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    Technical Talk : Compliant bridge mounted semi-rigid Road Safety Barriers

Technical Talk : Compliant bridge mounted semi-rigid Road Safety Barriers

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Roadside hazards, related to bridges and bridge approaches, are generally shielded by road safety barrier installations.  The most common bridge barrier systems are semi-rigid NCHRP 350 TL-3 W-section systems and TL-4 Thrie-Beam systems mounted on system specific surface mount  bridge posts.  These systems have been deemed to comply by the New Zealand Transport Agency at these containment levels based on in service performance and engineering reviews.

In New Zealand a bridge barrier performance level is generally the same as that of the same rail section mounted on ground planted posts.  A ground planted semi-rigid barrier system relies on two mechanisms for crash energy containment and redirection.  These are the ribbon strength and continuity of the rail section, and the resistance or stiffness developed through the lateral rotation of the length of post in the ground.  An equivalent bridge system will have the same rail ribbon strength and continuity.  However the equivalent lateral resistance is provided through the rotational resistance of the hold down bolts as the system tries to rotate backwards under impact.  Although the necked hold down bolts provide resistance, they are weakened to fracture and release the post at approximately the same lateral resistance that the equivalent ground planted posts break at. 

In new bridge installations the necked bolts will generally be threaded into cast-in-brackets placed during the forming of the deck or ground beams on the approaches.  In retrofit installations the necked bolts will generally be threaded into cast-in sockets that are epoxied into holes drilled into the concrete bridge deck or ground beam.

 Any modification to the standard hardware or installation details can affect the containment and redirection ability of the bridge barrier system.  Designers or installers should not modify the size or hole configuration of the base plate as the relative positioning of the post and hold down bolts on the base plate will affect the ability of the post to perform as expected.  Similarly the substitution of bolts, incorrect socketing or additional welding that is not part of the standard base plate detail can significantly affect performance.  Any hardware modification to the Transport Agency accepted barrier systems should be approved by the road controlling authority before installation commences.

CSP Pacific is available to provide assistance if there are situations where standard barrier hardware poses site specific installation challenges. We will ensure that a compliant barrier solution is achieved for the project.

For more information on New Zealand Transport Agency accepted bridge barrier systems click here.

For product information for CSP Pacific’s Flexrail W-Beam Guardrail  click here.

For product information for CSP Pacific’s Threi-Beam Guardrail  click here.

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