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?

 

[skeletron]

Assuming you are modelling this and want a starting point for discovery of concept, I would look at it this way:
1) Use minimum steel and determine relevant properties for deflection behaviour
2) Match the layout of the bottom steel and determine relevant properties for deflection behaviour


(1) would illustrate the minimum benefit you'll receive from top steel
(2) will illustrate a sort of upper bound on what you can expect
I would then consider the two basic results and go from there, perhaps choosing something where the top steel is half the bottom steel (ie. make it easy to place).

 

[HTURKAK]

14" reinforced concrete slab is too heavy.. You may try waffle slab to reduce the own weight and try to provide camber as an option. What are the span dimensions? If you provide a descriptive sketch ,it will be more helpful to get better responds..

 

[r13]

IMO, the most effective method to improve long term deflection is provide "camber" to the slab. The linked paper provides other areas of improvement, and note that, while I think additional reinforcing steel in highly stressed areas will help the long term performance, congestion of steel can hamper your effort. Also, why top mat in the mid span, to help compression? I think adding tension steel will be more helpful. Link

 

[dik]

As HTURKAK noted... either a waffle slab or flat slab with drop panels... I usually avoid camber with slab systems if possible.

Dik

 

[Agent666]

....good starting point for me to try

Try and see, really hard to say without calculating it out.

IMO, the most effective method to improve long term deflection is provide "camber" to the slab.

Adding camber just hides the potential issue given deflections can be quite variable (could be plus/minus 100% of some simplified checks in reality). It's certainly a strategy, but the slab is still deflecting over time the full calculated deflection so damage to any sensitive finishes is still there. It just looks like less deflection, so hides the issue. I wouldnt say it's that effective as it is only addressing a final deflected profile not the magnitude of the deflection.

Also, why top mat in the mid span, to help compression? I think adding tension steel will be more helpful.

I think this statement shows a possible lack of understanding of the mechanisms at play with creep under sustained loads. I'd think adding an appropriate amount of compression steel is far better bang for your buck than trying to solve a long term (creep) deflection issue by simply adding more tension reinforcement in isolation. Others feel free to chip in though.

 

[r13]

1. Creep causes serviceability problem, it does not directly linked to failure. The camber solves the long term deflection in the quantifiable manner, as opposed to other methods, while effective, but most are difficult to quantify. Note this is not to say camber every floor is desirable, but for cases it is a viable solution, and quite effective.
2. Holding the applied load constant, over reinforce tension steel reduces stresses/strains in the concrete directly. I admit, I've not looking into the effect of doubly reinforced section yet.

 

[jayrod12]

Having compression steel is much more effective than adding additional tension steel as the compression steel removes compression stress from the concrete, thereby reducing the magnitude of creep in the concrete.

Providing a cambered slab only improved the final in-service flatness of the floor, but as indicated previously by others does not change the total amount of deflection experienced by teh slab. And when talking long-term deflection, all of the finishes will be installed before the long-term portion kicks in and therefore you will still have the damage to the finishes that is undesirable.

The only way you're really going to fix high values of long-term deflection is either providing post-tensioning to the slab, or providing a thicker slab.

I'd have my own concerns with using a waffle slab or the like for a slab of this size. I can only imagine we're likely looking at some form of transfer slab, I highly doubt it is 14" thick on each floor.

 

[BAretired]

Reducing dead weight and adding prestress are likely more effective and more economical ways of reducing deflection than adding compression steel.

BA

 

[Agent666]

The camber solves the long term deflection in the quantifiable manner

No it doesn't address the magnitude of the deflection that occurs. It just addresses potentially the longer term sag. It does not prevent any issues with say building wall on cambered slab, then wall still sees the full deflection irrespective of the camber.

over reinforce tension steel reduces stresses/strains in the concrete directly.

Um, no it doesn't. Under serviceability loads the compression force is the same in the concrete. Tension reinforcement has lower stresses. Slab may be stiffer with more tension reinforcement, but you are not changing the creep behaviour and this is what this thread is about isn't it?

I admit, I've not looking into the effect of doubly reinforced section yet.

Well this is what this whole thread is about isn't it....

 

[r13]

In many cases, cambers are specified in construction documents to ensure level slabs and minimum slab infill. Since there are many uncertainties in long-term deflection estimation, a camber may be set as one-half of the computed total long-term deflections including the immediate deflections, as building owners and contractors prefer that the cambers are sized to accommodate only immediate deflection and a portion of the long-term deflection. It is also worth noting that cambers less than ¼ inch tend to be ignored by concrete contractors because the value is within construction tolerance and error range.

Link

 

[r13]

Below is a calculation on Icr for masonry. Maybe it is an over simplification, also the different material, but I think the concept is the same - increase in tension steel leads to increased Icr, which relates to deflection (immediate and long term). Another important property has not yet addressed in long term behavior of reinforced concrete is relaxation of steel (more pronounced in prestressed concrete though).

 

[hokie66]

The OP clearly stated this is a podium slab. So cambering it will do nothing for the supported structure above. A stupid idea, in short. The primary method of controlling deflection is to add depth. We weren't advised to details of the structure above, but a 14" slab is quite thin for a transfer slab.

 

[Retrograde]

Be careful relying on large amounts of compression steel to reduce long term deflections. Although it works, you can get a situation where if the shrinkage strain experienced on site is less than the value you used in your design, you can end up with more deflection. You need to run a sensitivity analysis with lower and upper values of design shrinkage strain.

 

[hokie66]

Retrograde,
While that may be technically correct, it is too complicated for actual construction. More depth and/or carefully designed PT can deal with deflection better than esoteric musings.

 

[r13]

Either method has its merit in deflection control, a combination of the two could be ideal, especially for a long spanned open space. I failed to see any stupidity about it.

 

[hokie66]

Just how do you think that camber will be of benefit? The deflection will still occur, and still have the same effect on the supported floors above.

 

[jittles]

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.

 

[JoshPlumSE]

The OP clearly stated this is a podium slab. So cambering it will do nothing for the supported structure above.

Thank you Hokie.... I knew I was missing something about the objections to camber. Your point is that even if you perfectly camber out the total long term deflection, then then the supported structure above is still going to see (and get damaged by) the deflection. The podium slab may be flat at the end, but the structure supported above has deflected the same amount.

We don't even get benefit from the supported structure not seeing the initial elastic deflection due to self weight, because that occurs (presumably) before the supported structure gets built on top of the podium slab.

I'm surprised I didn't see it sooner. But, thank you for so clearly stating that again. It genuinely helped me to better understand this thread.

 

[hokie66]

No problem, Josh. It is a shame that some, even after the explanation, still refuse to understand.