Under-designed beams, partially grouted joints and construction and material deficiencies were responsible for the damage to the evacuated A$165 million (£91million) Opal tower in Sydney, Australia, a final independent report has found.
It found a number of problems with the design and construction of the reinforced concrete building, which has post-tensioned concrete floors. The tower, designed by architect Bates Smart, was completed in August last year.
According to AJ sister publication New Civil Engineer, the report was carried out by consultant Unisearch.
The 36-storey tower was evacuated on Christmas Eve last year after large cracks appeared on the 10th floor. Engineers then discovered more cracks on the fourth floor and temporary props were installed as a precaution. The tower has remained unoccupied since.
The cracks were found in the precast panels and their supporting ‘hob’ beams, which clad the six-storey tall inset slots that form a feature of the building.
The report concludes that the as-constructed hob beams and panel assembly were ‘under-designed’ according to the National Construction Code (NCC) and the Australian Standard for concrete structures (AS3600) at a number of locations in the building. This, it says, left the beams susceptible to failure by shear compression and bursting.
It also concludes that the decision to only partially grout the joints between the hob beams and panels, ‘significantly raised the levels of stress’ in the hob beams on floors 4, 10, 16 and 26. This decision was taken after the initial design was completed.
‘Only inner surfaces, of approximately 50 per cent to 70 per cent of the joint width, appear to have been grouted, consistent with the shop drawings which show the grout extending over only the inner portion of the hob beam to panel connection,’ the report states.
‘Furthermore, coring of the level four hob beam revealed incomplete grout coverage in some places. This partial grout coverage led to an eccentric bearing load and elevated bearing and bursting stresses on the hob beams.’
Construction and material deficiencies ‘likely precipitated’ the cracking on level 10 where an electrical conduit and reinforcing steel were in the cover region of the concrete and where there had been a panel repair, says the report.
On the fourth floor, lower-strength concrete and the partial grout coverage had contributed to the failure of the beam.
‘Test records of materials used in construction revealed at least one case where concrete used in a hob beam may not have reached its specified 28 day strength,’ it adds.
Opal tower damaged hob beam on level 4 after concrete removed
Incomplete dowel bars, inadequate concrete cover and potentially inadequate tensile capacity in the horizontal direction in one of the panels were also listed as where construction had been different from the design or the standards.
Unisearch said it had drawn the conclusions after a number of activities including multiple inspections, reviews of the construction records, security camera footage and design documents.
The tower was designed with structural engineers WSP and built by contractor Icon.
An interim report into the cause of the damage was published in January. It said that despite the damage, the building was ‘overall structurally sound’. The new report said it found no evidence to contradict this.
Unisearch said it had not received complete details of the structural analysis for the proposed repairs to strengthen the structure and recommended that detailed plans should be checked by an independent engineer. But it said the structural principles behind the proposal were ‘sound’.
WSP and Icon have been contacted for comment.
Opal tower damaged hob beam on level 4 before concrete removed