base plate design

huangyhg

base plate design
i need to size a base plate for uplift, i have a small hvac unit to support, we want to use a 8*8*3/8" plate resting on a existing 3'*3'* 8" footing , to determine "fp" i need a value for ap: plate area and a2 : which by defin, is geometrically similar to ap ,i assume a2 usually refers to area of a  pier ,in this i can't find any defin. other than the above in this case would a2 be equal to ap since the footing is much larger than the base plate .  

that is true, it would be the area of the ftg if your baseplate rests on the footing.  the "geometrically similar to" part is addressing if the baseplate is centered on the pier/ftg.  if you have an 8"x8" baseplate and a 20'x20' ftg, but for some reason the baseplate is all the way at one corner of the footing such that the plate and footing have a corner in common, then your a2 is equal to a1 even though your ftg is much larger.  the reason is that you can't get an area that is "geometrically similar to" and whatever verbage they have in there in that instance.
the idea behind that is that the larger a2 helps to confine the concrete being beared on and will allow higher stresses.
in re-reading your post, i'm not sure how that applies to an uplift condition.
can you elaborate?