March/April 2021 - 73

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.

Code Evolution
The DFI committee members then
looked at the origin of this code
requirement and exception by
searching historical model building
codes. The current code closely mirrors
language in the 2009 IBC, which
included a completely reorganized
Chapter 18 - Soils and Foundations.
Before then, the IBC had different
requirements for piles versus drilled
shafts (piers), but blended the requirements for both into one category in the
2009 edition.

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.

deep foundation members, nor did it
limit the size and height of deep
foundation members. The 1993 BOCA
and the 1994 SBC both contained
b r a c i n g r e q u i r e m e n t s fo r p i l e s
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
s p e c i f i e d a r e p e r m i tt e d w h e r e t h e
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 Modeling

Formwork for grade beams to laterally support ACIP piles

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

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

The 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'c = 5 ksi, or 35 MPa) with a
DEEP FOUNDATIONS * MAR/APR 2021 * 73
March/April 2021

Table of Contents for the Digital Edition of March/April 2021
