Rotational Fixity of a Pad Footing

[asixth]

Hi guys, there has been a couple threads lately about semi-rigid design of columns, thread744-260012 and thread507-261922, and now I want to put the theory into practice. I have a steel-frame building which relies on frame action to brace the structure in one direction. I want to reduce the drifts for service conditions and reduce the column moments for ultimate conditions, so I am designing a semi-rigid base. Please note that I only intend on resisting a minor amount of moment using a rotational spring stiffness at the base of 0.1*4*E*Ic/Lc.

Because this is a steel-framed structure, there is not a great amount of dead load compression force on the column and thus uplift does result. What would be the most ethical approach to take out the base moment (referring to the attached sketches). Would it be best to assume an unbalanced slab-on-ground load or would it be assuming some passive soil pressure is developed on the pad footing to resist applied moment. My thoughts are that I should look at designing the slab-on-ground to span onto the pad to provide additional hold-down resistance. This will mean designing the slab on ground with bottom reinforcement locally around the column.

All help will be appreciated.

 

[SteelPE]

I am not sure if this is 100% applicable in this case but I have used the method found on page 3-41 and 3-42 of the PCI Design Handbook (5th edition). You can easily set up a spreadsheet to see what would happen if you change some of the parameters around. This way you can vary the column attachment stiffness the see how the parameters affect your frame.

 

[paddingtongreen]

Just as an aside, it was so easy to pop something like this into a Moment Distribution or a Slope Deflection calculation. I did it with STAAD too but that was an earlier, less sophisticated version than you have now.

Michael.
Timing has a lot to do with the outcome of a rain dance.