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Section 3: Structural Considerations of Guard Fence

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Overview

Post spacing, rail shape and thickness, splice strength and location, post embedment, and rail anchorage are all important factors that influence the structural integrity of guard fence.

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Post Spacing, Embedment, and Lateral Support

Typical post spacing is 6 ft- 3in [1905 mm] for guard fence. Where guard fence is to be placed at or near the shoulder edge, it is desirable that the roadway crown be widened, typically 2 ft [600 mm] from the back of the post location as shown in Figure A-2, to provide lateral support for the posts. Locating the roadway crown/side slope hinge point behind the rail also provides a platform that increases vehicular stability in the event of impacts that straddle the end section.

Embedment depth is shown on the standard detail sheet for both timber and steel posts.

Crown Widening to Accommodate Guard fence (click in image to see full-size image) Anchor: #i1001690grtop

Figure A-2. Crown Widening to Accommodate Guard fence

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Rail Element

Guard fence is fabricated in a deep beam shape to provide for bending strength. Nominal thickness of the rail is 10 or 12 gauge. End treatments, wingwalls, retaining walls, etc. provide firm rail anchorage. With full splice connections, the anchored rail has sufficient tensile and flexural strength to contain and redirect vehicles under nominal impact conditions.

To insure satisfactory performance for a range of vehicle sizes, rail should be mounted 25 in [635 mm] high as measured from shoulder surface, gutter pan, or widened crown to the center of the rail at the bolt. The rail element shall be spliced midspan between the posts.

Pavement overlays effectively reduce existing rail height. When rail height varies more than 1 in [25 mm] above and 3 in [75 mm] below the 31 in [787mm] top of rail standard height, steps should be taken to restore the rail to the standard dimension to reduce the possibility of vehicular vaulting or post snagging. For existing 27 in [686 mm] rail systems, the rail height shall not vary by more than 2 in [50 mm] above and 1/2 in [12 mm] below the 27 in [686 mm] top of rail.

When raising existing metal beam guard fence to the 31 in [787 mm] height, the railing will also need to be adjusted horizontally and an additional post will be needed to obtain the mid-span splicing location. Existing bridge transitions may need to be upgraded to current standards or adjusted with a new transition section to obtain the 31 in [787 mm] height. The end treatments may require new materials to adhere to the manufacturer’s specifications, such as the breakaway hole and angle strut locations.

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Blockouts

The guard fence is blocked out from the posts with routed timber or composite blockouts (6 in x 8 in [150 mm x 200 mm]). These blockouts minimize vehicle snagging on the posts and reduce the likelihood of a vehicle vaulting over the barrier by maintaining the rail height during the initial stages of post deflection.

It is acceptable to use double blockouts (up to 16 in. [406 mm]) to increase the post offset to avoid obstacles such as curbs. There is no limit to the number of posts that can have double blockouts installed, except terminals, unless approved by the manufacturer. Under special circumstances, such as avoiding buried obstacles that are not relocated, it is also acceptable to install additional blockouts to obtain up to 36 in [914 mm] of clearance for one or two posts in a section of guard fence.

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Deflection Considerations

Guard fence is a flexible 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 2.5 ft [750 mm] or more as diagrammed in Figure A-3. Where conditions permit, a barrier-to-obstacle distance of 5 ft [1500 mm] or more is desirable.

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

Figure A-3. Allowance for Deflection of Guard fence

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