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Introduction to Columns

Introduction to Columns. 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. Types of Compression Members. Short compression blocks - pedestals.

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Introduction to Columns

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  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 6 No 9 bars 15” 18”

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