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Re: two slabs

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I cannot comment on the first 2,

Normally the form would be pre-cambered before pouring but I have heard of the other method being used.

For spans in that range with RC I would not rely on the L/D rations to save me. If you like I will run it on RAPT and give you the estimated deflections if you give me the relevant spans, loading, concrete shapes , concrete strengths, reinforcement covers etc. RAPT will give you a more realistic estimate of the deflections taking into account creep and shrinkage (not as a multiplier) as well as cracking and shape changes such as drop panels.

Looking at deflection patterns, I would agree that grid lines also need pre-cambering for two way slabs as the column strip deflection is often more critical than the overall panel deflection.but most people probably do not think of this when specifying it.

At 05:50 AM 8/12/2004, you wrote:
Here are a few more questions-->
1. If using software like Adoss for the design of the slab, how would the deflection values given in the output relate to the amount you precamber?
2. I am not sure that Adoss calculates the cracked section properties..It shows a stiffness Ieff but this may only relate to the effect of the drop panel on the specific span.  The values of Ieff in my example seemed high. Does anyone have any info on this?? I am away from the manual right at the moment. I have version 7.01.
3. As mentioned by others how do they finish the slab to ensure it maintains thickness. Do they jack it up right after levelling and prior to final finishing or is the formwork jacked prior to concrete placement?
4. I have spans in the 28' range N-S and 24' E-W but I am using the code guidelines for minimum two-way slab thicknesses so that deflection does not have to be considered. Does this make sense that precambering is still required? 
5. It seems to be used quite often on flat slab designs that I have seen.  The value of precambering is noted in the center of the bays. Is there no precambering done on grid lines as well?

Regards  Gil Brock
Prestressed Concrete Design Consultants Pty. Ltd. (ABN 84 003 163 586)
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