Design of Spread Footings w/ Uplift AND Moment
Design of Spread Footings w/ Uplift AND Moment
(OP)
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)
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)





RE: Design of Spread Footings w/ Uplift AND Moment
RE: Design of Spread Footings w/ Uplift AND Moment
RE: Design of Spread Footings w/ Uplift AND Moment
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!
RE: Design of Spread Footings w/ Uplift AND Moment
RE: Design of Spread Footings w/ Uplift AND Moment
RE: Design of Spread Footings w/ Uplift AND Moment
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
RE: Design of Spread Footings w/ Uplift AND Moment
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.
RE: Design of Spread Footings w/ Uplift AND Moment