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 Short reinforced columns Long or slender reinforced columns

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

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

Slender Reinforced Column Bending deformations Secondary moments Instability or buckling

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

Types of Columns Tied Columns Spiral Columns Composite columns

Tied Column

Spiral Column

Composite Columns

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

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

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

Axial Load Capacity of Columns

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

Strength of Spiral Shell strength Spiral strength

Strength of Spiral ACI Code Equation 10-5

Spiral Steel Percentage

Spiral Parameters

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.

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

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

Supported Bars

Supported Bars

Supported Bars

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.

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

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

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

Column Design Examples

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.

Example 9.1 Determine the factored axial load

Example 9.1 Select the column dimensions

Example 9.1 Select the longitudinal steel

Example 9.1 Design the ties

Example 9.1 Other ACI Code requirements

Example 9.1 Other ACI Code requirements

Example 9.1

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.

Example 9.2 Determine the factored axial load

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

Example 9.2 Select the longitudinal steel

Example 9.2 Design of the spiral ties

Example 9.2 Design of Spiral

Example 9.2 18” 15” 6 No 9 bars