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‘Perfect storm’ of flaws caused cracks in Opal Tower, says report

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A combination of flaws in design, faulty pre-fabrication and inadequate installation of the pre-fabricated walls, plus damage to critical building elements by work done during construction, led to the cracking in the Opal Tower in Sydney’s Olympic Park.

And an independent report produced for the NSW government, delivered by Planning Minster Anthony Roberts today, says the remedial work already underway should be continued to prevent further damage and make the building safe, especially in case it is subject to extreme weather conditions.

Residents of the Opal Tower were evacuated after cracking was discovered in some load-bearing walls, initially on the building’s tenth floor, on Christmas Eve last year.

They were allowed to re-enter but were forced to re-evacuate three days later when the damage to the building was shown to be more extensive than first thought. The majority of residents have since returned to the building.

The Opal Tower report issued today, compiled by Professors John Carter, Mark Hoffman and Stephen Foster of the School Civil and Environmental Engineering at UNSW, identifies the failure of a relatively small number of under-designed, inadequately prefabricated and poorly installed construction elements in the building.

Their failure then put additional stress on other load-bearing sections, pushing them  beyond their designed capabilities. However, the professors reiterated their earlier belief that the Opal Tower is fundamentally sound and the problems can be fixed.

“We have found no evidence contradicting our interim assertion that the building is overall structurally sound, although there is significant damage to some elements,” says the independent report, compiled at the behest of the NSW Department of Planning and Environment.

“It should be noted that extreme environmental events, while rare, could precipitate further damage and consequently it would be prudent not to delay rectification works.”

The alert was initially raised on December 24 when Opal residents heard a loud bang and later observed cracks appearing in walls on the tenth floor, at the foot of a designed architectural feature “slot”.  Security cameras identified the first cracks appearing at precisely 2.18 pm, and cracking continuing for eight seconds.

The highly technical report released today focussed on hob beams and connected  wall panels, the latter being mostly constructed off-site and slotted into place.  The beams and panels together were intended to carry some of the “dead loading” of the 32-story building.

However, the beam and panel assembly was “under-designed, according to the National Construction Code and the Australian Standard for Concrete Structures, at a number of locations in the building”, says the report.  “This left the hob beams susceptible to failure by shear compression and bursting.”

The report then claims that a decision taken during the installation of the panels to only partially grout the joints between them and the hob beams “significantly raised the levels of stress in the hob beams on levels 4, 10, 16 and 26.”

The report also claims that “construction and material deficiencies likely precipitated the observed major damage to hob beams.” On Level 10-C, electrical conduit and reinforcing steel in the cover region and a panel repair were to blame, while on Level 4-A lower strength concrete than used in hobs elsewhere and partial grout coverage were at fault.

However, there is some good news for the apartment owners in the Opal. The report says the short-comings in the design and construction can be remedied, but they issued some words of caution.

“We recommended that a detailed analysis be undertaken of the potential redistribution of loads from the damaged elements to ensure that other newly loaded building elements, before and after rectification works, have suitable capacity and to avoid future damage,” they say, adding that checks on other undamaged parts of the building should be “robustly and independently verified.”

They also recommend that the rectification work and checks, followed by certification by qualified independent structural engineers, should be completed before the remaining residents were allowed to move back in.

At the end of January, the Opal’s builders Icon, saying that 65 percent of the apartments were safe for occupancy and could be re-occupied, stopped paying for emergency hotel accommodation for some residents.

As well as identifying the issues and remedies for the Opal, the trio of UNSW professors made a string of recommendations.

  • The creation of a government registered engineers database developed in partnership with an appropriate professional body.
  • Independent third party checking and certification of engineering designs and subsequent changes to the design of critical elements by a registered engineer, including confirmation of what are the critical elements for all major construction projects.
  • Critical stage, on-site checking and certification by a registered engineer that construction is as per the design for all major construction projects. All changes to identified critical structural elements that are proposed and made during construction should also be certified by an independent registered engineer.
  • An online database be created, where all certifications may be viewed by a broad range of stakeholders including owners and prospective owners; before, during and after construction. The aim is to increase transparency of the approval and certification process.
  • A Building Structure Review Board be formed, with the major purpose being to establish and publish the facts relating to structural damage of buildings arising from design and construction, investigate their causes and to recommend changes to codes and regulations where appropriate.

NSW Planning Minister Anthony Roberts has already flagged the creation of a Building Commissioner, should the Liberal Coalition retain government after next month’s election.

And Better Regulation Minister Matt Kean has announced a general tightening of the licensing and “duty of care” for all professionals in the building industry.

You can download the full report HERE.

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