strip footing

huangyhg

strip footing
how would you design a single strip footing,for moment and shear, which supports 2 parallel strip walls say 10 ft apart.(like a tunnel). lateral support is provided by a slab at the top of the walls. vertical load same on each wall.   earth pressure on exterior same for each wall.  
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moment diagram on the footing will look like an upside down suspension bridge.  you will need the thickness to provide for the one-way shear and you will need a full bottom layer of reinforcement with a top layer in-between the walls.
step 1 - find resultant load on foundation (center of the walls in this case)
step 2 - determine width of footing required based on allowable bearing capacity
step 3 - design combined or separate footings for walls based on required width (i'm assuming combined)
step 4 - free body diagram with a equally distrbuted soil reaction to the bottom of the footing
step 5 - shear, and moment diagrams
step 6 - calculate required slab thickness and reinforcement
i would design it on a per foot basis.  i would model it as a beam on elastic foundation with point loads (and moments if applicable) where the wall bears on it, and any other relevant loading.
joseph bowles, foundation analysis and design, has a good section on designing continuous footings supporting walls.  the section is in chapter 8 of thhe 4th edition.  not sure if it's in the same chapter in later editions.   
check the differential settlements along the wall if the wall is made of masonry. since supporting structural masonry in that case, better to keep settlement between two consecutive maxima or minima of the settlement under 1/1000 sag.