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CROSS Safety Report

Overloading metal decking in temporary state

Report ID: 919 Published: 1 August 2020 Region: CROSS-AUS

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Overview

During construction of steel structures with composite metal decking floors, a reporter has observed numerous occasions when the metal decking was close to being overloaded.

No visible damage appears to have occurred, but this appears to have been a series of three near misses on the same construction site within a fairly short period of time.

Key Learning Outcomes

For structural and civil engineers:

  • Check the load carrying capability of metal decking used as formwork for in-situ concrete construction

  • Ensure that all those working on construction sites where metal decking is being used as formwork understand the need not to overload the decking and not to apply loads to reinforcing bars awaiting placement of concrete

For construction personnel working on site:

  • Ensure loads applied to metal decking used as formwork in concrete construction do not exceed manufacturer’s recommendations, regardless of values given in standards or codes

For manufacturers of metal deck sheeting:

  • Consider displaying maximum (span-related) load-bearing capabilities on packaging delivered to site

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During construction of steel structures with composite concrete and metal decking floor slabs, a reporter has observed numerous occasions when the metal decking was close to being overloaded. The reporter has highlighted three examples from the same construction site at different times.

Propping applied to incomplete floor

In the first example, the concrete floor slab had been partially constructed leaving an area of exposed metal decking with reinforcement in place but not yet concreted. The propping for a concrete beam at the level above was placed partly on the concrete slab and partly on the reinforcing bars bearing directly on to the metal deck. It is the reporter's opinion that the subcontractor did not understand that without the hardened and cured concrete the metal decking would not have been able to transfer the propping loads back to the main steel structure.

Stack loading on incomplete slab

In the second example formwork had been temporarily stacked on the partially completed slab and was again partly supported on the concrete slab and partly supported on the reinforcement over the metal decking. The reporter notes that in addition to possible damage to the metal decking, these types of stacking loads can cause overloading of the structure resulting in cracking of the concrete structure or excessive deflection of the floor system.

Concentrated loading from concrete skip placed on metal decking

In the third example an area of metal decking and reinforcement were being prepared for concreting and a concrete skip had been placed directly on the reinforcing bars over the metal deck. The reporter is concerned that when the skip is placed and lifted off, there is a brief moment when the majority of the load will be concentrated at the edge of the skip and this could cause overloading on the metal decking or worse, punch through it. It is the reporter's view that these examples highlight the lack of understanding of how composite floor systems work and there needs to better management of construction material and stacking loads on site. Issues such as these can lead to incidents on site if not understood and managed correctly.

Expert Panel Comments

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It is common practice with composite floor systems for the profiled metal decking to be used to carry temporary construction loads but this must be done in accordance with the manufacturers’ recommendations for allowable loads and arrangement of temporary propping; and the parties involved in the construction should be familiar with these.

Significant difference between limitations on loads

For example, although AS3610 Formwork for concrete specifies a design load of 4.0 kPa for stacked materials prior to concreting, the allowable load for stacked material in the manufacturers’ published literature is generally much less – typically 1.0 or 1.5 kPa - and AS3610 further notes that any limitations on the magnitude and locations of stacked materials must be included in the project documentation.

Any departure from the manufacturer’s recommendations should be referred to the manufacturer for advice or be checked by a qualified structural engineer with experience of the system being used. The examples reported of concentrated loads from props and stacked materials being partly on the exposed metal decking would certainly fall into this latter category.

Near misses should be reported to CROSS-AUS

It would appear that in the reported cases, no damage occurred and there was no failure, and it therefore raises the question should cases such as this be reported to CROSS-AUS? The answer is yes, as the reporting of any concern for structural safety is important, and we can learn as much from near-misses as we can from actual failures. When failures do occur in such cases, it is often due to a lack of understanding of the products being used and/or the construction techniques required by the builder to use them.

In the event that damage, or a failure occurred, such an incident would likely be required to be reported to the relevant work health and safety regulator and an investigation would commence with enforcement action a probable consequence.   

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