PEMB foundation uplift

[jeffhed]

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

 

[JedClampett]

Concrete is very cheap. It's best to provide the code required uplift, even if it seems excessive. If it's just a few inches of concrete, put it in.
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.

 

[jeffhed]

It's not a problem of providing enough concrete, its more a question on what the intent of the code is. Slab weight has been included. My question is only regarding whether or not the concrete footing weight needs to provide 150% the required weight to resist the uplift force from 0.6D+W. Which in my opinion, the load combination has already provided the factor of safety. I don't really care about extra concrete if it is required, if that's what it is, then that's what it is. I want to design correctly, but do not want to be adding safety factors willy nilly and costing the client money that is unnecessary. Especially in this case where it is my opinion that it doesn't make sense to be adding safety factor on top of safety factor.

 

[slickdeals]

Have calculated the net uplift using 0.6 * the weight of the superstructure. If so, you should provide enough concrete in the footing such that 0.6*(footing Weight)/Net Uplift > 1.0.

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)

 

[a2mfk]

There have been a couple of VERY LONG posts on this very subject, and the opinions were pretty split.

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.

 

[jeffhed]

slickdeals,
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%.

 

[chrislaope]

My interpretation for IBC 1.5 safety factor for wind uplift is whole weight (including concrete, soil, superstructure)/wind uplift = or > 1.5.

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.

 

[slickdeals]

jeffhed,
I don't disagree with you at all. I had posted a similar question not too long ago. thread507-283074

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.

 

[jeffhed]

slickdeals,
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.

 

[JAE]

You use the 0.6D +/- W combination and design so that the net uplift < = 1.0.

The 1.5 factor no longer appears in the IBC. It is replaced by the 0.6 factor on the dead load.

 

[jeffhed]

JAE,
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?

 

[steellion]

@JAE: Agreed. Use the 0.6D+1.0W instead of FS=1.5. This will provide a FS of 1.0/0.6 = 1.67.

@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.

 

[WillisV]

jeffhed - see my post here for a slightly more longwinded version of JAE's statement: thread507-202887

 

[dcarr82775]

You use the load combination 0.6D +/- W or 0.7E. It inherently has the factor of safety worked into it.

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

 

[jeffhed]

Thanks everyone for the comments. I called the plan reviewer and talked to him. He accepted my argument and also stated that his firms standard practice is to use the load combinations and multiply the dead load of foundation, slab, etc by 0.9 to provide a F.S. of at least 1.1. My footing calculations show a F.S. of a little less than that in one case which triggered the comment. I still don't necessarily agree that a F.S. is required at all, but if I need to provide a F.S. of 1.1 to get a permit, I can live with that much more than a F.S. of 1.5.

Thanks again for everyones comments.

 

[gobsmacked]

I have followed this discussion with some interest, but, not being familiar with the codes used, did not contribute. However, looking at the totality of the discussion, it looks as though you are attributing factors of safety to specific things, such as uncertainty of loads, or uncertainty of material strengths. There are other uncertainties that these factors also have to take into account. For overturning calculations that cause uplift, you have an assumed centre of rotation. Now this is somewhat uncertain, and can be a moving point in the event of some uplift taking place; moreover, the point to which the centre of rotation may move to may also reduce the factor of safety against overturning. More generally, errors in any assumptions and analysis must be included in factors of safety.

 

[a2mfk]

jeff- can you clarify your conclusion. Are you going to include the footing/slab weight in the 0.6D + W equation? Or are you treating the footing as a separate dead load factor and reducing the weight by 0.9?

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...

 

[jeffhed]

a2mfk,
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?

 

[dcarr82775]

The load combination itself provides the 1.5 F.S. with 0.9*DL. It may not be explicitly stated anymore to avoid confusion

 

[JAE]

jeffhed - I think it is incorrect to use a 0.9 on the dead load of the footing and slab. Dead load is dead load - all dead load would fall under the 0.6 factor.