29 April 2014

Technical Talk – Rigid Columns vs Frangible Columns

Designers need to specify columns based on the purpose and the place of installation. To assist with this, designers should use information from New Zealand Transport Agency (NZTA) Specification M26 – Specification for Lighting Columns, click here for AS/NZS 1158: 2010 and NZTA : State Highway Geometric Design Manual.

Roadside columns are classified into two categories: Rigid and Frangible (see diagram 1). Frangible columns (safer in an impact) can be classified further into Shear Base and Impact Absorbing columns. The principle difference Rigid and Frangible columns is in their behavior during and after impact (see diagrams 2, 3 and 4). In both cases these columns can be Ground Planted, Flange Mounted with a Hold Down Assembly or Ground Stub.

Rigid Columns

  • Are designed to be impact resistant.
  • Are usually made of concrete or steel thicker than 4mm.
  • Are used as communication or transmission columns, where losing communication or falling electrical wires is unacceptable.


  • They should not be used where there is a reasonable likelihood of vehicular impact.
  • They should not be used within a clear zone without protection.

Frangible columns

There are two main types of frangible columns, namely: Shear Base and Impact Absorbing columns. The most common Frangible columns are made of steel less than 3mm thick.

Shear Base columns

  • They are generally dislodged from their original position when impacted at normal urban operating speeds (70 –100 km/h) and often do not significantly retard the progress of the impacting vehicle.
  • It is strongly recommended they not be used in high pedestrian activity areas.

Shear Base Installation Reminder …Please remember – when installing a Shear Base it is very important that the bolts be torqued to 250Nm initially and then slackened off to 90 –100Nm immediately or, at the very least, before the road is opened. If the bolts are left tighter than 90 –100Nm, the Shear Base will not perform as required in the event of an impact.

Impact Absorbing columns

  • As the name suggests, these columns absorb any impact energy by progressively deforming and entrapping the impacting vehicle. The column brings the impacting vehicle to a stop, generally within a distance of less than half the mounting height beyond the base.
  • They are suited to low vehicle speed and higher pedestrian activity areas where there may be concern about secondary accidents associated with, for example, a dislodged Shear Base column.