Footings with large eccentricities. 2

[SteelPE]

I have a question about designing a combined footing to support a concentrically braced frame.

Generally, I size the footing to provide a FOS against overturning and sliding > 1.5. Once I know the general geometry I then check the footing bearing pressures against the allowable bearing pressure in the soils report. This requires me to calculate e (which is based off the service loads).

Once everything is acceptable I then need to size the footing for strength. In order to be in compliance with the ACI I need to get my moments in to an “LRFD” format. This is where I get a little confused. Am I supposed to recalculate an eu base on the ultimate load combinations. Or can I calculate the required forces (moments and shears) based off the service load combinations and then factor the loads using a “Psuedo” load factor?

I have run into instances where during the initial design the footing will work (e < L/2) but when I use the factored loads the footing no longer works (eu > L/2).

Sorry if this is a simple question but I just can’t find an example of a footing with large eccentricities.

 

[Lion06]

This has been discussed before. I was of the opinion that for the concrete design you need to use the concrete load factors. Most others felt comfortable using a pseudo load factor on the service bearing pressures.

 

[DaveAtkins]

I agree with StructuralEIT. And yes, I have encountered the same thing--a footing which works under service loads but not under ultimate loads.

DaveAtkins

 

[BAretired]

How can a footing work under service loads but not under ultimate loads?

The proper way to perform the calculation is to use service loads to determine soil pressure, shear resistance of soil and resistance to overturning, applying whatever safety factor you wish.

Strength calculations of concrete and reinforcement should be based on factored loads in accordance with the code.

BA

 

[Lion06]

BA-

This happens if you have a high WL moment. The WL moment gets factored @ 1.6 and the DL axial load gets factored @ 0.9. There are other scenarios, but this is the most common that I've encountered. The load factors skew the location of the resultant (compared to the service loads).

 

[bootlegend]

This happens a lot to me. If you are designing foundations under service load with e very close to L/2 there is a good chance that it will be "unstable" (e>L/2) when you factor the loads for strength design. Like was mentioned above, it is usually the DL/WL cases that this happens to.

I guess the options are 1) increase footer so it isn't an issue, 2) reduce LF on overturning load, or 3) draw shear and bending diagrams based on service loads and apply some LF to that for strength design.

 

[Lion06]

What I typically do when this becomes an issue is start taking advantage of the soil overburden for additional axial load. That is typically not done for sizing the footing because the geotech typically gives "net" allowables, but you can benefit from the use of it when factoring loads and skewing your resultant location.

 

[Lion06]

BA-
Here is a possible scenario. Say you have a 4x4 ftg with service loads of Pd=83.33k (including ftg self-weight), and Mw=95k-ft. The service combination (0.6DL+1.0WL) will come up with e=1.9' (95k-ft/50k), and the footing will be stable - not by a lot, but stable nonetheless. When you use the concrete combinations (0.9DL + 1.6WL), you get e=2.03', which is outside the footprint of the footing and is, therefore, not stable.

 

[BAretired]

SEIT,

Okay, I see what you mean. And I would agree with your earlier comment that it is legitimate to consider earth load as part of the dead load.

But if a high water table is present, I think you would have to deduct buoyancy forces from dead load or, in other words, consider the submerged weight of the footing and soil.

The discrepancy appears to be a result of a roundoff error in a particular code and does not occur in all codes. If, for example the factor for concrete combinations was taken as (0.9DL + 1.5WL) instead of (0.9DL + 1.6WL), the two methods would agree. Alternatively, if the factor for service load was taken as (0.6DL + 1.0666WL) instead of (0.6DL + 1.0WL), the two methods would agree.

BA

 

[archeng59]

don't use factored loads for foundation design.

 

[BAretired]

Why not?

BA

 

[Lion06]

archeng-
How do you justify NOT using LRFD combinations for concrete?

 

[archeng59]

Perhaps I misunderstood the questions asked, but it appears to me that the question is regarding the pressure resultant location within the "kern". The concrete footing is designed using strength design (USD, not LRFD), but the soils portion is not. When you start sizing the footing width, length and checking soil pressures, use unfactored loads. After you size the footing for unfactored loads, use factored loads for moment and shear to determine reinforcing and footing thickness for the width and length calculated using allowable loads. At least that's how I was taught to design footings. If I misunderstood the question, my apologies.

 

[DaveAtkins]

archeng59,

I agree with your approach. But sometimes the resultant location is outside the "kern." When this occurs, the footing may work under service loads, but when using factored loads, the resultant location may be completely off the footing. And so the footing would need to be larger for the factored load case.

DaveAtkins

 

[csd72]

I check the bearing pressure, overturning and sliding using asd loads and a safety factor of 1.5 as applicable.

Then I check the concrete and rebar for factored loads, conservatively this is 1.6 times the allowable bearing pressure actions.

The additional factor is for the concrete design and not for the soil, typical allowable bearing pressures have a factor of safety of 2, or even more if settlement is an issue.

 

[asixth]

There will always be confusion when engineering checks for service and ultimate limit state needs to be performed. Soil-structure interaction has always been best engineered with an allowable stress approach since there are many uncertainties with the material (the soil) to begin with.

I am having the same problem with highway bridge design at the moment, where you need to consider lane factors, dynamic factors, ultimate limit state factors and service limit state factors which can become all so confusing when knowing combination applies with what and designing for actions on your structure which will never co-exist.

 

[archeng59]

Rechecking (or checking) the soil bearing pressure using factored loads will result in a conservative footing size. If the pressure resultant is within the kern, or even slightly outside the kern in some cases, when checking the soil pressures using service loads, the footing will be ok. There is no need to recheck with factored loads. The allowable soil pressures are not factored values, so why check the resultant location for factored loads? Not a comparison of apples-to-apples. In almost every case, you will have to upsize the footing. IMO, that's never bad, but it is unnecessary.

 

[DaveAtkins]

I disagree with both csd72 and archeng59.

The bearing pressure is checked using service loads.

The concrete design should be done using factored loads, which is not the same as multiplying the bearing pressure (calculated using service loads) by 1.6. This is because 0.6D + W is not proportional to 0.9D + 1.6W.

DaveAtkins

 

[archeng59]

What part of what I stated do you disagree with?

 

[Lion06]

I think there is a basic miscommunication happening. No one is saying that you should check soil bearing pressures using factored loads. The fact remains, however, that in order to do the footing design you need to determine what the soil bearing pressures are using factored loads. You don't need to compare that value to anything, but an issue does arise if the resultant falls outside of the footprint of the footing. I don't believe anyone is advocating using factored loads to CHECK soil bearing pressures, but you do need to use factored loads to GET soil bearing pressures to do the footing rebar design and shear checks.