Helical Piles and Design Considerations
Helical Piles and Design Considerations
2009 International Building Code Standard
The use of helical piles dates back almost 200 years. The earliest version of today’s helical pile was installed in 1886 by Alexander Mitchell, to support the Maplin Sands Lighthouse. Helical pile use continued to grow, focusing specifically on moorings and lighthouse foundations until the middle of the 20th century.
Due to technological advancements in hydraulic motors following World War II, helical pile use was expanded to tension applications. Today, it has become common practice to use helical piles in both compression and tension applications.
In 2007, the International Code Council Evaluation Service (ICC-ES) approved AC358 Acceptance Criteria for Helical Foundation Systems and Devices. AC358 provides helical pile manufacturers with standardized methods for the design and testing of helical piles, resulting in product capacity ratings that are generally considered conservative.
In the 2009 International Building Code (IBC), helical piles have been included with other deep foundation systems. According to the IBC, section 1802, helical piles are defined as, “manufactured steel deep foundation element consisting of a central shaft and one or more helical bearing plates. A helical pile is installed by rotating it into the ground. Each helical bearing plate is formed into a screw thread with a uniform defined pitch.”
This article briefly outlines sections in the International Building Code as they pertain to helical piles
Helical Piles and Design Considerations: Deep Foundations, Section 1810
This section of the IBC details all elements pertaining to deep foundations, now including helical piles.
Industry standards dictate that helical piles that are not defined as laterally braced must be designed in accordance to the IBC minimum unbraced length, as defined in 1810.2.1.
The first five feet of pile placed within stiff soil is considered unbraced, unless defined as laterally braced in all directions by section 1810.2.2.
The first 10 feet within soft soil (soil having an N-value of ≥4) is considered unbraced.
Additionally, underpinning performed on existing structures using helical piles are defined as an unbraced deep foundation. The pile must be designed to resist eccentricity when a side load bracket is used to transfer structural loads onto the pile.
Section 1810.2.1 Lateral Support states that, “Any soil other than fluid soil shall be deemed to afford sufficient lateral support to prevent buckling of deep foundation elements in accordance with accepted engineering practice and the applicable provisions of this code. Where deep foundation elements stand unbraced in air, water or fluid soils, it shall be permitted to consider them laterally supported at a point 5 feet (1542 mm) into stiff soil or 10 feet (3048 mm) into soft soil unless otherwise approved by the building official on the basis of a geotechnical investigation by a registered design professional.”
In section 1810.2.2 Stability, it is outlined that, “Deep foundation elements shall be braced to provide lateral stability in all directions. Three or more elements connected to a rigid cap shall be considered braced, provided that the elements are located in radial directions from the centroid of the group not less than 60 degrees (1 rad) apart. A two-element group in a rigid cap shall be considered to be braced along the axis connecting the two elements. Methods used to brace deep foundation elements shall be subject to the approval of the building official.
Deep foundation elements supporting walls shall be placed alternately in lines spaced at least 1 foot (305 mm) apart located symmetrically under the center of gravity of the wall load carried, unless effective measures are taken to provide for eccentricity and lateral forces, or the foundation elements are adequately braced to provide for lateral stability.
A single row of deep foundations without lateral bracing is permitted for one- and two-family dwellings and lightweight construction not exceeding two stories above grade plane or 35 feet (10 668 mm) in building height, provided the centers of the elements are located within the width of the supported wall.”
In section 1810.2.5 Group Effects, it is stated that “The analysis shall include group effects on lateral behavior where the center-to-center spacing of deep foundation elements in the direction of lateral force is less than eight times the least horizontal dimension of the element. The analysis shall include group effects on axial behavior where the center-to-center spacing of the deep foundation elements is less than three times the least horizontal dimension of an element.”
However it’s important to note that the the group effect on the minimum horizontal spacing is substantially larger than code allowance. This is due to the slender central shaft. In the case of the helical foundation systems installed by JES, multiple piles must have a center-to-center spacing, at the helix depth, of at least four times the diameter of the largest helix blade (ICC-ES-AC358). For tension applications, the uppermost helix blade shall be installed, at minimum, to a depth of at least 12 diameters below the ground surface.
According to section 1810.3.1.5 Helical piles, “Helical piles shall be designed and manufactured in accordance with accepted engineering practice to resist all stresses induced by installation into the ground and service loads.”
For information on mechanical strength, section properties and torque ratings, contact JES at 866-370-4816 to receive product tech specs.
Helical Piles and Design Considerations – Section 1810.3
JES installed FSI Helical Piles are made from ASTM A500 Grade B or C steel, with a minimum yield strength of 60 ksi and a minimum tensile strength of 70 ksi. The helix blades designed and tested in accordance with ICC-ES AC358, and are manufactured with ASTM A572 Grade 50 x 3/8″, ASTM A572 Grade 50 x 1/2″, or ASTM A36 x 3/8″ thick steel plate with a minimum yield strength of 101 kips. This exceeds the allowable individual helix plate capacity of 41 kips with a safety factor of two.
In section 1810.3.2.6 Allowable Stresses, it is outlined that the “allowable stresses for materials used in deep foundation elements shall not exceed” the following:
In compression applications: 0.6Fy ≤ 0.5 Fu
In tension applications: 0.6Fy ≤ 0.5 Fu
According to section 1810.3.3.1.9 Helical piles, “The allowable axial design load, Pa, of helical piles shall be determined as follows:
Pa = 0.5 Pu Where Pu is the least value of:
Sum of the areas of the helical bearing plates multiplied by the ultimate bearing capacity of the soil or rock comprising the bearing stratum.
Ultimate capacity determined from well-documented correlations with installation torque.
Ultimate capacity determined from load tests.
Ultimate axial capacity of pile shaft.
Ultimate axial capacity of pile shaft couplings.
Sum of the ultimate axial capacity of helical bearing plates affixed to pile.”
“Helical piles shall be installed to specified embedment depth and torsional resistance criteria as determined by a registered design professional. The torque applied during installation shall not exceed the maximum allowable installation torque of the helical pile.”
1810.4.11 Helical piles. For additional information about JES helical pile installation, read the helical pile technical specifications or call 866-370-4816 to schedule an appointment with one of our professional engineers.
1704.10 Helical pile foundations states that, “Special inspections shall be performed continuously during installation of helical pile foundations. The information recorded shall include installation equipment used, pile dimensions, tip elevations, final depth, final installation torque and other pertinent installation data as required by the registered design professional in charge. The approved geotechnical report and the construction documents prepared by the registered design professional shall be used to determine compliance.”
International Code Council. (2009). International Building Code