CROSS Safety Report
Steel balconies fixed to precast hollowcore floor planks
This report is over 2 years old
A reporter is concerned about the suitability of providing a retrofitted steel balcony requiring moment connections to the sides of typical hollowcore floor planks.
Key Learning Outcomes
For civil and structural design engineers:
The need for collaboration over boundaries should be recognised where multiple parties and / or systems are involved. Particularly for safety critical elements such as balconies.
Where specialist systems such as precast hollowcore units are used to support balconies, their structural capacity should be supported by design calculations
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A reporter's firm has been carrying out structural design calculations for a specialist steel fabricator on a housing project. They are concerned about the suitability of providing a retrofitted steel balcony requiring moment connections to the sides of typical hollowcore floor planks. There will clearly be a hogging moment at the junction of the balconies to the floors. If the hollowcore floor planks only have reinforcing tendons at the bottom of the planks, what will be resisting this negative moment and torsion in the slab and what will prevent uplift of the slabs asks the reporter?
The firm questioned a floor plank specialist who replied with ‘that is what we always do’. In the opinion of the reporter, this does not suffice as a justification. Similarly, there are scenarios where the steel brackets are fixed to the ends of the floor planks (i.e. they span the same direction as the cantilevered steel balconies) - but again, no thought appears to have been given to top reinforcement in the planks, and the only treatment is to infill the hollow voids in the planks with mass concrete at the locations of the fixings, says the reporter.
The firm questioned a floor plank specialist who replied with ‘that is what we always do’. In the opinion of the reporter, this does not suffice as a justification
Expert Panel Comments
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There have been numerous reports to CROSS about balcony collapses. You can search for safety information on balconies on the CROSS website. These show that cantilever balconies can and do collapse and must be designed and constructed with care.
There are also many cases reported in the media of balcony collapses. The laws of physics are immutable. A cantilever will induce bending at its termination point with tension on the top face. Such tension might be resisted by the tensile capacity of concrete but that is fundamentally unreliable and the mode of failure is brittle. Moreover, a cantilever has no redundancy so the described fixing methodologies are basically unsafe.
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