'Safety lower than required': What the Opal Tower report actually says


January 16, 2019 07:59:10

The independent investigation into the cracked Opal Tower apartment block confirmed yesterday what most residents were all-too-aware of: the 36-storey building has significant damage.

But as some residents head into their 21st day in a hotel room — forced out of their homes since Christmas Eve — there are still many questions yet to be answered.

The most potent being: ‘When will we be allowed back in our homes?’

The interim report, written by three of the state’s most senior independent engineers after a two-week investigation, went some way to giving them answers on the cause of the damage.

This is what it found.

The scope

The investigation focused on levels 3, 4, 9, 10, 16 and 26, as well as the basement level B3 — all the areas of known structural damage in the Opal Tower.

The areas of “significant structural damage” are located on levels 4 and 10.

The investigators watched security footage to “observe the cracking” on level 10 that triggered the initial evacuation on Christmas Eve.

The cracking occurred at 2:16pm and lasted for 8 seconds.

The three investigators found the three areas of damage on level 4 were all different, leading them to assume they are likely to be unrelated to each other.

“But this deduction requires further detailed examination before it could be confirmed,” the report said.

The cause

The team hypothesised five likely causes of the crack: the environment, such as heavy rainfall and storms, materials, the foundations, design and construction.

The enviroment and materials were effectively ruled out.

The investigators found the design, namely the connections between the beams and the columns on level 10 and level 4 — the two floors where there was significant damage — indicated factors of safety lower than required by standards”.

The foundations of the Opal Tower had been speculated by some industry insiders as the cause. It was delved into by the investigators, with “differential settlement” — known to the layman as an uneven finish because of a soft ground or the wetting of foundations — considered an “unlikely” cause.

On level 10 the team found a number of points where the construction differed from the design or industry standards.

It found a lack “grout coverage” led to an “eccentric bearing load” on the hob beam on level 10 (a hob beam sits on the edge of a slab of concrete or pillar, sometimes resembling the kerb on a road). They also questioned the location of reinforcing steel, a cut or incomplete steel bar in the area and overhanging precast panels.

“There is compelling evidence indicating that the wrong size reinforcing bars were placed in this area during manufacture of this particular panel – 20mm diameter bars were used instead of 28mm diameter bars,” the report found.

“It is likely that a combination of some of the above design and construction issues led to the observed structural damage on level 10.”

The engineers concluded that a “progressive build-up of load on the structure as [the] apartments became occupied” also culminated with the crack at level 10.

The takeaways

Three weeks after the incident, the investigators had not received all requested materials relating to the design and construction of the building, and were awaiting further information to enable “definitive conclusions”.

However, they found:

  • Significant rectification works are required ;
  • Independent and qualified structural engineers should be engaged to check the major rectification works;
  • Further analysis be undertaken on the structural design of the hob beams and associated structural members; and
  • Before residents re-occupy the building the designers must ensure no structure is overloaded as a result [of the] observed damage to the structure of the building.








First posted

January 16, 2019 06:08:36

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