Chp 12 Cont Examples to design Footings Example
- Slides: 60
Chp. 12 Cont. – Examples to design Footings
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/ft 2 Use fc = 3 ksi and fy = 60 ksi
Example 1 Assume a depth of footing. (2 ft or 24 in. ) The weight of concrete and the soil are:
Example 1 The effective soil pressure is given as:
Example 1 Calculate the size of the footing:
Example 1 Calculate net upward pressure:
Example 1 Calculate the depth of the reinforcement use # 8 bars with a crisscrossing layering.
Example 1 Calculate perimeter for two-way shear or punch out shear. The column is 18 in. square.
Example 1 Calculate the shear Vu The shape parameter
Example 1 Calculate d value from the shear capacity according to 11. 12. 2. 1 chose the largest value of d as is 40 for interior, 30 for edge and 20 for corner column
Example 1 The depth of the footing can be calculated by using two way shear
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
Example 1 The depth of the footing can be calculated by using one-way shear
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.
Example 1 Calculate the bending moment of the footing at the edge of the column
Example 1 Calculate Ru for the footing to find r of the footing.
Example 1 From Ru for the footing the r value can be found.
Example 1 Compute the area of steel needed The minimum amount of steel for shrinkage is The minimum amount of steel for flexure is
Example 1 Use a #7 bar (0. 60 in 2) Compute the number of bars need Determine the spacing between bars
Example 1 Check the bearing stress. The bearing strength N 1, at the base of the column, 18 in x 18 in. , f = 0. 7 The bearing strength, N 2, at the top of the footing is
Example 1 The bearing strength, N 2, at the top of the footing is
Example 1 Pu =683 k < N 1, 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.
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
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.
Example 1 The development length, ld for the #7 bars for the reinforcement of the footing. There is adequate development length provided.
Example 1 - Final Design
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/ft 2 Use fc = 3 ksi and fy = 60 ksi. Limit one side of the footing to 8. 5 ft.
Example 2 Assume a depth of footing. (2 ft or 24 in. ) The weight of concrete and the soil are:
Example 2 The effective soil pressure is given as:
Example 2 Calculate the size of the footing:
Example 2 Calculate net upward pressure:
Example 2 Calculate the depth of the reinforcement use # 8 bars with a crisscrossing layering.
Example 2 The depth of the footing can be calculated by using the one-way shear (long direction) Vu =150. 7 k in short direction
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.
Example 2 Calculate perimeter for two-way shear or punch out shear. The column is 18 in. square.
Example 2 Calculate the shear Vu The shape parameter
Example 2 Calculate d from the shear capacity according to 11. 12. 2. 1 chose the largest value of d. as is 40 for interior, 30 for edge and 20 for corner column
Example 2 The depth of the footing can be calculated for the two way shear
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
Example 2 The depth of the footing can be calculated by using the two way shear
Example 2 Calculate the bending moment of the footing at the edge of the column (long direction)
Example 2 Calculate Ru for the footing to find r of the footing.
Example 2 Use the Ru for the footing to find r.
Example 2 Compute the amount of steel needed The minimum amount of steel for shrinkage is The minimum amount of steel for flexure is
Example 2 Use As =8. 36 in 2 with #8 bars (0. 79 in 2). Compute the number of bars need Determine the spacing between bars
Example 2 Calculate the bending moment of the footing at the edge of the column for short length
Example 2 Calculate Ru for the footing to find r of the footing.
Example 2 Use Ru for the footing to find r.
Example 2 Compute the amount of steel needed The minimum amount of steel for shrinkage is The minimum amount of steel for flexure is
Example 2 Use As =9. 36 in 2 with #6 bar (0. 44 in 2) Compute the number of bars need Calculate the reinforcement bandwidth
Example 2 The number of bars in the 8. 5 ft band is 0. 83(22)=19 bars. So place 19 bars in 8. 5 ft section and 2 bars in each in (12 ft -8. 5 ft)/2 =1. 75 ft of the band.
Example 2 Determine the spacing between bars for the band of 8. 5 ft Determine the spacing between bars outside the band
Example 2 Check the bearing stress. The bearing strength N 1, at the base of the column, 18 in x 18 in. , f = 0. 7 The bearing strength, N 2, at the top of the footing is
Example 2 The bearing strength, N 2, at the top of the footing is
Example 2 Pu =683 k < N 1, 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.
Example 2 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
Example 2 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.
Example 2 The development length, ld for the #8 bars There is adequate development length provided.
Example 2 The development length, ld for the #6 bars There is adequate development length provided.
Example 2 - Final design 23 #6 12 #8