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 Spiral Column:

Several details must be met when designing a spiral 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 clear distance between spirals must be between 1 and 3 inches [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].
  • At least six longitudinal bars are required to be used in spiral columns [1d].
  • The concrete cover to the outermost surface of the tie steel must be at least 1.5 [1e]"
  • The minimum spiral wire diameter is 3/8 in, and the maximum spiral wire diameter is 5/8 in [1f].

Volume of Spiral Reinforcement to Column Core ratio:

ACI 318 requires that the volume of spiral reinforcement be greater than:

{V_{\hbox{Spiral Reinforcment}}\over{V_{\hbox{Column Core}}}} \ge \rho_s = 0.45\left({A_g\over{A_c}}-1\right)\left(f_c'\over{f_{yt}}\right)

where:

VSpiral Reinforcement = The volume of the spiral reinforcement in the column
VColumn Core = The volume of the columns core
ρs = spiral reinforcement ratio
Ac = Area of the column core, measured to the outside surface of the spiral section
Ag = the gross section area of the column
fc' = the compressive strength of the concrete
fyt = the yield strength of the rebar in the column

Spiral Pitch:

The following equation can be used to calculate the necessary spiral pitch. The spiral pitch will be used to ensure the clear distance between the spirals is between the 1" and 3" required.

To find spiral pitch:

s\sim {4A_{sp}\over {\rho_sD_c}}

where:

s = spiral pitch
Asp = the area of the spiral wire
ps = spiral reinforcement ratio (calculated in the previous section)
Dc = Diameter of the core measured to the outside surface of the spiral

To find the clear distance:

\hbox{clear distance} = s - d_{sp}

where:

s = spiral pitch
dsp = the diameter of the spiral reinforcment

Typical Spiral Wire Size:

Table 1: Typical Spiral Wire Sizes (based on column size)
Column Diameter (inches) Spiral Wire Size (in.)
15" or less (#10 bars or smaller) 3/8"
15" or less (Greater than #10 bars) 1/2"
16" to 22" 1/2"
23" & greater 5/8"

Note: This is not based on ACI code, just standard practice.

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 7.10.4.3
    3. Found in Section 7.6.3
    4. Found in Section 10.9.2
    5. Found in Section 7.7.1
    6. Found in Section 7.10.4.2
    7. Found in Section

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