STRUCTURAL ENGINEERS in Israel were warned five years ago that the type of floor which collapsed causing Jerusalem's wedding day tragedy last week did not comply with design standards.
Three floors of the Versailles Hall collapsed in the disaster, killing 23 of the 600 wedding guests. All had been dancing on the top floor when the concrete slab structure gave way, dropping them 25m through the building as the floors below collapsed in turn.
The disaster has prompted widespread emergency inspections by building owners across Israel. Investigations focus on the controversial floor construction system, known as Pal-Kal, which does not have conventional shear reinforcement (see box).
At least 500,000m floors are known to exist in Israel. The construction method was popular in the 1980s and major buildings such as the Bank of Israel, the Hebrew Union College and Tel Aviv University's main library all used it.
But in 1996, after a series of non-fatal failures, the Standards Institution of Israel set up a special committee to investigate Pal-Kal. It concluded that the system did not comply with the country's concrete design code, not least because of its lack of shear resistance.
The code was amended to highlight this non-compliance, and, despite furious protests from its inventor, both the Institution and the Ministry of the Interior circulated warnings to all structural engineers and building departments to inform them of Pal-Kal's shortcomings.
Committee member Asher Muller, a partner in Tel Avivbased structural engineer Muller Schnabel Tzacher, said on Tuesday that the Pal-Kal system 'definitely has a lower than normal coefficient of safety, especially in earthquakes'.
He added: 'It also appears that the slab that failed had not been designed for dance floor loading, and that water had been penetrating the concrete for some time, which could have corroded the steel.'
Ten people were arrested in the aftermath of the tragedy, including Pal-Kal's inventor and the building's owners.
Four were provisionally released on Monday when investigators ruled out the possibility that renovation works being carried out on the first floor could have caused the collapse.
Early reports that vital columns had been removed from the building have so far been discounted.
However, it has emerged that the building was operating as an entertainment facility without a permit from Jerusalem Municipality. Built in 1986 as an industrial unit, the Versailles Hall was granted a temporary permit for conversion to a banqueting suite in 1993, but this expired four years later.
It is not yet clear if the Municipality demanded a full structural analysis as a condition of a permanent approval, or if any such analysis was ever carried out.
Building dynamics expert Dr Aleksander Pavic of Sheffield University said that converting a building to dance floor use had special risks. 'With long span floors there can be a reaction to the third or fourth harmonic of dancing and jumping in time to music which will greatly increase the loads on the floor.
'This will not normally cause it to collapse, but the large deflections produced can cause all sorts of internal fracturing.'