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Combined footings
#1
. Combined footings are footings that support more than one column or wall.

. Combined footings can be used in case :

1. an exterior column that is immediately adjacent to a property line where it is
impossible to use an individual column footing.
2. two columns that are closely spaced, causing their individual footings to be closely
spaced.

. The choice of which shape (rectangular or trapezoidal) to be used is depends on the difference ¡n column loads and on physical/ dimensional limitations.

. The physical dimensions (except thickness) of the combined footing are generally established by the soil bearing capacity of the soil.

. In addition, the centroid of the footing area should coincide with the line of action of
the resultant of the two column loads.

. These dimensions are determined using service loads in combination with an
allowable soil bearing capacity.
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#2
DESIGN OF COMBINED FOOTINGS 

The design of a combined footing is more efficient and economical as well as the settlement of footing is uniform if the pressure distribution due to load is uniform. This condition may be achieved if the centroid of all applied loads and the centroid of the area of footing coincide. Generally, these footings may be of the following types :

i) a rectangular slab type with or without a beam connecting the columns,
ii) a trapezoidal slab type with or without a beam connecting the columns, and
iii) isolated footings connected by a beam (strap footing).
 
Steps for Design of Combined Footing


  1. Determine column loads : Locate the point of application of the column loads on the footing and self-weight of the footing.
  2. Loads passes through the center of footing : 
    The width of footing is fixed. Keep in mind that the length should always be more than distance between the external faces of extreme columns. The projections of footing beyond the columns in the longitudinal direction may he fixed in such a way that the C.G. of column loads must coincide with the C.G. of area of footing to have uniform distribution of soil pressure. 
  3. Determine area of footing for the above load : Compute the area of footing such that the allowable soil pressure is not exceeded.
  4. Calculate the shear forces and bending moments at the salient points and hence draw SFD and BMD.
  5. Fix the depth of footing from the maximum bending moment.
  6. Calculate the transverse bending moment and design the transverse section for depth and reinforcement. Check for anchorage and shear.
  7. Check the footing for longitudinal shear and hence design the longitudinal steel
  8. Design the reinforcement for the longitudinal moment and place them in the appropriate positions.
  9. Check the development length for longitudinal steel
  10. Curtail the longitudinal bars for economy
  11. Draw and detail the reinforcement
  12. Prepare the bar bending schedule
Manish Jain Luhadia 
B.Arch (hons.), M.Plan
Email: manish@frontdesk.co.in
Tel: +91 141 2743536 

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