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CROSS Safety Report

Failure of prestressed hollowcore units during lifting

Report ID: 749 Published: 1 October 2018 Region: CROSS-UK

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Overview

A reporter raises concerns about how prestressed hollowcore units can fail during lifting if not carefully designed, planned, and managed.

Key Learning Outcomes

For precast manufacturers and designers:

  • Additional reinforcement may be required in precast concrete units to accommodate forces generated during stripping from the mould, transportation, and final installation

  • Careful consideration should be given to the location of cast-in lifting anchors to ensure excessive cantilevers are not generated during lifting operations

  • Consider appointing a single entity (or Chartered Engineer) to have overall control of the design of the lifting system for stripping, transportation, and final installation

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Prestressed hollowcore units can fail during lifting if not carefully designed, planned, and managed, says a reporter. The units are prestressed on the tension face with none or minimal reinforcement in the compression face. Lifting can form cantilevers in the units which generates tension in the top surface. If this tensile stress exceeds the tensile capacity of the concrete, the concrete will crack and the unit may fail, with delamination likely.

The reporter has come across the following types of failures of prestressed hollowcore units during lifting:

  • Failure with lifting clamps (Figure 1): Excessive cantilever end due to use of wrong size of clamp for unit length and misposition of clamps in the unit length

  • Failure with choke hitched chains (Figure 2): Overstressing of unit bearing at chamfered end of plank due to inclined contact (low end makes first contact). Snagging of upper end of unit on adjacent steelwork. Snatch load from chains when unit is released on its upper end - due to the open cores, the snatch load crushed the upper end of the unit.

  • Failure with lifting hooks: Overstressing of unit during lifting due to either or a combination of; snagging of unit on adjacent steelwork, snagging of unit on adjacent units or pre-existing damage to the units

  • Failure with cast-in lifting anchors: Excessive cantilever end beyond lifting pins i.e. designed location of lifting pins not adequately considered. This can be due to separation of responsibility for design of lifting pins and the design of the unit to accommodate lifting operations.

Image
Figure 1: failure of prestressed unit with lifting clamps
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Figure 2: failure of prestressed unit with choke hitched chains

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