PEMB foundation uplift
PEMB foundation uplift
(OP)
I designed a foundation for a PEMB. I want to know peoples methods for designing the footings for uplift. I have typically designed the footing to weigh as much as the net uplift load (0.6D+(W or 0.7E)). The calculated net uplift force has a factor of safety of 1/0.6 = 1.67 in it from the reduction of the dead load. I received a plan check comment that states that with my calculations there is only a factor of safety of 1. But this is a factor of safety of 1 in regards to the net uplift force. Should we really be increasing the footing weight to maintain a factor of safety = 1.5 over the net uplift force? To me this is more like a factor of safety of 1.67*1.5 = 2.51. Not to mention we know what the actual weight of the footing will be. This is a commercial building and the city inspectors are very good in requiring construction matches the plans, especially foundations. Am I wrong? Do others design PEMB footings that weigh 1.5 times the net uplift force?
Thanks in advance for your replies
Thanks in advance for your replies






RE: PEMB foundation uplift
If you're still having problems getting sufficient weight, you can use some of the floor slab if it's over the footing or soil weight if you have a buried footing.
RE: PEMB foundation uplift
RE: PEMB foundation uplift
The intent of the code is that 0.6* resisting weight(which includes superstructure, slab on grade, footing, surcharge) should be greater than the gross uplift (with a FS = 1.0)
RE: PEMB foundation uplift
It seems that in order to abide to the letter of the code, you would have to reduce all dead loads by 0.6, including footings and slabs, there are no exceptions.
Many argue that its pretty ridiculous, there are no confirmed footing failures by being pulled out of the ground, there are too many other things that will fail first in a structure. And also we can very precisely predict the actual weight of the footing. I fall in this category, and believe in some instances codes have taken over good engineering judgment, maybe in part due to our litigious society and stringent code/plan reviewers. We have a really good history of structural engineering in this country prior to our current codes. I am not suggesting we don't need them and there hasn't be great progress...
I would suggest that ASCE/IBC consider a separate code combination that would have a separate footing dead load factor that maybe would only be reduce by 0.9/0.8, where the footing engineered (not prescriptive). This would realistically address the probability of uplift failure of this particular part of the structure.
RE: PEMB foundation uplift
a2mfk thinks more along the line as I do. Isn't the reason for a factor of safety to ensure that dead loads are not overestimated? We have taken care of this by multiplying the dead load of the structure by 0.6. The weight of the footings, especially in this case being a commercial building and will be inspected to ensure it constructed to plan, is very predictable. Another reason why I think that the footings only need be designed to match the weight of the net uplift, without being increased by 50%.
RE: PEMB foundation uplift
If you really want use 0.6*D in the load combination, then, your wind uplift need multiply directionality Factor Kd=0.85.
That's just my interpretation of the IBC and ASCE7 code.
RE: PEMB foundation uplift
I don't disagree with you at all. I had posted a similar question not too long ago. thread507-283074: Footing Uplift
I don't think you can design it per code using your logic (although it is not wrong). The building code does not distinguish dead loads differently because they are foundation weight. It all falls into the 0.6D pool unfortunately.
RE: PEMB foundation uplift
I read that thread and the opinions seem to be pretty split. In the end I also agree with you that if you read the code to the letter it certainly implies that the 0.6D factor would also apply to the footings. The reason for my question was the same as yours, it does not seem reasonable to provide this extra concrete. Especially when the steel building manufacturer has designed the columns, baseplates, and anchor bolt diameters based on the net tension from 0.6D+W. Why design the footing to resist more uplift than the baseplate connection can? Doesn't make sense to me.
RE: PEMB foundation uplift
The 1.5 factor no longer appears in the IBC. It is replaced by the 0.6 factor on the dead load.
RE: PEMB foundation uplift
I agree with you. It is my understanding that in the old days there were not any load combinations, full loads were used. So it was imperative to provide a 1.5 factor of safety. Reading a little more into the subject last night, the load combination in IBC 2009 and ASCE 7-05 does include H (0.6D + W + H), which I always used as lateral soil load, but it is also defined as bulk weight. In this case I would say the footing would be considered as bulk weight? But I noticed a strange discrepancy between the strength design and allowable stress design load combination. The strength design load combinations exception 2 states that H cannot be used in load combinations 6 and 7 where it counteracts E or W loads. This exception is not listed under the allowable stress design load combinations exceptions. Anyone know why that is?
RE: PEMB foundation uplift
@jeffhed: 'H' can also mean soil heave caused by expansive soil or buoyancy, which would put an uplift on your footings. Therefore, you would have to resist uplift forces from (W or E) + H with your dead load D.
RE: PEMB foundation uplift
RE: PEMB foundation uplift
the old method was use 0.9D with a factor of safety of at least 1.5.
1.5/.9=1.67 same as 1/0.6=1.67
RE: PEMB foundation uplift
Thanks again for everyones comments.
RE: PEMB foundation uplift
RE: PEMB foundation uplift
0.6D + W + 0.9 (ftg and slab) ??
Though I fundamentally agree with the approach of only reducing the footing weight by 0.9, I think we all agree that is not meeting the current letter of the code...
RE: PEMB foundation uplift
I am treating the footing and slab as a different dead load factor just like you stated "0.6D + W + 0.9 (ftg and slab)".
As far as it meeting the current letter of the code. Doing a search for uplift and 1.5 in the IBC 2009 commentary, the only place 1.5 was listed as a safety factor was section 1807.2.3 which is specifically for retaining walls. I cannot find in the code where it requires and factor of safety against upift of 1.5 must be maintained. That is where my argument stemmed from, if I must maintain a F.S. of 1.5, where is it specified in the code?
RE: PEMB foundation uplift
RE: PEMB foundation uplift
RE: PEMB foundation uplift
I am confused by your last post. Didnt you state earlier that the F.S. = 1.5 is no longer required since it is taken care of in the load combination? That you would expect the dead weight of the footing, slab, etc to be equal to the net uplift calculated from 0.6D+W? If so, I am being more conservative than that by reducing the footing weight by 10% and comparing to the net uplift from 0.6D+W. Maybe I am misunderstanding one of your posts.
RE: PEMB foundation uplift
RE: PEMB foundation uplift
You are checking the footing and column against overturning. Your column has dead loads coming down on the footing and the footing itself is a dead load too.
The check is against UPLIFT of the column/footing assembly.
At least that is what your original post said:
So you sum up ALL your dead loads - column dead load reaction, footing weight, column pedestal weight, any tributary slab on top, any earth load above the footing pad, etc. That is your total dead load D.
Then you take your wind uplift on the footing, W
The sum - 0.6D + W should not result in a "net" uplift.
In other words 0.6D > W
RE: PEMB foundation uplift
This struck me as very practical, however, in violation of the code. I've used it in medium spans where the footings where otherwise enormous. I have a hard time visualizing a 6x6x2 mass of concrete flying throught the air w/ the building intact. In small buildings I haven't worried about it. I don't design anything that's really large.
(I interperet the code to read 0.6D+W as the correct load combination for getting your required concete weight).
RE: PEMB foundation uplift
In Limit States Design, the 0.9 factor accounts for overestimating dead load. If 150 pcf is used in the calculations as the density of concrete, it is probably an overestimate. Concrete weighs between 140 and 145 pcf. The 0.9 factor accounts for such overestimating of dead load. The factor of safety against overturning is 1.5.
There have been a number of questionable arguments presented elsewhere on this forum suggesting that safety factors in the codes are combined in such a way as to provide an overly conservative design. In my view, those arguments are fallacious. The codes that I know provide a reasonable and proper way of providing an adequate level of safety to the users or occupants of the structures governed by those codes.
BA
RE: PEMB foundation uplift
RE: PEMB foundation uplift
If a plan reviewed gives me a comment in which I know he is not following the code (i.e. WillisV's example) then I will show him the error of his ways because I know he will not find a code provision that supports his argument. If that fails I go to his boss who hopefully is smarter. The code official always comes around when I can show they are wrong.
BA, I have always understood we use 150pcf to account for the heavier weight of steel than concrete. I have never bothered to do a calc to see if that is realistic or not.
RE: PEMB foundation uplift
My argument is not that the weight of the footing and slab is not dead load. It is. In the interest of getting the most economical design, my argument was that the dead load of the footing is predictable. Also, the steel building base plate and anchor bolts are designed to transfer the uplift load calculated from 0.6D+W of the STRUCTURE at the base plate. So I was wondering if the structure was designed to transfer the uplift force calculated from 0.6D+W of the structure (Anchor bolt design force), why increase the weight of the footing above and beyond the anchor bolt design force by multiplying footing weight by 0.6. If i should multiply the footing weight by 0.6 and have even more enormous footings than normal, that is fine. I only want to do the best economical design but also I want to do it correctly. I posted this question specifically to determine if I am crazy or not. It appears I may be crazy.
RE: PEMB foundation uplift
I think it is incorrect to provide base plate and anchor bolts to resist 0.6* superstructure weight. You should be consistent in your design assumptions. By doing the above, you have essentially created a weak link in your load path.
RE: PEMB foundation uplift
I made my statement regarding which load combination would result in the largest uplift force and anchor bolt tension force. Wouldnt 0.6D+W result in the largest uplift force?
RE: PEMB foundation uplift
1. Letter of the codes at the moment, all dead loads are put into D, including slab and footing weights, for stability analysis:
0.6D + W
2. Same goes with anchor bolt design, minus the weight of the footings and slab of course.
3. Many SEs (I know, not all BA ;) ) on this board and other see the need for ASCE/IBC to perhaps consider a separate, less conservative factor for slab and footing weights, that may be something like:
0.6D + W + (0.7->0.9) (footing and slab weights)
And those footings and slabs are engineered for uplift.
4. The flip side of #3 is that we have now reduced the OVERALL safety factor by reducing the DL reduction factor of the footing. Frankly, I think we would not be having this discussion if the equation were something like:
1.5/1.67W + D
D= all dead loads accurately calculated, up to you the engineer which to reduce slightly like roof MEP, ceilings, etc.
That we clearly see the factor of safety is for the unknowns of the wind and to provide an overall FS.
Is this the way you see it BA??
RE: PEMB foundation uplift
Thank you for doing what I could not, summarizing this thing into a more understandable format. We in our office agree that a different factor should apply to footing and slab weights as it is a much more reliable estimate than the dead load of the building. However there is no current code allowance for this. We further argue that if the anchor bolts and base plate are designed for a design uplift force of 10000 lbs (Resulting from 0.6D+W of the structure), why should the footing be designed for 10000/0.6 = 16667 lbs? Seems like a lot of extra concrete to me just for the hell of it when the anchor bolts and base plate have not been designed to lift 16667 lbs.
RE: PEMB foundation uplift
As you have said, the use of 0.6D in combination with 1.5W is simply wrong. We are fortunate in our area as the authority having jurisdiction does not review structural calculations, so it is not an issue I have experienced.
dcarr82775,
I have heard that too (150 pcf includes steel). I have never bothered to check it either, but it is certainly an approximation, particularly for lightly reinforced mass concrete as found in foundations which are being sized by weight alone.
jeffhed,
Using ASD, you do not design the anchor bolts to yield at the net uplift. You use their allowable stress. Thus the anchor bolts are capable of carrying 50 to 60 percent more at yield.
By the same token, if the total weight of foundation just balanced the uplift, there would be a factor of safety of 1.0 against overturning or pullout. That is clearly inadequate. The 0.6D term simply ensures a similar factor of safety as used in the anchor bolt design.
Think about it.
BA
RE: PEMB foundation uplift
Sorry, I missed your question due to my slow typing (and perhaps thinking). I'm not quite sure why you feel that a separate, less conservative value should be applied to the dead load of concrete slabs and footings.
In Canada, we use Limit States Design for stability calculations. We have a slightly different combination than you do. Ours is (0.9 or 1.25)D + 1.4W. When dead load contributes to the effect being considered, use 1.25. When dead load resists the effect, use 0.9. The 0.9 factor cannot reasonably be increased beyond that. When converted to ASD, 0.9 becomes 0.9/1.4 = 0.643.
jeffhed,
If the anchor bolts are designed using ASD for 10000#, they will yield at approximately 16000#. That is considered failure.
If the foundation weighs 10000#, failure will occur when the uplift is 10000#, long before the anchor bolts fail.
BA
RE: PEMB foundation uplift
I know what you say follows the letter of the code. I also know that multiplying the weight of the footing by 0.6 follows the letter or the code. I am just questioning it because in my opinion it does not seem reasonable to add 6667 lbs (1.5 yds) of concrete per footing when the footing weight and slab can be accurately estimated. As I have stated earlier today, it appears that I will need to multiply my footing weight by 0.6, like it or not.
RE: PEMB foundation uplift
Your post was just 2 minutes after mine, so you probably didn't have time to read it. Think about what I said. You are not adding 6667 lbs...that is where you are mistaken.
Certainly the footing weight can be accurately estimated. So can the yield strength of the bolts.
BA
RE: PEMB foundation uplift
The 0.6D+W combination provides about a 1.5 safety factor against uplift due to all sorts of variabilities such as:
1. Footings might be built smaller than actually designed.
2. Footings might be shallower than designed.
3. Wind loads might / WILL vary considerably
4. The wind tributary area on the building could be altered in the future.
I'm sure there are others. You have to realize that the 0.6 factor on D in this case is not there to just deal with the variability on the value of D. In most combinations the load factor is intended to directly correlate with the load type's variability.
In this particular combination, the 0.6 factor isn't just for dead load variability.
RE: PEMB foundation uplift
It stems from my opinion that 0.6 (FS=1.67) is an unreasonably high factor of safety for the uplift of a footing during a wind event (lets neglect overturning for this argument). I do not know if there is statistical data or testing that backs up this opinion, but I am just trying to get the ball rolling here that I would like to see a formal study done by people much smarter than me :)
I have been heavily involved in forensic engineering investigations since the hurricanes that hit Florida in 2004, and made several trips to the greater Biloxi, Mississippi area after Katrina. I do not recall either first hand observing or seeing in pictures, etc. of any uplift failures of a footing. I mean a footing being pulled out of the ground and relocated or at least being the root cause of a structural failure.
Now having read a lot of your posts and respecting your opinion, I think you could easily argue that is because load path has not been maintained so that these footings were ever even tested. I assure you most footings and slabs that were wiped clean of their wood framed structures were never tested, the roof cladding failed one piece at a time, the diaphragm failed, and then the walls easily failed. I saw so many bare slabs and foundations, it was really upsetting and sad to look at... At least I got to meet a lot of the homeowners who got the hell out of dodge.
Forgetting residential, with PEMB and single story retail box, you often get very large uplift loads resisted by a concentrically loaded pad footing. It seems hard to imagine that this column would be able to pull a footing out of the ground through the slab, even if undersized, because so many other things would fail first, ie, roof decking and then we have no uplift loads.
Ideally if everything were designed and constructed properly, the max uplift would see its way to the foundation, and then we'd have a bigger need for a larger factor of safety. You would probably argue that this means we need to improve the load path quality in design and construction so this happens, and I would agree. However, I think that a 0.7->0.9 reduction factor is a more realistic representation of how things actually react under real wind load max events.
But, like I said, I would like to see much smarter people discuss and research this topic. As of now I think we are wasting a lot of concrete to meet code...
RE: PEMB foundation uplift
On a related side note, I did once see a DOWNWARD pressure failure of a PEMB moment frame. BUCKLED THE FRAME but left the cladding intact. A house about 100 feet away with a shingle roof did not have a scratch on it!
It was a tornado in Daytona Beach, FL.. These things bounce around a lot in Florida and will cause all kinds of weird damage and then leave other structures untouched, very localized.
RE: PEMB foundation uplift
The uplift resistance of a foundation can include more than its own dead load. If there is soil above it, the weight of soil is part of the dead load. If there is a grade slab above that, the tributary weight of slab becomes part of the foundation dead load.
In the case of a pile, the dead load is usually a small part of its resistance to uplift, the main portion being carried by skin friction.
I agree that the foundation will not feel the full uplift of the wind if the roof blows off or other elements fail first but surely that cannot be the basis for design. Every structural element must be designed on the assumption that the rest of the building remains intact.
The OP in this thread and some of his co-workers seem to be under the misapprehension that the 0.6D term provides more resistance to uplift than the anchor bolts can carry. That is simply not true.
BA
RE: PEMB foundation uplift
I appreciate the time you guys have taken to argue against my post. This was the reason I posted it in the first place. We had designed footings in the past as stated above, but were always unsure if this was actually allowed or not. My feeling was more that it violated the letter of the code no matter what I personally felt. And I have to say, after thinking about this subject all weekend and reading through the arguments in this thread a few times, I have been convinced that the weight of the footings must be multiplied by 0.6.
RE: PEMB foundation uplift
That is good. And if there is any chance of a high water table, you must consider the submerged weight of concrete below water table. Adding dead weight to the foundation becomes a less attractive option when that is the case.
BA
RE: PEMB foundation uplift
I know its a case by case basis but at some point it may be more economical than just large blocks of concrete in the ground that have to sit around doing nothing until the next hurricane :)