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Section 3: Structural Considerations of Guard Fence

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Overview

Post spacing, rail shape and thickness, rail height, 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 3-in for guard fence. Where guard fence is to be placed at or near the shoulder edge, it is recommended that the cross slope of the shoulder be projected, typically 2-ft beyond the back of the post location as shown in Figure A-3, to provide lateral support for the posts. Locating the roadway cross slope/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.

Lateral Support to
Accommodate Guard Fence (click in image to see full-size image) Anchor: #i1001690grtop

Figure A-3. Lateral Support 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 ensure satisfactory performance for a range of vehicle sizes, rail should be mounted 25-in 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 above and 3-in below the 31-in 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 under riding the system. For existing 28-in rail systems, the rail height shall not vary by more than 2-in above and ¼-in below the 28-in top of rail. Existing systems installed with a top rail height less than 27 ¾-in should be upgraded to current standards whenever impacted, repairs needed, or when maintenance budgets permit.

When raising existing metal beam guard fence to the 31-in height, the railing will also need to be adjusted longitudinally and an additional post or 9-ft 4 ½-in rail length 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 height. The end treatments may require new materials to adhere to the manufacturer’s specifications, such as the breakaway hole and anglestrut 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). 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) 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 triple blockouts (up to 24-in) for one post for every 75-ft of guard fence.

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