Footing Uplift 5

[slickdeals]

Folks,
When you calculate the uplift load on a footing, the code requires the use 0.6D + W. Assume this gives a net uplift of 30 kips.

When you calculate your resistance to uplift in terms of the footing weight and the weight of a truncated soil pyramid (based on a 30 degree angle), do you use a 0.6 factor on the uplift resistance and compare it to the above calculated uplift of 30 kips?

It seems like double dipping (very conservative) to use a 0.6 factor on the resistance side also. Thoughts?

 

[BAretired]

abusementpark,

Where are these instances? I know of none.

BA

 

[abusementpark]

BAretired,

These instances occur, IMO, when you are applying the 0.6 factor to significant dead load that is GUARANTEED to be there, like in a foundation or concrete shear wall.

People argue that the 0.6D is still applicable because it implies a needed 1.67 factor of safety for overturning. But this is only true for shallow foundations where you are relying on material weight for overturning resistance. Consider a pile cap in which you are using piles for overturning resistance. The uplift capacity of the piles has an additional factor of safety. So, the way I see it, there are two factors of safety for overturning.

Personally, I think the code should be clarified to say that the 0.6 factor only applies to superstructure dead load. The code can stipulate a 1.5 or 1.67 required factor for cases where overturning is being resisted primarily by dead weight.

 

[BAretired]

abusementpark,

You stated:

I agree with ron9876 here. There are many instances where there is a doubling of safety factors.

Where is there a doubling of safety factors? I find your argument totally unconvincing. There is no doubling of safety factors and you are simply confusing young engineers who are trying to make some sense out of all of this. Why don't you simply admit that you don't know what you are talking about?

BA

 

[nutte]

I do not agree. The code seems reasonable.

Not one to disagree with BA, I also think the 0.6D case is reasonable. I don't believe the code gives you the option of increasing the factor (to say 0.9) just because you're sure of certain dead loads. Skin friction resistance provided by a pile is not a dead load. Again, I agree with BA's explanation above on this issue.

I found an abstract to an engineering journal article that deals with this topic, but I'm not able to access the paper itself. Perhaps it would further the discussion, if someone here can get hold of it.

Counteracting Structural Loads: Treatment in ASCE Standard 7-05
J. Struct. Engrg. Volume 135, Issue 1, pp. 94-97 (January 2009)
Bruce R. Ellingwood, F.ASCE and Yue Li, A.M.ASCE

http://scitation.aip.org/getabs/ser...0001000094000001&idtype=cvips&gifs=yes&ref=no

 

[dcarr82775]

@ Amusementpark:

I can not grasp the logic you and others are using. Why is the DL of the superstructure any less known than the weight of the foundation? The only thing that is unknown in the allowance for mechanical (or similar things).

- A W24x55 weights 55 pounds per foot, not 756, not 42.
- 1/2" gyp board weighs 2.2 psf, not 4, not 1.
- an 8" normal weight concrete slab weighs 100psf, not 75, not 150.

Assuming you have an 8" concrete slab type structure (apartment type construction). Well over 90% of your DL is known to the same degree of certainty as the weight of your footing. Yet for some reason you are perfectly willing to apply a 60% factor to this, but a 90% factor to the footing. Why, how would you defend this in a court of law? If you are carrying around an extra 10-20psf in your DL that is another story.

I have never seen a steel beam yield at 0.66Fy, so we should scrap that idea as well right? I agree parts of the code are conservative, but I don't go willy nilly ignoring them because they make my life to difficult.

 

[steellion]

All, it's hard to come to a consensus when we're discussing about 3 different things in one thread.

0.6D+1.0W is the uplift case. 1.0/0.6 = 1.67, that is your factor of safety. This includes a little bit of "fudge" factor because the DL is not exactly known, and engineers tend to overestimate DL for gravity forces. When this combination is used, it is NOT necessary to take on the additional 1.5 FS for uplift case, that's already covered. The 0.6 has much more to do with providing an FS against uplift than knowledge of dead load. If we really can't determine the DL within 40% accuracy, we've got much bigger problems on our hands.

I don't see why you would use this load combination at all for deep foundations. The geotech will tell you that each pile has X allowable capacity of uplift. Whether the uplift resistance comes from deadman weight or skin friction is irrelevant. This already has FS included, so it's simply 1.0W/pile capacity = # of piles.

 

[southard2]

OK as one structural engineer in Florida who has forever been criticized for oversized foundations here is what I do in my practice. I'm sure some of you will disagree with regard to code interpretation but this is what I do and I feel confident.

First my the .6DL for ASD design I think was intended to make sure the dead load wasn't overestimated with dealing with net uplift situations. Usually for a gravity load situation in ASD the factor is 1.0. For LRFD its 1.2. So clearly the more confident we are on the accuracy of our loads the less of a amplification factor is required. For live loads the amplifcation is much higher.

I think the 0.6 factor is extra low just because our base dead loads are usually overestimated for gravity load cases. So by using a 0.6 factor we are sure not to be using too heavy a dead load when calculation net uplift. Its not that all the sudden we are unsure of the dead load.

For single story buildings I actually calculate the net uplift at the base of the column using ASD's 0.6DL + WL. That is the acting force on the foundation. On the resistance side I will then use the soil weight, concrete footing weight, etc... then make sure that the this resistance is 1.5 times greater than the net uplift from above. My view is that the soil weight and the concrete weights are known and therefore don't have to have the 0.6 DL factor. If I were to do this than I wouldn't think any factor of safety would be required.

In addition I make sure that the concrete weight alone without any soil weight yields at least a factor of safety of 1.0. That way if the soil conditions are soaked with water I still really don't have a problem. Also if the foundation starts to lift out of the ground the water bouyancy issue goes to zero effect. If the area of the column is greater than 800 square feet I'll sometimes use the MFWRS uplift loads since they are lower. It just depends on my mood.

I'm sure some will disagree with this approach but it is substantial and I'm confident I'll never have a footing pull out of the ground. I never considered cohesion cause I don't have time for that. What I do know is that I get accused of over designing my foundations on a regular basis and it gets really old explaining to someone why. Every now and then you'll talk to a contractor whos seen overturned steel dumpsters from a hurricane and they get the idea. I always try to paint the picture for people by having them imagine the building upside down with the columns hanging from the foundations. Now hang about 10 cars on the columns, etc, etc... You start putting the loads in terms of how many cars and they start getting the idea. Still I see relative to mine plenty of light foundations out there that obviously have not considered uplift.

For the anchor bolts sometimes I'll use the proper LRFD factors but in most cases I'll just take the 0.6DL + WL and multiply by 1.6 and 2.0 which is almost always way conservative. I see no reason why connections shouldn't be over engineered since they are cheap.

I have to say though that I've never heard of a foundation pulling up out of the ground. Not even on old buildings where certainly the engineers of old didn't even consider this. My guess is that if one column starts to lift up all the sudden it carries a much larger areas of DL then just its normal tributary area. Other columns might not even be in uplift at the same time cause we are talking about gusting winds. So you need to have multiple columns foundation fail simultaneously. The ASCE wind loads are probably way conservative with respect to the higher tribuatary area situations. On top of that the cladding tends to probably fail first due to human error, higher localized loads, etc...

I've also seen those beat up metal building foundation engineers using some pretty low factor of safeties out there. I caught one guy using 1.1 while including slab weights far the columns. Be careful on those people. I'm so sick of this net uplift issue that I've come close to no longer even taking on metal building foundation projects. I probably would just start telling people I don't want them anymore because of liability issues only the economy is so slow with respect to new building construction right now.



John Southard, M.S., P.E.

http://www.pdhlibrary.com

 

[AVak]

I am not sure how you can be sure that your foundation concrete is 100% there when it is cast into an excavation that often has an uneven bottom surface and sides. It is no less variable than the building dead load itself.

As others have pointed out, the 0.6DL is intended as a replacement for the old 1.5 safety factor for uplift. It results in a larger factor of safety of 1.67, but it cannot be ignored.

Adam Vakiener, P.E.

 

[ron9876]

BA let's say you have a shearwall pilecap that has tension on a pile. You can reduce the tension by using 0.6D. That is the first safety factor. Then you need to compare the net tension to the pile capacity whose ultimte capacity has been reduced by a safety factor. That is the second safety factor. They are additive not multiplied as I said above.

southard2 I have also had a similar problem on metal building foundations. I had an architect tell me that he knew an engineer that told him that he would use 1.0. I explained the code to him and told him that was the way it had to be. It worked that time.

 

[slickdeals]

BA let's say you have a shearwall pilecap that has tension on a pile. You can reduce the tension by using 0.6D

Shearwall pile cap with overturning moment will have more tension if you use 0.6D.

 

[structuresguy]

Like others, I have never seen a foundation pull straight up out of the ground. I have been working in Florida for 10 years now, and did quite a bit of forensic work during the hurricane year. But one of the big reasons I think that we have not seen too many foundation failures isn't that they are over designed (well, any worse than a FS=1.5 anyway). It is that the cladding systems of old, and even fairly recently too, fail much sooner. This is why there has been such a big push in the recent past to ensure continuous tensile load paths from roof to foundation.

I surveyed some heavily damaged buildings in Ft Lauderdale in 2004, metal buildings at the airport in particular. The structure of the building was intact, but the roof panels were completely gone over most of the roof, and the wall panels were probably 50-75% gone. The foundations were never given a chance to fail, or even come close to it. Of course, everything inside was a complete write off. It's just like that video showing the entire roof of some house coming off the walls completely intact.

As cladding systems, and their connections to the structure, get better, and in-line with Code required load combinations, then the likelihood of foundation uplift failure will get much higher than in the past. Based on this assumption, it is more important to ensure adequate safety factors in the foundation now, then in the past.

 

[BAretired]

ron9876,

The safety factors are neither additive nor multiplied together. They are separate safety factors for separate types of resistance.

slickdeals,

I'm not sure what your last sentence says. Can you elaborate?

structuresguy,

I think you are correct in your assessment.

BA

 

[slickdeals]

Say you have a moment M due to overturning in combination with dead loads. The moment gets resolved into tension and compression on the piles in a pile cap.

Using a 0.6D+W instead of D+W will produce more tension in the piles.

I could have misinterpreted Ron9876, forgive me if that's the case.

 

[BAretired]

Okay, slick...I agree with that.

BA

 

[abusementpark]

Where is there a doubling of safety factors? I find your argument totally unconvincing. There is no doubling of safety factors and you are simply confusing young engineers who are trying to make some sense out of all of this. Why don't you simply admit that you don't know what you are talking about?

I'm not sure why you feel the need to continually condescend people on this site. I'm not going to respond to it. If there a specific statement of mine that you don't agree with, then I'd happy be to engage in civil discourse regarding the matter.

 

[abusementpark]

I can not grasp the logic you and others are using. Why is the DL of the superstructure any less known than the weight of the foundation? The only thing that is unknown in the allowance for mechanical (or similar things).

As others have said, I think most engineers tend to take conservative estimates of the equivalent uniform load to be added to account for architectural ceilings, electrical, mechanical, etc. I'm not sure how many sharpen their pencils to determine the true load added by your typical, light architectural coverings and finishes. Obviously, if you know something significant is being placed (i.e. granite flooring), then it should accounted for specifically.

That being said, I think the largest aspect of uncertainty associated with the superstructure is what can happen when the occupancy changes from its original use. A new tenant may decide that they want exposed framing members and they remove the entire architectural ceiling. Or a heavy floor covering may be removed and replaced with a much lighter floor covering. So, we could crack our knuckles and determine an fairly accurate estimate of the true dead load for the structure right after it is constructed, but who knows what the future holds and when an extreme wind or seismic event will occur. So, this is why I think the superstructure DL has more uncertainty then something you can pretty much guarantee is not going anywhere without extensive renovation and engineering, like all foundation elements or a major structural element like a concrete shear wall.

All, it's hard to come to a consensus when we're discussing about 3 different things in one thread.

Exactly. What exactly is the idea behind the 0.6 factor? Is it to account for DL overestimation, DL uncertainty in the future, or both? Or is it to provide a factor of safety for overturning when you are relying solely on the weight of foundation?

For light single story structures, I often do gravity design based on a DL,max and uplift design based on a DL,min, which is what I think the minimum weight of the roof structure could be. Is it overly conservative that I multiply my DL,min by 0.6, since the uncertainty may have already been accounted for in the 0.6 factor? Someone may say I don't need to take the "fat" out of my dead load for uplift calculations, since that is what the 0.6 factor is there for.

 

[BAretired]

What exactly is the idea behind the 0.6 factor? Is it to account for DL overestimation, DL uncertainty in the future, or both? Or is it to provide a factor of safety for overturning when you are relying solely on the weight of foundation?

It is all three of the above. If you knew precisely the magnitude of the dead load and knew it would never change, would you change the factor to 1.0? That would provide no safety factor against overturning.

BA

 

[BAretired]

abusementpark,

This is what you stated on Oct. 13, 2010 at 20:45

I agree with ron9876 here. There are many instances where there is a doubling of safety factors.

Would you be so kind as to elucidate on where these many instances occur? As I mentioned earlier, I know of none, but if there are some instances then I think they should be addressed. Now is the time to speak up.

BA

 

[ron9876]

@slickdeals that is what I meant.

I have always thought that the need for a safety factor is the potential for the wind uplift to be larger that code minimums. Say one of those 155-160 mph storms makes it way on shore.

 

[abusementpark]

Would you be so kind as to elucidate on where these many instances occur? As I mentioned earlier, I know of none, but if there are some instances then I think they should be addressed. Now is the time to speak up.

See Ron's post on October 15.

Basically, anytime you have uplift being reduced due to the 0.6D factor on something that is guaranteed to be there (i.e. footing, pile cap, concrete shear wall, etc.) along with a geotechnical factor of safety on the uplift resistance of a foundation element (i.e. friction piles, drilled shafts, adhesion on footings). For all other service loadings, we are comfortable relying on the geotechnical factor of safety alone. Here we are factoring up the load by virtue of this 0.6D factor in addition to the geotechnical safety factor. That seems inconsistent to me. If the geotechnical factor of safety alone is good enough in other scenarios, then why isn't it good enough here?

It is all three of the above.

Then that is the problem in my opinion. We are trying to use one number to cover a multitude of scenarios. It seems inconsistent to me. The differing levels of structural reliability depending on your situation could be over the place.