li0ngalahad
Structural
- May 10, 2013
- 89
Clause 5.5.2 of AS3600-2018 specifies that
- for solid slabs supported by walls or beams, the minimum thickness requirement is not applicable
- for flat slabs, the minimum slab thickness must be as per table 5.5.2(A) "over a rectangular area extending 0.16 of the larger span length either side of a column in each direction"
Now, my quesitons are:
- I have a 200 flat slab with desired FRP of 90minutes so my minimum is 200mm. Slab is supported by 2000x200 blade columns/walls and there are 30mm deep setdown wet areas near them, bringing thickness locally to 170mm. This brings back the old philosophical question of what is considered a wall and what a column. My interpreatation of the clause is that the failure they're trying to prevent is punching shear, and when they refer to a "wall" they mean it should be continuousm such a boundary wall supporting the whole extend of the slab, for which punching shear is not a failure mode. So if this interpretation is correct I cannot consider these as walls but as columns, even though punching shear utilisation is very low for them. What's you take on this? Would you consider these as walls or not?
- With the 2009 code, one way we could deal with this issue was to check the slab with EC2 ("methods of calculation" of Clause 5.3.1 (b)), however the new code specifically excludes this option for this particulat check ("The depths in this table cannot be reduced by calculation methods in accordance with Clause 5.3.1(b)). So I was thinking to treat these wet areas effectively as openings in the slab, i.e. still designing them for strength but create a model in RAM with voids where the setdown areas are and checking the remaining slab for bending and shear under the fire case (G+0.4Q), and do a temperature distribution analysis into the section to check the reduced reinforcement strength at 90minutes, and distribute the wet area self weight as a dead line load at its edges. It should be conservative because we are discounting the wet areas for strentgh completely and making sure the slab doesn't fail. Obvioulsy we still need the setdown slab to comply with the minimum thickness for isolation. Anyone sees any issues with this method?
- for solid slabs supported by walls or beams, the minimum thickness requirement is not applicable
- for flat slabs, the minimum slab thickness must be as per table 5.5.2(A) "over a rectangular area extending 0.16 of the larger span length either side of a column in each direction"
Now, my quesitons are:
- I have a 200 flat slab with desired FRP of 90minutes so my minimum is 200mm. Slab is supported by 2000x200 blade columns/walls and there are 30mm deep setdown wet areas near them, bringing thickness locally to 170mm. This brings back the old philosophical question of what is considered a wall and what a column. My interpreatation of the clause is that the failure they're trying to prevent is punching shear, and when they refer to a "wall" they mean it should be continuousm such a boundary wall supporting the whole extend of the slab, for which punching shear is not a failure mode. So if this interpretation is correct I cannot consider these as walls but as columns, even though punching shear utilisation is very low for them. What's you take on this? Would you consider these as walls or not?
- With the 2009 code, one way we could deal with this issue was to check the slab with EC2 ("methods of calculation" of Clause 5.3.1 (b)), however the new code specifically excludes this option for this particulat check ("The depths in this table cannot be reduced by calculation methods in accordance with Clause 5.3.1(b)). So I was thinking to treat these wet areas effectively as openings in the slab, i.e. still designing them for strength but create a model in RAM with voids where the setdown areas are and checking the remaining slab for bending and shear under the fire case (G+0.4Q), and do a temperature distribution analysis into the section to check the reduced reinforcement strength at 90minutes, and distribute the wet area self weight as a dead line load at its edges. It should be conservative because we are discounting the wet areas for strentgh completely and making sure the slab doesn't fail. Obvioulsy we still need the setdown slab to comply with the minimum thickness for isolation. Anyone sees any issues with this method?
