Investigators have said design flaws and poor quality construction are likely to have contibuted to structural cracks in Sydney, Australia’s 36 storey Opal tower which was evacuated on Christmas eve.
The tower was evacuated after large cracks appeared on the tenth floor. Engineers then discovered more cracks on the fourth floor which is now supported on temporary props as a precaution.
The tower was designed by WSP and built by contractor Icon. The results were published in an interim report produced by Unisearch.
The cracks were found to be in the precast panels and their supporting beams which clad the six-storey tall inset slots which form a feature of the building. The building is constructed from reinforced concrete with post-tensioned floor slabs.
Although the interim report says that it is not possible to establish a definitive cause for the cracking of the beams and panels, it said flaws in the design and poor quality construction were likely to have contributed.
The report adds that a preliminary evaluation of the beams indicated that a lower safety factor than is required in the standard had been applied, although further tests are being carried out to confirm this.
During construction, partial rather than full grouting of beams to panels had been found and there was inadequate cover of the concrete on beams and columns due to the presence of reinforcement in the joint and an electrical conduit.
A doweled connection between the beam and panel was found to be incomplete and the report says that there was evidence that the wrong reinforcement had been used in the beams. This meant that they had inadequate tensile capacity. It also says that the panels had been specified to be 180mm thick, but the manufactured thickness was actually 200mm thick.
The report concludes that differential settlement within the building was unlikely to be a contributing factor, saying had this been the case, it would have expected to see more cracks in the nearby floor slabs and columns.
It adds that the timing of the failure was probably due to a progressive build-up of load on the structure as residents moved into the flats.
The report says that the building was “structurally sound and not in danger of collapse”. But it says that designers must check that structural member were not overloaded as a result of load redistribution caused by the damage to the structure before residents were allowed to move back in,
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