A balcony collapse

Jul 1, 2007 by

We read an interesting article in the morning paper about a balcony collapse. Later on, we were assigned the investigation.

The insured’s property was a four-story condominium complex with an in-ground swimming pool at the first floor level.Walking up to the building, the fourth floor balcony was clearly visible.  While the walls of the balcony were still in place, the main body of the balcony floor had fallen down onto the floor of the third floor balcony, directly underneath.    Were one to step out onto the 4th floor balcony, one would have a quick trip, through the gaping hole with jagged edges, onto the third floor balcony.

The partially collapsed balcony floor was supported by three 6-inch deep I-beam section cantilever steel beams projecting from the building.   A 9.5-inch by 3-inch outer edge C-channel steel was bolted to the ends of the I-beams.  The middle I-beam (off-center from the balcony sides) appeared to have been embedded in the chimneystack, but the extreme side beams could not be discerned as to their point or method of attachment to the building, and may have been similarly cantilevered or may have been attached using a bolted mechanism.  A framed wall surround to the balcony sat on top of the outer edge C-channel steel and the side I-beams.  The wall was coated with stucco.  The floor of the balcony was formed using a 2-inch thick concrete slab poured integral with formed steel trough members approximately 1 – 2 mm thick, with those members spanning between the building and the channel edge beam.

The balcony floor concrete was reinforced with steel reinforcing bars (rebar) in the 2-inch thickness using ½-inch rebar.  Beneath the trough members, forming an enclosed space, was a further former layer of rebar, under and attached to which had been laid a steel wire mesh, to which the stucco ceiling surface below the balcony had been applied.  The rebar appeared to have spanned between the edge beam and the building in support of that mesh.  Drainage from the slab was taken through a 2-inch by 4-inch scupper to a rainwater hopper attached to the outside edge of the balcony and then to a down pipe.

The observed condition of the rebar, the trough members and the steel cantilever beams and edge beam was that of corroded steel.  The trough members appeared to have disintegrated and to be almost completely comprised now of rust, with the rebar and steel beams having an unknown lateral extent of corrosion, but directly observable as at least 1 mm depth of corrosion product to the rebar and light corrosion to the beams.  It was our expert’s opinion that such corrosion had taken at least five years to form in the confines of the enclosed floor space of the balcony structure.  What remained of the edges of the concrete floor of the balcony had no waterproofing finish to it.  The unit below the affected unit had suffered penetration of water from the balcony level above, as evidenced by staining and efflorescence around their chimneystack.

The balcony floor fell because the floor of the balcony had been exposed, without sufficient protection, to the elements of the weather, allowing penetration of the permeable concrete slab by accumulating rainwater.  The rainwater percolated through the concrete to the unprotected steel trough and rebar and caused corrosion to occur in that steel.   Once the trough and rebar were sufficiently corroded, they could no longer support the weight of the concrete floor.  The floor then fell to the balcony below it.  The direct cause of the failure was the lack of waterproofing on the surface of the concrete deck.