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# AS3600 Property modifier (Cracked moment of inertia)

## AS3600 Property modifier (Cracked moment of inertia)

(OP)
Sir,

I am reading the AS3600 code and can't seem to find the modification factor for the inertia of an element to consider its cracked behavior.

in ACI it is given as:
Column = 0.7I
Beam = 0.35I

For complete list see ACI 10.10.4.1

Am I just having no luck searching or is AS3600 has different version of it?

### RE: AS3600 Property modifier (Cracked moment of inertia)

Clause 7.7 might help!

### RE: AS3600 Property modifier (Cracked moment of inertia)

Clause 7.7 is for AS3600-2001

Or Clause 6.3 for AS3600-2009

Oops!

### RE: AS3600 Property modifier (Cracked moment of inertia)

Guys sorry for interupting (dont wanna open new thread, couse this is somewhat also concerned to cracked sections)
but in AS3600-2001 section 9.4.3. "Crack control for shrinkage and temperature effects" there is a value called sigma,cp wich is mentioned in formulas for controling cracks with various degrees of control eg:

(3.5-2.5 * sigma,cp)*D*b*0.0001

How do you determine value for sigma,cp?
Thanx!

### RE: AS3600 Property modifier (Cracked moment of inertia)

That is the effective amount of prestress. Is your slab prestressed? If not, it is zero.

hokie66 thanx!

### RE: AS3600 Property modifier (Cracked moment of inertia)

Forgot, to say, look here:

http://www.engineersaustralia.org.au/sites/default...

page 52-59...it seems that for one way slabs you should provide reinforcement of equal amount in secondary direction to control the shrinkage cracks.

### RE: AS3600 Property modifier (Cracked moment of inertia)

#### Quote ((mar2805)

page 52-59...it seems that for one way slabs you should provide reinforcement of equal amount in secondary direction to control the shrinkage cracks.

Depending on the exposure classification and amount of restraint-to-shrinkage, yes.

Interesting.

### RE: AS3600 Property modifier (Cracked moment of inertia)

Not sure what you mean by "equal amount".

In a one way slab, the Temp & Shrinkage reinforcement requirement for the secondary direction is actually higher than the T & S reinforcement requirement for the primary bending direction if you read the whole clause. But the reinforcement in the secondary direction is divided between the 2 faces, while in the primary direction it will normally be at the tension face only unless there is a high degree of restraint!

The other thing that you have to watch out for is that this clause actually allows you to provide full crack control against restraint effects with prestress only. Firstly, you would need to make sure that the prestress actually gets into the slab, so that there is no restraint while it is being stressed. secondly, PT at wide spacing does nom give good crack control. You really need to enforce a maximum spacing rule as well, probably 200 - 300mm maximum, so normally un-tensioned reinforcement would also be needed. This not currently explicitly mentioned in this rule but the general crack control rules do require a maximum spacing of 300m with reduced maximums depending on the stress in stress in the reinforcement and bar size.

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