How to distinguish between a Spiral and Tied Column:

A Tied Column will have horizontal reinforcement (aka closed ties or hoops). This is different than a Spiral Column that will have reinforcement in the form of a continuous spiral that circles up and down the column. Note that this definition has nothing to do with whether the column is square or circular, and in fact you can have a tied circular column or spiral square column if you so choose.

Design specifications for a Tied Column:

Several details must be met when designing a tied column including:

  • The ratio of the longitudinal steel area to the gross column area must be between 0.01 and 0.08 [1a]min and ρmax respectively).
  • The concrete cover to the outermost surface of the tie steel must be at least 1.5 [1b]"
  • The longitudinal bars must have a space between the bars of at least 1.5 times the bar diameter but must be greater than at least 1.5" [1c].
  • Ties must be at least no. 3 bars if the longitudinal reinforcement is no. 10 bars in size or smaller. If the longitudinal reinforcement is greater than no. 10 bars then the ties must be at least no. 4 bars [1d]. The maximum practical size is no. 5, since after that point the pars become too difficult to bend to shape.
  • The center-to-center spacing of ties must not exceen the smallest of {16 longitudinal bar diameters, 48 diameters of the tie, the least column dimension} [1e].
  • At least four longitudinal bars are need for columns with square or circular ties [1f]. This helps ensure that you'll always have longitudinal support along any bending axis.
  • The ties must be arranged so that every corner and alternate longitudinal bar shall have lateral support provided by the corner of a tie [1g]. The corner of the tie must have an included angle of not more than 135 degrees.
    • Also, no bar shall be farther than 6" clear on each side along the tie from such a laterally supported bar. Simplified that means that a non-tied longitudinal bar must be less than 6" from a tied-bar, or additional ties are necessary to properly support the longitudinal bar.
    • When longitudinal bars are located around the perimeter of a circle, a complete circular tie is permitted.

References

  1. American Concrete Institute, "Building Code Requirements for Structural Concrete (ACI 318-05) and Commentary (ACI 318R-05)", 2005
    1. Found in Section 10.9.1
    2. Found in Section 7.7.1
    3. Found in Section 7.6.3
    4. Found in Section 7.10.5.1
    5. Found in Section 7.10.5.2
    6. Found in Section 10.9.2
    7. Found in Section 7.10.5.3

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