Reinforcement Design of Isolated Footing with Tengs Charts 1

[SouradeepGuha]

Hi All,

This is regarding a confusion that me and my colleagues faced regarding design of reinforcement of isolated footing.

Sizing of footing was done as per unfactored LCs. Since there was uplift, the bearing pressure was adjusted with the help of Teng's Charts.
So far so good but the problem arises when we use the factored LCs to calculate the reinforcements. In this case the uplift being higher, Tengs Chart requires the use of higher factors to adjust the bearing pressure and so the moment at face of pedestal significantly increases. Ast required is considerably high.

Is this the correct approach?

One of my colleagues suggested to multiply the net bearing pressure (with unfactored LCs) by 1.6 and design reinforcement accordingly.
This approach gives considerably lower Ast.

What is the correct approach in such cases?

Thanks,
Souradeep

 

[r13]

The way your colleague is the old timer practice. He is on the conservative side for positive downward pressure. Keep an eye on the uplift load case, you are unclear, and I don't know Teng's chart, how it affects the moment?

 

[SouradeepGuha]

From the Tengs chart we take the modified bearing pressure(w') for the uplift case and calculate the moment w'L^2/2.
This approach gives higher design moment than what we get by multiplying the unfactored design moment by 1.6.
And that is where my question lies, which is the correct approach irrespective of which is conservative.

 

[r13]

Which face is critical, top or bottom?

 

[SouradeepGuha]

Bottom.
Loss of contact for my case is 8% for unfactored LCs whereas it is 41% in case of factored LCs. This high increase in loss of contact area in factored LCs is also due to the fact that the foundation is eccentric.

 

[r13]

If you have accounted for the cracked base (loss of contact area) in the un-factored load analysis, this is how would handle this situation:

For the uplift case, multiply the maximum un-factored bearing stress by 1.6, the resulting stress should be less than the ultimate bearing stress, if so, design the reinforcement use the resulting pressure. Otherwise, you need to increase the footing area to bring down the pressure.

 

[SouradeepGuha]

So if my understanding is correct then our unfactored bearing pressure should be less than (ultimate bearing capacity/1.6) and if that is so one should design the reinforcement with w=1.6 X unfactored bearing pressure. There is no need for any calculation of factored bearing pressure.

Thanks for the help!!

 

[r13]

Yes, at least it in the old way. The important things are - 1) is the footing stable under all load cases, factored and unfactored; and 2) did you account for the cracked base in the analysis (this is the area that Teng's chart was used for, I guess); 3)top face reinforcing.

 

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