huangyhg

what exactly is harmful about a little net uplift at a frame
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
lawyers!!!
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
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.

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