Quick Search

620 Steel balconies fixed to precast hollowcore floor planks

Report ID: 620

Published: Newsletter No 47 - July 2017

Report Overview

A reporter is concerned about the suitability of providing a retro-fitted steel balcony requiring moment connections to the sides of typical hollowcore floor planks.

Report Content

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 retro-fitted 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

Comments

There have been reports to CROSS about balcony collapses and entering "balcony" into the Quick Search box on the home page of our website gives a number of cases (see below). 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.

Cantilever balconies on domestic structures are safety critical and lives are at risk if they fail. There should always be a lead designer taking responsibility for a coordinated structural design. The design should result in a robust structure which should be executed and checked by competent persons. In a case such as this, has the capacity of the precast units to take the loads been proven?

CROSS reports on balconies:

109 Balcony collapses (news)

341 Balcony collapse at block of flats

435 Balcony strengths of blocks of flats - further experiences

613 Collapse of reinforced concrete balcony (news)

616 Balcony collapse in France (news)


Email Updates

How to Report

Online submission:
Submit by post: