Section 3: Structural Considerations

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

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Post Spacing, Embedment, and Lateral Support

Typical post spacing is 6 ft- 3in [1905 mm] for guardrail. Where guardrail 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 inFigure 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 Guardrail (click in image to see full-size image) Anchor: #i1001690grtop

Figure A-3. Crown Widening to Accommodate Guardrail

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Rail Element

Guardrail 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 21 in [550 mm] high as measured from shoulder surface, gutter pan, or widened crown to the center of the rail at the bolt.

Pavement overlays effectively reduce existing rail height. When rail height varies more than 3 in [75 mm] from standard height, steps should be taken to restore the rail to the standard dimension to reduce the possibility of vehicular vaulting or post snagging.

When raising existing rail, existing timber posts should be removed and replaced if rotted or otherwise deteriorated. Where existing timber posts are in good condition and embedded in cohesionless soils, the posts may be pulled (jacked) up to 4 in [100 mm]. These partially extracted posts should be restrained at ground line to preclude settlement, and the resultant rail height is increased up to 4 in [100 mm]. For cohesive soil conditions or where more than 4 in [100 mm] height increase is desired, timber posts should be removed and replaced.

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Deflection Considerations

Guardrail is a flexible barrier system. The amount of dynamic deflection varies primarily with weight of impacting vehicle, its speed, and its encroachment angle. Guardrail 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 Guardrail (click in image to see full-size image) Anchor: #i1001708grtop

Figure A-4. Allowance for Deflection of Guardrail

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