lightly loaded foundation design

huangyhg

lightly loaded foundation design
for a two story wood framed home i am designing a spread footing per the soils report.  the maximum p(dead)+p(live)=1500 plf.  the maximum point loads at the corners of the shear walls due to lateral loads is a 1300lb couple and occur at various loations along the foundation. how do these point loads factor into the asd equations p dead + p live + (p wind or seismic) in order to detrermine the governing condition.
thanks  
digrazi,
i don't know how stringent your local plans checkers are. i would make the wall footing about 20" - 22" wide, reinforce the footing and stemwall with 0.002 x ag, and blow off the point loads due to shearwall reactions. the code reqirements are geared more toward commercial buildings. in fact, the irc residential code gives prescriptive requirements, but those usually don't apply unless you have all the qualifying elements in place.
i agree with sacrebleu.  i would add that if those points loads are due to wind, you put them in with the oad combinations you are using under wind.  i would check the shear in the footing local to the point loads and make sure you provide enough thickness of concrete so that you don't need shear reinforcing.  for those small loads i doubt you'll have to do anything.  around here 2-story residences have 10"x20" or 12"x24" footings typically.
it totally depends on what the soils report recommends as a maximum soil bearing pressure. if you are on dense sandstone with a allowable bearing of 5000 psf, you probably don't even need a footing (an 8" wall will give you 3333 pounds per lineal foot allowable - you have 1500). if you have expansive clay its a totally different design. maybe there is a minimum dead load requirement and you need voided footings...
no footing reinforcing is required if the soil is not expansive or is densely packed.
pray like everything depends on god
work like everything depends on you.
cegg,
i was referring to the continuous wall footing, to take the shear wall reaction without providing a spread footing at the hold down.
if you don't use reinforcing in continuous wall footing/stem, i hope you have your e&o premiums all paid up.