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Pre-Eng. Steel Bldg Moment Ftg.w/ Tie-Rods;& Base PL. Shear

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We are using dropped & encased tie-rods in combination with a moment footing to resist 55k horizontal.  The rods are dropped about 2', so the pier and footing have to resist the moment from the couple.  There is not floor.

On asking the question if the footing will act as a lever, increasing the force in the tie-rods beyond the outward thrust of the column, we calculated that the rods will have to elongate 1" to develop 24 ksi.  So it appears that the footing will provide the moment before the rods will be over stressed...ok provided the footing & pier can take it, and the soil will accommodate as required until the rods engage.  We are only allowing 2ksf at the toe.  Without any tie-rods it's around 4 ksf.

Assuming that all works, so much movement of a frame seems excessive.  On the other hand, if it were a (more common) 80' building, we would still have .79".  Has that been a problem?


On the connection of the base plate to the pier, AISC Steel Design Series #7, Industrial Buildings Roofs to Column Anchorage, a method is outlined whereby the vertical load creates base plate friction to resist shear.  Even without the clamping action of the anchor bolts, the horizontal thrust would nearly be accounted for by the simultaneous vertical loads.  With 55 kips, it seems a little out of common engineering practice to rely mainly on friction.  By this method, I'm getting less than 8 kips/bolt, assuming 6 of 8 engage.

I ran the numbers on a shear lug, and that method would require 1" thick steel for the same force.  The text mentioned previously states you can't use the lug in combination with the friction procedure.  Would there be any merit to adding a 3/4" lug for added safety, or might it be counter-productive?


Regards,
Ed Fasula, E.I.T.





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