Ultimately, all lifting insert tension bars may not be equal

Rod Mackay Sim1, Andreas Boomkamp2 and Farnaz Alinoori31Principle, Hillside Engineering Pty Ltd

2National Technical Manager, Ancon Building Products Pty Ltd T/A Leviat3Structural Engineer, Ancon Building Products Pty Ltd T/A Leviat

Abstract : The characteristic strength of several concrete lifting and fixing insert systems relies on component reinforcement (tension bars) made from N class bars as defined in Australia/New Zealand Standard AS/NZS 4671:2019 , “ Steel for the reinforcement of concrete ” . Type tests are conducted in accordance with Australian Standard AS 3850.1:2015 “ Prefabricated c oncrete elements ” , to determine the characteristic strength of inserts and insert systems and the Working Load Limit (WLL) for their design.

Where inserts are dependent on component reinforcement, the tests may not always provide a reliable measure of the characteristic (sy stem) strength. The standard does not require the manufacturing process to be disclosed to users. In addition, shear strength is not defined and m any bars available in the market have an ultimate tensile strength exceeding the minimum value which is indirectly defined by the standard.

This project investigated tension bars for lifting insert s made from quench and tempered (QT) and cold- stretched (cold- worked ) (C S ) N16 reinforcing steel using four different Australian anchors . The tests demonstrate substantial risk for unconservative design when tension bar strengths are based on tests of one particular type of bar which cannot be guaranteed to be representative of all bars conforming to AS/NZS 4671:2019. A safe design method is proposed for the update of standards based on the test results.

Keywords: tension bar, precast concrete, working load limit, characteristic strength, WLL.

1. IntroductionD esigners require larger and heavier panels to take advantage of the benefits o f prefabrication. Erectors demand simple 2-point rigging systems to avoid lifting and handling hazards. This has increase d the need for lifting inserts with higher Working Load Limits (WLL).

The requirements for inserts used to lift prefabricated concrete elements are set out in AS 3850 .1 :2015 [1] . A lifting insert may be single component or multiple components working as a system.

The WLL can be written as

(1)

Where

R u is Critical Characteristic (Ultimate) Strength of an element or assemblage (a value with a 95% probability of being exceeded, with a confidence of 90%).

And

FoS is the Factor of Safety = 2.25 for lifting inserts

Many lifting inserts are designed to interlock with the concrete and the ultimate strength ( R u ) of the insert is limited to the characteristic breakout strength of the concrete . The characteristic strength is principally determ ined by the effective insert embedment depth, concrete compressive strength at the time of loading and the proximity to the edges. Inserts may be designed with the Concrete Capacity Design (CCD) method defined in Australian Standards [1, 2] and called up in AS 3600:2018 [3].

The most critical lifting condition normally arises when lifting elements from their molds at concrete strengths, f ’ c.age of 15MPa and sometimes less. This is especially so for “ edge-lift insert s ” located in the edges of thin panels.

W here the concrete capacity R u of the insert is insufficient to achieve the desired WLL, AS 3850 .1 :2015 [1] allows the use of “ component reinforcemen t ” to enhance the characteristic capacity of a lifting system.

Component reinforcement is defined in AS 3850 .1 :2015 as “ reinforcement placed in conjunction with lifting, brace and fixing inserts and required to achieve the nominated capacities of the insert s ” (emphasis added) and shall be made from N class bars in accordance with AS/NZS 4671:2019 [4].

AS 3850 .1 : 2015 mandates the use of tension bars for edge-lift inserts in panels. The need for tension bars arose as a consequence of failures, some causing injury and damage to plant. A tension bar has a U or V shape which is passed through an ape rture in the insert . The insert is located in the bend of the bar which is supported by the “support bridge” of the insert (Figure 1).

Figure 11. Effect of bar diameter on lapped splice lengths.

Australian Standard, AS 3850.1:2015 (1) requires the strength test of each component and the assemb led insert-tension system by suppliers to determine the critical “ R u ” from which the WLL of the system is derived. The standard provides a framework for testing and sta tistical evaluation of results but does not include a detailed description of a standard tests method. The test methods are left to the discretion of insert suppliers.

During the last 5 years, a number of tests with randomly selected specimens from differ ent sources, including precast manufacturer s ’ stock, showed variable failure strengths, some below expectations, attributed to differences in the steel manuf acturing processes [5] . Incorrect detailing of lifting inserts and tension bars have led to accidents.

(a) Lifting Insert pullout failure (b) Undersized tension bar failure

Figure 2. Failures resulting from incorrect tension bar specification.

Reinforcing steels which comply with AS/NZ S 4671:2019 [4] may made by different metallurgical processes with different microstructural and mechanical properties.

The three principal mechanisms for achieving the required properties are classed as below:

Micro-Alloy (MA): Hot rolled with special alloy additions. Quenched and Tempered (QT): Hot rolled bars surface quenched after rolling and self-tempered by

heat in the core. Continuously Stretched (CS) or Twisted (CT): Where the bars are cold worked to increase the yield

strength at the expense of ductility.

The characteristic properties are statistically determined and vary from manufacturing plant to plant.

In order of ductility, ultimate strength and manufacturing cost: MA>QT>CS or CT

1 The general design principles are set out in the Appendix.

The method of manufacture is not generally disclosed to cust omers unless spe cified. The characteristic yield strength (YS) of N class bars may range from 500-650MPa. The ultimate tensile strength (UTS) is only indirectly defined by the minimum R m /R e of 1.08. The maximum ultimate strength is not defined nor limited but can be expected to be in the range 540-700+ MPa for N Class bars.

A supplier may unknowingly test an insert - tension bar with an N class bar of ultimate strength in the high range of possible strengths and assume that the WLL derived from the tests is v alid for all N c lass bars. Suppliers publish WLLs which vary from 7 to 10t lifting insert systems that include N16 tension bars.

The described testing was conducted to develop a safe design method tension bars.

2. Project Aims I nvestigation of the failur e modes and the Characteristic strength ( R u ) of N16 tension bars with

different inserts. Evaluation of the effect of different manufacturing methods on Ru Provide a design method based on the minimum properties of N class bars

3. Test program and setupTest inserts were randomly selected from inserts supplied by three different manufacturers.

Test Samples were tested from selected heats of QT and CS bars with mill test certification.

Tension bar test (shear-bend) with the insert to determine Ultimate Shear-Bend Strength (U-SB). Control tensile tests of the plain bars to confirm mill certification and determine UTS. Control double-shear tests of plain bars to determine the Ultimate Shear Strength (USS)

Bars were bent to comply with AS 3600:2018 (3) to diam eter 4d b and the ends threaded for fixing to the tool. The bars were passed through the insert aperture and retained in the lower tool with a washer and nut. The tool was retained in the lower jaws of a standard Universal Testing machine. The insert was c onnected to its compatible clutch (pickup device) which was attached to a clevis in the upper crosshead of the test machine and the assembly loaded until failure.

(a) Arrangement Elevation (b) Insert-tension bar under test

Figure 3. Test setup.

4. ObservationsS everal specimens failed by thread stripping or insert damage prior to failure of the tension bar and these results were discounted. The remainder failed by a shear-bend failure at the point at the support web of the in sert. The failure surface of the QT bars was typical ductile tensile overload. A detailed metallurgical examination of the failures was not undertaken as it was outside the scope of this project.The higher ductility QT bars necked significantly. The less ductile CS bars showed some necking with fracture features which tended toward those of a more brittle material.

(a) Bar failure at the insert support web (b) Failed tension bar after testing

Figure 4. Ultimate tension bar failures during testing.

All tension bars plastically deformed and “ necke d ” in the bent region adjacent to the insert support web prior to failure. It was clear that the bar manufacturing method influenced the shear-bend failures.

(a) QT bars showing distinct necking (b) CS failure with some necking

Figure 5. Comparison of area reduction (necking).

5. Results

5.1 Ultimate Tensile Strength (UTS) and Ultimate Shear Strength (USS) relationshipsFigure 6 shows the breaking force distributions for ultimate tensile and double shear failure of the control bars. The average UTS control results of CS and QT were 1.5-2% higher than the mill presumably as a result of statistical variations i n the batch and small number of tests. The variance between the UTS of the CS bar mill tests and our tests were identical 0.5%.

(a) Ultimate tensile breaking force (b) Ultimate double-shear breaking force

Figure 6. Control bar breaking strengths.

The variance between mill tests of UTS for QT was higher than CS reflecting the tighter distribution of properties expected from controlled cold work than from hot working processes. The UTS variance of the QT control bars was higher (3%) co mpare d to the CS results which can be seen in see Figure 6 (a). The ratio of characteristic values of USS/UTS (0.68 - 0.73) was within the ranges expec ted for steel shear without bend effects and the lower value for CS is consistent with the reference for a cold-worked steel (Table 1).

5.2 Tension bar shear bend The ultimate forces required for a WLL of 7t and 10t are shown against the test data. Neith er QT n or CS N16 bars can provide a WLL of 10t. CS N16 bars claimed to provide 7t WLL would be unconservative.

(a) Comparison of test results & required forces (b) Frequency distribution of results

Figure 72. Shear-bend breaking force tested with insert.

5.3 Ultimate Shear Bending (U-SB), USS and UTS relationshipsDespite lower USS variance (1.6%), bending increased CS U-SB variance (11%), compared to QT (4%).

The shear ratio with and without bending, U-SB/USS (0.7) ind icates that CS was highly bend-sensitive which was not the case for QT. This increased variance is reflected in the ratio of shear-bend to UTS, U-SB / UTS CS:0.48 QT:0.73 (Table 1).

Table 13. Summary of the results.Steel Manufacturing Process CS QT

Mill Certificate Heat Number 598037 768518Number of samples 3 3UTS Average MPa 624 649Variance % 0.5 0.9Rm/Re Average 1.11 1.19Ductility Average % 6.9 9.3

Control tests NATA Laboratory

Number of samples 5 5UTS Average MPa 634 662UTS Variance % 0.5 3.0UTS Characteristic MPa 623 594Number of samples 5 5USS by (D Shear) Average MPa 449 470USS by (D Shear) Variance % 1.6 2.1USS by (D Shear) Characteristic MPa 425 437

Tension bar tests NATA Laboratory

Number of samples 34 17Average Breaking Force kN 154.3 192.4Characteristic Breaking Force kN 120 175U-SB Average MPa 384 479U-SB Variance % 11 4U-SB Characteristic MPa 298 435Average: USS / UTS 0.71 0.71Average: U-SB / D Shear 0.86 1.02Average: U-SB / UTS 0.61 0.72Characteristic: USS / UTS 0.68 0.73Characteristic: U-SB / USS 0.70 1.00Characteristic: U-SB / UTS 0.48 0.73

2 Whilst the results for tension bars tested with different inserts did vary the data was combined to provide representative values for the insert population in current use.3 The nominal N16 cross-sectional area (201mm2 – tension; 402mm2 – Double shear) was used to convert the breaking forces to strength (MPa) and characteristic values calculated in accordance with AS 3850.1.

6. Discussion

6.1 Shear failureQT bars showed more ductile failures, less variability and higher characteristic strengths than CS bars.

To better understand the mechanisms of failure and effects on tensio n bar “ R u ” and WLL it is useful to examine the strain effects resulting from bendin g reinforced bars, the causes of brittle and ductile shear, the expected shear/tensile strength ratios for steel and influence of steel production methods.

6.2 Brittle and ductile shear without bending

Brittle shear

Tresca predicted the shear strength (onset of yielding) for brittle materials [6],

(3)

Ductile shear

Von Mises predicted that as a result of plastic deformation the onset of yielding is higher [6],

(4)

Whilst these equations are useful to predict the onset of yielding, they cannot be used for the prediction of the ultimate shear strength (USS) or ultimate tensile strength (UTS) in more ductile materia ls which strain harden, where the USS and UTS increase with plastic deformation prior to failure. Ultimate tensile failure of ductile materials is understood to result from processes of internal shearing so one could expect the two to be related however no direct relationship has been established. It is reaso nable to infer that “ all other things being equa l ” , the degree of ductility of the material should determine the onset and final ultimate shear strength.

For brittle materials, the yield and ultimate s hear strengths are equal and agree with the Tresca val ue whilst ductile materials of low ductility would have values approaching the Von Mises prediction.

The ultimate shear strength of more ductile materials can be expected to range from Von Mises 0.577Y to some higher value depending upon ductility and strain-hardening properties.

Machinery handbook [7] indicates USS/UTS strength ratios of 0.73 for cold worked steels to 0.75 for hot worked or hardened steels. Torsion testing of AISI 4140 [8] bars showed a mean of 0.71 and characteristic of 0.61. For materials of the same tensile strength, the ultimate shear strength of low ductility materials could be expected to tend towards the Von Mises limit with more ductile materials reaching higher values. QT tension bar tests indicated a U-SB/UTS ratio of 0.71 [9].

6.3 Effects of reinforcement on bendingErasmus and Pussegoda [10] demonstrated that bending reinforced bars results in high plastic strain rates at the interior surface of the bends. Furthermore, the def ormations lead to the development of notches at the intersection with the plain bar. The notches concentrate stress and increase the resulting plastic strain by 25-30%. In the case of tension bars, the position of the deformations is indeterminate with res pect to the support web of the insert, but it could be expected that at least one load-concentrating deformation would rest on the web. Figure 8 shows the deformation imprint on the insert support web.

Figure 8. Rebar deformation imprint on the insert support web.

The actual load distributions are complex and variable strain demand can be expected over the loaded area. It is likely that the stress state is complicated by plane strain effect s because the compression side of the bar is locked by the deformations.

Crack initiation can be expected in the regions of high strain demand. While stress redistribution may suppress crack initiation in high ductility steels, failures of low ductility steels are likely to be more brittle.

6.4 Reinforcing steel manufacture and conformance to standardsThe results show that CS is more sensitive to the combined bending and shearing compared to QT. The QT bars have sufficient ductility to re-distribute the s tresses so the ultimate shear strength val ues with and without bending converge. The less ductile CS steel is unable to redistribute the stresses and so the bending reduces the ultimate strength from to below the shear strength without bending.

When bendi ng, the CS being less ductile and therefor e more strain sensitive could also be expected to be more susceptible to localised strain variations arising from the number and orientation of the bar deformations on the insert support web. These variations may b e the reason for wider variance in the fai lure strengths than for QT steel.

6.5 The need for tension barsTension bars have two functions:

1. To provide security by a secondary load path. Edge anchorage is susceptible to variations in panel geometry, manuf acturing errors, proximity to embedded ite ms, poor concrete compaction, type and distribution of reinforcement [11, 12], aggregate type and the concrete strength at the time of lifting.

2. To provide sufficient “ R u ” to develop the design Working Load in ci rcumstances where the concrete anchorage of the insert alone is insufficient to develop the required WLL.

With increasing panel mass, the insert capacity controlled by concrete breakout strength is often not sufficient to provide the required insert WLL. P refabricated concrete wall panels of 150mm thick ness with minimum central reinforcement and a mass of 10-12t are common in Australia. Insert working loads of 7-8.5t are required to lift these panels on two points, after accounting for dynamic and slinging effects.

Figure 9 compares the characteristic strength “ R u .CCD ” , calculated by the CCD method to “ R u .test ” and WLL obtained from test s with typical 150mm thick panels . The panels included a minimum reinforcement whi ch improved the concrete breakout capacity [11, 12, 13].

At 15MPa the WLL for concrete breakout of the insert is only 3t rising to 5t at 25MPa.

Figure 9. Comparison of Ru for 150mm thick panels.

The shaded area shows the require d working load of 7-8.5t which not only exceed s the concrete breakout WLL at a ll concrete strengths but at 15MPa equals R u which would, if attempted reduce the FoS to 1 . The inserts need tension bar reinforcement to provide the required 7-8.5t working load.

6.6 Failure modes of inserts with tension bars, tested in concreteInsert l oad-displacement with and without N16 tension bars is shown in Figure 10 . The concrete resist ed the load until it crack ed at 59 -102kN , wit h a typical cone failure and brittle load-response curve. These val ues lie within the range for the characteristic R u shown in Fig. 9. After the concrete cracked, the inserts with N16 tension bars (C1, C2), transferred load to the tension bar which failed by a shear-bend mechanism at 167kN, i.e. with in the range of tests discussed in 5.2 above . The shear-bend failure is highlighted . The ultimate load response shows strain hardening and ductile deformation prior to steel failure.

The Load-disp lacement curve s demonstrate that when inserts are reinforced with tension bars, the surrounding (cracked) concrete contributes no further resistance and plays no part in the ultimate failure. The cracked concrete surrounding the insert has been removed to reveal the ten sion bar failure . The steel failure and bar capacity may therefore be evaluated by standard mechanical tests in air.

Figure 104. Load-displacement for concrete and tension bar failures [14].

6.7 Working Load Limit & Safe Design methodAccording to AS3850 .1:2015, the characteristic breaking force “ R u ” and F oS of 2.25 re quires an R u of 154.3 kN and 220.5 kN, for working load limits of 7 and 10t respectively . However, the characteristic breaking force from 34 CS N16 t e sts was only 1 20 kN and 175 kN for the 17 QT N16 bars . N either the QT nor CS N16 bars met the minimum F oS of 2.25 for a WLL=10t, required by AS3850(1 ) . QT N16 bars provide a WLL of 7t (FoS=2.55) whilst CS bars do not (FoS=1.79) (Table 2).

Table 2 also demonstrates t he consequences of WLL “ over-specification ” for insert s with N16 tension bar s. The FoS (values in bol d ) are reduced to below the minimum 2.25. I f a designer specifies a WLL of 10t for an N16 bar, the safety factor could drop to as low as 1.22.

Table 2. The influence of N16 tension bar WLLs on the factor of safety.WLL (ton) 7 10

Bar type Ru.test (kN) FoS calculated from the tests

CS 120 1.75 1.22

QT 175 2.55 1.79

The maximum value of the UTS is not specified in AS/NZS 4671 and nor is the s hear strength. Currently, there is no method of determining a WLL from the minimum properties of AS/NZS 4671 and neither is it possible to relate the properties of the bar used to make tension bars with those used for the insert supplie r ’ s proprietary test s. A safe desig n method based on AS/NZS 4671 minimum properties, is required to resolve the current lack of information.

4 The WLL at 15MPa is only 3.5t, rising to 5t at 25MPa.

6.8 Proposed method for the Safe Design of tension barsThe proposed approach calculates “ R u ” from the minimum ultimate strength defined in AS/NZS 467 1 by applying a reduction factor derived from these tests, to account for the shear-bend material properties:

(4)

(5)

(1)

Where:

= capacity reduction factor to account for shear with bending

= limit state tensile strength

= number of shear planes

= cross – sectional area of a bar

= the lower characteristic value of the yield stress (e.g. N class bar = 500MPa)

= ratio of maximum ultimate tensile strength and yield strength (or 0.2% offset) in any one test

Example: N16 QT and CS tension bars, Ab = 201mm2

Table 3. Shear-bend Ru and WLLs of N16 tension bars using the proposed method.Bar Manufacturing method

(MPa)

Ru

(kN)WLL(tons)

QT: Hot-Rolled,Quenched and Tempered bars 0.73 540 158.4 7.2

CS: Cold-Worked, Continuously Stretched barsor bars of unknown manufacturing method 0.48 540 104.1 4.7

WLLs calculated from the proposed design method are shown in Figure 11 . It is evident that the proposed

method provides values for the consistent with the requirement for safety critical lifting systems.

Figure 11. Test results with WLLs for N16 bars.

7. Conclusions Lifting insert-tension bar systems made with CS or QT steels have different WLLs. Neither QT nor CS N16 bars provide a WLL of 10t. The capacity design factors for QT and CS steels derived from the tests may be applied to t he

minimum ultimate tensile strength of AS/NZS 4671 to provide safe design WLLs of tension bars. Further tests with other diameters and MA bars would provide assurance of the method. Tension bar users should require their supplier declare the method of steel manufacture. Where the steel type is uncertain, the (lower) shear factors for CS bars should be used to preserve

the minimum factor of safety.

8. AcknowledgementAncon Building Products Pty Ltd trading as Leviat for their financial supports in this project.

9. References

1. Standards Australia, “AS 3850.1:2015, Prefabricated concrete element, Sydney, Australia.

2. Standards Australia/Standard New Zealand, "AS/NZS 4671:2019, Steel Reinforcing Materials, Sydney, Australia & Wellington, New Zealand.

3. Standards Australia, "AS 3600:2018, Concrete Structures, Sydney, Australia".4. Standard Australia, "AS 5216:2018, Design of post-installed and cast-in fastenings in

concrete, Sydney, Australia.5. Construction, Forestry, Maritime, Mining and Energy Union (CFMEU), "Incident involving

the incorrect use of lifter inserts in precast concrete panels, August 2011. 6. Cottrell A, Arnold Edward. An introduction to metallurgy,1967, pp 433-434. 7. Machinery’s Handbook 14th Edition,1953. p347. 8. Steiner S, et al. Shear Spacer Tests. Engineering Report E2016-19. (Unpublished)9. Mackay Sim R.Type Testing of Anchors to AS3850, 2006. (unpublished)10. Erasmus LA, Pussegoda N. Safe bend radii for deformed reinforcing bar to avoid failure

by strain age embrittlement. New Zealand Engineering. 1978;33(8):170-7. 11. Al-Deen S, Ranzi G, Gilbert RI, Mackay-Sim R. Tensile tests on edge-lifting anchors

inserted in precast concrete panels. 12. Barraclough A, Lloyd N, editors. A plate type edge-lift anchor: Panel reinforcement

influence on failure loads. Australasian Structural Engineering Conference 2012: The past, present and future of Structural Engineering; 2012: Engineers Australia.

13. Mackay Sim R. Tension testing of anchors in 150mm thick panel edges, 2010:ACR:005&6. (unpublished)

14. Ranzi A, et all. Behaviour of lifting inserts for precast concrete construction, ARC Project LP110100008, 2011.