CROSS Safety Report
Balcony collapse at block of flats
This report is over 2 years old
Overview
An in-situ reinforced concrete balcony collapsed at a block of flats which are believed to have been built in the 1960s.
Key Learning Outcomes
For clients and the construction team:
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Cantilever balconies deserve particular attention during design and construction
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Quality control and competent supervision on site can help to ensure that the structure is built in accordance with the design
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Effective communication of essential design information in an accessible form to tradespeople working on site can also ensure the works are in accordance with the design intent
Full Report
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An in-situ reinforced concrete balcony collapsed at a block of flats which are believed to have been built in the 1960s. Fortunately no one was injured. An inspector found that the external balcony appeared to be a continuation of the floor slab and served as the residents' only access to their individual flats. It was approximately 150mm thick and cantilevered by approximately 1.3m.
The majority of the reinforcement, of approximately 8mm in diameter, was located in the bottom half of the slab (compression zone) and some of the reinforcement had next to no cover. The closest the reinforcement appeared to get to the tension zone was around the neutral axis.
The reporter says that the most likely reason for the collapse was the lack of reinforcement in the tension zone. However, the balcony had clearly managed to remain in place for the last 40 years or so, relying only on what little tension capacity was afforded by the unreinforced concrete.
There are a number of other similar balconies in the area which have now been propped and work is underway to assess whether they too are susceptible to a similar type of failure. The balconies repairs are being planned in a risk based priority order.
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Feedback
This describes a form of ductile failure. I think the "all at once & nothing fust" of Wendell Holmes is now well discredited, although one could (unfairly?) describe that as an accountant’s method/principle. (v. the "wonderful one-hoss shay", as described in an old design book). Report 365 Alterations to existing buildings with no site visits brings to mind an experience that I had. Report 411 Quick & cheap design calculations: –astounding, perhaps should not be. See BSI news:- BS 7000-4:2013 Design Management Systems: Guide to managing design in construction. I am often reminded of the tragic Aberfan principle/fallacy, including – Nothing gets done until there is a tragedy – often, sadly, loss of life. There was a very similar tip collapse to the Aberfan landslide, a few miles away during WW2. Witnessed by a lone cyclist, I seem to remember from the parliamentary report. And one of their other conclusions, that all necessary knowledge was available, but not to those responsible for the tip. A tragic case of door-shutting after the horse.
Expert Panel Comments
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It is not known why the bars were in the wrong face of the slab. They may have been placed correctly but without adequate support, and site operatives could then have walked on the bars and displaced them whilst placing concrete. Or perhaps there was a continuous shutter under the slab for the interior of the building and the balcony, with no recognition that the last section was a cantilever. If this was the case, the bottom rebar for the interior span would simply be carried on into the cantilever.
In any event, there was poor control on site and for reasons of quality and safety there has to be competent and resourced supervision. Since this case was brought to the attention of the Health and Safety Executive (HSE), there have been reports to them of other cases, so the problem may be systemic.
Structural safety relies not just on adequate strength but also on controlling modes of failure such that they are ductile, do not cause harm and give warning of impending collapse. Rapid brittle failures of any kind are to be avoided. No doubt this slab was mostly relying on tensile capacity of the concrete to survive, but even if that had been adequate, the slab would have been ‘unsafe’.
Great care is also required if percentages of rebar are small such that members have less bending capacity than they would have based on the tensile strength of the concrete. The danger is that under increasing load, the member fails at the higher moment and load transfer to rebar is so rapid it just snaps rendering it useless.
The incident once again shows the importance of an experienced person just walking around sites and seeing that gross mistakes are avoided.