Deflection Limits for Australian Flooring

[James_Gleeson]

Hey guys,

Guidance for deflection limits seems to be extremely scarce, even when considering the suggested deflection limits provided in AS1170-2020 Appendix C. Also, when comparing limits and load combinations adopted by manufactures, they seem to be contradictory even with things that you would assume would be so standardised throughout the industry such as floor framing supporting flexible flooring (carpet, flexible underlay etc) and floor framing supporting brittle finishes (tiles, grout etc)

Does anyone have a more extensive summary of reasonable deflection limits and which load combinations they apply to?

What is reasonable for the floor framing for the following load combos:
Flexible floor finish
- Short term - G + ΨsQ
- Long term - G + ΨlQ
- Live - Q
Brittle floor finish
- Short term - G + ΨsQ
- Long term - G + ΨlQ
- Live - Q

In trying to rationalise everything I was thinking of settling for short and long term for flexible finishes being span/300 and for brittle finishes being span/500 or maybe 600 (However some manufacturers adopt span/360 based off AS 3958.1—2007 guide to installation of ceremic tiles which confuses me though). I think Live by itself can be limited to span/360?

Would be grateful for any feedback because I am starting to go crazy.

 

[mrlm]

See attached a paper from the ASI for steel deflections, extracts from a portal frame design textbook, recommendations from Timber design handbook HB108-2013 and AS 1720.3:2016. I find the criteria from AS 1720.3 confusing as most other references allow L/360 for dead load and a less strict deflection for live load. This appears to be the other way around.

 

[James_Gleeson]

Thanks for your response.

Do you have any documentation regarding brittle floor finishes like tiled floors?

Thanks.

 

[mrlm]

I'd be looking towards the concrete code where they recommend designing for an incremental deflection of L/500 or 15mm (industry standard) for brittle finishes. Incremental deflection is total - initial (where initial is deflection under dead load and prestress). Obviously this is for a concrete floor.

You also want to ensure that the slab is unpropped when the brittle finish is added. Otherwise your incremental deflections are your total deflections as the slab has not yet experienced its deflection under dead load.

In my experience, if you design your slab to deflect L/250 or 25mm for long term total deflections, your incremental deflection come out to be about L/500 or 15mm.

 

[Smoulder]

@mrlm Some limits are for G+Q which has to permit more deflection than just G. Some are for Q only, can be more strict than G only because you feel it not just see it. L/800 for concrete Q only.

 

[rapt]

What is wrong with AS3600 table 2.3.2 for this?

 

[James_Gleeson]

I did not think to refer to AS3600 but yes table 2.3.3 is a great table for this so thank you very much I appreciate it. Just to make sure I am not making any mistakes. Would the incremental deflection load combination generally just be creep + long term live load?

 

[James_Gleeson]

Now that I think about it, for the incremental deflection would you need to check
1. short term Q (given Ψs>Ψl)
2. Creep + long term Q (given that creep + ΨlQ may be > ΨsQ)

Both of which would have to achieve span/500?

 

[mrlm]

Are you designing a concrete floor, steel, timber, something else?

Incremental deflections aren't really a load combination, it's more of a load condition. It is the difference between the total deflections and the initial deflections:

Delta.Inc = Delta.Total - Delta.Initial

Your total deflections are long term deflections. G + Q (with the relevant factor) + shrinkage and creep
Initial deflections are instantaneous deflections that occur due to dead load (and any prestress). This excludes any live load and typically any SDL as they have not yet been applied.

Provided the difference between these two deflections (your incremental deflections) are less than L/500, you should be ok for your brittle floor finishes.

For example: under dead load (essentially SW only) you have a deflection of 5mm.
Your long term deflections are 19mm.
Therefore your incremental deflections are 14mm. Provided this span was >7m in length, this deflection should be fine for your brittle finishes.

Have a look at As 3600 Clause 8.5.4.
Clause a) is your long term total deflections
Clause b) is your incremental deflections. The only difference between these 2 equations is the 1.0g factor is removed as your initial deflections are subtracted from your total deflections.

Hopefully that clarifies it for you.