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
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 
Embedded part failure 


AS4100 and AS3850 WLL=ΦN_{tf }/2.5 
AS3850 Appendix 2. Clutch+Anchor Tension Test 
Hanger attachment point failure “pullthrough” of the bar 


No reliable method to calculate the bendingshear failure strength of the anchor. 
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 
Design Calculation 
Test method 

Hanger bar shear failure 


No reliable method to calculate the bendingshear failure strength of the reinforcing bar. 
Appendix 2. Clutch+Anchor 
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

use characteristic strength 
CONCRETE FAILURE
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 
Appendix 2. Embedded anchor Tension test 

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 }=k_{1} k_{1 }f_{sy} A_{b} / (2*a + d_{b}) √ f’_{c} and L_{sy.t }≥ 25k_{1}d_{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! 

DO NOT USE HORIZONTAL e.g. Trimmer bars! Horizontal bars do not increase the pullout strength of the anchor !


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).
© 2007 Hillside Engineering