Concrete Lifting Anchors: Strength Limit States

An anchor can fail by:

Anchor Failure failure of the anchor itself or its reinforcing elements (e.g. hanger bars)

Concrete Failure where the concrete surrounding and supporting the anchor fails

Safe design requires that all strength limit states for each type of failure be considered

Anchor Failure

The anchors may fail in a number of ways and the limit states must be assessed by design and / or verified by test for anchors to comply with AS3850 (AS4100, AS3600 as applicable).


Spherical Head Anchor Systems		Hairpin Anchor Systems

STRENGTH LIMIT STATE	FAILURE Spherical Head anchors	FAILURE Hairpin anchors	Design Calculation	Test method
Anchor body failure			AS4100 and AS3850 WLL=ΦN_tf/2.5	AS3850 Appendix 2. Clutch+Anchor Tension Test (out of concrete!)

STRENGTH

LIMIT STATE

FAILURE

Spherical Head anchors

FAILURE

Hairpin anchors

Design Calculation

Test method

Anchor body

failure

AS4100 and AS3850

WLL=ΦN_tf/2.5

AS3850 Appendix 2.

Clutch+Anchor Tension Test (out of concrete!)

STRENGTH

LIMIT STATE

STRENGTH LIMIT STATE	Design Calculation	Test method
Lifting attachment point failure	No reliable method	AS3850 Appendix 2. Clutch+Anchor Tension Test (out of concrete!)
Embedded part failure	AS4100 and AS3850 WLL=ΦN_tf/2.5	AS3850 Appendix 2. Clutch+Anchor Tension Test (out of concrete!)
Hanger attachment point failure “pull-through” of the bar	No reliable method to calculate the bending-shear failure strength of the anchor.	AS3850 Appendix 2. Clutch+Anchor+Rebar Tension Test (out of concrete!)

STRENGTH

LIMIT STATE

FAILURE

Spherical Head anchors

FAILURE

Hairpin anchors

Design Calculation

Test method

Lifting attachment point failure

No reliable method

AS3850 Appendix 2.

Clutch+Anchor Tension Test
(out of concrete!)

Embedded part failure

AS4100

and AS3850

WLL=ΦN_tf/2.5

AS3850 Appendix 2.

Clutch+Anchor Tension Test
(out of concrete!)

Hanger attachment point failure

“pull-through” of the bar

No reliable method to calculate the bending-shear failure strength of the anchor.

AS3850 Appendix 2.

Clutch+Anchor+Rebar Tension Test (out of concrete!)

Anchor Failure: Hanger Bar failure

Hanger bars are required when the lifting load exceeds the WLL of the Concrete Strength.

STRENGTH LIMIT STATE	FAILURE Spherical Head “Eye” anchors with hanger	FAILURE Hairpin anchors with hanger	Design Calculation	Test method
Hanger bar shear failure			No reliable method to calculate the bending-shear failure strength of the reinforcing bar.	AS3850 Appendix 2. Clutch+Anchor +Rebar Tension Test (out of concrete!)
Hanger bar tension failure		AS3600 and AS3850 WLL = ΦR_u / 2.5 The Limit state is the ultimate bar strength N_tf = A_b x1.05 x f_sy since R_u = N_tf and Φ=0.8 for steel Total WLL for 2 legs: WLL_{hanger bar}* = 2 ΦN_tf / 2.5	Not required: use characteristic strength

STRENGTH

LIMIT STATE

FAILURE

Spherical Head “Eye” anchors with hanger

FAILURE

Hairpin anchors with hanger

Design Calculation

Test method

Hanger bar

shear failure

No reliable method to calculate the bending-shear failure strength of the reinforcing bar.

AS3850

Appendix 2.

Clutch+Anchor
+Rebar Tension Test
(out of concrete!)

Hanger bar

tension failure

AS3600 and AS3850

WLL = Φ*R_u / 2.5

The Limit state is the ultimate bar strength N_tf = A_b x1.05 x f_sy

since R_u = N_tf

and Φ=0.8 for steel

Total WLL for 2 legs:

WLL_{hanger bar}

= 2 ΦN_tf / 2.5

Not required:

use characteristic strength

CONCRETE FAILURE

STRENGTH LIMIT STATE	Anchors placed in edges - “pie” shaped partial cone	Design Calculation	Test method
Concrete Cone failure		Empirical from tests	AS3850 Appendix 2. Embedded anchor Tension test
			AS3850 Appendix 2. Embedded anchor Tension test
Hanger bar pullout	AS3850 and AS3600. Length to develop bar in the concrete below the crack from the foot of the anchor. Leg L_st = L_syt where L_sy.t=k₁ k₁f_sy A_b / (2*a + d_b) √ f’_c and L_sy.t≥ 25k₁d_b	Not Required

STRENGTH

LIMIT STATE

All Anchors placed well away from edges - “cone” failure

Anchors placed in edges - “pie” shaped partial cone

Design Calculation

Test method

Concrete Cone failure

Empirical from tests

AS3850

Appendix 2.

Embedded anchor Tension test

AS3850

Appendix 2.

Embedded anchor Tension test

Hanger bar pullout

AS3850 and AS3600.

Length to develop

bar in the concrete below the crack from the foot of the anchor.

Leg L_st = L_syt

where

L_sy.t=k₁ k₁f_sy A_b / (2*a + d_b) √ f’_c and L_sy.t≥ 25k₁d_b

Not Required

HANGER REINFORCING BAR DETAILING

Hangers must extend downwards below the crack to shed the load deep within the panel.

The required “leg length” is calculated according to AS3600 to develop the strength of the bar.

Note:

AS3850 requires that the strength of the anchor exceeds 2.5 X WLL of the anchor.

The bar forms part of the anchor itself and therefore its strength must meet this requirement for all its strength limit states.

NB: the limit state strength of the bar may not be limited by its tensile strength only!
The bar strength is normally limited by shear at its attachment point, which must be checked!

DO NOT USE HORIZONTAL e.g. Trimmer bars!

Horizontal bars do not increase the pullout strength of the anchor !

After the concrete cracks, horizontal reinforcing progressively strips out of the panel edge at little or no increased load!

Typical hanger bar dimensions and how they are calculated.

The development depth required for each hanger leg to share the load is calculated from the using AS3600 clause 13.1.2.1 to develop the strength of the bar.

An N16 bar is required for anchor working loads up to 7.1Tonnes
An N20 bar is required for anchor working loads up to 9Tonnes

The following diagram shows a hanger bar detail designed for lifting 150mm thick panels when demoulding at 10MPa.

This is the recommended standard detail for all 7, 8 and 9 tonne hanger bars, regardless of anchor type or make (applicable for both hairpin and spherical head “eye” anchors).