Min. Slab thickness

[Enhineyero]

Structural codes have this criteria of minimum thickness of slab in relation to its unsupported length (like L/20, L/28, etc depending on support conditions). ACI says this does not apply to "members carrying partitions or other construction likely to be damaged by large deflections". Does this clause have a limit on applied live load?

Some engineers tend to use this to clause to justify a thin slab, and if the slab deflect too much they will point that "the code says we can use ____ thickness". say i have 10kPa as a live load, i dont think this clause will be applicable. does anyone have any insight on what live load limit can be applied to this criteria?

 

[TXStructural]

It is the engineer's duty to design in compliance with codes and standards, AND to design a structure which will be fit for the intended use:
ACI 318-11 9.5 Control of Deflections
9.5.1 - Reinforced concrete members subjected to flexure shall be designed to have adequate stiffness to limit deflections or any deformations that adversely affect strength or serviceability of a structure.

The provisions of Table 9.5(a) for one-way slabs do not release an engineer from the required diligence. The L/## values are useful where deflections are not a critical factor in serviceability. The commentary only mentions that the minimum thicknesses control under service load levels. The PCA Notes for 318-08 states that these controls "will normally be satisfactory" for "loads commonly experienced in buildings."

The 2009 CRSI Design Handbook states that the tabulated designs have live load, elastic deflection less than L/360. It also states that where long term deflections are critical they must be computed. Span to thickness ratios exceed 45 in some cases (at higher reinforcement ratios, where service load deflection controls.) At reinforcement ratios just above minimums, strength controls, and deflection should be no issue at all. Use of higher strength steel in design computations will result in wider cracks and larger deflections, so be careful if you try to minimize reinforcement based on Fy above 60 ksi.

One additional data point: the 1950 edition of the CRSI Design Handbook shows a minimum slab thickness of L/32 for the tabulated values.

 

[fcu45]

Simply your thickness will be controlled by the allowed max deflection in your structure.

 

[DST148]

@Enhineyero: Ref H'Book of Concrete Engg. Edited by Mark Fintel - The allowable deflection depends on the human activities carried out in the structure , the sensitivity of the persons using it and the amount of structural damping. The allowable span / deflection ratio in a prestigious public building should be as high as 1000 or 2000. In a low cost office building where deflections are damped by many partitions, a span / deflection ratio of 360 or 400 would be adequate. In a warehouse floor, the ratio could be as low as 180. These ratios are based on live load deflection and not the total load.

Type of partitions need to be clearly identified. Masonry partitions are fragile and susceptible to deflections. Gypsum board partitions are relatively insensitive to deflections.

10 KPa (200 psf) seems warehouse / industrial loading. ACI span /depth ratios would not be directly applicable in your case. You will have to calculate deflections and limit them to ratios acceptable for use and occupancy of your structure. Also a lot depends on what portion of 10 KPa is sustained load.

 

[asixth]

The span to deflection rules always seems to be a good way to get into trouble. They are a great starting point but I don't think it will hold up to well when trying to explain why a structure has deflected excessively. Most textbooks and publications for the concrete institute will provide typical span to depths which are great to start with for a design. I am guessing that the span to depth are based on 3-5.0kPa ranges. For a 10kPa range best to take a couple off the span to depth ratio, say Span/20 for a RC band single span, I would try Span/18 for a 10kPa live load as a starting point before verifying the design.

 

[DST148]

@asixth: Span / depth ratio is simplified (equivalent) way of limiting span / deflection ratios.
For example: Span is Simply supported, LL = 40 psf(2 KPa), f'c = 3000 psi(21 MPa), Ieff ~ 0.5 Ig, L/deflection = 360 translates to span / depth ratio of about 20.
For the loading of 10 KPa (200 psf) mentioned in OP, and for the assumptions made above, L / deflection = 360 translates to span / depth ratio of about 12, and
for L / deflection = 180 translates to span / depth ratio of about 15.
Although Span / depth ratios (which are are based on live load deflections) meet codal requirements for normal loading conditions encountered in buildings, they are not adequate for an unusual loading of 10 KPa(200 psf) and long term effects of sustained loading. Detailed deflection calculations are required so as not to get into trouble.

 

[rowingengineer]

What are normal conditions?

Would normal cover be 20mm or .8 inch? what if I had 40mm or 1.6 inches or 60mm or 2.4 inches? are span/depths still applicable?
What would the normal concrete strength? what if I used low strength normal concrete.
What if I remove the props early, what is the normal removal time?
What if the slab is exposed to the weather is this normal?

Minimum is minimum not calculated, I think engineers need to preform some type of deflection analysis, preferably establishing an upper and lower bound for deflection.

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[Enhineyero]

Thank you all. I guess code makers should put a note on the live load limit in using the span depth ratio. Sometimes engineers forget how these tables come up, and use them without the knowledge of what are the limits of using such tables.