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what exactly is harmful about a little net uplift at a frame column? 4

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hippo11

Structural
Mar 21, 2003
161
I always hear that when you are doing preliminary lateral design for a bldg, you should check that there's no net uplift at the frame column foundation, in other words, make sure that

0.9 D - 1.0 E > 0 and
0.9 D - 1.3 W > 0

But my question is, what's wrong if you have a bit more than zero? What's so evil about 2 kips of net uplift, heck, what's wrong with 30 kips of net uplift?

It's not like your frame column will fly up into space at 1 kip net uplift.

What's wrong with the foundation losing contact with the soil in an earthquake or hurricane, as long as your structure is still stable?

A frame column that has lifted up 0.05" off the soil still has about the same amount of tension in it as a frame column that is still in contact with the soil. It just seems like an arbitrary threshold...as long as you can show thru a P-delta analysis that you are still stable, you should be okay even with net uplift occuring...right?
 
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LOL, I've asked myself the same question. I have a hard time visualizing the column and footing ripping through the slab on grade and flying through the air. I think it's just a good, but perhaps a bit arbitrary, limit to keep things reasonable.
 
Even better, can someone point me to a section in the code that says "NO NET UPLIFT"
 
hippo11,

IBC 2000:

Section 1604.4 - fifth paragraph
"Every structure shall be designed to resist the oveturning effects caused by the lateral forces specified in this chapter."

See also:

1801.2 (references 1605.3 which includes the .6D+W combo)
1801.2.1
1807.2.8.5
1805.4.1.1 (also ref to 1605.3)

 
umm, this seems like an easy one but isnt net uplift on your column-footing assemblage by definition an unstable condition? also, it seems to me that you arent satisfying overturning.

 
The column can have net uplift but after you calculate the column and footing together you can't have any net uplift. That seems self-evident. If there if net uplift including the foundation resistance, you have a net unbalanced force. That's a no-no for buildings. that means movement.
 
UcfSE...yes, net uplift including everything, that's precisely what I'm questioning.

If I have net uplift of 1 kip, including everything, what's going to happen? It's not going to fly off into space. It's going to detach from the soil a few thousandths of an inch, depending on how unbalanced we are.

Movement is not a no-no for buildings, I disagree. Nothing is infinitely stiff, so we talk instead about limits on those movements.
 
We have limits on slab deflection so partitions won't crack. We have limits on lateral acceleration so tentats won't get seasick on the 60th floor...why then not have a limit on the foundation lifting up a fraction of an inch, as long as we're still stable?
 
cuz there's things out there (real creepy things) called

[red]LAWYERS!!![/red]

 
What do you have do justify your estimation of a few thousandths of an inch? Anything quantitative?

Try it with a balance. Put 1kg on one side and 1kg plus 1g on the other, 0.1% of net uplift, or unbalance. How many thousandths of an inch does each tray move? One, two or several? Does it only move a finite amount or does it keep moving until the physical limits of the scale make it quit? What does that correspond to in a building?


Obviously I need to define what I meant by movement. Elastic and inelastic deformations, both movements of points along a member's length or cross section, is not only acceptable but unavoidable.
 
If think you're confusing stability with serviceability.
0.9 D - 1.3 W = -0.1 does not mean that the foundation will move. It means that the Global Safety Factor has decreased and therefore the probability of failure has increased.
We are told that these Global Safety Factors are determined using probabilistic analysis methods. I don't think any proposal to reduce them on an ad hoc basis will be received with much enthusiasm. Further, if you can argue for a reduction of GSF for stability, why not the same for strength?
BTW, I agree with UcfSE. Once you have an unbalanced force the movement will continue (actually accelerate) until a counter-acting reaction occurs, eg. due to shear/racking in the cladding in the example presented. The weight and strength of any slab-on-grade etc. should be utilised in the total resisting weight of course.
 
I think what hippo11 might be thinking of (correct me hippo) is that despite our best calculations, there are always many secondary elements that add to safety that aren't accounted for in the design.

One example would be interior stud walls with gypsum board sheathing adding to lateral stability.

For the uplift situation we are discussing here, I can see that the slab-on-grade that surrounds a column/footing might add an uncertain amount of additional "hold-down" capacity that we don't account for.

But even with all these secondary effects, as an engineer I don't feel good about counting on them, or diminishing my levels of safety just because they are there.

 
UofCSE, that's a good analogy with the balance, but in a 3-D building, there are many more load paths due to redundancy than in a pulley contraption....if not from other frames/shearwalls around the building then from the rotational resistance of plain old shear connections, reverse bending of composite slabs, etc.

I agree with apsix that SOG strength should be utilized as well.

To answer your question, UCFSE, about what your scale corresponds to in a building, I say nothing in any real building... don't you think the unbalanced upward force would redistribute to upward pressure on the underside of the slab on grade, to adjacent frames, to frames on the upper stories, etc? Basically wherever significant stiffness is provided.


The slab on grade would have to be punched through upwards to have the foundation fly up like in your balance example (unless of course there's an isolation joint around the column, then you couldn't depend on the SOG holding the fdn down)


It just seems like in an earthquake or hurricane situation there's a lot of paths where the unbalanced force could flow if the bottom of the foundation separates from the soil upon which it bears.


ANYWAY...My point is that in a typical building, if you can prove that your unbalanced load flows to other parts of the building, without causing any individual element to collapse, and that you are still stable in 3D after load redistribution, what's the big deal if you have lifted off the soil a bit? ...you can use upward pressure on your SOG underside, frames on the upper stories to redistribute to other frame columns and therefore other foundations, etc. No?
 
If the unbalanced force is resisted by other parts of the building then they are not unbalanced; you have stability. It is a slightly separate exercise to determine if any damage caused is acceptable.

I have no trouble in including portions of the SOG, edge beam, wall etc that can be reasonably assumed to contribute to the total weight. I would be amazed if this approach would result in failure, there is no need to be over-conservative in my opinion.
 
You would have to show that these other systems that withstand the redistributed forces or imposed deformations, for one. An owner would not be pleased with you if the floor was bunged up because the footing was too small to do its job. Proving that a system works is more than a sophomore exercise, which is what I have seen attempted and incorrectly assumed in most cases. Punching shear, for instance, is not the only failure mode for a sog that is receiving uplift from its underside. So the floor didn't punch through. It's cracked all to [heck] though, so what is the owner supposed to think?

How often do you have uplift on a frame that would not also occur in an adjacent frame? Yes, it's possible, we know this, but how typical? Did you really design all the members to redistribute forces and deformations, some perhaps excessive? That may require some inelastic deformation in some members or connections. This is acceptable in seismic events for energy dissipation and because designing members to be elastic is not our typical construction in the US, but would you want to allow this for any uplift at all no matter the source? That implies repairs are necessary.

There are many things that add to increased capacity just because of their presence. I'm aware of these of course like most others. The sog is an example, interior walls are another, as noted by JAE. Exactly how reliable are these things, unless you have gone to the effort and expense to design and detail these extra systems and the owner has gone to the expense to pay for it, and the contractor installed it as you intended? IMO we are not in the business of counting dubious items which have an unsubstantiated reliability, especially with rudimentary and insufficient "engineering" to back up their use. If it's there, great, that's one more unworried "z" I'll get at night. How can you quantitatively describe the effect of these systems, including a prescribed amount of reliability or confidence?

At the end of the day, if you allow a net uplift on your column, you have violated code and your standard of care. If you are willing to accept all of that and the benefits outweigh the risk, by all means, do what you want. I just don't agree. It's good to discuss things though :)


 
I mostly design buildings where there are frost depth requirements. As such, I include the weight of soil over my frame footings, concrete frost walls above the footing, any concrete slab-on-grades. Its easier and cheaper to use these items to resist uplift than to pour additional yards of concrete in the footing. I design my frames so there is no net uplift, and I sleep well at night. It just seems to me that if I have a frame, and I push on it, it will topple over without being held down on an end. I'm not sure where that 1 kip net uplift goes to after the frame rises a fraction of an inch.
 
UCFSE, if we are talking about ultimate quake/wind level forces, then we ultimate failure modes. So if the SOG cracks as you suggest in an ultimate-level earthquake or hurricane, I would say to the owner, yes, that's what it's supposed to do, as long as it doesn't fail. All the structural elements in the building are designed to behave inelastically under ultimate loads.

Now if the SOG cracks badly due to uplift from everyday service loads, then no, that's not okay.

How often do you have uplift in a frame but not in an adjacent frame? That can happen very easily. If you have a torsional irregularity in your building, the code tells you to use a certain eccentricity for your seismic load application, so you could have your lateral point load a lot closer to one braced frame than the other braced frame... also, the adjacent braced frame may be supporting more dead load, especially if we are talking edge frame vs. interior frame. So a frame in one bay could pretty easily have net uplift while a frame a couple bays over might have no net uplift

I too wouldn't suggest using partition walls, etc for stiffness because it's so hard to quantify, but the SOG, other frames, etc., yes, I think you could use those pretty easily to justify load redistribution, and it would take some work but it wouldn't be that bad... If you could save adding a frame or two it could be worth the effort.
 
I see we just have a difference of opinion. I would not consider the sog significantly cracking due to wind forces as acceptable. We should be able to remain elastic for these loads.

I think detailing a structure to transmit forces between bays would be excessive and unusual and would probably cause more issues than it would solve. Typical construction in my area is not detailed such that this would work without excessive deflection and damage. Who knows though, when you try it on your building, come back and let us know how it worked out. It's an interesting thought at least.
 
Hang on...do you not agree that the SOG should remain elastic for service-level forces, but you should go inelastic for ultimate-level forces?

I said in my previous post that cracks @ ultimate level wind are acceptable. If you don't let things go inelastic at ultimate level that's a huge overdesign.

 
I believe, it all comes down to the soil bearing capacity. Uplift is a tension load on the soil. Since soil cannot carry any tension, the tension on the foundation can be concluded as soil bearing failure.
 
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