Preventing structural collapse during demolition

WorkCover NSW has issued a safety alert following an incident involving the collapse of a suspended concrete slab during the demolition of a multistorey building. An excavator and its operator fell with the slab as it collapsed. The operator and some passers-by received minor injuries and the excavator was badly damaged. Preliminary findings indicate the concrete slab was overloaded, resulting in the collapse of the slab. The collapse was initiated by ‘punching shear’, where an internal column ‘punched’ through the slab resulting in a rapid, progressive collapse of the entire slab.

WorkCover NSW has reminded employers and those in control of demolition sites to ensure they verify the load capacity of suspended concrete slabs, before loading them with demolition machinery and/or rubble. Additional temporary support, eg, propping - may be needed to enable the floors to hold the demolition loads.

Before commencing demolition, ensure the following:

• A sequential demolition plan is to be prepared that is specific to the site and is approved by a competent structural engineer. Specifically, the plan should consider the rate of demolition and outline the load capacity limits of floors, ramps or other suspended slabs. If temporary supports are required, their specification and positioning should clearly be shown in the plan.
• Consideration must be given to the method for moving plant from floor to floor. If using ramps, the loads on the structure should be verified. In particular, the ramp’s specifications, location and the placement of temporary supports should be documented in the demolition procedure.
• Consideration must also be given to the means for clearing rubble from floors or other suspended slabs to prevent a build-up of rubble in excess of that specified in the demolition plan.
• Exclusion zones are to be established to keep unauthorised people outside of potential collapse zones.
• During demolition, the person in control of the work must ensure the following: work is to be supervised by a competent person(s) and done safely; the documented demolition procedure is to be followed. If changes to the procedure are proposed, they are not to be implemented until they are verified, in writing, by the structural engineer. Such changes can concern the following: the installation of temporary supports according to manufacturer’s instructions and their positioning according to specifications in the demolition procedure; instructions on when to remove rubble to prevent slabs being overloaded; instructions on where a ramp is to be used and whether it needs temporary supports, and the prescribed distances between machines when they’re located on slab.

The Occupational Health and Safety Act 2000 requires employers and those who control workplaces to take all necessary steps to ensure that safe systems of work are established and maintained and that all staff are adequately informed, trained and supervised. Clauses 234 and 247 of the Occupational Health and Safety Regulation 2001 provide more specific obligations to prevent structural collapse of a building during a temporary state of weakness or instability, and to undertake demolition work as required by AS 2601 - 1991 the demolition of structures.