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Section 4: Placement of Guard Fence

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Overview

The placement of guard fence pertains to the lateral and longitudinal position.

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Lateral Placement at Shoulder Edge or Curb Face

Typically the face of rail is placed at the shoulder edge or curb face throughout most of its length as shown in Figure A-4.

Placement at Shoulder Edge or Curb Face (click in image to see full-size image) Anchor: #i1001744grtop

Figure A-4. Placement at Shoulder Edge or Curb Face

Guard fence placed in the vicinity of curbs should be blocked out so that the face of curb is located directly below or behind the face of rail. Rail placed over curbs should be installed so that the post bolt is located 25 inches [635 mm] above the gutter pan or roadway surface.

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Lateral Placement Away From the Shoulder Edge

In certain instances it is desirable to place guard fence closer to the obstacle rather than at the shoulder edge or curb face as shown in Figure A-5. Placement in this manner can substantially reduce the length of rail required to shield a given obstacle and minimize the probability of impact, but undesirably, encroachment angles may increase. This manner of placement is most applicable to small areas of concern such as point type obstacles, overhead sign bridge supports, bridge piers, etc.

To preclude vaulting or impacting at an undesirable position by errant vehicles; care should be exercised in selecting placement location of guard fence with respect to slope conditions. Guard fence may be placed at any lateral location on a side slope only if the slope rate between the edge of the pavement and the face of the barrier is 1V:10H or flatter.

 Location of Roadside Guard Fence. (click in image to see full-size image) Anchor: #i1001754grtop

Figure A-5. Location of Roadside Guard Fence.

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Deflection Considerations

Guard fence is a semi rigid barrier system. The amount of dynamic deflection varies primarily with weight of impacting vehicle, its speed, and its encroachment angle. Guard fence should be laterally positioned to provide a clear shoulder width while maintaining a distance from a fixed object that is greater than the dynamic deflection of the rail. Based on crash test data, this barrier-to-object distance should be 4’-0” [1219 mm] minimum from the back of the post to fixed object or more as diagrammed in Figure A-6. Where conditions permit, a barrier-to-obstacle distance of 5 ft [1524 mm] or more is desirable.

Allowance for Deflection of Guard fence (click in image to see full-size image) Anchor: #i1001708grtop

Figure A-6. Allowance for Deflection of Guard fence

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