Shake-up following Kaikoura

This Issue This is a part of the Seismic learnings feature

By - , Build 164

A construction detail is under the microscope and changes have been made to the standard for concrete construction after the poor performance of some mid-level buildings in the Kaikoura earthquake.

Figure 1: Double tee flooring system.
Figure 2: Section A–A through double tee unit.

THE MAGNITUDE 7.8 Kaikoura earthquake in November 2016 produced severe ground shaking in Wellington and resulted in substantial damage to a number of relatively modern commercial buildings. Some of these have already been demolished, and others will need to be.

Statistics House investigation

One of the most significant structural failures occurred at Statistics House in the CentrePort Harbour Quays business park where the building suffered the partial collapse of two floors.

Responding to what was deemed unacceptable performance, the Ministry of Business, Innovation and Employment (MBIE) commissioned an independent investigation into the building performance to find any implications for the building regulatory system. The expert panel noted vulnerabilities in the design of buildings with precast concrete floor systems in combination with frames that might be affected by beam elongation.

Before the Kaikoura earthquake, remedial work was under way at Statistics House to provide additional support to the precast floor units. Where this retrofit solution had been installed, the floor units performed as expected, without collapse.

Projects to address findings

On the back of the Statistics House investigation, MBIE began several projects to address the panel’s recommendations (see Table 1).

This article focuses on the amendments to NZS 3101.1&2:2006 Concrete structures standard and details of the proposed ban on the loop bar hanger detail in new construction.

Table 1 Changes prompted by the Kaikoura earthquake

 
DESCRIPTION OF WORKPROGRESS TO DATEEXPECTED COMPLETION DATE
A proposal to ban loop bar hanger details, also known as pigtails, for use in new precast double tee floor units. (The loop bar connection detail was a contributing factor in the partial floor collapses in Statistics House.) MBIE has recently completed the consultation process on this ban. A decision on the implementation of the proposed ban will be made in early 2018. Early 2018
Amendments to NZS 3101.1&2:2006 Concrete structures standard. This work began following the Canterbury earthquakes and was further developed after the Kaikoura earthquake. Amendments are complete. MBIE has recently completed the consultation process to amend B1/VM1 to cite this standard. A decision on the implementation of the proposed amendments will be made in early 2018. Early 2018
Development of guidance on the assessment of precast concrete floor systems in existing buildings. Project under way. Early 2018
Amendments to the New Zealand earthquake loadings standard (NZS 1170.5:2004) to address, in the short term, amplification of ground shaking in the Wellington basin. Project under way. Early 2018
Research programme into basin edge effects in the Wellington region and the effect of ground motion duration on structural performance. Project under way. 2020

Proposed ban of loop bar hangers

Loop bar hangers have been used to reinforce the projecting flange of precast concrete double tee floor units in situations where a flangehung seating is supporting the flooring (see Figure 1). Flange-hung seating describes the situation where the flange of the floor unit rests on the supporting beam. This contrasts with rib-supported seating, where the double tee ribs rest on the supporting beam.

Figure 1: Double tee flooring system.

The loop bar hanger detail (commonly referred to as a pigtail) typically comprises a pair of plain round bars looped at each rib and embedded back into the ribs (see Figure 2). A short length of round reinforcing bar is inserted through the loop bars. The loop bars project from the double tee precast units and are embedded into the concrete topping, although typically without a positive connection between the topping and the double tee.

Figure 2: Section A–A through double tee unit.

The history of loop bar hangers

The loop bar detail was developed in the early 1970s. It was originally covered by patents and treated as a proprietary system. Use of the loop bar was validated by early testing, and limitations were placed on its use in respect of span lengths and loading conditions.

International observations of the sometimes poor performance of precast floor systems triggered a review of precast floor support details. For loop bars, conventional analysis did not support the load capacity that was claimed by precast concrete floor manufacturers.

In 2008, the Structural Engineering Society New Zealand (SESOC) issued a warning to the structural engineering community around the use of loop bars.

Kaikoura earthquake damage prompted proposed ban

Until the Kaikoura earthquake, there was insufficient evidence to support a ban of loop bar hangers. Now, following the earthquake, several factors have contributed to MBIE’s decision to propose a ban on the loop bar hanger connection.

Firstly, the expert investigation into the cause of failure at Statistics House concluded that, among other things, the shortening of the floor units caused by concrete breaking away from the loop bars at the units’ ends contributed to the collapse.

Additionally, the typical loop bar detail does not comply with NZS 3101.1&2:2006. Compliance of the detail is usually sought as an Alternative Solution based on laboratory tests and available published test data. The original design methodology is now not considered valid in light of current concrete design practice.

More robust solutions available

MBIE expects that the proposed ban on loop bar hangars will not affect a large number of new building projects as use of the detail has decreased significantly since 2008. The purpose of the ban is to ensure that any designers who are unaware of the vulner–abilities of this detail will instead utilise a more robust solution. This ban is a precautionary approach for the construction of new buildings, noting that other, more robust connection solutions are available for flange-hung double tees.

Guidance coming for existing buildings

Research and development of guidance to support the detailed assessment of existing precast flooring systems is under way and is expected to provide more definitive criteria for the assessment of existing buildings.

The ban should not be construed as an indication that all buildings with loop bar hangers are dangerous. Building owners should seek advice from a structural engineer if they are concerned that their building may have potential issues.

Updating NZS 3101.1&2:2006

The Statistics House expert panel recommended that NZS 3101.1&2:2006 be reviewed to ensure that matters highlighted by the unacceptable structural performance of Statistics House are addressed within the design standard.

NZS 3101.1&2:2006 incorporating Amendment 3 includes:

  • a section on beam elongation
  • provisions for earthquake actions greater than the ultimate limit state
  • significant revisions to precast concrete floor seating requirements.

The revised standard was published in August 2017, and soon after, MBIE began the consultation process to cite the latest revision of the standard in the Verification Method for compliance with Building Code clause B1 Structure.

Always learning

New Zealand is at the cutting edge of international seismic design standards, but we have not yet solved all the potential ways a building can fail. Learning from the impact of earthquakes on our built environment is an ongoing process.

The Building Code, its supporting documents and design standards are regularly updated to incorporate new lessons, such as those from the investigation into the performance of Statistics House. MBIE will continue to monitor New Zealand’s building system to ensure the performance settings are working.

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Articles are correct at the time of publication but may have since become outdated.

Figure 1: Double tee flooring system.
Figure 2: Section A–A through double tee unit.

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