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230 Defective imported scaffold ties

Report ID: 230

Published: Newsletter No 22 - April 2011

Report Overview

On a contract where multi lift access scaffolds had been installed for a period of up to 18 months in an exposed coastal location, it was found by a reporter on one of the regular inspections that some of the scaffold ties had failed.

Report Content

On a contract where multi lift access scaffolds had been installed for a period of up to 18 months in an exposed coastal location, it was found by a reporter on one of the regular inspections that some of the scaffold ties had failed. The ties, including the anchor bolts had been installed in accordance with the scaffold design and load tested in accordance with TG4:04. The anchor bolts had been sourced from a reputable supplier. The failures were first identified when several heads of the screw anchors used for fixings into both concrete and brickwork were found broken off and lying on the scaffolding. Visual inspections over the following two weeks identified additional heads breaking away, with approximately 5% of over 440 fixings failing.

The anchors were of small diameter (6mm) case hardened boran steel with a zinc chromate coating. After testing was carried out the fixings were found to be made from substandard material. It was subsequently identified that they had been imported from the Far East, through a Mediterranean intermediary. Also, due to the diameter of the anchors, aggressive exposure conditions and the time that the scaffold had been erected there was little margin in the fixings to compensate for any deterioration of the fixings. Metallurgical testing indicated that the probable cause of failure was a combination of rolling and heat treatment (quenching) defects in conjunction with hydrogen from corrosion. The flexible nature of scaffold structures, with the slight movements resulting from wind and other loading appear to have caused the fixings to break.

Additional stresses may have been incurred during installation.

When the failures were identified the scaffold was taken out of use and remedial works were carried out. These consisted of re-tying the scaffold using alternative tie arrangements or larger diameter anchors.

Learning Points, says the reporter are:

  • Regular inspection of scaffolds is essential (especially for scaffolds that are to be erected for a long period of time). The failures were identified through the regular inspection regime.
  • The incident defines that it essential that anchors are correctly load tested to achieve confidence at installation stage.
  • Designers need to consider anchor installation methods, and whether that may affect the effectiveness of the design.
  • Designers must consider the exposure of fixings and the potential reduction in performance over time.
  • Designers need to ensure that they know and take into account the design life of the scaffold and exposure conditions.
  • Selection of anchors may require the exercise of judgment as to the robustness of the fixings, for example to accept installation as well as service loads, as against the manufacturers’ quoted performance.
  • Designers should incorporate additional redundancy into fixings or tie arrangements for large scaffolds, those with long duration use or subject to high cyclical loadings.
  • The source of the fixings (as far as possible) needs to be checked and the technical data noted on the boxes needs to be reviewed to ensure the anchors comply with the designer’s recommendations.

Comments

One of the key issues is the need for the designer to consider how fixings are to be installed and used, making sure that they are suitably robust to accommodate the environment, installation and imposed loads with a margin beyond the manufacturer’s recommended loads given in their literature. These considerations should feature as part of the designer’s hazard and risk management process.  Whilst CROSS does not have access to the full reports this sounds as though the problem might be hydrogen embrittlement of higher strength steel. Such a degradation mechanism has been found in the past when galvanising high strength bars and CROSS is aware of failure of similar small diameter fixings in a roof deck diaphragm due to hydrogen embrittlement. In all metals strength is not the only acceptance criterion. The metal must retain toughness and not become brittle. Brittleness can be promoted by faulty quenching, leaving the steel too hard, and free hydrogen can promote cracking (as it does in welding certain steels). In a detail such as that shown, the use of a 6mm fixing is determined by the hole diameter and the size of the bracket. Recommendations from bracket suppliers should be consulted for fixing to the relevant substrate. If there are no recommendations then the bracket design may not be suitable.

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