MYTH: Shrinkage steel increases anchor loads

1. Shrinkage reinforcing is only designed to control stresses developed in the concrete when it shrinks.

2. Reinforcing expected to support Lifting loads must be specifically designed to meet the following conditions:
   • The reinforcing must be fully anchored at each end and of sufficient development length to develop the load at the point at which the concrete fails (from the crack developed at the bottom of the anchor)
   • If the load is designed to be transferred from the anchor to the reinforcing by normal development principles then the development length of the anchor (the length of the anchor leg) must be sufficiently long to transfer the load through the concrete to the steel reinforcing.
   • AS3850 requires a Design Factor of 2.5 between the the Working Load Limit and the ultimate strength of the anchor. The additional steel required to provide anchorage exceeding the concrete strength limit state (cracking strength) must be designed with Design Factor 2.5 for every limit state. Normal shrinkage reinforcing is not designed for and cannot be relied upon to meet these conditions.

NOTE!

The most efficient manner of directly reinforcing for loads exceeding the concrete WLL is with a properly designed and detailed hanger bar.

A worked example is helpful to understand why:

An anchor is required to provide 7 tonnes WLL in a concrete panel of 150mm thickness and a compressive strength of 15MPa at the time of lift.

The panel is normally reinforced with central shrinkage reinforcing of SL82 mesh and N16 edge trimmer bars.

Calculate load transferred to the panel at the anchor: AS 3850 requires that the Working Load Limit contain a factor of 2.5 against the ultimate load capacity of the anchor resisting the lifting load:

i.e. 7*2.5 tonnes = 17.5tonnes force = 171.5kN (AS3850)

Now, the concrete strength limit state ~ 7 tonnes = 69kN
(load supported by the concrete before cracking)

Conclusion: additional reinforcing is required! This could be achieved either by:

• a properly detailed hanger bar or
• by adding additional steel to reinforce the concrete across the crack.

Preferred method: Use a hanger bar directly attached to the anchor (eye anchor).

The load is transferred directly from the lifting anchor to the hanger bar to shed the load directly to the concrete away from the edges, deep within the panel and.

The required hanger bar is N16 of total length 1400mm (see HOW TO of hanger bars).

Total additional mass steel: 2.2kg

Now check the non-preferred method: Add panel reinforcing specifically to resist the lifting loads

Suppose it is decided to add additional reinforcing to enable the panel to withstand the additional stresses without relying upon the shrinkage or other reinforcing.

In this case the lifting loads are transferred to the additional reinforcing steel bars through the (cracked) concrete itself, relying upon adhesion and concrete interlock between the anchor, cracked concrete and the reinforcing steel.

Place additional steel bars parallel to the anchor in the vicinity of the anchor to trap the load and shed it into the panel.

Additional steel area required:

A_st = 171.5kN * 1000 / 500MPa / _s

= 171.5*1000/500/0.8

= 343 Squ mm.

Suppose N12 bars (113mm² cross section)

Require 4 bars to meet the requirement.

Calculate the required Development Length from AS3600

From AS3600

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

Now, for k₁=1, k₂ = 2.4,

Compressive strength f’_c=15MPa (at the time of lift),

Bar diamenter d_b = 12, Bar Area A_b = 113,

Cover or distance between bars a=30mm

L_sy.t = 486mm ≥ 25k₁d_b (300mm)

Note 1: AS3600 requires this embedded length either side of the point at which the stress exists i.e. either side of where the concrete cracks at the base of the lifting anchor.

This is to ensure that the reinforcing bar cannot pull out of either the edge strip where the anchor is located or the body of the panel below the crack.

Since the typical anchor lengths are 250-450mm long and so with required concrete cover of 30mm, it is not possible to guarantee sufficient embedment in the edge strip unless the bars are cogged or terminated with standard hooks.

Distance to the bottom of the hook or cog = 243mm, Required cover = 30mm

Check whether it is possible to develop the bars in the lifting strip:
Total Distance from the crack to the edge of the concrete = 243+30 = 273mm

It is possible (but not efficient!) to provide additional steel reinforcing around the anchor specifically to reinforce it against the additional loads generated during lifting provided that 4 additional N12 bars are used 486mm long with a standard hook on the end nearest the edge of the concrete (overall length of each bar including the hook = 486+273+75 = 834mm)

i.e total additional length of reinforcing bar = 3336mm (2.96kg)!

Conclusion: a hanger is more efficient, not only saving .75kg steel but less cost to manufacture and place and in addition, the load is shed deeper into the panel and further from the panel edges.