Minimum Footing thickness as per ACI 318

[pjyb1962]

Dear expert engineers, I have a query.
For RC footing design as per ACI 318, we use development for tension (Ldt) as L bar and for compression (Ldc) for the straight part of the same bar coming as column rebar/ dowel. Bot Ldt and Ldc can be proportionately reduced as per the induced tension/ compression stresses in the bar. But there are minimum requirement of Ldc = 200 mm in ACI 318M-11. Eventually I have to decide the minimum thickness of the footing based on this 200 mm value.
Whereas in 15.7 clause of the same code, the minimum footing thickness recommended as 150 mm.
Various books stae that compressive stree distribution in compression transfer throgh bearing and passes to footing in 1V:2H slope. So why do we need to retain 200 mm minimum Ldc requirement?

Can anybody put some light on it?

 

[TehMightyEngineer]

I usually hook the bars to get around this problem but I'd also say that you're right and compression development of the bars at the footing shouldn't be a concern.

Maine Professional and Structural Engineer
American Concrete Industries

http://www.americanconcrete.com

 

[KootK]

It sounds as though your concern is mostly for the compression case since you're proposing bearing as a solution. Hooks won't help for compression.

I prefer to make my footings thick enough to develop my column bars in compression. While plain bearing may well work numerically, mobilizing it with undeveloped dowels would imply bond failure of those dowels as the rebar load path gives way and the bearing path takes over. And once that happens, the dowels instantly become useless for tension, compression, and shear friction.

One solution is to use smaller dowels. They don't necessarily have to match your column verticals in size.

It's also worth noting that as columns creep, axial stress in the cross section tends to migrate towards the reinforcing steel.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[TehMightyEngineer]

I always forget that hooks don't help in compression...

Maine Professional and Structural Engineer
American Concrete Industries

http://www.americanconcrete.com

 

[KootK]

I'm sure they kinda do.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[pjyb1962]

Yes, my concern is compression, as it is ultimately guiding the thickness. If the footing is supporting a very minor load, and my Ldc calculated is 75 mm (3"), say, still I have to use Ldc, min = 200 mm which will land up to minimum 325 mm overall thickness. In the cut-throat EPC business environment, every mm counts money. Code does not explicitly tells you to comply this L,dc for footing. Books are also giving the dispersion theory in a way that Ldc, requirement is not at all concern.
I do not have any reservation to provide Ldc, calculated, by why minimum is 200 mm?
Eurocode 2 -1-1 gives Ldc, min = Ldt, min = 100 mm which is more logical.

 

[KootK]

In the cut-throat EPC business environment, every mm counts money

Please. EPC is frenzy of frivolous spending compared to residential.

Eurocode 2 -1-1 gives Ldc, min = Ldt, min = 100 mm which is more logical.

What makes the Eurocode value more logical? Simply the fact that you like the answer better?

I do not have any reservation to provide Ldc, calculated, by why minimum is 200 mm?

Because some smart people put their heads together and set some reasonable limits to help keep folks from doing silly things. Are you really so gung go to install a 6" footing, with 3" bottom cover in an industrial facility? What if someone drives over it with a bicycle or a skateboard?



I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.