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CROSS Safety Report

The use of cast-in ferrules as structural connections

Report ID: 993 Published: 30 April 2021 Region: CROSS-AUS


Overview

This report highlights several problems that can arise with structural connections between elements when cast-in ferrules (also known as threaded inserts) are used for the anchorage of reinforcing bars.

It demonstrates that, if not designed, detailed, and installed correctly, brittle failure can occur rather than ductile behaviour as required by Building Codes and Australian and New Zealand Standards.

Key Learning Outcomes

For structural and civil design engineers:

  • Be aware that when cast-in ferrules and similar threaded inserts are used to make structural connections using threaded reinforcing bar, the failure mode may be brittle, and the design may not comply with relevant AS/NZS Standards

  • If cast-in ferrules and similar threaded inserts are to be used when attaching other elements, ensure there is adequate development length and pay particular attention to the load transfer and failure mechanisms

  • There are proprietary systems available that will comply with the relevant AS/NZS Standards for ductile behaviour if installed in accordance with the manufacturer’s specification. Talk to the suppliers who can provide technical advice on the tensile and shear capacity of their threaded inserts.

  • Check if the structural connection is required to have a fire rating; in which case there may be reduced load capacity

For construction professionals:

  • Where proprietary products are specified, these should be installed strictly in accordance with the manufacturer’s technical specification

  • Any proposed changes to the specified connection detail should be approved by the designer

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Cast-in ferrules, or threaded inserts, in both in-situ and precast concrete have been used successfully for many years and initially these were for fixing light steelwork and other structures to prefabricated (precast) concrete, and for temporary bracing and propping. In recent years, the reporter has noted a trend to using cast-in ferrules in precast concrete members for the anchorage of dowel bars and starter bars into in-situ concrete. The threaded dowel bar or starter bar is screwed into a cast-in ferrule within the precast concrete and then the in-situ concrete is poured around the threaded bar.

The reporter is aware of several failures with this arrangement where the floors were post-tensioned and the axial shortening of the floor has pulled the ferrule with its associated threaded bar out of the prefabricated concrete walls. The reporter has also seen the failure of cast-in ferrules when impact wrenches have been used with high-strength bolts to fasten steelwork and similar elements to prefabricated concrete members.

Threaded bar

For detailed advice on the use of threaded bars, the reporter refers to a paper published by the Australian Steel Institute in July 2014: Specification of threaded bar in structural applications where it suggests that threaded bar is not an ideal structural member as the cut threads act as notches. Reinforcing bars are not designed to have a thread on them and typically need to be of sufficient diameter to achieve a full thread cut. This means for example, an N24 bar is threaded to fit an M20 ferrule.

Reinforcing bars are not designed to have a thread on them and typically need to be of sufficient diameter to achieve a full thread cut

Australian Standards

The basic premise of AS3600: Concrete Structures is that in an overload condition, the reinforcement must yield in a ductile manner before the failure of the concrete. The Standard states the following: ‘the standard sets out minimum requirements for the design and construction of concrete structures and members that contain reinforcing steel or tendons or both.’ Clause 19.3 of AS3600 covers the design of fixings including ferrules and requires the fixings to be designed to yield before ultimate failure in the event of an overload. And Clause 13.1.4 of AS3600 requires the area of the anchor head to be greater than, or equal to, 4 times the cross-section of the bar.

The reporter notes that in AS3850.1: Prefabricated concrete elements, a ferrule is defined as an element to take a threaded bolt, not a threaded bar.

The reporter recommends that cast-in ferrules be designed in accordance with the principles in AS3850.1, Appendix B. Cast-in ferrules are available from several suppliers in Australia together with technical data on their tensile and shear capacities. It is important for designers to ensure that the cast-in ferrule complies with Australian Standards and to specify the supplier together with details including length, finish for durability and to ensure that adequate edge distances are achieved so that the full strength of the ferrule can be achieved. Imported ferrules may not comply. Similarly, as many contracts require contractors to construct a structure in compliance with a relevant Australian Standard, it is important for contractors to review a given design to identify and eliminate any potential non-compliance.

As many contracts require contractors to construct a structure in compliance with a relevant Australian Standard, it is important for contractors to review a given design to identify and eliminate any potential non-compliance

As an example of the dilemma facing designers, if one considers an N20 reinforcing bar into an M20 ferrule (and the bar should be an N24 as noted above), the bar has an ultimate tensile strength of at least 500 MPa or about 155 kN at yield. However, a commonly used M20 ferrule has an ultimate capacity of about 100 kN and it will fail before the reinforcing bar in a non-ductile manner. Accordingly, this combination does not comply with AS3600.

Connections of steel members and other elements to prefabricated concrete

Where connections are bolted into a cast-in ferrule, then a Grade 4.6 bolt must be specified rather than a Grade 8.8 bolt so that the bolt will fail in tension in a ductile manner before the ferrule fails. In other words, the capacity of the bolt must not exceed the capacity of the ferrule.

The reporter is aware of several examples where the use of over-torquing using impact wrenches has resulted in cone failure of the cast-in ferrules and emphasises that impact wrenches must not be used for tightening bolts. The ferrule suppliers can provide information on the torque that needs to be applied, and a calibrated torque wrench must be used. The reporter also notes there have been problems with the temporary bracing of precast concrete elements and draws attention to the safety alert issued by WorkSafe Victoria: Brace footing installations on concrete.

Connections between in-situ concrete members and precast concrete

When fully anchored starter bars are used to connect in-situ concrete and precast concrete, if correctly designed the reinforcement will yield in a ductile manner in an overload situation. However, where a cast-in ferrule with a threaded starter bar is used, a typical failure scenario is shown in Figure 1. Where connection loads are minimal and tying together is only nominal, then it may be satisfactory to use a cast-in ferrule with a screwed-in starter bar, but where reinforcement is required for structural strength, then cast-in ferrules should not be used. A typical connection without ferrules is shown in Figure 2.

A cross-section of a concrete slab to wall connection made with a ferrule
Figure 1: Typical failure scenario where a cast-in ferrule with a threaded starter bar is used
A cross-section of a concrete slab to wall connection made with conventional rebar links
Figure 2: Typical connection without ferrules

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This report highlights the importance of following basic structural engineering design principles such as ensuring that failure modes of elements remain ductile and avoiding the risk of brittle failure.

When cast-in ferrules are being used, it is important to ensure adequate development length and to assess correctly the local load transfer mechanism between the elements. Otherwise, a local shear failure (pull-out) is likely to occur as shown in Figure 1.

One point often overlooked, when using a connection such as shown in Figure 1, is that although it may be the designer’s intention that the dowel bar operates purely in shear, precast walls are often relatively stiff elements that will attract bending moments from the slabs they support. These bending moments are transferred to the walls by the dowels and ferrules and can subject the ferrules to significant axial loads that may be additional to the shrinkage and prestress loads noted by the reporter. There have been reported cases of the slab-to-wall connection ‘unzipping’ as these brittle failures progress along the wall. While this problem may occur with a single row of dowels, even if the connection is assumed to act as a hinge, it will be exacerbated in the case when two rows are used resulting in some degree of fixity.

There have been reported cases of the slab-to-wall connection ‘unzipping’ as these brittle failures progress along the wall

Proprietary systems

There are proprietary types of cast-in anchor systems available that are designed to fail by plastic yielding of the anchor before brittle failure of the concrete. Provided these are used strictly in accordance with the manufacturer’s specification, then the failure mechanism should be ductile. Depending on the loads involved, this type of anchor may be an acceptable alternative to that shown in Figure 2. Where there is doubt, modelling the dowel axial loads in wall-to-slab connections is possible using finite element analysis.

New Zealand experience

The use of cast-in threaded inserts has been an issue in New Zealand especially when used in precast panels to connect starter bars into floor diaphragms. Research into these types of connections has found that they perform poorly under seismic load and has concluded that such connections that have a potential breakout failure mode should not be allowed as a structural load path.

Research into these types of connections has found that they perform poorly under seismic load

Further details of this research can be found in the paper Performance of panel-to-foundation connections in low-rise precast concrete buildings by Hogan, Henry & Ingham, published in the Journal of the Structural Engineering Society of New Zealand (SESOC), April 2018, where it states: It was found that in the out-of-plane direction, current connection details utilising shallow embedded threaded inserts resulted in brittle joint failure and as such do not meet performance criteria in NZS3101:2006.

Durability and behaviour in fire

For exposed structures or where there is a corrosive environment, the durability of cast-in inserts may be an issue that needs to be considered. In this case, ferrules and threaded inserts may need to be hot-dipped galvanized or stainless steel.

It will often be the case that the structure will be required to have a specific Fire Resistance Level (FRL), and this can influence the connection design. Deformations of the main structural elements when exposed to high temperatures are likely to impose additional loads on the connection. In addition, there may be a direct effect on the behaviour of a cast-in anchor itself if there is a risk that it will be exposed to fire. In such cases the advice of a fire specialist should be sought.

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