1. Introduction to Columns

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. Types of Compression Members
Short compression blocks - pedestals
Short reinforced columns
Long or slender reinforced columns

4. Short Compression Block
Height less than three times least lateral dimension
May be designed as unreinforced or plain member
Maximum force is:

5. Short Reinforced Column
Stocky members
Material failure
Maximum load supported is controlled by section dimensions and strength of materials

6. Slender Reinforced Column
Bending deformations
Secondary moments
Instability or buckling

7. Secondary Moments
P is the axial force
M is the primary
moment

8. Types of Columns
Tied Columns
Spiral Columns
Composite columns

9. Tied Column

10. Spiral Column

11. Composite Columns

12. Tied Columns Ties increase column strength
Ties hold longitudinal bars in place during construction
Ties prevent longitudinal bars from buckling after cover spalls
Tied columns are usually, but not always, rectangular in shape

13. Spiral Columns
Spiral columns are usually, but not always, circular in shape
Helical spirals
Spirals are more effective than ties in increasing column strength
Spirals are loaded in hoop tension when compressive load is place on a column

14. Spiral Columns Spiral columns are more expensive than tied columns
Spiral columns are better for seismic applications

15. Axial Load Capacity of Columns

16. Failure of Columns
Tied columns – cover spalls and, unless ties are closely space, longitudinal bars buckle
Spiral columns – cover spalls but longitudinal bars and concrete core are confined by spirals and remain intact
Spiral cage is designed to be have a strength equal to the spalled cover

17. Strength of Spiral
Shell strength
Spiral strength

18. Strength of Spiral
ACI Code Equation 10-5

19. Spiral Steel Percentage

20. Spiral Parameters

21. ACI Code Requirements for CIP Columns
Percentage of longitudinal steel may not be less than 1% nor more than 8%
At least four longitudinal bars must be used within rectangular or circular ties
At least six longitudinal bars must be used within spiral ties
The practical minimum column dimension is about 8 to 10 in.

22. ACI Code Requirements for CIP Columns
For tied columns with No 10 and smaller long-itudinal bars, the minimum size tie is No 3
For tied columns with longitudinal bars larger than No 10 , and for bundled bars, the minimum size tie is No 4
For tied columns, the maximum ties spacing is the smallest of:
48 tie bar diameters,
16 longitudinal bar diameters, or
the least lateral column dimension

23. ACI Code Requirements for CIP Columns
For tied columns ,ties must be arranged so that every corner and every alternate longitudinal bar has lateral support provided by a tie bent around the longitudinal bar with an included angle not greater than 135o. No longitudinal bar can be located more than 6 in. from such a laterally supported bar

24. Supported Bars

25. Supported Bars

26. Supported Bars

27. ACI Code Requirements for CIP Columns
Spirals may not have diameters less than 3/8 in.
The clear spacing between spirals may not be less than 1 in. or greater than 3 in.

28. Capacity Reduction Factor
Failure of a column is more significant than failure of a beam
For tied columns, f = 0.65
For spiral columns, f = 0.75

29. Eccentricity of Axial Load
To account for (accidental) eccentricity, the ACI Code uses a factor a
= 0.80 for tied columns
= 0.85 for spiral columns

30. ACI Column Design Capacity Equations
(spiral)
(tied)
ACI Code Equation 10-1 (spiral)
ACI Code Equation 10-2 (tied)

31. Column Design Examples

32. Example 9.1
Design a square tied column to support an axial dead load o 130 k and an axial live load of 180 k. Begin using approximately 2 percent longitudinal steel, a concrete strength of 4,000 psi and Grade 60 steel.

33. Example 9.1
Determine the factored axial load

34. Example 9.1
Select the column dimensions

35. Example 9.1
Select the longitudinal steel

36. Example 9.1
Design the ties

37. Example 9.1
Other ACI Code requirements

38. Example 9.1
Other ACI Code requirements

39. Example 9.1

40. Example 9.2
Design a round spiral column to support an axial dead load of 240 k and an axial live load of 300 k. Begin using 2 percent longitudinal steel, a concrete strength of 4,000 psi and Grade 60 steel.

41. Example 9.2
Determine the factored axial load

42. Example 9.2 Select the column dimensions
Since 255 < 266 in2, the reinforcing steel percentage will be greater than 2%

43. Example 9.2
Select
the
longitudinal
steel

44. Example 9.2
Design of the spiral ties

45. Example 9.2
Design of
Spiral

46. Example 9.2
18”
15”
6 No 9 bars