Min reinf in equipment foundation

[SHBH]

Hi,

I am to provide main (vertical) reinf in an equipment foundation which is octagonal foundation with each side =1574mm. Depth of this fdn is 1.5m out of which 300mm above ground.
I am struggling to provide min reinf into this foundation. ACI318 cl 10.9 says that ther should be min reinf = 0.01Ag whic result in huge no of bars of very big dia. Is there any other ref clause which can actually reduce this value.
BTW this is high seismic zone area so actually cl 10.8.4 is not applicable in reduciog the gross area.

thanks

 

[shin25]

Looks like you are defining it as foundation element but, providing minimum required reinforcement for column. For footing the min. required reinf. is = .002Ag.

 

[shin25]

And this minimum is for temp and shrinkage.

 

[SHBH]

but, shouldn't this pedestal be treated as a compression element, rather than a flexural element.

 

[shin25]

Then all the footing should be as they carry compression force from columns.

The definition of column per ACI-318 is-

"members with ratio of height to least lateral dimension exceeding 3 used primarily to support axial compressive load."

 

[SHBH]

Alright, so which clause is the one which define reinf area as 0.002Ag, (blame it to my limited knowledge of ACI)
Moreiver do i need to provide 3d reinf cage to avoid any shrinkage cracks within the core. IS there any clause on this issue. I have see this kind of 3d cage for m/c foundation blocks. Is it not valid here ?

 

[shin25]

ACI-318- 10.5.4.

 

[49merc]

What kind of equipment is this? There may be significant dynamic loads involved.

 

[SHBH]

but what about the clause in chapter 15, "Footing"(sorry , the code is not in front of me)of ACI318 which says that at least reinf equal to 0.005Ag should be extended from supporting element(bsae slab) to the supported slab.


49merc, there is no dynamic load on this foundation. It is just a vertical vessel.

 

[MarcbSE]

The 0.005Ag provision would be for a pedestal extending above the footing to the column (typically) bearing elevation. It does not apply to the footing itself.

 

[shin25]

That's the amount of vertical reinforcement required at the interface between column and footing to transfer force from column to footing.

 

[SHBH]

What is the use of this clause then...? when it is followed...?
I am getting confused now. Moreover cl 10.5.3 says we don't need to follow 10.5.1 reinf provided is more then 1.3 time the required fromanalysis. In such a big pedestal literally there is no reinf required. So whatever you proved should be sufficient..?

 

[MarcbSE]

As previously posted, the 0.005Ag reinforcing is the minimum vertical reinforcing required to transfer forces between a column (or pedestal) into the footing. It would not apply whenever you do not have a concrete to concrete interface.

The minimum horizontal reinforcing for a flexural member such as the footing is 0.0033bd, but that can be waived when you provide at least 133% of the required reinforcing for the design loads.

In your case, the footing is so thick that it will almost assuredly work as a Structural Plain Concrete section - i.e. unreinforced. However, you should still provide at least the minimum horizontal reinforcing required for temperature and shrinkage.

 

[SHBH]

MaecbSE,
can you explain,"It would not apply whenever you do not have a concrete to concrete interface"

For normal case is it not valid..? Concrete to concrete transfer..

 

[MarcbSE]

SHBH,
What I meant was that if you have a steel column bearing directly on the footing then the vertical reinforcing is not required by code since the force transfer is accomplished through the anchor bolts.

Say you have a masonry wall sitting on the footing. The vertical wall reinforcing would need to be extended into the footing for the development length, but the footing would not be required to be reinforced vertically.

Even when you have a concrete column or pedestal the footing itself does not require vertical reinforcing throughout. Section 15.8.2.1 of ACI 318-02 says:

"For cast-in-place columns and pedestals, area of reinforcement across the interface shall not be less than 0.005 times gross area of supported member."

That means you have to provide at least 0.005Ag from the column or pedestal into the footing, but the Ag used is the area of the column not the footing.

 

[SHBH]

Thanks MarcbSE for the explanation.
Thanks everyone for the valuable contribution to enhance my understanding on using ACI code.