Occasionally a guardrail system must be strengthened to reduce the design deflection distance. For example, a transition section is needed where a guardrail attaches to the approach end of a rigid concrete bridge rail (See the HKP article, Barrier Classifications (coming soon)). A guardrail must be stiffened if a rigid object obstacle is present within the design deflection distance. Standard installations can be modified using the following techniques to strengthen the guardrail:
• Add a rubrail
• Install additional posts (reducing the post spacing) (See MGS Working Width Table in Section 3.11.)
• Use extra length posts (e.g., 7-foot or 9-foot posts, not the standard post length of 6 feet)
• Use thicker gauge posts (e.g., 10-gauge posts instead of 12-gauge posts)
• Double nest the rail
• Bolt a W-beam to back of the posts
For extra length posts and nested guardrail, quantities should be measured and bid items provided. Consult Section 719 of the Standard Specifications for Road and Bridge Construction for additional information. Where extra strength is needed different methods may be combined (e.g., when the area of concern is near the back of rail).
A guardrail is classified as a semi-rigid barrier due to its deflection distance upon crash impact (see the HKP article, Roadside Safety (coming soon), for a discussion on barrier categories). A transition section is needed when a guardrail connects to a rigid barrier (e.g., bridge railing or concrete barrier) or rigid object (e.g., bridge pier or sign structure). The transition should provide a gradual, continuous stiffening of the guardrail system from a less rigid to more rigid system. This reduces or prevents a vehicle from snagging or punching through the barrier. Use of a cast-in-place anchor or through-bolt connection is recommended to ensure the connections are as strong as the barrier itself.
Use TL-2 or TL-3 Thrie-Beam Guardrail Transitions to connect W-beam guardrail to concrete bridge rails, rigid barriers, or rigid objects. The TL-2 railing transition should be used when speeds are 45 mph or less. The TL-3 railing transition must be used when speeds are over 45 mph. For more details, see Standard Drawings Nos. BHS-013 and BHS-014.
Historically, the Guardrail Connector to Bridge End Type A was used on both bridge ends on undivided highways and on the approach bridge ends of divided highways. A Guardrail Connector to Bridge End Type A1 was used on the exit bridge ends of divided highways. A Guardrail Connector to Bridge End Type D was applied on each end of a bridge on which a sidewalk was present or its installation was proposed on the structure (and not on the roadway). This only applied to rural structures having two-direction traffic with a sidewalk.
Type A, A1, and D connectors are no longer installed on new highway construction. When repairing or restoring existing concrete barrier wall and these guardrail connectors, see Standard Drawings RBC-004, RBC-005, RBC-05N, and RBC-006. Refer to the RBB and RBC series of Standard Drawings in the HKP article, Bridge Railings and Transitions (coming soon), for more details.
On some retrofit projects use of standard guardrail connectors may not be appropriate. Contact Central Office Division of Maintenance for guidance.
Delineators must be installed on new guardrail. Standard Drawing No. RBR-055 (Delineators for Guardrail) illustrates delineator elements, placement, and spacing. Section 830 of the Standard Specifications addresses retroreflective requirements for guardrail delineators.
The District is responsible for deciding (1) whether to install new delineators on existing guardrail systems which lack delineation or (2) whether it is necessary to replace existing guardrail delineation due to non-reflective and/or missing delineators. High-priority locations for guardrail delineation include curves and ramps on freeways and expressways, as well as guardrail sections located in the median. The District may elect to place delineation to indicate openings at entrances on highways.
Where a guardrail end treatment adjacent to the roadway is marked, a Type 3 object marker is typically used. Alternating stripes of Type 3 object markers should slope downward, toward the side on which traffic is to pass. Generally, the end treatment manufacturer supplies Type 3 object markers. Use retroreflective sheeting that conforms to the size of the approach end.
Double-faced guardrail has two W-beam rails configured as a single beam on each side of post. It is typically used in the median in place of concrete barriers, particularly when the required shielding length is relatively short. Double-faced guardrail is also used when a guardrail may be exposed to impacts from either side. When installed near the pavement edge, it is critical for the area between the posts to be free of asphalt.
On longitudinal guardrail installations, double blockouts (up to 16 inches deep) may be used to increase the post offset to avoid obstacles such as curbs. There is no limit on the number of posts that can have double blockouts installed.
Do not employ double blockouts for transitions and terminals, unless approved by the manufacturer. Under special circumstances (e.g., avoiding buried obstacles that are not relocated), additional blockouts may be installed to obtain up to 24 inches of clearance (three 8-inch blockouts or two 12-inch blockouts) for one or two posts in a section of guardrail. Standard Drawing RBR-031 (Guardrail End Treatment Type 3 Pipe Drainage Detail) offers an example of using extra blockouts.
Omit a post only under special circumstances and only on a standard run of guardrail. Omitting a post requires the construction engineer’s approval.
Single posts may be omitted along runs of MGS W-beam guardrail without the system needing modification (i.e., no weakened posts, no nested rail elements, no special posts). Always consider the following issues before taking this step:
1. At least 50 feet is required between omitted posts, a terminal, or other special design.
2. Omitted posts cannot be used within transitions, terminals, or special designs.
3. No curbs are present where guardrails are installed.
4. Additional deflection resulting from the omitted post.
Occasionally, an intersecting side road or driveway is located so close to a side obstacle that installing a minimum length MASH TL-3 guardrail is infeasible. This most occurs often at bridge ends. A short radius curved guardrail installation may accommodate this situation. See Standard Drawing RBI-001 for more details.
Keep the following in mind when designing a short-radius curved guardrail:
• Installations at intersections and driveways can create nuisance hit problems for turning traffic (e.g., tractor trailers turning off of or onto side roads, farm implements turning into and out of field entrances). Check turning patterns to ensure a guardrail installation will not significantly affect turning movements.
• Installations at intersections and driveways can obstruct motorist sight, especially if the side road or entrance is at or near the crest of a vertical curve. Perform sight distance analysis for both plan and profile views to determine what effects guardrail installation may have on motorist line-of-sight.
On roadways where the design speed is between 30 and 45 mph and there is a curb adjacent to the traveled way (e.g., urban and suburban roadways), keep the border area free of obstacles. Any decision to use a guardrail within the border area should be informed by careful evaluation. A guardrail used in the border area should present less of a hazard than the obstacle it shields. See Chapter 10 of AASHTO’s Roadside Design Guide for further guidance on clear zones in urban areas.
Use the appropriate omitted-post design (as discussed above) if rock is present where a single guardrail post is to be installed. If rock is present at several points where posts will be installed, place the posts in cored holes (see Section 719.03.01 of the Standard Specifications for Road and Bridge Construction).
When shales are used in embankment fill sections, acidic shale is generally encased inside the embankment. At least 4 feet of nondurable shale or clay soil should be placed atop the embankment to control the corrosion of guardrail posts. It may be appropriate to install corrosion-resistant guardrail posts. See Special Note 8N Corrosion Resistant Guardrail and the Geotechnical Guidance Manual GT-609-5 on Acid-Producing Shales.
Horizontal lagging walls are often used for temporary or permanent retention of soil material and other backfill. Lagging may consist of wood, guardrail, or geogrid. When backfill is placed or compacted behind the guardrail wall, it is imperative that rails not to be damaged.
Lagging has traditionally been designed based on experience or empirical methods. (e.g., the Federal Highway Administration uses the Goldberg Zoino chart). Contact the Geotechnical Branch for more information.
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