Fire - Structural adequacy for flat slabs 1

[li0ngalahad]

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?

 

[blihpandgeorge]

i agree that it is still a flat slab supported by columns, so need 200mm thick. also agree on proposed alternative of treating as voids under fire case but maintaining insultation requirement. My experience with the latter is that often these wetareas are near columns so this creates other problems.

 

[rscassar]

Not necessarily, the 1/6 max span length is important. It has come up a lot for us recently particularly with placing columns away from setdowns otherwise a drop panel is required to rectify the 200mm deemed-to-satisfy. I don't like the justification that a 2000x200 blade is a "wall" for the slab design fire condition. I don't believe this is the intent of the code as the load re-distribution available is minimal and should be treated as a two-way flat slab supported on columns.

Unless you can justify and detail for the fire condition this portion of the slab under fire condition to have zero-stiffness and zero-strength.

 

[rapt]

For an engineer to even think of this as a possibility is really worrying.

Why cant you have a punching shear problem on a 2000 * 200 blade?

Its "definition" as a wall is for combined bending and compression design of the column/wall. Not how it interacts with the slab.

 

[dik]

I used to, but haven't done it lately... just not the projects... treat a 200 wide wall as a 200 x (5x200) for punching shear and moment and would further treat the slab as being totally fixed at the face of the wall + d. I'd also add 2-25M in the bottom of the slab to intercept any punching shear.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[li0ngalahad]

@rapt Thanks for the sweet words mate, unfortunately a real word engineer faces these kind of issues all the time and interperting the AS3600 words is always a challenge, wish the quality and completeness of the Australian Standard were somewhat similar to EC2 and ACI but can't have everything I guess

 

[rapt]

Have you ever read ACI or EC2?

Where in those 2 codes is this explained any differently to AS3600?

Where in AS3600 does it say punching shear only applies to columns. The word "column" is only used once in the whole clause in a NOTE in 9.3.3. And Notes are not actually part of the code, they are advisory.

In the definition of perimeter it uses the word "support", not column or wall.

If you read any of the good theory works on punching shear, they will show the punching effects at a blade column. They will explain why the Betah shape factor is there and that punching failure occurs on the ends of the blade column with little to no load transfer through the majority of the long sides, except for the length at the end approximately equal to the width of the blade, so it effectively becomes approximately a 2:1 shape. That is what the Betah factor does in the formula.

But it is not the codes job to teach you the theory. That is what text books and research papers are for.

The differentiation between a column and a wall based pm 4:1 ratio was removed from the code in the 1980's because it was very arbitrary and was abused by the design community. 800 * 200 was a column and 800 * 199 was a wall.

It was reintroduced in about 2001 in the Fire rules only in clause 5.6.2 to differentiate between the fire effects because corner bars lose a lot more strength in fire than face bars and there are a lot more face bars than corner bars in a wall compared to a column. This came from the Eurocode Fire rules.

In 2018 it was reluctantly introduced into the earthquake rules in section 13 as it was necessary to treat walls differently for minimum reinforcement rules and boundary elements.

From there designers seem to have decided it applies for all clauses in AS3600, even though it is not a general definition.

 

[Retrograde]

RAPT - I agree with your points above, but I'm more interested in your opinion on the design philosophy of checking the punching shear while assuming the wet area set down is a void. I know a lot of consultants adopt this approach.

 

[Agent666]

It sounds like a conservative compromise to me regarding treating the set down as a void for the structural integrity check, exercising some engineering judgement essentially.

Provided you are working out the properties of the critical perimeter accounting for the pseudo opening, and reinforcing the remainder appropriately, and it works for strength using this approach, then I don't see any immediate issues with it.

Reality is you could have an actual opening the size of the set down and it would satisfy this fire requirement if the slab was 200mm. Just don't go less thickness than the insulation minimum thickness or violate the axis distance to reinforcement.

https://engineervsheep.com

 

[rscassar]

Yes, but perform both cases, reinforce for strength under 1.2DL+1.5LL for the full slab cross-section, and then again for fire case treating the setdown as a void (allowing for the load on this void as line loads around the perimeter), with the additional reinforcement requirements for the fire case to be superimposed over the strength requirements. Don't want to under-reinforced a cross-section on the basis that it is being treated as a void.

 

[rapt]

I thought I had commented on this on other threads.

I see no problem with doing the full ultimate strength check including punching shear assuming that the set down is a full depth penetration so a void in the slab. Then use the deemed to comply punching shear acceptance for fire, so no extra calculations are required based on fire loading.

Or you could try to use the Eurocode design method for fire. That is not as easy as it sounds. You have to base it on a reduced depth due to the soffit concrete loss of strength. That is defined in Eurocodes. You have to use a reduced tensile strength of concrete over the depth. How you do that no-one has really defined as far as I have seen.

You have to allow for concrete spalling. Again how do you do that? I have not seen a method defined.

 

[rscassar]

Thanks Rapt, insight from a committee member is always appreciated.

 

[rapt]

Remember, it is my opinion. Not a committee opinion.

 

[rapt]

One other thing on this,

AS3600 and the Eurocode documents specifically say that the fire design methods cannot be used to reduce the allowable minimum slab thickness below 200 for 90min fire!