Design of Spread Footings w/ Uplift AND Moment

[pcronin]

I am in the process of designing a tall and slender structure with cross braced frames up high and chevron braces down low with the bottom of the Chevron braces about 15 ft above finished floor. Without getting into all the details, I had deflection concerns so I decided to fix the base of the frames to limit the lateral drift.

I am desiging the structure based on the IBC 2000 and ASCE 7 and for uplift, the controlling load case is 0.6DL+WL. I planned to size the spread footings based on uplift and then check the bearing pressure and overturning based on the applied moment. While performing this check, I realized that the footing will have to be substantially larger than I anticipated based on the load combination of 0.6DL+WL.

My questions is as follows, with the foundation designed for uplift, do I still use the load combination of 0.6DL+WL when I check the footing for bearing and overturning? Basically I will need to double the size of the footing for adequate weight to resist the overturning forces. If anyone has experience with this load case, please let me know.

I am unable to sink the footing deeper or use driven piles, because the entire area is within an existing industrial building. As a last resort, I may need to use helical auger piles for uplift or release the fixity at the base and increas the columns for added stiffness. (I think I already know the answer to my question, I just don't like the result)

 

[UcfSE]

That's what I've done in the past, unless the uplift occurs at a different time than the moment. If they occur at the same time, it seems to me you'd have design the foundation for both acting at once.

 

[JAE]

You must design your entire structure for all the required load combinations.

 

[pcronin]

Thanks JAE. Such a simple and correct response.

I have decided to stiffen the columns and release the fixity of the bases. It seems like the best design option at this point. At least I can tell my client I explored all my options.

Thanks again!

 

[Ron]

pcronin...I agree with JAE and UcfSE (you do have to design for both at once). I do these a lot where I have a lightweight aluminum structure with high wind loads being held down by a footing to keep it from flying away...but the frames are usually fixed to do as you indicated...limit lateral drift (and distribute stresses a bit better in the frame), so my footings take uplift and moment at the same time. The net result is a larger footing, keeping in mind that the contact area for the moment is usually only about a 1/3 of the bearing area, so its bearing stress contribution can be relatively high. The OT requirements push the size up.

 

[mcutchi]

Knowing that the footings cannot "actually" overturn if they are moment-connected to the columns of a rigid structure without breaking that connection, it is probably valid to 'push' the theory, and use soil frustrum counterweights or whatever helps to avoid unnecessary overdesign. As for bearing stress, the peak is pretty artificial, and in reality does not exist, unless you are bearing on rubber. Hence this too can be 'pushed'.

 

[chichuck]

I think that the uplift will occur concurrent with the overturning moment. For the load case he cited, 0.6*D + W, the overturning comes from two components:
1) wind shear * (top pedestal elev - bott footing elev)
2) moment from base of col (if fixed connection is designed)

If there is a net uplift from the wind loads (very possible with a wide, low building with light roof), then there is a net upward load on the footing.

This is resisted by the weight of footing and overburden. The footing must bee sized to make sure that max bearing pressure under toe is < allowable bearing as specified by geotech. That toe pressure is calculated simply by
P/A +/- M/S.

Another load case you may have to examine is D+L+W. (refer to your governing code) For this load case, the downward load may be greater than the above case. For this one, make another calculation of P/A +/-M/S and see which one governs the size/design.


regards,


chichuck

 

[bjb]

Back before we adopted the IBC our building code did not require checking the combination .6D+W. Instead, minimum factors of safety were specified for overturning and for axial uplift. Using the basic ASD combinations of 1605.3.1 of the 2000 IBC requires .6D+W; this combination tends to make our footings much bigger than they were before.

If you can still use the 2000IBC the alternate basic load combinations of 1605.3.2 are allowable, and there is no requirement for .6D+W. In section 1805.4.1.1 reference is made to design loads per section 1605.3, so presumably either 1605.3.1 or 1605.3.2 are acceptable. I have found this very confusing (among many other things) since having to use the IBC. I wonder if the intent really was to require checking soil bearing pressure for .6D+W. Someone here refered to the IBC as the incomprehensible building code, and I totally agree, but I can't remember who it was.

 

[ihateibc]

This is one of the many problems I have with the IBC. One used to design the soil pressure for the .9DL+WL combo and then do a stability check to make sure that you had a 1.5 safety factor(up to 2.0 depending on preference). Every text book I have also shows this same method for retaining walls. You did not incorporate the safety factor into the soil pressure because the allowable soil pressure already had safety factors built into it when the geotech provided it. The IBC combination is fine for the anchors but makes no sense for the acutal soil pressure check as it results in footings being too large. I am also still having a problem with the .6DL+.7E (allowable stress design) as I fail to understand why I am discounting the deadweight to such an extent when the seismic force is a direct function of it - I understand this with wind as it has no relationship but if my DL is high so is my E.