17.5.2 Structural Considerations of Guard Fence
Post spacing, rail shape and thickness, rail height, splice strength and location, post embedment, and rail anchorage are all key factors that influence the structural integrity of guard fence.
17.5.2.1 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 there should be no slope break within 2-ft beyond the back of the post location as shown in
, 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 guard fence.
Embedment depth is shown on the standard detail sheet for both timber and steel posts.
Figure 17-3: Lateral Support to Accommodate Guard Fence
17.5.2.2 Rail Element
Guard fence is fabricated in a w-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. The standard length of the rail element is 25-ft, or 12.5-ft.
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 must be spliced midspan between the posts.
Pavement overlays effectively reduce existing rail height. The following guidelines must be followed:
- When rail height varies more than 3-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 instability or vaulting or under riding the system;
- For existing 28-in rail systems, the rail height must 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 (when splice is at the post location), 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 midspan splicing location; and
- 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 angle strut locations.