uplift of trench footing

huangyhg

uplift of trench footing
what is the rule of thumb for uplift regarding a trench footing?  i have a 24"w x 36" deep x 40'long trench footing with 20kip of uplift & compression at the ends (from shear wall calcs). what length of the trench footing can i consider resisting uplift?  i've heard that you can only consider 8' contributing to uplift resistance. the shear wall is in line with this trench footing and is not load bearing since the trusses also run parallel with the footing. the shear wall is 15'long. the wall is located in a high wind zone. would it be prudent to analyze the footing as a beam? any suggestions would be greatly appreciated.  
is the trench footing unreinforced (plain concrete)? is it cracked? does it have undoweled cold joints? if so, i can see your reasoning.
if it is reinforced then why would you not use the whole length and figure out the soil pressure underneath from the applied loads as you would a combined footing? then proceed to check for shear and bending.  
there is no rule of thumb--you have to analyze the footing.  you have a 2' wide x 40' long footing with dead load and an overturning moment.  check soil bearing pressure for 0.6dl + wl, then check the footing and wall above it for 0.9dl + 1.6wl.  i am assuming you have a foundation wall dowelled to the footing that can help resist the bending along the footing.
daveatkins
jike,
the footing is reinforced and currently w/o any noted joints.
daveatkins,
if i were to analyze as a beam.  would a pin-pin condition be accurate?   
no, you must analyze it as a rectangular footing, with a uniform dead load and a point load at each end (that's what jike is saying, as well).  the soil bearing pressure pushing up from underneath will be trapezoidal or triangular.
using just the weight of the footing, 0.6dl = 21.6 kips.
your overturning moment = 800 kip-feet.
therefore, e = 37' >> 40'/2.
your footing is not big enough.
daveatkins
fos = mr/mot = 21.6*20/(20*40) = 0.54 < 1.5, (n.g.)
need more weight, or anchors to resist the uplift.
kslee1000,
the 21.6 kips is based on 0.6dl, so it already has the 1.5 fs in it.
but the footing still does not work.
daveatkins
dave: i concur. (too used to the old fashion way without that many embed factors)  
thanks i appreciate it.