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PEMB Column Footing

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I’m looking for insight in the design of Pre-Engineered Metal Building column footing.  I’m not sure if anyone can give me any insight or not beyond what I already know.  I’ve already designed dozens of these systems.  I’m looking for a ‘better way’.  Maybe there are some tricks to the trade that I’m not aware of.  I really can’t find much information on the design of these foundations in print.  I have seen a little blurb in the MBMA system design manual.  It is not of much help.

 

I typically design foundations for PEMB as turned down perimeter slab systems.  At the rigid frame columns a footing is created below the frame.  I always try to get the column inside the perimeter grade beam.  If I have a heavy crane loaded column this usually can’t be done and the column footing is center and below the perimeter grade beam.  I design the footing as an eccentrically loaded footing.  I add dead weight from the surrounding slab and grade beams to help resist wind uplift forces.  I’m always finding myself fighting the eccentric loads on the soil bearing pressure, especially on projects with inset wall girts.  On those projects the column load is less than a foot from the edge of footing and the soil bearing stresses shoot sky high.  I usually end up with rectangular size footing with the long dimension parallel to the perimeter wall.

 

From my perspective, this is the correct way to design the footing, but maybe my perspective is not inclusive enough.  Are there ‘numbers games’ that can be made to reduce the eccentricity, which in turn distributes the stresses more evenly under the footing?  I can’t picture any.  I use hairpins and tie rods, but I see these as only being ‘equal and opposite’ to the rigid frame thrust.  I can’t see how these would help induce any loads to reduce the physical eccentricity of the column on the footing.

 

Are there other methods of analysis out there others use to get more reasonable footing design below rigid frames?  Does anyone know of literature that addresses these types of foundations in a comprehensive way?  I understand there is an old Butler design manual but it is out of print and I haven’t been able to see a copy.

 

Thanks for any insight.

 

Rich