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Technical Note - Nesting of semi-rigid guardrail
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A non-proprietary guardrail system comprises of either W-Beam or Thriebeam on timber or steel posts. These systems achieve crash containment and redirection through a combination of longitudinal strength of the guardrail and rotation of the posts.

Sometimes site limitations can mean that one or two posts cannot be installed due to obstructions or a low fill situation such as a culvert. When this situation occurs "nesting" of the guardrail panels can compensate for the loss of strength and stiffness due to the missing posts.

The New Zealand Transport Agency (NZTA) has prepared a technical memo and drawing detailing procedures to be followed for “nesting” of non-proprietary guardrail systems. This covers all new installations and, where possible maintenance situations. The memo and drawing will soon be available from the NZTA website. If you have any enquiries regarding these details please contact the National Traffic & Safety Engineer, Highways & Network Operations, NZTA National Office or

The main points of this memo are as follows:

  • Covers non-Proprietary W-Beam or Thriebeam on timber or steel posts only.
  • Additional lengths of guardrail should be installed behind the main guardrail to ensure correct lapping is maintained.
  • Recommends that the two layers of guardrail are nested and splice bolted together.
  • Detail covers spans up to 5.72 metres maximum.
  • If factory drilled guardrail is not available then the additional holes required for nesting are to be formed by drilling only. Do not use oxy-acetylene equipment (gas-axe) or similar flame cutting methods.
  • Once the holes are formed they should be carefully filed to remove rough edges or drilling swarf and painted with a single pack zinc-rich primer that meets AS/NZS3750.9

Note the above information does not include proprietary systems such as NuGuard 31TM. For nesting of these systems details are available from the system supplier.

View NZTA memo >
View technical drawing >

Auckland, Northland and Central North Island Lower Nth Island Sth Island