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1
Breakaway Cable Anchorage

• Youll review the basics for installing a given
  type of terminal for a strong post system,
  breakaway cable anchorage, in this chapter.

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CHAPTER 10 OBJECTIVES

• You will be able to perform the following tasks
  after finishing this chapter
• determine when a breakaway cable anchorage is to
  be used
• describe the standards used in installing this
  type of terminal

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GUARDRAIL TERMINALS

• The main purposes of a terminal are to help
• prevent injuries to a vehicle's occupant and
• minimize vehicle damage.
• You will ensure a terminal will achieve this by
• installing it as tested and approved under NCHRP
  350 test criteria
• developing the necessary tension at the end of
  the system to redirect a vehicle

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STRONG POST W-BEAM SYSTEM(BREAKAWAY CABLE
ANCHORAGE)

• Terminal systems with exposed ends, such as the
  Std. GR-7 and GR-9, use an anchored cable to
  develop the tension required to handle
  downstream impacts.

• The terminals vertical end is broken away upon
  end-on impacts, thus allowing the vehicle to
  continue downstream.

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STRONG POST W-BEAM SYSTEM(BREAKAWAY CABLE
ANCHORAGE)

• The exposed end in a breakaway cable anchorage,
  with the exception of the Brakemaster system, is
  a wooden post in a steel foundation tube. The
  anchor cable passes through a hole and steel
  sleeve in the post near ground level.

BREAKAWAY CABLE TERMINAL 4' FLARE 501.10
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STRONG POST W-BEAM SYSTEM(BREAKAWAY CABLE
ANCHORAGE)

• The cable is restrained with a bearing plate on
  the upstream side of the post and steel tube, and
  is attached to the rail element with a cable
  anchor bracket just in advance of the second
  post.
• The bearing plate must be restrained from
  rotating by using a restraining device.
• Some systems provide for a nail through the
  bearing plate to prevent rotation. Others use a
  nail bent over the bearing plate for the same
  purpose.

BREAKAWAY CABLE TERMINAL 4' FLARE 501.10
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IMPROPERLY INSTALLED EXAMPLE(BREAKAWAY CABLE
ANCHORAGE)

• This bearing plate has rotated. The standards
  require that bearing plates used with breakaway
  terminals be nailed to the post to prevent
  rotation.

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STRONG POST W-BEAM SYSTEM(BREAKAWAY CABLE
ANCHORAGE)

• The anchorage cable must be taut. The cable, when
  properly tightened, cannot be easily lifted more
  than 1 inch.

• When tightening the nut to make the cable taut,
  the cable must be restrained from twisting. This
  can be accomplished by clamping the cable with
  pliers or vice grips while the nut is being
  turned.

BREAKAWAY CABLE TERMINAL 4' FLARE 501.10
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IMPROPERLY INSTALLED EXAMPLE(BREAKAWAY CABLE
ANCHORAGE)

• This cable is sagging more than two inches.
• Cable anchorges should be taut.

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STRONG POST W-BEAM SYSTEM(BREAKAWAY CABLE
ANCHORAGE)

• A steel strut and yoke is used to connect the
  first two foundation tubes in order to spread the
  tension developed in the cable anchor.

• The strut can be used as an indication of proper
  installation with respect to site grading.
• The measurement between the ground and the bottom
  of the strut must be no more than 2 inches.
  Preferably, the strut will be flush with the
  ground.

BREAKAWAY CABLE TERMINAL 4' FLARE 501.10
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STRONG POST W-BEAM SYSTEM(BREAKAWAY CABLE
ANCHORAGE)
HEIGHT EXCEEDS 4"MAXIMUM

• Soil foundation tubes, when used in conjunction
  with breakaway wood posts, should be installed so
  that they are 2 inches above the ground. After
  installation, the top of tube should never be
  more than 4 inches above the ground.

• These tubes are installed incorrectly the
  exposed tube is more than 4 inches above ground
  level.

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IMPROPERLY INSTALLED EXAMPLE(BREAKAWAY CABLE
ANCHORAGE)

• The exposed height of the steel strut is
  excessive. It should be flush with the ground or
  no more than 2 inches above the ground.

• The top of the soil plate should be below the
  ground line.

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IMPROPERLY INSTALLED EXAMPLE(BREAKAWAY CABLE
ANCHORAGE)

• This blockout is nailed to the post however, the
  nail does not penetrate enough of the wood
  before going into the post. VDOT standards call
  for the nail to be placed within 2 inches from
  the blockouts top or bottom in order to provide
  more resistance to rotation.

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STRONG POST W-BEAM SYSTEM(BREAKAWAY CABLE
ANCHORAGE)

• For utility conflicts where the soil plate tube
  will cause damage, a Controlled Release Terminal
  (CRT) post may be used for posts 3 and 4 with
  the approval of the project engineer.

STANDARD BLOCKED-OUT W-BEAM GUARDRAIL (STRONG
POST SYSTEM) 501.04
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STRONG POST W-BEAM SYSTEM(BREAKAWAY CABLE
ANCHORAGE)

• Use CRT (controlled release terminal) weakened
  wood posts for all posts in the terminal system
  that do not sit in steel foundation tubes.
• Weaken the wood by drilling two holes, 3½ inches
  in diameter, in the post. Place one hole where it
  will be 16 inches below ground after
  installation place the other where its center
  will be at ground level after installation.
• Orient the holes parallel to the roadways
  center line.

STANDARD BLOCKED-OUT W-BEAM GUARDRAIL (STRONG
POST SYSTEM) 501.04
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IMPROPERLY INSTALLED EXAMPLE(BREAKAWAY CABLE
ANCHORAGE)

• These end terminal posts were not installed
  correctly. The centers of the top holes are above
  ground level. The standards require that when
  installing the centers of the top holes be at
  ground level when installing Std. GR-7 or GR-9
  terminals that incorporate wood posts designed to
  break upon impact.

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STRONG POST W-BEAM SYSTEM(BREAKAWAY CABLE
ANCHORAGE)

• Toenail the blockout to the post within 2 inches
  of the blockouts top or bottom to prevent
  rotation when installing a wood blockout on a CRT
  post or a shortened wood post in a foundation
  tube.

68 WOOD POST ROUND WOOD POST 501.05
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STRONG POST W-BEAM SYSTEM(BREAKAWAY CABLE
ANCHORAGE)

• The wood blockouts on this Std. GR-2 guardrail
  have rotated over time. The system may not
  function as expected when hit. The blockouts are
  to be toenailed within 2 inches of the top or
  bottom of the blockout to the wooden post to
  prevent rotation and to meet VDOT standards.

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SUMMARYBREAKAWAY CABLE ANCHORAGE

• Develop the tension for downstream guardrail
  impacts

• Post breaks away and releases cable for end on
  impacts
• Uses soil plates or long steel foundation tubes
  with the first two posts tied together with a
  strut to distribute cable tension to soil
• Cable must be taut (cannot be lifted more than 1
  inch)

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3 SELF-CHECK QUESTIONS

• 1. Terminals with an exposed end use an anchored
  _____ to develop the necessary tension to absorb
  the energy of a hit. This cable must be _____ .
• 2. A blockout must be toenailed at _____ from its
  top or _____ from its bottom when attaching it to
  a wooden _____ .
• 3. Toenailing the blockout at this location keeps
  the blockout from _____ .

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ANSWERS

• 1. Terminals with an exposed end use an anchored
  cable to develop the necessary tension to absorb
  the energy of a hit. This cable must be tight
  (or taut).
• 2. A blockout must be toenailed at 2 inches from
  its top or 2 inches from its bottom when
  attaching it to a wooden post.
• Toenailing the blockout at this location keeps
  the blockout from rotating.