
Federal Highway Administration
Par.
PURPOSE. To transmit information pertaining to the design and installation of the transition from an approach W-beam or \thrie beam guardrail system to concrete bridge rail, wingwall or parapet, or other concrete barrier or rigid wall.
BACKGROUND.
Nationwide, most bridge guardrail transitions consist of a semi-rigid W-beam connecting to a rigid bridge rail. (Field reviews show that the concrete safety shape bridge rail is the most common bridge rail being constructed.) These transitions vary in detail with the most common generally conforming to the design principles contained in the 1977 American Association of State Highway and Transportation Officials' "Guide for Selecting, Locating and Designing Traffic Barriers" (Barrier Guide).
Some of these guardrail transitions have been recently crash tested in accordance with the guidelines in "National Cooperative Highway Research Program (NCHRP) Report 230." The initial test results were unsatisfactory, marginal or inconclusive. Further testsof improved designs were satisfactory. Common elements of the failed systems included the following:
(1) A vertical concrete face or the toe of a concrete safety shape barrier which came in contact with the vehicle wheel resulted in snagging.
(2) Inadequate guardrail stiffening (such as standard guardrail posts at 3 foot, 1½-inch spacing) resulted in substantial deflection of guardrail and subsequent snagging or poor redirection of the vehicle.
OBJECTIVE To provide information which will help identify existing substandard guardrail transition designs and to describe new and updated transition systems which have been successfully crash tested and are acceptable for both retrofit and new construction.
Almost all existing W-beam guardrail systems that connect directly to a bridge rail without adequate blockouts or a rubrail 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 the guardrail and bridge rail common, but many State standards still detail the transition without adequate blockouts, rubrail, or other 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, doubled (nested) rail elements and other special features. Both new and retrofitted transitions have been tested and several have proven satisfactory. Examples of crashworthy designs are shown in Figures 1A, 1B, 2, 3A, 3B, 4 and 5 (links to these graphics are provided at the end of this document).
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 and upgraded or replaced, as needed, to prevent future construction of known transition deficiencies.
All the transitions presented in this Technical Advisory are guardrail attachments to vertical or nearly vertical concrete barrier ends. Successful crash tests of guardrail transitions connected directly to the concrete safety shape barrier have not been conducted. Consequently, the transitions included in this Technical Advisory should not be connected directly to a concrete safety shape barrier. To use the transitions tested to date, the concrete safety shape barrier must be transitioned to a vertical or flared back vertical wall. Transitioning from the concrete safety shape barrier to avertical wall should not be abrupt. A 10-foot transition (as shown in Figure 3C) is recommended.
Most of the crashworthy transitions discussed in this Technical Advisory require that special attention be given to designing drainage features consistent with the transition selected. Coordination will be required among those responsible for bridge rail, guardrail and drainage design. For example, the use of more closely spaced posts, as recommended, may require that special attention be given to drainage inlet and pipe locations.
Many of the existing transitions previously reviewed are substandard as compared to the treatments recommended in this Technical Advisory. Consideration should be given to replacing or upgrading these existing transitions as the opportunity becomes available.
New transition designs, modifications to existing designs or untested existing designs should be verified as crashworthy by testing in accordance with NCHRP Report 230 before implementation in the field.
The following bridge guardrail transitions have been successfully crash tested under the required conditions for a passenger car (4,500-pound car at 60 mph and 25 degrees).
(1) W-Beam Guardrail Transitions Three W-beam transitions were crash tested with satisfactory results.
(a) Vertical Concrete Bridge Rail End (Figures 1A and 1B) An older design similar to that shown in Figure 1A, except lacking the rubrail, the double section of W-beam on the top rail, and extra posts, was crash tested with catastrophic results. The retrofit designs shown in Figures 1A and 1B were tested and produced satisfactory results. The critical features of these transitions include:
Use of a rubrail.
Use of a double section of W-beam (one W-beam nested inside the other) on the top rail near the guardrail to bridge rail connection.
Additional posts.
Construction of a vertical face bridge rail end at the guardrail connection.
(b)Vertical Curved Back Concrete Bridge Rail End (Figure 2) Bridge rails or bridge parapets that terminate by curving or flaring back are also relatively common. The bridge guardrail transition shown in Figure 2 was tested and proven satisfactory.
The critical features of this transition include:
Use of a rubrail.
Additional posts.
Construction of a vertical face bridge rail end flared or curved back at the guardrail connection.
These additional features reduce possible snag points and gradually increase the strength and stiffness of the guardrail as it approaches the bridge rail end, thus reducing vehicle snagging and improving redirection characteristics.
(c) Vertical Flared Back Concrete Bridge Parapet (Figures 3A and 3B) The bridge guardrail transitions shown in Figures 3A and 3B provide another method of connecting guardrail to a vertical bridge parapet. The blocked out W-beam guardrail transitions shown in Figure 3A and 3B include the following critical features:
Additional posts.
Use of a double section of W-beam (one W-beam nested inside the other) on the top rail near the guardrail to bridge rail connection.
Flared back parapet wall as detailed in Figure 3C.
Use of a steel pipe section as a crushable block to absorb energy and facilitate redirection of the vehicle. (A wood block was found to be undesirable in this transition because it was too rigid.)
(2) Thrie Beam Guardrail Transitions Two thrie beam transitions were crash tested with excellent results. Each system smoothly redirected the vehicle.
(a) Vertical Tapered Concrete Bridge Rail End (Figure 4) This transition provides a large open space between the bridge rail end and the first post which is suitable for a drainage structure. The following features are critical:
Larger posts are used to compensate for the loss of strength from not using closer spaced standard wood posts.
Use of a double section of thrie beam (onethrie beam nested inside the other) near the guardrail to bridge rail connection.
Tapered parapet wall.
(b) Vertical Flared Back Concrete Bridge Rail End (Figure 5) This transition includes the following critical features:
Additional posts.
Use of a double section of thrie beam (one thrie beam nested inside the other) near the guardrail to bridge rail connection.
Use of a steel pipe section as a crushable block to absorb energy and facilitate redirection.
Flared back parapet wall as detailed in Figure 3C.
The following general features are also important in the performance of the guardrail transition to the bridge rail or parapet end.
(1) Adequate grading is essential to good safety performance. It is recommended the area in front of all guardrail sections and especially the area at guardrail transition to the bridge rail end be relatively flat and clear.
(a) The roadway cross section slope in front of the guardrail transition should be 10:1 or flatter.
(b) Curbs, raised drop inlets, depressed inlets or any other features vehicle or initiate instability should be avoided in this area. Flush drop inlets are recommended. Inlets should be constructed to minimize conflicts with the post spacing and the subsurface drainage system.
(2) Adequate strength of the concrete bridge rail or parapet terminal to which the guardrail is connectedis essential. This will ensure that upon impact the concrete section does not dislocate or rotate.
(3) Proper anchorage of the guardrail to the concrete bridge rail or parapet terminal is essential. Use of a cast-in-place anchor insert or a through bolt connection is recommended.
The attached drawings, in a format suitable for use on the Intergraph CAD System, are available from the Federal Highway Administration, Office of Engineering, Geometric and Roadside Design Branch, HNG-14, 400 Seventh Street, S.W., Washington, D.C. 20590.
/s/
Thomas O. Willett
Director, Office of Engineering
/s/
R. Clarke Bennett
Director, Office of Highway Safety
Links to the 8 attachments are provided below.
Each thumbnail graphic is a link to the larger technical drawing on a separatepage.
(The attachment and figure numbers are provided above each thumbnail graphic.)
