Uplift Stability

[haynewp]

SBC states essentially that a 1.5 safety factor shall be applied when checking for lateral stability of the building. This lateral stability is described by them to be sliding and overturning moment. But what about uplift by itself? In the case of a shear wall box building with pinned interior columns, there will be ftg uplift to be resisted by the dead weight of the footing and slab if available. I have noticed that lots of other engineers only use a 1.0 safety factor applied for this type of situation due to SBC's explanation. How is that justified? There will be material safety factors when analyzing the slab to help in resistance but none if there is no slab available.

Also, how much slab should be taken for resistance? It would seem to me to depend greatly on the stiffness of the slab and reinforcing. I have heard a lot of different opinions on this also. And to what point would you stop considering the slab to be useful? Some believe it should be before the cracking point of the slab,however in a catastrophic event, besides not falling down, what else should really be expected of the structure? Is add'l slab cracking considered unacceptable? Does anyone consider suction between the slab and the ground as the slab is being lifted to contribute to this resistance?

 

[ishvaaag]

I am not sure if I understand the questions you place completely, but I can point that in Spain safey factors 1 to 0.8 are used when in loadcases and where the considered action leads to stability. Uplift is not a condition leading to stability, but something to be prevented, and so I think your view of considering a safety factor of say 1.5 (or 1.4 to 1.6) is the correct view from the practice here.

Other question is that small buildings with a cellar may be a bit compromised in feasibility by this clause, so maybe some designers and reviewing parties are closing their eyes to this, specially where uplift is something contingent (flood?) and buildings are not seen being put afloat (er, some we have seen at floods, or so, in the tv). Suction, or even better, friction/adhesion against the walls that even in the submerged state counteract flotation may be accounted for in a part sure to stay, but this being difficult to determine a better choice would be not to.

Respect slabs, subpressure etc, all this comes to proper calculation and detailing. Once well engineered this needs not be a problem.

 

[bengineer]

Just like a sumo wrestlers shorts, you want to have your building firmly supported.

There is a load combination that considers the wind uplift case. In this load case you combine 0.75 DL + Wind Uplift.
This case provides a factor safety of 1.33 against uplift and is fairly common.

 

[mjmce]

A S.F. of 1.5 to 2.0 should be used for any building structure which could be severely damaged by hydrostatic uplift forces. For non-building type structures (i.e., underground tanks, transformer foundation containment pits, etc.) for which uplift effects are not as detrimental a S.F. of 1.2 to 1.5 should be utilized. These values are well documented in some soil mechanics and foundation text books. One that comes to mind is a book on the subject by Chester Duncan; pub., Van Nostrand Reinhold.

When calculating the safety factor for the situation described above, the full permanent dead weight of the structure should be considered.

 

[dlew]

Haynewp,
I have always checked the base plates, anchor bolts and footings of columns subject to uplift to insure that there is a safety factor of at least 1.5 (based on unfactored service loads) against uplift.
I normally consider a portion of the ground slab as adding to the safety factor against uplift. It is clear that the portion of the ground floor slab immediately on top of the footing is resisting uplift. If you want to include something more than that, I suggest you check that portion of the slab as a cantilever subject to his own weight.
With respect to consider suction between slab and the ground, I would not do that. Vacuum is a very large force, (14.7 psi = 2100 psf), but it is very unreliable too. If a small crack develops between soil and slab... it is gone. Besides soil is a porous medium.
A common mistake made when calculating the safety factor of a footing is to base the calculations on the NET wind uplift (wind uplift less weight of the superstructure). The gross wind uplift must be used, and the weight of the superstructure added to the weight of the foundation.

AEF

 

Bengineer, SBC does not provide a load combination using 0.75D + WL as an option.

 

[mgg]

haynewp,

When one considers lateral stability, one must look at the bigger picture as well. Yes, some interior columns of the structural system you mentioned will be governed by uplift alone. However, the uplift is a component of the overall structure performing against sliding and overturning forces. I advise that you should satisfy the required 1.5 safety factor. After all, when a structure has been uplifted, all lateral stability will have been diminished.

 

[Ron]

haynewp....Overturning and uplift are done in the same vein. Both are functions of wind loading and the same factor is applied. My interpretation for SBC is (and has been for a long time) that the 1.50 factor applies to either/both.

 

[haynewp]

Thanks for responses.

 

[Taro]

A factor of safety against overurning of 1.5 is appropriate for wind loads, earth pressures, and other essentially monodirectional loads. Many engineers use a factor of safety of 1.0 for seismic loads because of the short-term cyclic nature of the loading and because there is very little evidence of building structures failing in global overturning.