Concrete Lifting Anchors: Strength Limit States

An anchor can fail by:

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=ΦNtf /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
(out of concrete!)

Embedded part failure





AS4100

and AS3850

WLL=ΦNtf /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 = Φ*Ru  / 2.5

The Limit state is the ultimate bar strength Ntf = Ab x1.05 x fsy

since Ru = Ntf

and  Φ=0.8 for steel

Total WLL for 2 legs:

WLLhanger bar

= 2 ΦNtf / 2.5

 


Not required:

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


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 Lst = Lsyt

where

Lsy.t =k1 k1 fsy Ab / (2*a + db) √ f’c and Lsy.t ≥ 25k1db

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.

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 Revision 26th June.