Footings

1. Footings

2. Acknowledgement This Powerpoint presentation was prepared by Dr. Terry Weigel, University of Louisville. This work and other contributions to the text by Dr. Weigel are gratefully acknowledged.

3. Footings • Support structural members and transfer loads to the soil • Structural members are usually columns or walls • Design for load transfer to soil uses unfactored loads • Structural design of footing is done with factored loads

4. Footings • Footings must be designed to prevent bearing failure, sliding and overturning • Footings must be designed to prevent excessive settlement or tilting • Typically, bottom of footing must be located below frost line • Excavation may be required to reach a depth where satisfactory bearing material is located

5. Wall Footing • Wall footings – enlargement of the bottom of the wall

6. Isolated Square Footing • Isolated or single column square footing – loads relatively light and columns not closely spaced

7. Combined Footing • Combined footings – support two or more columns – heavily loaded columns; closely spaced columns; columns near property line

8. Mat Footing • Mat or raft foundation – continuous concrete slab supporting many columns; soil strength relatively low; large column loads; isolated spread footings would cover more than 50 percent of area; reduce differential settlement

9. Pile Cap • Pile caps – distribute column loads to groups of piles

10. Soil Pressure • Soil pressure is assumed to be uniformly distributed beneath footing if column load is applied at the center of gravity of the footing • Footings supported by sandy soils • Footings supported by clayey soils • Footings supported eccentric loads

11. Assumed Soil Pressure

12. Soil Pressure - Sandy Soil

13. Soil Pressure - Clayey Soil

14. Allowable Soil Pressure • Actual soil pressure is based on unfactored loads • Allowable soil pressure may be determined by a geotechnical engineer • When soil exploration is not feasible, values provided by building codes may be used • Factor of safety is typically 3

15. Allowable Soil Pressure (Table 12.1)

16. Design of Wall Footings • Generally, beam design theory is used • Shear strength almost always controls footing depth • Compute moment at the face of the wall (concrete wall) or halfway between wall face and its centerline (masonry walls)

17. Design of Wall Footings

18. Design of Wall Footings

19. Design of Wall Footings

20. Design of Wall Footings

21. Design of Wall Footings • Shear may be calculated at distance d from face of the wall • Use of stirrups is not economical – set d so that concrete carries all the shear

22. Design of Wall Footings • Design a 12-in wide strip • Section 15.7 of ACI Code: • Depth of footing above bottom reinforcement not less than 6 in for footings on soil and not less than 12 in for footings on piles • Minimum practical depth of footing is 10 in and 16 in for pile caps

23. Wall Footing Design Examples

24. Example 12.1 • Design a wall footing to support a 12-in. wide reinforced concrete wall with a dead load of 20 k/ft and a live load of 15 k/ft. The bottom of the footing is to be 4 foot below final grade, the soil weighs 100 lb/ft3 the allowable soil pressure is 4 ksf. The concrete strength is 3,000 psi and the steel is Grade 60.

25. Example 12.1

26. Example 12.1 • Assume a footing thickness of 12 in. With a minimum cover of 3 in., this gives a d value of about 8.5 in. Compute the footing weight and • soil weight:

27. Example 12.1 • Effective soil pressure and required width of footing:

28. Example 12.1 • Factored bearing pressure for design of concrete:

29. Example 12.1 • Compute design shear (at distance d from face of wall):

30. Example 12.1

31. Example 12.1

32. Example 12.1

33. Example 12.1 • Appendix Table 4.12, r = 0.00345 < 0.0136, section is tension controlled; f = 0.9 • Use No 7 at 10 in (As = 0.72 in2 / ft from Table A.6)

34. Example 12.1 • Development length:

35. Example 12.1

36. Example 12.1 • Available length for development

37. Example 12.1 • Temperature and shrinkage steel • Use No 5 at 8 in (As = 0.465 in2 / ft)

38. Design of Isolated Square Footings • Most isolated square footings have a constant thickness • For very thick footings, it may be economical to step or taper footing • Two types of shear must be considered – one-way shear and two-way shear

39. Design of Isolated Square Footings Constant thickness

40. Design of Isolated Square Footings Stepped

41. Design of Isolated Square Footings Tapered

42. One-way Shear • Same as for wall footings

43. One-way Shear

44. Two-way Shear • ACI Code Section 11.11.1.2 states that critical section is at a distance d/2 from face of support

45. Two-way Shear

46. Two-way Shear

47. Two-way Shear • <- ACI Code Equation 11-33 • <- ACI Code Equation 11-35 • <- ACI Code Equation 11-34

48. Two-way Shear • as = 40 for interior columns • as = 30 for exterior columns • as = 20 for corner columns

49. Flexural Design – Isolated Square Footings • Flexural reinforcement is required in two directions • The values of d for the layers of steel in the two directions will be different • For square footings, design using the value of d for the upper layer is typical • For square footings supporting non-square columns, moments are larger in the shorter direction of the column

50. Flexural Design – Isolated Square Footings • Reinforcing steel areas required to resist moment are often less than minimum required steel: • Code Section 10.5.4 states that minimum area and maximum spacing need only be equal to values required for temperature and shrinkage steel