Using Commercial Helical Anchors/Tiebacks

Determination of Capacity

The ultimate capacity of a helical anchor may be calculated using the traditional bearing capacity equation:

Q_u = ∑ [A_h (cN_c + qN_q)]

Total stress parameters should be used for short-term and transient load applications and effective stress parameters should be used for long-term, permanent load applications. A factor of safety of 2 is typically used to determine the allowable soil bearing capacity, especially if torque is monitored during the helical anchor installation.

Like other deep foundation alternatives, there are many factors to be considered in designing a helical anchor foundation. Supportworks recommends that helical anchor design be completed by an experienced geotechnical engineer or other qualified professional.

Another well-documented and accepted method for estimating helical anchor capacity is by correlation to installation torque. In simple terms, the torsional resistance generated during helical anchor installation is a measure of soil shear strength and can be related to the bearing capacity of the anchor.

Q_u = KT

The capacity to torque ratio is not a constant and varies with soil conditions and the size of the anchor shaft. Load testing using the proposed helical anchor and helix blade configuration is the best way to determine project-specific K-values. However, ICC-ES AC358 provides default K-values for varying anchor shaft sizes, which may be used conservatively for most soil conditions. The default value for the Model 150 Helical Anchor System (1.50" square shaft) is K = 10 ft^-1.

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Anchor Capacity

The cross section of a square shaft is very compact which can allow the anchor to penetrate more easily through the soil. This compact shape also reduces the stiffness of the cross section and introduces more potential for buckling. These two factors make square shaft helical anchors better suited for tension loads. Supportworks, Inc. therefore recommends their use mainly for these types of applications. Square shaft helical anchors (piles) used in compression should be evaluated on a case by case basis by the project engineer.

Mechanical Axial Capacity (see note):

• Allowable Tension = 26.5 kips*

* The mechanical tensile capacity of the Model 150 Helical Anchor System is limited by the allowable stress levels dictated by AISC for a high strength bolt in double shear. The allowable tensile capacity of the shaft is actually much higher than this Allowable Tension value.

Torque Limited Axial Design Capacities based on Ultimate Torsional
Resistance of Anchor Shaft = 6,340 ft-lbs**:

• Ultimate Soil Capacity = 63.4 kips** (with K = 10 ft-1, see note)
• Allowable Soil Capacity = 31.7 kips (FOS = 2, Allowable System Capacity therefore governed by mechanical capacity = 26.5 kips*)

** This Ultimate Torsional Resistance and its corresponding Torque Limited Capacities are based on laboratory test results from an IAS accredited facility and may only be approached in idealized conditions. Plastic torsional deformations can begin in the anchor shaft near 4,600 ft-lbs. This value may be reached and exceeded in the field by maintaining alignment between the anchor and the drive head, limiting impact forces and torque reversal, and reducing the tendency to "crowd" (push down on) the anchor. Installation through soils with obstructions or high variability may result in impact loading on the anchor. In these cases, achieving high torque values becomes more difficult and a further reduction in the Design Torque Limit may be appropriate.

Note

K = 10 ft^-1 is a default value as published in ICC-ES AC358 which can, in many cases, be considered conservative. Higher capacities can often be achieved with site-specific load testing. Allowable capacities based on site testing shall not exceed the Mechanical Axial Capacity.

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Model 150 Helical Anchor System

• Outer Dimensions = 1.50" x 1.50"
• Anchor Shaft Yield Strength = 90 ksi (min.)
• Coupling Hardware: ¾" Grade 8 Bolt with Nut
• Available Helix Blade Diameters = 6", 8", 10", 12" and 14"
• Helix Blade Thickness = 0.375"
• Termination Hardware: 1" Threaded Rod, Tensile Strength = 120 ksi (min.)

-- View Our Helical Anchors and Tiebacks Product Specifications document --

Model 175 Helical Anchor System

• Outer Dimensions = 1.75" Round Corner Square Bar
• Anchor Shaft Yield Strength = 90 ksi (min.)
• Coupling Hardware: (2) ¾" Grade 8 Bolt with Nut
• Available Helix Blade Diameters = 6", 8", 10", 12" and 14"
• Helix Blade Thickness = 0.375"
• Termination Hardware: 1" Threaded Rod, Tensile Strength = 120 ksi (min.)

-- View Our Helical Anchors and Tiebacks Product Specifications document --