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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 clear zone 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-6. Areas of Concern

Figure A-6 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.

Lu is the length of guard fence needed to protect traffic adjacent to proposed guard fence. Upstream refers to the guard fence upstream of traffic adjacent to proposed guard fence. While Ld is the length of guard fence needed to protect the opposing traffic. 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 clear zone of opposing (northbound in Figure A-6) 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-7. 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 justify minimum guard fence treatment where located slightly outside the clear zone 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 clear zone 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 clear zone required is less than 16 ft [4.9 m] and the present ADT is 750 or less, use Equation A-1 for calculating the guard fence length of need.

Anchor: #i1036763Table A-2: Equations for Upstream and Downstream Length of Need

ADT < 750

Equation A-1.

Equation A-2.

ADT > 750

Equation A-3.

Equation A-4.



Where:

  • Lu = Length of guard fence needed (upstream of area of concern), ft
  • Ld = Length of guard fence needed (downstream of area of concern), ft
  • Du = Distance from edge of travel lane to far side of area of concern or to outside edge of clear zone, whichever is least, ft (for upstream direction of traffic)
  • Dd = Distance from edge of travel lane to far side of area of concern or to outside edge of clear zone, whichever is least, ft (for opposing direction of traffic)
  • Gu = Guard fence offset from edge of travel lane adjacent to proposed guard fence, ft
  • Gd = Guard fence offset from edge of opposing direction of travel lane (centerline)

Click here for Metric.

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

For higher volumes, a clear zone 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 clear zone is greater than 16 ft [4.9 m]):

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

The length of need for guard fence, as illustrated below in equation A-5, is equal to the sum of the required upstream length (Lu), the guard fence length parallel to the area of concern Lp, and the required downstream length.

Equation A-5.

Where:

Ltotal = Length of guard fence needed

Lu = Guard fence Length Upstream of Area of Concern

Lp = Guard fence Length Parallel to Area of Concern

Ld = Guard fence length Downstream from area of concern

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Using Design Equations to Determine Length of Guard Fence

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.

Where the prescribed length of the guard fence cannot be installed at a bridge end due to an intervening access point such as an intersecting roadway or driveway, the length of guard fence may be interrupted or reduced. This change in length is acceptable only in locations where the Department must meet the obligation to provide access and this access cannot be reasonably relocated. Alternative treatments in these situations include wrapping the guard fence around the radius of the access location, terminating the guard fence prior to the access location with an appropriate end treatment and continuing the guard fence beyond the access location if necessary or using an alternate bridge end treatment. The selected treatment should consider potential sight line obstructions, cost and maintenance associated with the selected treatment and any accident history at the site. Reduced guard fence length to accommodate access points will not require a design exception or a design waiver.

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

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