The “new” Australian Concrete code AS3600 was released in 2018. Although we are over three years past its release date, we are only now starting to see how Structural Engineers have interpreted some of the AS3600 changes. A quirky and often missed change to AS3600 has been the interpretation of how to treat the fire rating considerations for flat plate slabs with set-downs. Lets jump in and take a closer look…

When it comes to a thin floor structure, you can’t beat a post-tensioned 200mm thick (7.8 inch) two-way flat plate slab. In a residential apartment setting, spans of 8.4m (27.5 feet) can comfortably be reached with this thickness. This grid span of 8.4m works wonderfully with most car park arrangements as well (incidentally, for more on what post-tensioning is, check out THIS article).

So for a multi-level apartment building sitting above a car park, the 200mm thick two-way flat plate has been a popular solution for most high-rise apartment buildings in Australia.

Set-down in a Two-Way flat Slab

Lets set the scene a little bit first. Once a column grid has been established to allow a 200mm two-way slab to work comfortably, the devil is then in the detail…

There are two details which always come to the surface during concept design of an apartment building. These are…

1. Detail of the threshold going from inside the apartment to external balcony.
2. Bathroom and other wet-area levels and falls for drainage.

Both of these details have a knock-on effect to the primary structure i.e. the 200mm thick flat plate slab. Lets take a look at the detail for an external balcony…

Option 1

With the question of a balcony detail, there are two options which are commonly used. Option 1 is to keep the primary slab flat. The balcony is then built-up with a pod and paver arrangement (paving units supported by legs). Drainage is then provided in the gap between the paving and the primary slab. A secondary hob is then provided at the threshold point and slab edge for water proofing purposes. Here is a section indicating this arrangement…

This arrangement has its positives and negatives. The first obvious negative is that tenants of the apartment need to step up to get out onto the balcony. This is a bit of a cheaper feel.

A great positive is that the primary slab remains very simple and easy to construct with flat formwork (resulting in a cheaper structure).

Option 2

Option two results in a better product. This involves keeping all elements outlined above with the exception of the primary slab. In this case, the primary slab is stepped down to allow the pod and paver system and associated drainage zone. This allows a smooth flat transition when walking from your apartment to the balcony.

There is a bit of an increase in cost due to the higher complexity of the primary slab formwork, reinforcement, detailing and construction. Here is what that arrangement looks like…

With both of these details the 200mm two-way flat slab thickness is maintained. Therefore we aren’t triggering any issues with fire rating in the eyes of AS3600 (both the previous 2009 iteration and current 2018 revision).

Things begin to get trickier once we enter the bathroom wet area. Here the primary slab needs to be set-down again to account for the falls of the floor for water drainage as well as the thickness of the tiles themselves.

The tile thickness can be finessed with a threshold detail at the bathroom door. However there is no escaping the drainage issue within the shower itself. Here there is often no choice but to provide a set-down in the slab to accommodate the drainage of the shower.

Considering the small plan area of the shower, the previous “solution” was to keep the slab soffit flat and provide a minimal set-down in the top surface of the slab (usually 30mm is sufficient). Here is what this arrangement looks like…

In order to keep the formwork simple, Structural Engineers have allowed a local reduction in the 200mm thick slab at the shower location. This means that for a small portion of floor area, the slab thickness is actually circa 170mm.

This triggers some issues with fire rating (for both the older version of AS3600 and AS3600-2018, we will cover this in the next section). There were some work-arounds in the 2009 version of AS3600 which allowed this to be acceptable, however things may have changed with the newer revision AS3600-2018…

Fire Rating Requirements of a Two-way Flat Slab to AS3600

The concrete code requires that slabs are designed with consideration to the event of a fire. The given fire rating period of a structural element is dependant on the buildings classification.

The fire rating period is essentially the period of time (in minutes) a structural element is required to withstand the heat effects of fire before it collapses. This gives occupants time to evacuate or even time for the fire to be put out before something catastrophic occurs to the structure.

Structural elements are requried to be deisgned for three aspects of fire…

1. Integrity: Ability of a fire-separating member to resist the passage of flames or hot gases through the member when exposed to fire on one side.
2. Insulation: The ability of a fire-separating member, as as a wall or floor, to limit the surface temperature on one side of the member when exposed to fire on the other side.
3. Structural Adequacy: The ability of a member to maintain its structural function in the event of a fire.

The fire rating requirement for a slab in an apartment building is 90/90/90 (that is 90 minutes for structural adequacy, integrity and insulation respectively). This is found in the building code…

Lets now look at the fire design requirements of our beloved 200 thick two-way slab in accordance with AS3600…

Design for insulation of a slab in fire conditions is found in AS3600-2018 table 5.5.1. The requirement is simply read directly off this table…

So far so good, we only need 100mm thickness in our slab to tick the box on insulation, our 200 thick sab passes with flying colours. Design for adequacy under fire however is a different story…

Design for adequacy of a two-way flat plate slab is found in AS3600-2018 table 5.5.2(A). Again simply read directly from the table…

In this case, our 200 thick slab is just sufficient. However, as you may remember, there is that tricky detail at the shower set-down which leaves us with a 170mm thick slab. Its important to note at this point that the equivalent table found in AS3600-2009 was largely the same as the current iteration. So how was this issue addressed under the “old” concrete code?…

Fire Design of Flat Slab for Adequacy to AS3600-2009

The way engineers overcame this minor issue of a flat plate slab thinner than 200mm locally at shower set-downs was quite a practical one…

The approach has been to simply assume this localised shower zone to be a penetration in the slab and run an analysis on this arrangement for the fire condition.

This largely worked in most instances as the safety factor for applied loads under fire conditions is less than that for room temperature (nominally 1.1G + 0.4Q as apposed to 1.2G + 1.5Q). The reduced loading coupled with the fact that large deflection on a slab in the event of a fire is nothing to get too hung up about meant that this has been the long standing solution for the last few decades.

However things may have changed with the current revision of the Australian Concrete Code…

Fire Design of Flat Slab for Adequacy to AS3600-2018

There is a little change which can be found in AS3600-2018. The section in question lies within clause 5.5.2. The most important part which is applicable to our detail in question is highlighted in yellow below…

The highlighted portion makes reference to the table we previously looked at for adequacy. The same table which showed that a 200mm thick flat plate was just adequate for a 90 minute fire rating. The most important part of this highlighted section which changes everything is…

“The depths in this table cannot be reduced by calculation methods in accordance with Clause 5.3.1(b)”

This very clear direction from the code applies to a footprint area over the support column equivalent to 0.16 times the span in that direction. There are a couple of things to unpack here…

It appears now that there is no longer any opportunity to calculate your way out of justifying a thinner slab to work in the fire case. However it appears that this stricter restriction exists only within this plan footprint immediately surrounding the column. There still may be opportunity to run the assumption of “set-down=penetration” in the fire case elsewhere. It would appear to me that the new interpretation looks a little like this…

The green hatched zones indicated within the image above are the areas where the current revision of AS3600 is telling us that a reduction in the thickness values outlined in table 5.5.2(A) cannot be reduced by any means or calculation trickery.

It does appear however that the remaining zones are still fair game for adopting the fire mode check. The unfortunate fact however is that due to apartment planning and layouts, the bathroom locations are more often than not adjacent to column locations. This would mean that adopting the penetration assumption for a shower set-down now has quite limited application, especially when apartment layouts may not be locked in until later in design.

How has the Industry Interpreted this Change to AS3600

Design codes can sometimes be a lot like the bible… open to interpretation. This particular clause is no exception. I have heard a number of interpretations that engineers are having on this particular section of the concrete code. I believe there is however a majority which take the view that:

• 30mm Shower set-down cannot be accommodated near columns for a 200 thick flat plate slab without introducing a corresponding soffit step to maintain full 200mm thickness.
• The slab thickness can still be locally reduced and still be adequate for fire in zones some distance away from columns. This can be justified by running a fire mode check, adopting the reduced design fire load and assuming a local penetration around the effected shower set-down (similar to the “old way”).

However I have heard a mixture of interpretations which go against this. This has come from a relatively large cross-section of engineers, all the way from one man/woman operations to engineers in some firms which would be regarded as “Tier 1”. These are generally along the lines of…

“this clause is strictly applicable to columns. If support is provided by a blade column resembling more a wall than a column (assuming larger than a 4:1 aspect ratio) then the clause is no longer applicable in AS3600-2018”

This I believe is drawing a very long bow and reading quite a bit between the lines of AS3600. I believe what AS3600-2018 is trying to do is protect the most critical zone of a two-way flat plate (where all the action happens as it were). Immediately surrounding the column is the point of highest stress due to negative bending moment as well as critical shear due to punching of the column (see more on punching over at THIS article). This section remains critical whether the slab is supported by a square, rectangle or blade column. This leads to the second justification I’ve heard a few times…

“The code is mostly concerned with punching shear. As long as you run a punching shear check in the fire mode with the penetration in place at the shower set-down location, it is fine”

There is some truth to this. As I outlined earlier, I believe AS36000-2018 is trying to better protect the critical shear zone immediately adjacent to a column with this clause update. However justifying this away by using the “old” method is going against what I believe the intended spirit of the clause. I believe it says it clear enough in black and white that no calculations or justification can allow a reduction below 200mm for a 90 minute fire rating…

“The depths in this table cannot be reduced by calculation methods” – AS3600 circa 2018.

Final Thoughts

Although the new revision of AS3600 came out over three years ago. There is often a transition period which takes a while for code updates to fully take effect. A principle reason for this is that the germination period of the design phase on most projects mixed with town planning approval periods means that it can take a project 1.5 to 3 years to go from concept to construction.

With most significant code updates, it takes the industry a while to gain a majority consensus on things. This is usually over a period of several generations of projects. It is often through the push-pull process between designers and certifiers that issues such as the one covered in this article can be fully teased out.

Currently with this specific issue of a 200 thick flat plate slab in fire case I see the current scenario playing out right now:

• A Structural Engineer who is unsure on the interpretation of this code update reverts to the Fire Engineer
• The Fire Engineer then reverts to the project building surveyor for their direction
• The project Building Surveyor then reverts back to the Structural Engineer for their interpretation of AS3600 (which is fair enough, the concrete code is the Structural Engineers domain and bible after all).

So it is really left to the Structural Engineers interpretation of what the code is trying to tell us on this specific occasion. Therefore any assumption on this clause in either direction needs to be well thought out and backed by engineering logic.

AS3600-2018 has also seen a revamp of the formulas and approach to designing shear capacity of beams. For an in-depth article on this with a direct comparison between AS3600-2009 and AS3600-2018, which also covers a potential anomaly (maybe a mistake?) in the current version of AS3600, take a look at THIS article.

The source for the feature image of this article is Masterform Systems. They are an Australian supplier who provide formwork systems (including shower set downs in slabs!!).