CROSS Safety Report
Balcony construction defects of residential flats
This report is over 2 years old
A reporter's firm have been undertaking structural assessments of cantilevered reinforced concrete walkways, which in the majority of cases have been found to be under strength.
Key Learning Outcomes
For clients and the construction team:
Cantilever balconies deserve particular attention during design and construction
Quality control and competent supervision on site can help to ensure that the structure is built in accordance with the design
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
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The following report has been sent in response to Report 341 which was about a balcony collapse at a block of flats. The reporter’s firm has had experiences of a similar nature in a UK and wishes to make others aware of what may be a widespread design issue. In recent years, the reporter's firm have been undertaking structural assessments of cantilevered reinforced concrete walkways on a large number of two storey blocks of local authority-owned residential flats. In the majority of cases, the walkways have been found to be under strength.
The flats were all constructed between the mid 1950’s and mid 1970’s and have cavity masonry walls supporting a reinforced concrete first floor slab with a pitched tiled timber roof above. The first floor slab is cantilevered by around 1.3m externally beyond one of the main elevations, in order to provide an access walkway to the entrances of the upper storey flats.
In the first group of blocks examined it was found that in each walkway, the reinforcement was only nominal in size and was lying close to the bottom surface of the slab, even at its support at the face of the building. No reinforcement was located in the upper half of the slabs where it would be expected for cantilevers. Fortunately, both the reinforcement and the concrete were in good condition with only local instances of surface corrosion. The original asphalt waterproofing to the slab was now in need of full replacement. The subsequent strength assessment undertaken found that the theoretical resistance in bending was well below that required for full imposed and dead loading, and in some cases below that required for dead load alone.
The findings were reported to the local authority who undertook precautionary temporary propping of the outer edge of the cantilevers, as recommended by the reporter’s firm, whilst action was considered for both long-term strengthening of the blocks and assessment of other similar designed stock. The Council has nearly 200 of these blocks containing between 6 and 16 occupied flats each. Following the original discovery, a risk assessment was undertaken to identify priorities for further intrusive investigations. Intrusive investigations and structural assessments were then started in tranches, beginning with the highest priority blocks.
The first tranche also included some more recently constructed blocks as a comparison. As each tranche of investigations progressed, the risk assessment was refined to update the risk priority, taking into account any patterns discovered from the findings of the structural assessments. This was especially relevant with regards to the construction quality related to particular individual contracts or individual contractor’s work, because when the blocks were built, the contracts had been let in groups. The investigations disclosed some lesser quality ‘contracts’ or ‘contractors’ which then reprioritised the order of the remaining work. The risk assessment was also revisited following the reporting of a collapse of an apparently similar second floor walkway to a three storey block of flats that occurred elsewhere.
Inadequate reinforcement of balconies
To date, the firm has assessed over half of the blocks and of these, 80% have been found to be inadequately reinforced generally, having the rebar in the lower part of the slabs. Blocks constructed from mid-1970’s appear adequate; all having a structural engineer involved in their design.
Quality of reinforcement was found very poor in a very few cases and included random mixing of square twist and circular bars, and also cases of ‘homemade’ site-welded mesh of small diameter (4mm max) bars at varying centres. The concrete itself has fortunately been found to be a reasonable quality throughout. Visible deflection of slabs has not been a good indicator of potential under strength as it has been found that perfectly good slabs can appear deflected (probably cast like it, possibly due to failings in formwork support during construction). On the other hand severely under strength slabs have appeared to be level.
To date, the firm has assessed over half of the blocks and of these, 80% have been found to be inadequately reinforced generally, having the rebar in the lower part of the slabs.
Structural assessments to date have been undertaken using two checks, one to BS 8110-1 Structural use of concrete and the other to CP 114 The structural use of reinforced concrete in buildings, but in both cases, using loadings as recommended by BS 6399-1 Loading for buildings. Precautionary temporary propping has been undertaken where one or both of these assessments have shown a significant weakness of the slab.
A programme of initial investigations and assessments into all other cantilevered concrete elements in the same local authority’s remaining low, medium and high rise stock is being undertaken. Initial findings are giving examples of poorly positioned/designed reinforcement, with the need in one case of having to urgently prop a pair of public accessible balconies immediately above a main entrance of a three storey block of flats.
Lack of design checking
The firm now advises clients to consider full structural assessment of such cantilevers rather than just checking their condition alone. Experience so far shows that structures built from the 1950 to mid-1970’s may be a greater risk regarding correct positioning of reinforcement. In this era, many contracts for houses/blocks of flats would appear to have been designed, in the case of a large Local Authorities, by a ‘City Architect’ with any structural design of its elements being made the responsibility of the Contractor (or a sub-Contractor) and without the requirement for an independent check of this structural design. There is a wide range to the quality of design, checking, construction and maintenance of the 1950’s/1960’s housing stock and with much of this stock now approaching its originally intended design life of 50/60 years, then there needs to be more than just a visual inspection conducted, says the reporter.
Expert Panel Comments
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The built environment will be left with many legacies, some remarkably good, some equally bad. As a consequence, the issue of ageing infrastructure is the possibility that poor construction or poor design exists, and the engineering community must respond.
In this instance, there is a clear and widespread breach where the structures relied on concrete tensile strength only and as a result there could have been brittle failure with no warning. As the reporter says, visual inspections are not sufficient and a more rigorous inspection with, for example, cover meters is required.
There are also generic issues here which can apply to other cases. Firstly, all structures go into service with a notional safety margin and thereafter degrade, so their safety generally reduces with time. Secondly, with any design, there is always a question for the designer of assuring that what they thought was being built was actually built.
Even with records that claim to show what happened during construction, there may be inaccuracies. When there is doubt about a generic form of structure, then inspections are necessary to protect those residents who have a right to expect cantilevers to be sufficiently safe.
There is a duty to make these findings as widespread as possible. Cantilevers are always potentially vulnerable and this is particularly so for thin slab cantilevers.
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