March/April 2021 - 72

Exposed CIP elements at NUCOR facility in Kentucky

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 modeling
to evaluate the language's impact.

Current Code Application
While the article authors acknowledge
that arguments can be made about the
impact of element diameter with
respect to lateral stability, this article
focuses specifically on the limiting
ratio of length (or height) to diameter,
herein called the L/D ratio, as explicitly
stated in the provision. Also, as the code
language in question is part of IBC 2018,
the focus is on building construction,
not transportation projects typically
governed by other design codes.
The full text of the code provision in
question reads:
72 * DEEP FOUNDATIONS * MAR/APR 2021

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
the height 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
March/April 2021

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