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# AS3600-2018 Shear: Calculating 'A_ct'

## AS3600-2018 Shear: Calculating 'A_ct'

(OP)
Hi all,

Clause 8.2.4.2.2 of the new AS3600 provides us with an option to calculate 'A_ct' if the strain at 'mid-depth' is less than zero. However, the term 'mid-depth' is never clarified in the code. Based on my cursory reading of the FIB 2010 model code - which also uses MCFT, it appears that mid-depth should really be the middle of the depth between centroids of tension and compression chords (and not the section depth). This would imply that the engineer should iterate to find the elastic neutral axis under the applied loads, calculate the centroids of tension and compression and then finally calculate A_ct. On the other hand, the code does use the words 'mid-depth of the section' but using half the depth of the section doesn't seem mathematically correct

Am I overthinking this ? Or is the code in the wrong here ?
Replies continue below

### RE: AS3600-2018 Shear: Calculating 'A_ct'

The calculation of Act is intended to be an approximation to avoid the need for a cracked section analysis. The Canadian codes simply defines it as the area of concrete on the flexural tension side of the member (as in AS3600-2018). FIB is referring to the middle of the effective shear depth. Either way it would be about the same.

### RE: AS3600-2018 Shear: Calculating 'A_ct'

You are over thinking it.

There are several areas of the calculation where the "tension half" of the section are referred to. They are based on D / 2. Similar to the Canadian Code.

### RE: AS3600-2018 Shear: Calculating 'A_ct'

This is using Act in Eq 8.2.4.2(2)? You can just take εx = 0. Is that satisfactory?

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