What Is Spiral Rebar

  • Author: Fazal Umer
  • Posted On: January 9, 2024
  • Updated On: January 9, 2024

Reinforcements play a vital role in enhancing the structural capacity of concrete structures by providing tensile strength to concrete members as concrete is weak in tension while much stronger in compression. 

Along with the main longitudinal steel, transverse steel is also provided in RCC (Reinforced Cement Concrete) members. While transverse steel is termed as stirrups in case of beams, for columns it is referred to as: 

  1. Lateral Ties 
  2. Spiral Reinforcement or Helical Reinforcement

Spiral Reinforcement

Spiral reinforcement is provided in a column to provide ductility, sustain maximum loads at extreme deformations and controls the lateral expansion of concrete.

Figure 1: Column Failure with spiral rebar
Figure 2: Column failure with lateral ties

Notice how in case of spiral reinforcements, due to increased ductility the reinforcement absorbed and dissipated large amount of energy before column failure. However, in case of column with lateral ties, there is significantly less deformation seen even when the column has failed.  

Furthermore, it increases the load-carrying capacity of a column by providing confinement to the inner core of the column; confinement is provided to the inner core until the spiral reinforcement yields, afterwards the concrete core fails at excessive deformations.

Spiral Reinforcement is also a crucial element in construction of structures in seismic zones.

Types of spiral columns

There are three types of spiral columns (columns with spiral reinforcement):

Light Spiral 

As the name refers, less transverse steel is required and this type is generally used in areas of less seismic activity.

ACI Spiral (Balanced Spiral)

For this type of transverse reinforcement steel is provided such that a balance is attained between ductility and strength for resistance against earthquakes. It is used where there is medium seismic activity.

Heavy Spiral

As the name suggests, maximum amount of spiral reinforcement is used as it is used for construction in high seismic activity areas.

While spiral reinforcement enhances the load-bearing capacity and as such, there are still a few drawbacks for this type of transverse reinforcement, such as: 

  1. Required skilled labor as its installation is quite complex.
  2. Makes the formwork more complex.
  3. Limited code guidance for spiral reinforcement.

Difference Between Helical And Spiral Reinforcement 

Generally, these two terms are often used interchangeably, however it is important to consider a minor difference between the two terms.

Helical reinforcement is a continuous circular reinforcement that is provided as transverse reinforcement it columns. It provides more confinement of inner concrete core comparatively.

Due to the superior load bearing capacity provided by the helical reinforcement, it is mostly favored in pile construction as well.

Figure 3: Column with Helical Reinforcement

On the other hand, spiral reinforcement is provided as a non-continuous circular reinforcement around longitudinal bars placed in a circular manner in a column. 

Figure 4: Column with Spiral Reinforcement [Source ACI 318]

Both, helical and spiral reinforcements, have the same primary functions however they may be used as per the engineering requirements.

Difference Between Spiral Reinforcement And Lateral Ties

Both of these are used in columns around the longitudinal reinforcements in order to hold the bars in place and to provide stability.

Figure 5: Cross-section of a rectangular column with a lateral tie around 4- bars

Figure 6: Cross-section of a spiral column with spiral reinforcements around six longitudinal bars

  1. The main difference between the two transverse reinforcements is about their configuration.
  2. Spiral reinforcement increases the ductility of column as well as provides resistance against buckling while lateral ties only provide stability against buckling of the column.
  3. Spiral reinforcement are used in structures in earthquake prone areas whereas, lateral ties are used in conventional construction.
  4. According to ACI 318, the total strength reduction factor for spiral columns isφ = 0.85 x 0.75” while, for tied columns (columns with lateral ties) the strength reduction factor is φ = 0.80 x 0.65”.

Figure 7: Difference in Performance of Spiral Reinforcement and Lateral Ties

Codes And Standards

The limitations and design considerations for Spiral Reinforced Columns are provided by American Concrete Institute (ACI 318-19(22)) and International Building Code (IBC).

Limitations And Considerations As Per Aci-318

When spiral reinforcements are used in columns it is vital to keep the following in mind:

  1. At least six longitudinal (main bars) are to be provided when spiral reinforcement is to be used.
  2. Since spiral reinforcement provides increased splitting resistance therefore, the code allows us to reduce the splice length in compression.
  3. In multi-story buildings, for the purpose of lateral support of main bars provided by spiral reinforcement, the bottom of spiral should be at the top of footing or slab for any story level. And the top of spiral should be positioned in accordance to the following table:
Source: ACI 318-19 (22)

Terms Related To Design Of Spiral Reinforcement 

Clear Pitch

The clear distance between the adjacent faces of two consecutive spirals.


Center-to-center distance between two consecutive spirals. 

Figure 8: Circular Column with Spiral Reinforcement

In reference to the above figure;

Dc = Diameter of the core of concrete column

dsp = Diameter of spiral reinforcement 

Core of Column

It is the main load bearing component of a column that is surrounded by reinforcement, as illustrated in figure 4.

Diameter of Core

It is the center-to-center distance between the spirals as shown in figure 4.


  1. Diameter of spiral bar should not exceed 10mm.
  2. Smin (minimum clear pitch) = larger of
    1. 4/3 of maximum aggregate size
    2. 25 mm
  3. Clear pitch maximum = not larger than 75 mm


fc’ = 28-days compressive strength of concrete cylinder

fy= yield strength of concrete

Ag= Gross Area

Ac= Area of concrete

In conclusion, spiral reinforcement should be used in seismic-prone areas and where there are high strength requirements instead of lateral ties as they provide ample warning before failing while enhancing strength of column core. The design of this reinforcement should be according to the guidelines provided by the code.

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Author: Fazal Umer

Fazal is a dedicated industry expert in the field of civil engineering. As an Editor at ConstructionHow, he leverages his experience as a civil engineer to enrich the readers looking to learn a thing or two in detail in the respective field. Over the years he has provided written verdicts to publications and exhibited a deep-seated value in providing informative pieces on infrastructure, construction, and design.