In IBC 2018 1810.2.2, deep foundations are required to be braced such that lateral stability is provided in all directions. An exception is provided for isolated, cast-in-place elements that have a diameter (D) of at least 2 ft (610 mm) and a length (L) not more than 12 times the diameter. Large diameter drilled shafts, anecdotally, are sometimes not held to this length limitation. As a geotechnical community, we intuitively understand that a 3 ft (900 mm) diameter shaft is no less stable at lengths of 35 ft than 37 ft (10 m or 12 m), or just above and below the limiting ratio presented in the code. However, larger diameter augercast piles are becoming more common, and the authors see this limitation applied more stringently by engineers and building officials, even when it doesn’t appear to have a rational basis.
In response to member concerns, the DFI committee looked closer at the code applications and the historical development of the code language, as well as performing analytical modelling to evaluate the language’s impact.
Current Code Application
The full text of the code provision in question reads: 1810.2.2 Stability: Deep foundation elements shall be braced to provide lateral stability in all directions. Three or more elements connected by a rigid cap shall be considered to be 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.
The code provides an exception to the bracing. It states that: [Exception] 1. Isolated cast-in-place deep foundation elements without lateral bracing shall be permitted where the least horizontal dimension is not less than 2 feet adequate lateral support in accordance with Section 1810.2.1 is provided for the entire height and it does not exceed 12 times the least horizontal dimension.
Examination of the code and commentary suggests buckling as the IBC code writer’s concern, with this commentary provided to explain the limiting ratio in C1810.2.2: “This is an empirical requirement that is intended to offset the concerns that typically require consideration in more slender elements.” Because of the basic mechanics of materials, unbraced slender elements have axial capacities that are governed by elastic buckling behavior. However, the code allows the soil to restrain against buckling, as noted in section 1810.2.1: “Any soil other than fluid soil shall be deemed to afford sufficient lateral support to prevent buckling of deep foundation elements.” The lateral bracing could also provide support against moments due to eccentrically applied loads, pushover failures, and force effects generated when piles are installed out of tolerance (verticality). These loads can translate into second-order effects on the shaft, thereby reducing the element’s axial capacity.
Currently in construction, engineers use varying approaches to design isolated, CIP elements, and for the interpretation of 1810.2.2. It is clear from Exception 1 of Section 1810.2.2 that any isolated CIP element’s diameter must be greater than or equal to 24 in (610 mm). Many engineers will perform soil-structure interaction analyses, using project-specific loading, fixities and soil properties. If the resulting deflections and stresses fall within allowable limits, the design proceeds without requiring added lateral bracing because the soil provides sufficient lateral resistance. The engineers that employ this approach generally interpret the language in 1810.2.2 as limiting the portion of the element that can be cast “unbraced,” either in potentially fluid soil or above grade. Some engineers will also consider contributions to lateral stability from the slab-on-grade if it is cast directly against the pile cap or column above. Other designers provide additional lateral bracing through grade beams tied into the pile cap in orthogonal directions, as suggested in the code. Rarely will the L/D ratio of the designed element impact any one of these approaches. When engineers rely on grade beams to brace elements, they will not typically eliminate the grade beams if the L/D ratio of the pile is less than 12.
IBC 2006 had virtually the same lateral bracing requirements for piles as IBC 2018, without the exceptions. It also had a separate code provision that limited the diameter of drilled shafts (hereto referred to as “piers,” as in IBC 2006 and previous codes) to not less than 2 ft (610 mm); it also limited pier heights to not more than 12 times the diameter. The 2018 code provisions for lateral bracing thus appears to have combined separate provisions for piles and piers from IBC 2006.
Looking further back , IBC 2000 (the first edition) contained the same restrictions for isolated piers, but allowed exceptions for residential and utility use; piers constructed of reinforced concrete, structural steel or encased in a steel shell; and scenarios where the surrounding materials (soils) provide adequate lateral support. The third IBC 2000 exception has similar verbiage to the IBC 2018 exception (“adequate lateral support ... is provided for the entire height”), but does not limit the diameter or height as does IBC 2018.
Prior to IBC 2000, there were three model building codes in use nationally: the Standard Building Code (SBC), the Uniform Building Code (UBC) and the Building Officials and Code Administrators International (BOCA). The 1997 UBC omitted bracing requirements for deep foundation members, nor did it limit the size and height of deep foundation members. The 1993 BOCA and the 1994 SBC both contained bracing requirements for piles resembling IBC 2018. However, the 1994 SBC omitted limits for isolated piers based on diameter and height. Regarding isolated piers, 1993 BOCA contained the following provision:
Except for occupancies in Use Group R-3 and light structures, the minimum dimension of isolated piers used as foundations shall be 2 feet (610 mm), and the height shall not exceed 12 times the least horizontal dimension, unless constructed of reinforced concrete or structural steel, or where entirely encased in a steel shell at least ¼ in (6 mm) thick. Approved heights greater than herein specified are permitted where the surrounding foundation materials furnish adequate lateral support.
The code requirements for isolated piers in BOCA 1993 and IBC 2000 are nearly identical, suggesting that the limiting ratio in IBC 2018 can be traced to previous BOCA codes. However, the additional exceptions that existed in earlier codes are no longer in the IBC 2018. The question is whether the limiting ratio included in the IBC 2018 code provisions is justified, or whether different criteria should be used to determine when isolated deep foundation members are allowed.
Finite Element ModelingThe commentary associated with 1810.2.2 implies that the limiting ratio is empirically derived. Based on the engineering intuition of this paper’s authors and the historical code review, this limiting ratio needed additional consideration. Therefore, we performed analyses using FB Multipier, a nonlinear finite element soil-structure interaction program from Bridge Software Institute (BSI). A simple geotechnical model was created that involves a homogenous, cohesionless soil profile with the water table well below the pile tip. The model assumed a concrete deep foundation element (f’ = 5 ksi, or 35 MPa) with a steel reinforcement ratio (ρ) of 0.5% (f =60 ksi, or 415 MPa). The element had s no lateral bracing (i.e., it was under free head conditions), with the element head at grade. Because buckling was addressed by IBC 2018, alternative needs for lateral bracing were evaluated, first by the moments generated by eccentrically applied loads. For the eccentricity analyses, three element diameters were considered (24 in [610 mm], 36 in [900 mm], and 48 in [1.2 m]) and the maximum geotechnical capacity applied as determined by the shortest element (lowest L/D ratio) scenario for each element diameter. A moment equivalent was also applied to the axial load at a 3 in (75 mm) eccentricity (the allowable construction tolerance dictated by IBC 2018). For all models, the L/D ratio was varied from 8 to 24. By plotting the values of pile head deflection and pile head rotation against the L/D ratio, the impact of the L/D ratio on the pile’s behavior could be determined. An unstable pile tended to deflect or rotate, thus impacting the structure above.
Further exploration occurred of another mechanical reason for providing lateral bracing: the force effects generated when piles are installed out of tolerance (verticality). The above model was duplicated (with all three diameters) and with a pile installed at the verticality limit imposed by IBC 2018 (2 degrees from vertical). This modeling again involved applying the maximum geotechnical axial load with an eccentric load. The model results indicate that increasing the ratio of length to diameter has no detrimental effect on the element’s behavior under the force effects. Therefore, the current IBC code limitation that requires unbraced elements to have a length-to-diameter ratio not more than 12 does not appear to be justified by analysis.
Note: DFI’s Augered Cast-in-Place and Drilled Displacement Pile Committee white paper on the subject of International Building Code (IBC) Section 1810.2.2 is still under technical and editorial review. This article regarding that IBC section’s focus on the lateral stability of cast-in-place deep foundation elements includes some of the data to be presented in the white paper, and is meant to inform the magazine readership. However, the more rigorously reviewed white paper, to be published by DFI, will be the long-term reference document.
AcknowledgementsMorgan NeSmith, P.E. (Berkel and Company), Noah Miner, P.E. (Malcom Drilling Company) and Christophe Locussol, P.E. (GEI Consultants), assisted the authors with research, review and FE modeling.
Jeffrey Donville, P.E., has over 20 years of geotechnical engineering experience. For the past 13 years, he has led the design of deep foundations and earth retention systems at Beaty Construction.
Hannah Iezzoni, P.E., is a design engineer for Keller North America with 8 years of experience in contractor support design and geostructural construction. She is the secretary of the Augered Cast-In-Place and Drilled Displacement Pile Committee and an active member of DFI’s Women in Deep Foundations Committee.
Dan Stevenson, P.E., is the chief structural engineer for Berkel and Company Contractors. He has over 30 years of experience in the design, execution and monitoring of geostructural solutions and has published technical papers for geotechnical conferences and periodicals. Stevenson is chair of the DFI Codes and Standards Committee.