Chp.12 Cont. – Examples to design Footings

1. Chp.12 Cont. – Examplesto design Footings

2. Example Design a square footing to support a 18 in. square column tied interior column reinforced with 8 #9 bars. The column carries an unfactored axial dead load of 245 k and an axial live load of 200 k. The base of the footing is 4 ft. below final grade and allowable soil pressure is 5 k/ft2 Use fc = 3 ksi and fy = 60 ksi

3. Example 1 Assume a depth of footing. (2 ft or 24 in.) The weight of concrete and the soil are:

4. Example 1 The effective soil pressure is given as:

5. Example 1 Calculate the size of the footing:

6. Example 1 Calculate net upward pressure:

7. Example 1 Calculate the depth of the reinforcement use # 8 bars with a crisscrossing layering.

8. Example 1 Calculate perimeter for two-way shear or punch out shear. The column is 18 in. square.

9. Example 1 Calculate the shear Vu The shape parameter

10. as is 40 for interior, 30 for edge and 20 for corner column Example 1 Calculate d value from the shear capacity according to 11.12.2.1 chose the largest value of d

11. Example 1 The depth of the footing can be calculated by using two way shear

12. Example 1 The second equation bo is dependent on d so use the assumed values and you will find that d is smaller and a = 40 Actual (d =14.02324 in.) bo=128.93 in

13. Example 1 The depth of the footing can be calculated by using one-way shear

14. Example 1 The depth of the footing can be calculated by using one-way shear The footing is 19.5 in. > 13.9 in. so it will work.

15. Example 1 Calculate the bending moment of the footing at the edge of the column

16. Example 1 Calculate Ru for the footing to find r of the footing.

17. Example 1 From Ru for the footing the r value can be found.

18. Example 1 Compute the area of steel needed The minimum amount of steel for shrinkage is The minimum amount of steel for flexure is

19. Example 1 Use a #7 bar (0.60 in2) Compute the number of bars need Determine the spacing between bars

20. Example 1 Check the bearing stress. The bearing strength N1, at the base of the column, 18 in x 18 in., f = 0.7 The bearing strength, N2, at the top of the footing is

21. Example 1 The bearing strength, N2, at the top of the footing is

22. Example 1 Pu =683 k < N1, bearing stress is adequate. The minimum area of dowels is required. Use minimum number of bars is 4, so use 4 # 8 bars placed at the four corners of the column.

23. Example 1 The development length of the dowels in compression from ACI Code 12.3.2 for compression. The minimum ld , which has to be greater than 8 in., is

24. Example 1 Therefore, use 4#8 dowels in the corners of the column extending 19 in. into the column and the footing. Note that ld is less than the given d = 19.5 in., which is sufficient development length.

25. Example 1 The development length, ld for the #7 bars for the reinforcement of the footing. There is adequate development length provided.

26. Example 1 - Final Design

27. Example 2 Design a footing to support a 18 in. square column tied interior column reinforced with 8 #9 bars. The column carries an unfactored axial dead load of 245 k and an axial live load of 200 k. The base of the footing is 4 ft. below final grade and allowable soil pressure is 5 k/ft2 Use fc = 3 ksi and fy = 60 ksi. Limit one side of the footing to 8.5 ft.

28. Example 2 Assume a depth of footing. (2 ft or 24 in.) The weight of concrete and the soil are:

29. Example 2 The effective soil pressure is given as:

30. Example 2 Calculate the size of the footing:

31. Example 2 Calculate net upward pressure:

32. Example 2 Calculate the depth of the reinforcement use # 8 bars with a crisscrossing layering.

33. Vu =150.7 k in short direction Example 2 The depth of the footing can be calculated by using the one-way shear (long direction)

34. Example 2 The depth of the footing can be calculated by using one-way shear design The footing is 19.5 in. > 18.8 in. so it will work.

35. Example 2 Calculate perimeter for two-way shear or punch out shear. The column is 18 in. square.

36. Example 2 Calculate the shear Vu The shape parameter

37. as is 40 for interior, 30 for edge and 20 for corner column Example 2 Calculate d from the shear capacity according to 11.12.2.1 chose the largest value of d.

38. Example 2 The depth of the footing can be calculated for the two way shear

39. Example 2 The third equation bo is dependent on d so use the assumed values and you will find that d is smaller and a = 40 Actual (d =14.032 in.) bo=128.173 in

40. Example 2 The depth of the footing can be calculated by using the two way shear

41. Example 2 Calculate the bending moment of the footing at the edge of the column (long direction)

42. Example 2 Calculate Ru for the footing to find r of the footing.

43. Example 2 Use the Ru for the footing to find r.

44. Example 2 Compute the amount of steel needed The minimum amount of steel for shrinkage is The minimum amount of steel for flexure is

45. Example 2 Use As =8.36 in2 with #8 bars (0.79 in2). Compute the number of bars need Determine the spacing between bars

46. Example 2 Calculate the bending moment of the footing at the edge of the column for short length

47. Example 2 Calculate Ru for the footing to find r of the footing.

48. Example 2 Use Ru for the footing to find r.

49. Example 2 Compute the amount of steel needed The minimum amount of steel for shrinkage is The minimum amount of steel for flexure is

50. Example 2 Use As =9.36 in2 with #6 bar (0.44 in2) Compute the number of bars need Calculate the reinforcement bandwidth