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CROSS Safety Report

Structural integrity reinforcement in post-tensioned concrete slabs

Report ID: 1277 Published: 12 June 2024 Region: CROSS-AUS


Overview

A reporter has found that structural integrity reinforcement in post-tensioned concrete slabs is not being installed correctly by some contractors. They believe that this could result in catastrophic failure of slabs if subjected to earthquake action.

Key Learning Outcomes

For Structural Engineers and designers:

  • Recognize the need for appropriate structural integrity reinforcement near the bottom face of concrete slabs and beams to be continuous through column cores to ensure integrity of these structural items in the event of severe events, such as earthquakes
  • Check that designs for post-tensioned slabs and beams have sufficient flexural reinforcement continuous through supporting columns and are fully compliant with design standards, even after construction tolerances have been taken into account
  • Wide, shallow “band beams” that are susceptible to punching shear should be considered as slabs and slab integrity reinforcement provided in two directions in accordance with Clause 9.2.2 of AS3600

Full Report

Find out more about the Full Report

This section contains the Full Report submitted to CROSS and describes the reporter’s concerns or experiences. However, the text has been edited for clarity, and identifiable details have been removed to ensure anonymity and confidentiality. If you would like to know more about our secure reporting process, or submit a report yourself, please visit the reporting to CROSS-UK page.

 

A reporter has found that structural integrity reinforcement in post-tensioned concrete slabs is not being installed correctly by some post-tensioning (PT) contractors. In addition, several PT contractors known to the reporter do not agree that slab bands should have structural integrity reinforcement in both directions.

Structural integrity reinforcement through column cores is required in both directions within a slab band as stated in AS3600-2018 Clause 9.2.2. This requirement is clearly described in the Steel Reinforcement Institute of Australia Technical Note 8 (SRIA TN8) published in November 2022, which states:

"In 2018, based on the lessons learnt from the Christchurch, New Zealand earthquake events in 2011, AS3600 was revised to incorporate many new provisions to safeguard future Australian buildings from seismic events and provide important life safety to the occupants if the buildings are subjected to one of these extreme events. One simple reinforcement detailing requirement that was incorporated into the Standard was structural integrity reinforcement for both beams and slabs. This nominal amount of reinforcement was found to be very effective at preventing the collapse of slabs following punching shear failures in Christchurch, improving the life safety of the building."

The reporter's concern is that some PT contractors are treating the "slab band" as a "beam" in design. The reporter notes that SRIA TN8 suggests the rule of thumb for a beam is a member with a depth of one and a half to two times its width. Therefore, the majority of these slab bands, in the reporter's opinion should not be considered as beams and should not be designed as such.

The reporter is concerned that without the correct installation of the structural integrity reinforcement, there could be catastrophic failure of slabs under earthquake action.

It is the reporter's opinion that there is a lack of understanding of the importance of this requirement within the post-tensioning industry, coupled with an unwillingness to alter design methods as it adversely affects cost, suggesting little consideration for safety.

The reporter would like to see this issue broadcast as widely as possible so that current design methods change, and that it is factored into the cost by PT contractors when initially pricing jobs.

Expert Panel Comments

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Expert Panels comment on the reports we receive. They use their experience to help you understand what can be learned from the reports. If you would like to know more, please visit the CROSS-UK Expert Panels page.

Although it is assumed that this report refers in the first instance to PT contractors who are providing a design and construct service, it applies equally to any design entity documenting post-tensioned slab solutions.

For completeness it should be noted in the context of this report that "integrity reinforcement” refers to reinforcement at the bottom of a slab running continuously through the slab/column interfaces in all spans framing into columns, as well as shear reinforcement.

Integrity reinforcement is required to increase resistance to progressive collapse in accordance with the National Construction Code and AS/NZS1170.0. As reflected in the extract from SRIA TN8 quoted above, experience has shown that the overall integrity of a structure can be substantially enhanced by minor changes in the detailing of reinforcement; in this particular case the provision of “structural integrity reinforcement for both beams and slabs”. It is the responsibility of the designer of post-tensioned slabs to comply with the NCC and, in turn, with AS3600-2018, assuming the deemed to comply approach is taken.    

As the reporter says, there appears to be a lack of understanding of the requirement for structural integrity reinforcement within the PT industry. It may be that the significant benefit of a relatively small amount of bottom face continuous reinforcement through the column core in preventing catastrophic collapse may not be intuitively obvious to many designers.

some confusion exists amongst design engineers regarding the design of wide, shallow “band beams”

The effect of the structural integrity reinforcement is to produce a much more ductile punching shear mechanism, thus preventing the redistribution of load to adjacent columns (which could initiate progressive collapse) in the event of a punching shear failure. In this sense it contributes to robustness, although it does not satisfy normal robustness requirements which must also allow for the loss of the support. On the other hand, while the PT tendons in the top of the slab over the column cannot be considered to contribute to the integrity solution, they can be used to contribute to the robustness requirements if they are placed within the column core.

It is noted that some confusion exists amongst design engineers regarding the design of wide, shallow “band beams”. Some designers and PT contractors assume that these members can be designed as beams, when in the majority of cases they should be designed as thickened slabs. Although AS3600 does not provide a definition for a "beam", in its discussion on slabs the commentary to AS3600:2018 states in Cl. 9.2.2:

"Integrity reinforcement should be supplied at column or wall connections in all slabs which are susceptible to possible punching shear failure. This includes flat plates, flat slabs with drop panels and band beam and slab connections."

The implication for band beam systems is that if the band beam is wide enough relative to its depth so that a punching perimeter can form within the width of the band beam, then the band beam is susceptible to possible punching shear failure, and should be treated as a slab in the evaluation of structural integrity reinforcement.

In the absence of a more detailed analysis, SRIA's Technical Note 8 is a good summary of the requirements for structural integrity reinforcement in slabs, as read in conjunction with the AS3600 commentary Cl.9.2.2.

ensure that an adequate proportion of the tendons and unstressed reinforcement at the top of the slab are also located within the punching zone

It is further noted that slabs, including band beams, must also be designed for beam shear in accordance with Clause 9.3.2 (a), which refers to Clause 8.2 of AS3600, where shear failure can occur across the width of the slab or band. Where shear reinforcement is required, the use of open fitments is undesirable. These are often used to assist in steel fixing and laying of post-tensioning ducts. Where open fitments are permitted, closing ties are required to provide effectively closed fitments and to meet the detailing requirements.

Note: The term "fitments" is used here for shear reinforcement that otherwise may be termed ligatures, stirrups, links or ties.

It is worth bearing in mind the need to ensure that an adequate proportion of the tendons and unstressed reinforcement at the top of the slab are also located within the punching zone as required by Clause 9.1.2 of AS3600.  This is an issue that is often addressed on the document only to be ignored on site.  An additional consideration is that whilst the tolerance for the horizontal location of tendons may be ten percent of the spacing (Cl17.5.3 b) iii), the required proportion may not be met if that ten percent causes tendons to fall outside this zone.

In the interim (until such time as compliance with this requirement becomes commonplace) it may be necessary for the principal designer to place a hold or check point in the construction specifications that requires the contractor to check and record specific reinforcing details including integrity reinforcement.

there have been several previous CROSS reports on the subject of punching shear

Finally, it is noted that this issue should be applied to all structures, not only those subject to seismicity. Failure to provide integrity reinforcement can lead to progressive slab collapses, whether due to accidental overload, fire, or other extreme events. Indeed, catastrophic slab collapses have occurred due to crowd events such as dancing at wedding receptions. 

It would appear that requirements for punching shear in slabs are not well understood and there have been several previous CROSS reports on this subject, including:

CROSS-AUS is considering possible further publication on this topic, including a discussion on “beams” in slabs, in forthcoming communications. If you have experienced any similar issues or would like to contribute to this topic, we invite your response via the Feedback page on our website.

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