NSW's apartment construction reform set to follow the devastating eight seconds of Opal Tower low strength concrete cracking

NSW's apartment construction reform set to follow the devastating eight seconds of Opal Tower low strength concrete cracking
NSW's apartment construction reform set to follow the devastating eight seconds of Opal Tower low strength concrete cracking

The NSW Government today released the final report from the independent investigation into Opal Tower

It has recommendations about ensuring qualified people design buildings and that buildings are built to those designs.

The biggest recommendation in today’s report is about registering engineers.

“We will be working through the detail of today’s report with our new Building Commissioner to make sure that when homeowners are handed over the keys to a new building, it is safe, structurally sound and free from major defects," Minister for Planning and Housing Anthony Roberts said.

The report came after Professors Mark Hoffman, John Carter and Stephen Foster were asked to consider the cause of the structural damage to the Opal Tower on Christmas Eve.

The report advised security camera footage showed the cracking in the bottom corners of the bottom panel (Panel A) on Level 10 with the time stamp on this video recording indicated that the cracking of the panel commenced at approximately 2.16 pm on Monday 24 December 2018 and continued for approximately 8 seconds.

NSW's apartment construction reform set to follow the devastating eight seconds of Opal Tower low strength concrete cracking

“Our final report confirms that a number of structural design and construction issues, including non-compliance with national codes and standards were responsible for the observed damage at Opal Tower.

“We found some of the as-constructed hob beams and panel assemblies were under designed according to the National Construction Code and Australian Standards, leaving the beams prone to failure.

"We also found construction and material deficiencies likely contributed to the damage to the hob beams on levels 4 and 10.

NSW's apartment construction reform set to follow the devastating eight seconds of Opal Tower low strength concrete cracking

“We consider that the building is overall structurally sound and the localised damage to the building can be rectified to ensure the building is compliant with the National Construction Code.

"We agree in principle with the rectification works planned to date, noting that these have advanced considerably since our interim report was released.

"However, we repeat our call for qualified structural engineers to check the final design and construction proposals in detail before major rectification works begin and before the building is deemed completely safe for occupancy, while noting the reoccupation of the building extends beyond the scope of our investigation and is a matter for residents and the builder."

NSW's apartment construction reform set to follow the devastating eight seconds of Opal Tower low strength concrete cracking

It confirmed most, if not all, major columns in the building had been designed so that their maximum axial load carrying capacity far exceeded the design loading imposed on them, indicating a very high factor of safety to these critical elements. 

However, a preliminary analysis of the splitting forces near the interface with the two hob beams reviewed (Level 4 – grid line A and Level C -grid line C) indicates that this aspect of the design should be confirmed by a qualified engineer for all such features.

NSW's apartment construction reform set to follow the devastating eight seconds of Opal Tower low strength concrete cracking 

It found no evidence contradicting their interim assertion that the building is overall structurally sound, although there is significant damage to some elements. 

It advised it was notable that the observed damage was constrained to the inside facing of the hob beam; the outside of the hob beam at the critical section was supported by a lateral garden bed wall (Figure 16) built integrally with the slab and against the hob beam.

This wall likely provided support to the outer side of the hob beam forcing the damage inward where such support was lacking.

The damage remained “hidden” until exposed on 27 December 2018 after removal of an internal wall in the adjacent apartment. 

The site observations of the damaged beam are consistent with a bursting failure.

"There are contradicting views and documentation as to the design strength of the hob beam concrete on Level 4.

"Notwithstanding, 65 MPa concrete was understood to have been poured during construction as it was specified for the puddle pours in the slab around the columns and it was considered expedient to use the same batch of concrete when pouring the hob.

"During our investigations, and subsequent to the issue of our interim report, records of strength for the concrete used to construct the hob beam were provided to us.

"These reveal 28 day strengths of these concrete samples as 50 MPa, where 65 MPa concrete was ordered for supply, indicating that the concrete in the hob may also have been of a lower strength.

"Our independent analysis of concrete core testing samples, extracted and tested in January 2019 by Mahaffey Associates, did not contradict this observation.

"Clearly, this mis-match in design strengths points to a possible and unfortunate ambiguity in the interpretation of the design documentation."

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

It formally noted: 

  1. The as-constructed hob beam / panel assembly was under-designed, according to the National Construction Code (NCC) and the Australian Standard for Concrete Structures (AS36001), at a number of locations in the building. This left the hob beams susceptible to failure by shear compression and bursting.

  2. The decision, taken after the initial design, to grout only partially the joints between the hob beams and panels, significantly raised the levels of stress in the hob beams on levels 4, 10, 16 and 26.

  3. 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) and Level 4-A (lower strength concrete than used in hobs elsewhere and partial grout coverage).

  4. The observed damage in the concrete panel at Level 10 and in the Level 10 floor slab was likely a consequence of the adjacent hob beam failures and not the original cause of the damage observed at Level 10.

It formally advised making the Opal Tower safe for occupancy

  1. Appropriate rectification works can address deficiencies in the original, as- constructed, structural design and ensure the building is compliant with the NCC.

  2. Significant rectification works are necessary to ensure that the building and all its structural components satisfy the NCC and specifically the current AS3600- 2018

  3. The damaged hob beams should be rectified to provide the required load carrying capacity.

  1. Other hob beam / panel elements of similar as-constructed, structural design may not comply with the NCC and AS3600-2009 and, if so, will require rectification works. We also recommend checking of the forces in other structural elements adjacent to the hob beams, such as the columns.

  2. We agree in principle with the rectification works planned to date, noting that these have advanced considerably since our interim report was released but have yet to be agreed by all parties and independently certified.

     10. 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. A preliminary analysis has been carried out and indicated structural loads satisfied the NCC in the non-damaged parts of the building structure. Nevertheless, this finding should be robustly and independently verified.

     11.We recommend that all designs and construction associated with the rectification works be checked and certified as safe for building occupancy by qualified independent structural engineers.

     12. The viability of residents re-entering the building extends beyond the structural issues considered here and hence beyond the scope of this investigation. Nevertheless, we would recommend that items 9-11 listed above be completed prior to re-occupation.

A full copy of the final report is available at: https://www.planning.nsw.gov.au/finalopalreport

 

Jonathan Chancellor

Jonathan Chancellor

Jonathan Chancellor is one of our authors. Jonathan has been writing about property since the early 1980s and is editor-at-large of Property Observer.

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