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'9' Appendix A -Metal Beam Guardrails
Appendix A -Metal Beam Guardrails
Section 1. Overview
Introduction
The objectives of this appendix are to make available data and guidelines for the use of roadside traffic
barriers (typically metal beam guard fence) in a consolidated and understandable form . These guidelines
should be supplemented by sound engineering judgment.
The area adjacent to the traveled way plays an important role in the safe operation of a high speed facility.
Accident statistics show that a significant portion of accidents on rural roads are the single vehicle, run-off-
the-road type. Provision of an obstacle free zone and the effective use of barriers to shield obstacles that
cannot otherwise be removed or safety treated are important considerations for enhancing safety
performance.
The Appendix also contains the following sections:
Section 2 -Barrier Need
Section 3 -Strucwral Consideration
Section 4 -Placement of Guardrail
Section 5 -End Treatment of Guardrail
Section 6 -Determ ining Length of Need of Barrier
Section 7 -Example Problems.
Section 2. Barrier Need
Overview
Traffic barriers are needed only when the obstacle without the barrier is greater than the obstacle of the
barrier itself.
Should a roadside obstacle exist, treatment should be considered in the fol lowing priority:
1. Eliminate the obstacle.
2. Redesign the obstacl e so it can be safely traversed.
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3. Relocate the obstacle outside the obstruction free zone to reduce the likelihood that it will be struck.
4. Treat the obstacle to reduce accident severity, i.e., use fl ush or yielding designs.
5. Shield the obstacle with a barrier (median barrier, roadside barrier, or crash cushion).
6. Delineate the obstacle if the above alternatives are not appropriate.
The three basic types of obstacl es that are commo nly shield ed using roadside barriers are as follows:
+ slopes, lateral drop-offs, or terrain features
+ bridge ends and the areas alongside bridges
+ other roadside obstacles that cannot be eliminated , made breakaway or otherwise traversable, or
relocated.
T bl A 1 h a e -s ows a summary o f roa d "d f t SI e ea ures th t a are commonly s h. Id d "th 1e e WI guar d ·1 ra1
I Table A-1: General Applications of Conditions for Roadside Barriers
I Roadside Feature II Applications
!Terrain Features:
!steep Embankment Slope llhc8 , See Figure A-1
JRough Rock Cut Ii he
JBoulders llhc, dia. Exceeds 6 in [150 mm)
!water Body llhc, width exceeds 2 ft [600 mm], permanent
!Lateral Drop-off llhc & steeper than 1V:1 Hand depth exceeds 2 ft [600 mm)
!Side Ditches llhc & unsafe cross section b
-
Bridges:
!Parapet Wall/Wingwall/Bridge Rail End IJhc & approaching traffic I
!Area Alongside Bridges IJhc & approaching traffic I
-
JRoadside Obstacles: I
!Trees llhc & dia. Exceeds 6 in [150 mm] I
!culvert Headwall IJhc & size of opening exceeds 3 ft [900 mm] (w .o. safety grates only)I
!Wood Poles , Posts llhc & cross section/area exceeds 50 in2 (32000 mm2l I
!Bridge Piers, Abutments at Underpasseslihc
!Retaining Wal ls llhc & not parallel to travelway I
a he -Within horizontal clearance for highway class and traffic volume conditions .
b For preferred ditch cross sections, see Side Ditches in Chapter 2
Where the prescribed length of the guardrail cannot be installed at a bridge end due to an intervening
access point such as an intersecting roadway or driveway, the length of guardrail may be interrupted or
reduced. This change in length is acceptable only in locations where the Department must meet the
obligation to provide access and this access cannot be reasonably relocated. Alternative treatments in
these situations include wrapping the guardrail around the radius of the access location, terminating the
guardrail prior to the access location with an appropriate end treatment and continuing the guardrail beyond
the access location if necessary or using an alternate bridge end treatment. The selected treatment should
consider potential sight line obstructions, cost and maintenance associated with the selected treatment and
any accident history at the site. Reduced guardrail length to accommodate access points will not require a
design exception or a design waiver.
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The combination of embankment height and side slope rate may indicate barrier protection consideration
as shown inFigure A-1. For low fill heights a more abrupt slope rate is tolerable than at high fill heights.
Because steeper than 1V:4H side slopes provide little opportunity for drivers to redirect vehicles at high
speeds, in the absence of guardrail, an area free of obstructions should be provided by the designer
beyond the toe of slope.
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""1v:3H ... z la.I ~ z
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IV: 4H
1'I:5H
lV~ 6H /
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R.6.IL IS
NEEDED
20
CUIDE roR USE OF GUARDRAll FOR
EMBANK~ENT HE[CHTS AND SLOPES ~us CUSTCfoJARY}
Figure A -1 (US). Guide for Use of Guardrail for Embankment Heights and Slopes (US Customary)
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