Top mat reinforcement to help reduce creep in a two way slab. 2

[tonyiggy]

I have a 14" conventionally reinforced concrete podium slab with a large amount of long term deflection in a middle span. I want to try and add a top mat to help reduce the creep. what percentage of steel e.g (1%) or layout e.g.(#6@12" O.C) is a good starting point for me to try?

 

[rapt]

From my investigations into the different effects for RC slabs

- tension reinforcement will help for more lightly reinforced sections. As the tension reinforcement ratio increases, the addition of extra tension reinforcement has less effect.

- compression reinforcement will help for deeper more heavily reinforced slabs and beams.

- adding reinforcement is probably the least efficient way to reduce deflections, especially compression reinforcement.

- do not believe the effect simplified methods in different design codes use to allow for compression reinforcement effects (eg kcs in Australian code and the equivalent in ACI code. They are really based on the optimum case I mentioned, deeper very heavily reinforced members.

- as mentioned above precamber is useless if you are worried about movement effects on the structure above. It only changes visual deflection profile, not movement.

 

[r13]

hokie66 is correct, cambering does not have any merit for deflection control. It might change the final deflected shape, but it doesn't change the amount of change in that shape. Whether you start flat and deflect downward, or start with an upward camber and deflect to flat, the deflection experienced by finishes and the structure above is the same and the potential issues are the same.

The assessment is correct, nobody argues that. We are talking about long term deflection for service concerns, isn't it? See this thread. Link

 

[hokie66]

We are talking about a podium (transfer) slab. Short term or long term, your approach of cambering the slab will not help with the issue of what happens to the structure above the podium.

 

[tonyiggy]

Thanks for al the input. This podium slab supports a 5 story wood structure above. due to arch issues, there is a column span of 35 feet that is giving me long term deflection issues.

 

[jayrod12]

You need more depth over that span. There's no magic way around it. Maybe you need to add a deeper beam between those two columns.

 

[HTURKAK]

Dear Mr hokie66 ;

I suggested the use of waffle and also suggested pre-cambering without knowing the podium slab supports a 5 story wood structure above. We are on east side of the Atlantic and in general , partition wall material is masonry block work. If necessary , the use of Pre-camber may be used to compensate for some the deflection and the common practice is offset by pre-cambering for deflection due to dead load.

For this thread, the use of pre cambering not for the long therm deflection but for the own weight dead load of the transfer slab will be of benefit.