AS3600 - Fire Design of Slab - Axis distance and FRL for integrity ?

[Huong83]

In Section 5.5, there is no FRL (Fire resistance level) for integrity ?

My slab is 110mm thick, and 20mm cover at bottom reinforcement of SL82. Therefore, axis distance is 24mm => FRL for structural adequacy < 30 min (Table 5.5.2 (B)) ?

Section 5.5.2(b)(i) say : "the axis distance is determined in accordance with the amount of moment redistribution used in the analysis" . This is meant that I have to input the load applied to the slab to work out the moment and neutral axis distance using rectangular stress block ?

Thanks

 

[rapt]

Huong83,

5.5.2(b)(i) was deleted from the code in the amendment released in 2010! You are not using the latest code.

The insulation and integrity requirements are covered by table 5.5.1 giving minimum slab thicknesses.

Table 5.5.2(B) gives minimum slab thicknesses for strength for flat slabs. 110mm does not work, 150mm is minimum.

The code recommends against using mesh in flat slabs!

Also, how are you going to fit mesh into the slab at the lapping points. There is a possibility of 4 layers (16mm each) at an overlapping point. You cannot build it.

I have real doubts about a 110mm thick flat slab anyway, but with mesh, impossible.

 

[Huong83]

We investigate FRL of the existing structure.
The slab thickness was increased at the lapping point of reinforcement.