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CROSS Safety Report

Incorrect lifting system used for precast concrete stair elements

Report ID: 883 Published: 13 December 2022 Region: CROSS-UK


Overview

A reporter was asked to inspect a precast concrete stair unit which was exhibiting cracking around the lifting anchor locations after being de-moulded.

Key Learning Outcomes

For precast concrete manufacturers and designers:

  • The lifting system for precast concrete units should be suitably designed to ensure the manufacturer's lifting anchor specifications are met
  • Lifting anchors should be installed and used in accordance with the manufacturer's specifications. The manufacturer should be consulted if their specifications cannot be met.
  • Be aware that spreader beams may be needed to achieve the correct sling angles of the lifting chains
  • Consider appointing a single entity (or Chartered Engineer) to have overall control of the design of the lifting system for de-moulding, transportation, and final installation of precast concrete units

For contractors and the construction team:

  • Consider introducing a quality control procedure for the inspection of precast concrete units that are delivered to site to ensure they have not been damaged during transportation 

Full Report

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Whilst working for a precast concrete manufacturer, the reporter was asked to inspect a stair unit which was exhibiting cracking around the lifter after being de-moulded and placed into storage in the yard at the production facility. The particular stair unit was exceptionally wide and long compared to flights typically produced; it was approximately 2.5m wide, almost 6m in length, and weighed 10.5t. The lifting anchors specified by the designer were 4 No. 10t Spherical Head type lifting anchors of 250mm in length.

The reporter firstly highlights that this lifter had a 10t capacity, however, this capacity is only achieved in certain lifting conditions and relates to the geometry of the element. Therefore, they note that a 10t capacity should not automatically be assumed to satisfy the design just because that is the load rating stamped on the lifting anchor.

Shear cone failure of lifting anchor

On examining the unit, the cracking around the lifter was indicative of a shear cone failure towards the face of the tread. When witnessing a similar unit being lifted the reporter observed that due to the geometry of the unit, when the stair was laid on its back and lifted during storage and delivery, the chain angles relative to the lifter at the top of the flight is exceptionally acute as the lifter itself is inclined away from the angle of pull of the chains.

By calculation, based on the sling lengths used, the angle of the chain relative to the lifter was just 15°. Manufacturer's literature for the anchors said that a minimum angle of 45° must be observed. The lift which resulted in the cracking around the lifter was therefore outside of the scope allowed by the manufacturer. From further consideration of other stair units, this minimum angle was often not achieved on a range of stair units and geometry. Long chains are required in order to achieve the necessary angle, even on short flights, which are often not possible given the allowable headroom within the precast factory.

the lift which resulted in the cracking around the lifter was therefore outside of the scope allowed by the manufacturer

Concerns over future failures

Although these lifters have been used extensively across the industry for many years without report of incident as far as the reporter is aware, they are concerned that as the shear capacity of the lifters is unknown, the designer is unable to predict when a further failure could occur which could have significant risk to life. The precast manufacturer concerned now uses an alternative lifting anchor type, but the reporter believes that other manufacturers in the industry may continue to use the Spherical Head style anchor.

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