Existing Footing Capacity 1

[RPMG]

I have a project assessing an existing structure from the 40's where the square spread footings are under-reinforced (<0.0020*Ag). If I apply the old "one-third greater area of steel" rule, then the entire building is in distress. The footings have a monolithic pad on top, and I'm curious as to the design methodology of the footings in the 1940's. It appears the designer intended much greater capacities, and there have been no problems.

 

[JoshPlumSE]

Let me see if I understand what you're saying.

1) The footing doesn't have the 0.002*Agross minimum steel.
2) The footing does have sufficient reinforcing to satisfy the amount required for flexure resistance.
3) The footing does NOT have sufficient reinforcing to satisfy the 4/3*As required to ensure a ductile failure.

Is that correct?

Also, in the image you shared where are you taking the moment demand B-B or A-A? I would take it as at B-B.

 

[JAE]

Not sure about the 1940's. Is this in the US?
In ACI 318, the min. footing reinforcement isn't supposedly governed by the 1/3 greater rule....only the 0.002Ag (assuming fy = 40 psi).
But back in 1940, I don't think the min. 0.002Ag was present but would have to check.

 

[jayrod12]

What kind of dimensions and reinforcing are we talking about here.

 

[RPMG]

Josh, I took the moment about both locations. A-A always governs, and all the statements are correct.

JAE, this is in the US. The 1/3 rule is not applicable to spread footings per ACI, but it was commonly applied in the past.

jayrod, 11x11x2 ft footing & 4x4x1 ft monolithic pad, for example. I did check them as plain footings.

As I understand it, the minimum area of steel is when you compare the lowest factored reinforced to the highest unfactored plain strength. And the rational for the minimum is that under-reinforced concrete might behave non-ductile as plain concrete. To me, that's just a thought experiment, and someone should have been breaking concrete.

 

[r13]

You should calculate moment at section A-A, as well as B-B (usually will not govern), with d varies. Then check stress at steel level, by working stress method with uncrack section, and transformed steel area. Compare the result with allowable permitted in ACI 318 Alternative Design Method. I don't think the allowable stress has changed much from the 40s. Note that the old foundations are usually stacked in layers, with increase in d to meet increased demand towards the column face.

 

[JoshPlumSE]

Retired13 -
I have always taken the moment demand for a footing at the face of the pedestal, not the face of the column. Some may not want to call this thickened area a pedestal, I guess. Because the dowels are interior to the face of concrete. Is that your rationale?

RPMG -
So, when you're looking at AA, are you considering the total capacity of the section. Including an increase for the portion where d is increased?

I'm not familiar with the 1940's code. But, if the As_required < As Provided, I'm willing to bet that explains why you haven't seen any problems. Granted, if there were to be problems, they would not be ductile.

My assessment of this footing would like be something like "it probably meets the design codes in place at the time. (Though I would like to obtain a copy of that design code before concluding that). While the footings do not meet current design codes / standard of safety, absent any observable damage, there is little reason to suspect them to be unsafe for the current occupancy. However, any attempt to add additional loading to the structure would likely require significant retrofit / improvement."

 

[jayrod12]

From a handy dandy cheat sheet I keep from an old coworker done to ACI 318-63, for a footing that size, I'd expect to see 9 or 10 #10 bars each way. this cheat sheet was based off 2000 PSF allowable bearing pressure, 20 ksi steel, 3ksi concrete. Not sure what you have on site, but your proportions don't really seem to worry me too much honestly. How big is the column on top of the 4ftx4ft pad?

 

[r13]

JoshPlum,

This situation is similar to design of flat slab with drop panels. Moment and shear need to be checked on varies critical locations. Another popular form of footing at that time is truncate cone, or pyramid (similar to capital of flat slab).

 

[RPMG]

Thanks all. I'm moving forward with working stress results. ACI 318-14 adopts 318-99 Appendix A: Alternative Design Method, which does not require a minimum area of steel. It produces higher results than the 1/3 rule, but more importantly, these are the only provisions that are applicable.

 

[r13]

JoshPlum,

Actually you have a very interesting point - why not consider the upper concrete block a pedestal? I think it can be, if its free edge to column face distance is very small, and its thickness is relatively large, so flexural behavior is not likely to occur. However, there is no rigid line drawn between large and small. At the end, prudence and engineering judgement shall prevail.

 

[JLNJ]

If you don't need the footing's full capacity, maybe you can consider the bearing stresses to be higher under the middle of the footing so that the outer regions see less load. This will reduce the flexure at BB.

Said another way, if a smaller footing will work, how could there be a penalty to adding concrete to the perimeter?

 

[JAE]

RPMG - the min. area of flexural reinforcement of 1.33 x (required As) was not included in early 318 specs...i.e. 318-47 for example.

 

[wannabeSE]

Have you checked the capacity of the plain concrete ignoring the steel completely. With so little steel, the plain concrete might have more capacity.