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Section 6: Determining Length of Need of Barrier

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Overview

The shape of the obstacle, its location with respect to travel lanes, the volume of traffic and its corresponding horizontal clearance width are the primary variables influencing length of barrier need.

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Variables

After all practical means to free the roadside of obstacles have been exhausted, certain areas may remain which constitute an obstacle to errant vehicles. These areas, as illustrated in Figure A-6, will be referred to an “area of concern.”

Areas of Concern (click in image to see full-size image) Anchor: #i1001814grtop

Figure A-7. Areas of Concern

Figure A-7 illustrates the variables of interest in the layout of approach barrier to shield an area of concern. Length of need is equal to the sum of the following variables:

  • length of upstream barrier, Lu,
  • length of barrier parallel to the area of concern, Lp,
  • the length of downstream barrier, Ld

For roadways serving one-way traffic operations, Ld=0. Ld is greater than zero for two-way operations when the area of concern lies within the horizontal clearance of opposing (northbound in Figure A-7) traffic as measured from the centerline pavement markings.

Variables Involved in Barrier Layout. Click here to
see a PDF of the image. (click in image to see full-size image) Anchor: #i1001816grtop

Figure A-8. Variables Involved in Barrier Layout. Click here to see a PDF of the image.

In certain instances judgment should be exercised to supplement design chart solutions and provide for public safety. For example, high severity fixed objects (e.g., bridge columns) may warrant minimum guardrail treatment where located slightly outside the horizontal clearance if geometric conditions (i.e., steep fill slope, outside of horizontal curvature, etc.) increase the likelihood of roadside encroachments.

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Design Equations

To determine needed length of guard fence for a given obstacle, design equations have been formulated for low volume (ADT 750 or less) and higher volume (ADT more than 750) conditions. A horizontal clearance width of 16 ft [4.9 m] and length of roadside travel of 200 ft [61 m] are incorporated in the low volume design equation (for use on roadways when the present ADT volume is 750 or less). Also, if the horizontal clearance required is less than 16 ft [4.9 m] and the present ADT is 750 or less, use Equation A-1 for calculating the guardrail length of need.

US Customary:

Where:

  • L = Length of guardrail needed, ft
  • D = Distance from edge of travel lane to far side of area of concern or to outside edge of horizontal clearance, whichever is least, ft
  • G = Guardrail offset from edge of travel lane, ft

Metric:

Where:

  • L = Length of guardrail needed, m
  • D = Distance from edge of travel lane to far side of area of concern or to outside edge of horizontal clearance, whichever is least, m
  • G = Guardrail offset from edge of travel lane, m

Equation A-1

For low volume conditions, if the horizontal clearance width (16 ft [4.9 m]) is met or exceeded, L=0.

For higher volumes, a horizontal clearance width of 30 ft [9 m] and length of roadside travel of 250 ft [76 m] are incorporated into the design equation (for use on roadways when the present ADT volume is more than 750 or the recommended horizontal clearance is greater than 16 ft [4.9 m]):

(US Customary):

(Metric):

Equation A-2

For high volume conditions, if the horizontal clearance width (30 ft [9 m]) is met or exceeded, L=0.

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Using Design Equations to Determine Length of Guardrail

Before determining length of guard fence, the designer should assemble the following pertinent data:

  • present ADT volume
  • clear zone (horizontal clearance)
  • traffic operations (one-way or two-way)
  • lateral and longitudinal dimension of the area of concern
  • shoulder width
  • offset distance of the area of concern from the edge of travel lane (including from the centerline markings for two-way traffic operations)
  • design slope conditions, (i.e. will slopes be 1V:10H or flatter?)
  • placement location (alongside shoulder vs. near object, flared, etc.)
  • presence of other nearby areas of concern which should be considered simultaneously.

Once this design data has been assembled, the appropriate equation can be used.

The Example Problems section provides example problems and solutions using the design equations. The guardrail lengths produced by the equations should be rounded up to an even length of guardrail.

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