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Footings. 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 . Footings. Support structural members and transfer loads to the soil.
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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.
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
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
Wall Footing • Wall footings – enlargement of the bottom of the wall
Isolated Square Footing • Isolated or single column square footing – loads relatively light and columns not closely spaced
Combined Footing • Combined footings – support two or more columns – heavily loaded columns; closely spaced columns; columns near property line
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
Pile Cap • Pile caps – distribute column loads to groups of piles
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
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
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)
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
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
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.
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:
Example 12.1 • Effective soil pressure and required width of footing:
Example 12.1 • Factored bearing pressure for design of concrete:
Example 12.1 • Compute design shear (at distance d from face of wall):
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)
Example 12.1 • Development length:
Example 12.1 • Available length for development
Example 12.1 • Temperature and shrinkage steel • Use No 5 at 8 in (As = 0.465 in2 / ft)
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
Design of Isolated Square Footings Constant thickness
One-way Shear • Same as for wall footings
Two-way Shear • ACI Code Section 11.11.1.2 states that critical section is at a distance d/2 from face of support
Two-way Shear • <- ACI Code Equation 11-33 • <- ACI Code Equation 11-35 • <- ACI Code Equation 11-34
Two-way Shear • as = 40 for interior columns • as = 30 for exterior columns • as = 20 for corner columns
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
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
