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CROSS Safety Report

Demolition risks

Report ID: 96 Published: 1 January 2008 Region: CROSS-UK

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Overview

This report is about the collapse of a flat slab building that occurred overseas whilst the top floor of the building was being demolished onto the next floor down.

Key Learning Outcomes

For clients and the construction team:

  • Where demolition works are required it is good practice to provide knowledge and history of the structure, including form, materials of construction, structural interactions, and location

  • Be aware that structures may have been susceptible to poor detailing and construction, as well as degradation over time which may need to be accounted for prior to demolition

  • Stockpiling of demolition material and access ramps formed from crushed concrete should be adequately controlled to ensure the structure is not overloaded

Full Report

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A reporter has written about the collapse of a flat slab building that occurred overseas whilst the top floor of the building was being demolished onto the next floor down. They say that this process was being undertaken by three tracked vehicles operating on the floor. The collapse was a classic ‘pancaking’ of the slabs as they effectively slid down the supporting concrete columns. Several employees were killed in the accident and a number of serious defects were identified.

Cover to Reinforcement

According to the reporter engineers familiar with the design of flat slab construction will know that CP114 (which was the relevant design document when the building was constructed) and other Codes and Standards, allocate flat slab design bending moments between column and middle strips. Heavy concentrations of shear occur around column heads and these are often strengthened by thickening the slab locally.

The design must ensure that sufficient of the column strip steel located over the column area is correctly located. Investigation of the construction, after the collapse of this building, showed that top reinforcement was not located where it should have been but had been placed closer to the geometrical neutral axis than was originally intended in the design.

Temporary Propping

To cater for the potential additional load of machines and debris, says the reporter, the three floors immediately below any floor that was being demolished were provided with temporary screw jack propping. The reporter says that it will be appreciated that it is extremely difficult to assess the contribution which such propping makes at any individual floor level, because each floor can deflect either during the propping operation and/or during the demolition stage.

An individual floor may have substantially more or less load imposed upon it. A secondary consideration was that the props were designed to be tied at mid height position to add to their load carrying capacity and also to have some cross bracing. It was thought highly probable that no such intermediate tying was used, or, if it was, then only on an ad-hoc and casual basis. Essential cross bracing was thought to have been omitted.

It was thought highly probable that no such intermediate tying was used, or, if it was, then only on an ad-hoc and casual basis. Essential cross bracing was thought to have been omitted.

Loads and vibration from demolition machinery and from debris

The assessment of the building loads and the proposed temporary propping arrangement before demolition did not take into account the fact that heavy tracked vehicles not only imposed static loads, but dynamic loads. There was also no specified maximum height of demolition material which could be allowed to accumulate. Stock piling of broken concrete occurred to considerable heights and heavy machines ran onto and over these stockpiles. Demolition machinery was supposed to be restricted to machines of not greater than 20 tonnes. In fact, one machine was over 30 tonnes.

So, says the reporter, ‘what do we learn from this?’

  • Never assume that because a building has been standing for 30 years that it must be OK!

  • Never assume that a building has been constructed in accordance with the original drawings and specification

  • Temporary propping - indeed any propping - should commence from a firm, solid and static base unless the risks of not doing so are assessed

  • Machinery can result in both static and dynamic loading

  • There may be risks because nobody has explained to the operatives doing the job, why it should be done in a particular way

Expert Panel Comments

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This example clearly demonstrates the need to design for demolition with the same thoroughness as one would design for construction. It also illustrates the need (and requirement) to take an holistic look at the existing structure in order to eliminate all foreseeable hazards and reduce associated risks, so far as is reasonably practicable. The designer i.e. the person specifying the demolition must be satisfied there is a safe way to demolish the structure, and provide adequate information for the contractor.

The current state of the structure must be assessed allowing for deterioration, not the state of the structure as built. A contractor undertaking such work must have the requisite competence and the requirements of CDM regulations apply. It appears that punching shear failure may have been involved, and there have been other examples of the failure of flat slab car parks and similar buildings. Shear failures often give no early indication of impending failure, so demolition, or indeed alteration, of flat slab structures demands care.

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