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Volume-7, Issue-3 (September-December, 2022)
A Comprehensive Analysis of Foundation Design Approaches
Samirsinh P Parmar*
Assistant Professor, Department of Civil Engineering, Dharmsinh Desai University, Nadiad, Gujarat,
India
*
Corresponding Author: spp.cl@ddu.ac.in
ABSTRACT
Foundation design is an iterative process
irrespective of the type of foundation. A
design approach for critical structure
foundations needs a precise estimation of
bearing capacity and settlement criteria. In
the modern foundation design process,
consideration of load combination is
important. The present paper discusses the
steps involved in foundation design, various
design approaches, respective load
combinations, and factors of safety. It also
focuses on recent development in the LRFD
method. The key points are listed for
respective design approaches along with their
pros and cons. It also delineates exact
guidelines for the selection of a foundation
design approach in combination with field
conditions.
Keywords- ASD (allowable stress design),
Foundation design approaches, LRFD (load and
resistance factor design), LS, Serviceability limit
states (SLS), Ultimate limit states (ULS)
INTRODUCTION
Catastrophic failures of civil engineering
structures are uncommon, but when they do
occur, the consequences can be severe, resulting
in multiple deaths or injuries. Less severe
failures, which cause inconvenience and some
repair costs, are more common. Designers and
code drafters strive to avoid failures of all types
and severity levels. This is typically
accomplished by demonstrating that a design
would not fail even if the parameters and
conditions were significantly worse than those
expected to take precedence.
The parameters under consideration may
be either basic input to the design calculations
(for example, actions and material strengths) or
values derived within the calculations (e.g.
action effects and resistances). In this paper,
severe failures that result in danger or gross
economic loss will be referred to as ultimate
limit states (ULS). Serviceability limit states are
less severe failures that cause inconvenience,
disappointment, or relatively minor costs (SLS).
To ensure that severe failures (ULS) are
extremely unlikely, recent code drafting has
primarily used a partial factor approach in some
form. The factors are applied to parameter
values that are thought to be reasonably likely to
occur to derive parameter values for the
calculations that seem to be extremely unlikely
to occur. This method is widely used in
structural design and has been adopted by the
geotechnical community to attain compatibility
in the analysis of ground and structures as they
interact and rely on one another.
Two broad approaches are used for
serviceability (SLS): (a) direct calculations of
displacements & deformations, crack widths,
and damage, and (b) limits on the mobilization
of strength allowed, to limit displacements and
damage. In both cases, it is a common procedure
to base calculations on probable parameter
values. Approach (a) is ideal in theory but may
be difficult to implement in practice. Approach
(b) limits the proportion of strength mobilized by
applying a factor to the strength that is
sometimes referred to as a "mobilization factor,"
but in practice is difficult to distinguish from a
partial factor applied to material strength or
resistance, as might be used for ULS
calculations.
The "highly plausible" values could be
deliberately careful ("characteristic" in
Eurocodes, "conservatively assessed means" in
some US publications, "moderately
conservative" in some UK practice) or mean
values - the most likely to occur. The author
contends that good designers would not use
mean values (the most likely values) on instinct
in circumstances of considerable uncertainty,
except in safety formats that allow the design

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engineer to vary the factors applied based on his
interpretation of variability. This was explicitly
the case in earlier Swedish practice, for example.
A new approach, accepted by Eurocode
7 (EC7), is "direct appraisal of design values," in
which the designer consciously evaluates a value
severe enough that a worse value is extremely
unlikely to take place. It is not straightforward to
define this value, and EC7 resorts to
comparisons with factored values by saying "If
design values of geotechnical actions are
evaluated directly, the values of the partial
factors suggested in [the code] should be utilized
as a guide to the requisite degree of safety".
Some further options available, not yet
adopted in standards of practice, could be to
perform a reliability calculation, in which the
failure probability is calculated, or an index for it
such as the "reliability index". This is typically
accomplished by taking into account a stochastic
spread of parameter values, including some that
are extremely severe. As a result, the intention
here is to allow for an acceptable range of severe
values.
REVIEW
George Goble (1999) [1] published a
book in which he explained the applicability of
the LRFD method in geotechnical engineering
for foundations, earth retaining systems,
culverts, etc. Foye K.C.et al. (2006) [2] studied
methodology for the estimation of soil
parameters for the design of shallow foundations
using the LRFD method as shown in Fig.1.
Robert L.P. (2006) [3] published a special
edition, in which he discussed limit state design,
as well as Christian J.T. (2007) [4], described
LRFD for geotechnical engineering applications
considering the probably based design to achieve
more precise results to design geosystems.
Schuppener B. et al. (2009) [5] proposed
Eurocode 7 for geotechnical design codes for
non – European Union countries. Kotadia R. and
Malvania A. (2021) [6] made rationalization of
the LRFD method for the estimation of the safe
bearing capacity of shallow footings. In their
analysis, they proposed to use partial safety
factors instead of bearing capacity factors to
derive SBC to incorporate shear failure criteria
in Eurocode 07-based design approach.
Dodigovi´c, F.; Ivandi´c, K. (2021) [7]; studied
modified reality-based geotechnical design
methods.
Figure 1: Classification of different types of foundations.
Procedure for Designing Foundations
The following basic steps must be
followed while designing foundations:
 A soil investigation should be conducted.
 The total load (both dead and live load)
must be computed, and the distribution
must be evaluated.

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 It is to assess both the total and differential
settlement that the structure may
experience.
 The type of foundation is determined by the
type of soil, structure, and load, as
discussed in previous posts.
 The allowable soil pressure for the
preferred type of foundation must be
determined.
 The material for the foundation must be
selected.
 Before final approval, alternate designs
must be created.
 A cost estimate must be made, and any
further changes must be made with the
economy and life of the structure in mind.
FOUNDATION DESIGN STEPS
Design is an iterative process that
requires the integration of
 The requirements of the consumer
 Analyses of geosystem
 Understanding and decision
 Economics
 Constructability
 Safety criteria
 Ecological concerns
The foundation design process usually starts
with conceptualization, which is a
comprehension of the client's (person,
organization, government, etc.) needs. The
designer will then use his/her engineering
information to construct an
innocuous, constructible, and cost-effective
system to meet the client's needs while limiting
negative environmental impact. Safety
throughout the construction and life of a
geosystem is a critical design factor. One should
never negotiate in safety for economics in
design. A designer may be involved in only a
limited part of the design process and may not
know the needs of the client. It may be possible
that the designer may have diminutive or no
understanding, and are unlikely to know price,
constructability, and safety issues. The following
section of the paper focuses on the step-by-step
decision-making for the selection of the design
approach with the factors concerned with the
design of foundation systems [8].
LIMITSTATES
Create a geosystem that meets the
requirements for stability and serviceability.
These are known as limit states, and they occur
when a system begins to respond unfavorable or
fails to satisfy the desired design function. You
must ensure as a geotechnical engineer that your
system will not reach a limited state under any
expected loading or environmental conditions.
Two limit states serve as the foundation of
geotechnical design criteria; both must be
satisfied. The section that follows describes
them.
Ultimate-Limit State
The ultimate limit state (stability
requirement) specifies the strength of a
geosystem or component that must not be
exceeded by any conceivable loading during its
design life. At the ultimate limit state (ULS), the
geosystem is expected to become unstable,
resulting in structural damage (local or global-
the latter is called collapse).
In most geosystems, various types of
instability are possible, and each of these modes
implies an ultimate limit state. A retaining wall,
for example, could fail due to sliding, rotating,
or soil load-bearing insufficiency, or as a result
of a mass failure event in which the wall is part
of a large mass or soil that slips, such as in a
slope failure or a landslide. Each of these,
known as the mode of failure or failure mode,
must be investigated to ensure that the ultimate
limit state is not reached under the anticipated
loading and climatic conditions. The ultimate
limit state disregards the strains or displacements
required to achieve instability. The serviceability
limit state specifies these.
Serviceability Limit State
A geosystem or component's
serviceability limit state (serviceability
requirement) delineates a limiting deformation
(displacement, rotation, and settlement) that, if
exceeded, will degrade its operation. A few
examples of exceeding serviceability limit states
(SLSs) include intolerable vibrations, obnoxious
cracks, differential settlement, and excessive
total settlement. Unacceptable lateral
displacements and rotations are also included.

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Volume-7, Issue-3 (September-December, 2022)
Limit State Provisions
Limit state guidelines are typically
specified in codes (for example, the International
Building Code, Eurocode, Canadian National
Building Code, and the American Concrete
Institute). Based on real-world experience with
existing systems, these codes establish minimum
standards and recommend best practices. Codes
strive to streamline analyses to perform
consistent design calculations regularly. In most
cases, codes specify how a system or component
should function but do not specify the method or
procedure for carrying out the required
functionality. The design must satisfy both limit
states (ULS) and (SLS). Both requirements
cannot be met by the designer. These limit states
are also known as design criteria [9].
The key points are:
 Foundation design is an iterative procedure
that integrates the client's requirements,
analyses, expertise and decision,
economics, constructability, safety, and
environmental effects.
 The ultimate limit state denotes a strength
that, if exceeded, will fail the geosystem or
component.
 A serviceability limit state is a deformation
of a geosystem or element that, if exceeded,
will impair its intended function.
 Foundation design necessitates the
fulfillment of both ultimate and
serviceability limit states.
 Codes establish professional standards (but
not necessarily processes or methodologies)
for creating a safe, constructible, and cost-
effective geosystem.
DESIGN METHODS
Two design methodologies are used in
foundation analysis. One method is the
allowable stress design (ASD), which has long
been used in geotechnical practice. The other is
load and resistance factor design (LRFD), which
is quickly replacing ASD. Designers must
understand the implications of these strategies as
well as the distinctions between them. This
section will go over these strategies briefly.
Allowable Stress Design to Satisfy ULS
Allowable stress design (ASD)
calculates a structure's ultimate resistance and
divides it by a factor greater than one, known as
the factor of safety, to determine the allowable
load or stress.
(1)
Where Qa and Qult are the allowable and ultimate
loads and FS is the safety factor. Table 1 lists
typical safety factors for foundations and earth
structures.
Table 1: Typical factor of safety values for various geosystems.
Sr. No. Foundation/ Earth Structures FoS
1 Foundations – Bearing capacity 2 to 3 (normally 3)
2 Retaining walls 1.5 to 2
3 Earthworks 1.3 to 1.5
4 Seepage- Uplift 1.5 to 2
5 Piping 2 to 3
6 Slopes 1.25 to 1.75
The key points for ASD are:
 In ASD, the loads and resistances are
conclusive; such that, the dead load, live
load, earthquake load, and so forth are
assumed to be known a predetermined
during the design life of a system, and any
variability is ignored.
 All loads are considered equal and
combined.
 The analysis's inaccuracies and risks are not
explicitly considered.
 The various degrees of danger associated
with various structures and their
components are not explicitly considered.
 The FS has no core principle; it is based on
previous experience and decisions
regarding existing structures' performance.

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When performing a geosystem analysis, it is a
must to combine loads in such a way that they
have the greatest negative effect on the system.
Load combinations are often recommended by
codes for the worst-case scenario. Design for
allowable stress. The following load
combinations are recommended by IBC (2006):
 QDL + QFL
 QDL + QHL + QFL + QLL + QTL
 QDL + QHL + QFL + (QRoL or QSL or QRL)
 QDL + QHL + QFL +0.75 (QLL + QTL) +
0.75(QRol or QSL or QRL)
 QDL + QHL + QFL + (QWL or 0.7QEL)
 QDL + QHL + QFL + 0.75(QWL or 0.7QEL) +
0.75QLL + 0.75(QRoL or QSL or QRL)
 0.6QDL + QWL + QHL
 0.6QDL + 0.7QEL + QHL
Where QDL is dead load. QLL is live load. QRoL is
the roof load, and QSL is the snow load. QWL is
wind load. QEL is earthquake load, and QHL is
lateral loads due to earth pressures. Groundwater
pressures or pressure from bulk materials, QFL is
loads from fluids with well-defined pressures
and maximum heights, QTL is loads due to
temperature changes, and QRL is rain load. These
load combinations apply only to structural
components and must be used with extreme
caution when applied to soils. Structural
components are examined as either linearly
elastic or linearly elastic-rigid plastic materials.
Nonlinearity exists in soils. The stress path is
anisotropic, and stress-history dependent
materials, where the size and direction of
loadings are critical. The positive sign in load
combination expressions should be read as load
components functioning concurrently rather than
arithmetic addition.
Figure 2: Steel column to foundation design.
The responses at supports that rest on
foundations are estimated during structural
design. These supports may not be visible from
the ground. For shallow foundations, structural
loads at the supports must be shifted to the
foundation's base. The structural loads must be
transferred to the pile head at the ground surface
in the case of pile foundations. Fig. 2 depicts a
connection detail for a steel column to the
foundation. As indicated in Fig. 3a, the loads at
the bottom of the column are a vertical-centric
load, a clockwise moment, and a horizontal load.

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Volume-7, Issue-3 (September-December, 2022)
Fig. 3b depicts the equivalent loads at the
footing's base for foundation design. Designers
should be conversant with the applicable codes
because there are exceptions or alternate loads
that should be considered.
Figure 3: (a) Load at column base (b) Equivalent load at the column base.
Load and Resistance Factor Design (LRFD)
to Satisfy ULS
The load and resistance factor design
(LRFD) examines the uncertainties of various
loads and soil resistances using reliability theory.
Each load type is evaluated independently, with
factors based on its uncertainty applied. The
ultimate resistance provided by the soil is
adjusted based on the uncertainty of the soil
parameters (each parameter is treated
separately), the sample and testing method, the
scope of the soil investigation, the analytical
method, the amount of risk associated with the
structure, the consequences of failure, and
construction practice. LRFD is defined
mathematically as
ΣϕR Rult ≥ Σρi Qi (2)
Where ФR is the performance or resistance
factor, Rult is the ultimate soil resistance, and ρ is
the magnitude of the nominal load (estimated
actual load) for load type i. Codes recommend
various load and resistance factors. Table 2
compares the load factors in IBC (2006).
Eurocode 7 and the Canadian Foundation
Engineering Manual (CFEM) (1993).
Table 2: Load factors for different codes.
Sr. No. Loads IBC (2006) CFEM Eurocode-7
1 Dead Load (QDL) 1.4 1.25 1.1
0.9 0.8 0.9
2 Live Load (QLL) 1.6 1.5 1.5
0
3 Fluid Load (QFL) 1.4 1.25 1.0
0 0.8 1.0
4 Earthquake load (QEL) 1.0
5 Wind Load (QWL) 1.6
0.8
6 Lateral Loads (QHL) 1.6
0
The top values are Maximums and the lower values are minimums.
AASHTO (2004) recommends a
different set of load factors, as shown in Table 3.
AASHTO (2004) also considers a modification
factor to the load factors in Table 3 to account
for ductility, redundancy, and operational
importance.

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To produce the worst-case scenario, the loads
are combined in specific ways. The following
load combinations are recommended by IBC
(2006):
The top values are maximums and the lower
values are minimums.
 1.4{QDL + QFL)
 1.2(QDL + QFL QTL) + 1.6(QLL + QH1) +
O.5{QRoL or QSL or QRL)
 1.2QDL + 1.6 (QRoL or QSL or QRL) + f1 (QLL
or 0.8QWL)
 1 .2QDL + l .6QWL + f1QLL + 0.5 (QRoL or
QSL or QWL)
 1.2QDL + 1.6QEL + f1 QLL+ f2QSL
 0.9QDL + (1.0 QEL or 1.6QHL)
where:
f1 = 1 for floors in places of public assembly
for live loads above 4.8 k Pa and parking garage
live load
= 0.5 for other live loads
f2 = 0.7 for roof configurations (such as saw
tooth) that do not shed snow off the structure
= 0.5 for another roof configuration.
Table 3: Load factors for different types of load and geosystems.
Sr. No. Type of Loads
Load Factor
Maximum Minimum
1 Live Load (QLL) 1.75 1.35
2 Dead Load (QDL) 1.25 0.90
3 Down drag QDD 1.80 0.45
4 Wearing surface and utilities QDW 1.50 0.65
5 Horizontal Earth Pressure (QEL)
Active 1.5 0.90
Passive 1.35 0.90
6 Vertical Earth Pressure (QEV)
 Retaining structure 1.35 1.00
 Rigid buried structure 1.30 0.90
 Rigid frames 1.35 0.90
 Flexible Buried Structure 1.95 0.90
 Flexible metal box culvert 1.50 0.90
7 Earth Surcharge (QES) 1.50 0.75
Data Source: AASHTO, 2004
Because there is insufficient data to use
reliability theory effectively, current LRFD
issues focus on resistance factors (also known as
performance factors). As a result, resistance is
proposed using experience and judgment, as well
as reliability theory and limited data sets. The
performance factors may change as more high-
quality field and laboratory test data becomes
available. Furthermore, in reliability theory, the
correct or expected value is assumed to be
known a priori, which is not the case for soils.
An alternative method to LRFD called
the factored strength method (FSM) or partial
factor method (PFM) is used in some countries.
In FSM or PFM. a factor (< 1) is applied to the
soil strength parameters rather than the
calculated soil resistance.
ASD and LRFD to Satisfy SLS
When calculating geosystem movements
to satisfy the serviceability limit slate, a load
factor of one is used for all types of loads,
regardless of whether ASD or LRFD is used.
Meyerhof (1995) proposed a preliminary
serviceability limit state for structure rotation
(Table 4).

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Table 4: Rotation limits of different structures.
S
r.
N
o.
Relative
rotation
δ/L2
Type of Geosystem
1 1/100 The safety limit for statically determinate structures and retaining walls.
2 1/150
The safe limit for Statically determinate structures with flexible cladding and
retaining walls. Danger limit for open steel and reinforced conc. Frames, offshore
platforms, steel storage tanks, etc.
3 1/250
The safe limit for open steel and reinf. Conc. Frames, offshore platforms, etc.
danger limit for panel walls of frame structures, and tilt of bridge abutments.
Tilting of high-rise buildings may become visible.
4 1/300 Limit when difficulties with overhead cranes are to be expected
5 1/500 Panel walls of frame buildings and tilt of bridge abutments
6 1/1000 Sagging of unreinforced load-bearing walls.
7 1/2000 Hogging of unreinforced load-bearing walls.
δ is the differential settlement along the length of L
Reference: Canadian Foundation Engineering Manual
https://www.academia.edu/43064024/CANADIAN_FOUNDATION_ENGNEERING_MANUA
L_4th_ED_TION_CANADIAN_GEOTECHNICAL_SOCIETY_2006
DISCUSSION
Which Design Approach should be HIGHLY
applicable?
ASD is also known as a conservative
design. Because it has been the method used in
geotechnical practice for the past five decades.
In structural engineering, LRFD is a popular
choice. LRFD is becoming increasingly popular
in geotechnical design, owing to the desire for
consistent, coherent, and compatible design
methodologies in structures and geotechnical
engineering. On numerous large projects, a
structural engineer would provide the structural
loads to the geotechnical engineer, and the
geotechnical engineer's main focus would be the
resistance factors, stresses in soil, and water
pressures. Although the designer intended to
take into account analysis uncertainties, soil
properties, methods for obtaining them,
construction practice, and other factors, the
resistance factors in LRFD have been changed to
match the design attained by ASD using safety
factors consistent with good engineering practice
and experience. As a result, designs produced
using LRFD and ASD are probably similar.
Since LRFD makes an effort to logically
address the numerous unpredictable aspects of
materials, construction, and analysis, it is
expected to develop into the preferred approach
of design as it gets more advanced. The stresses
or loads applied to the structural system must not
be more than the strength or load-carrying
capability (collapse load) of the earth, which
applies to both the ASD and LRFD design
approaches. To ensure that the serviceability
limit state is not exceeded, displacements
necessitate a separate study. The design of many
geosystems is governed by serviceability limit
states rather than ultimate limit states (e.g.,
shallow foundations and retaining walls).
For geosystem design, a displacement-
based method is preferable to a strength-based
method. However. A fundamental understanding
of soil-structure interactions is necessary for a
displacement-based design. Precise
measurement of soil parameters, accurate
modeling of soil stress-strain behavior, and close
recording of actual earth system displacements.
These studies are still in progress and are not yet
ready for use in customary engineering design
and norms. Numerical approaches such as the
finite difference method or the finite element
method are appropriate for displacement-based
design, but they necessitate a thorough
understanding of the numerical method used as
well as precise input parameters (especially soil
properties). To evaluate the results, an accurate
soil model and experience are required. In the
preliminary design stage, equations based on
limit equilibrium are typically employed to
determine the ultimate limit load, followed by
numerical approaches if the relevance and risk

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connected with the structure merit involved
analyses.
The Key Points are:
 The allowable stress design approach does
not explicitly account for uncertainty in
load and soil parameters. To reduce the
possibility of failure, a factor of safety (an
arbitrary, subjective value based on
experience) is applied to the ultimate
resistance.
 The load and resistance factor design
method take into account the uncertainties
of the load and soil resistance.
 LRFD is calibrated against ASD using
safety factors following good engineering
practices. LRFD and ASD with the
appropriate safety factor should produce the
same design.
DETAIL PROCEDURE TO START WITH
THE BEST SUITABLE DESIGN
APPROACH
How can I begin a design? Here is a list of
potential answers to this question:
 Collect as much information about the
project as possible, such as its location,
purpose, loads, importance, and
surrounding buildings. Environmental
consequences, cost projections, and so on.
 Go to the location and dig a few test pits or
shallow borings.
 Preliminary investigations should be
performed by utilizing existing geotechnical
data and the presumed soil parameters in
reference books, as well as those in codes
or technical manuals. Consider a variety of
soil factors that you believe characterize
your soil. For example, if the soil at the site
is soft clay and you only have water content
data from the test pits, you can estimate
values of undrained shear strength and
friction angle using empirical equations.
When you finish the preliminary analysis,
you should be able to limit your selection of
geosystems and appreciate the geosystem's
sensitivity to changes in soil values. If there
is a structure similar to the one you are
planning on the site or nearby, you can run
back analyses to obtain preliminary soil
parameters and evaluate the structure for
any apparent concerns such as cracks,
differential settlement, and so on. This will
assist you in determining the scope of the
soil research and the sort of geosystem that
may be necessary.
 Prepare and carry out a soil investigation.
Your preliminary assessments should assist
you in determining the soil parameters to
gather as well as the scope of the soil
inquiry. You must choose the soil
parameters that will be used in a design.
This is a crucial phase since the soil
strength (friction angle and undrained shear
strength) and anti-deformation (elastic and
shear moduli) characteristics directly affect
the ultimate and serviceability limit states.
 Poor soil parameter estimation could lead to
(a) an imperfect system: (b) failure-ultimate
limit state reached or exceeded: (c)
structure failing to meet its design function-
serviceability limits state exceeded: and (d)
loss of property and life. Design the
geosystem to comply with the appropriate
code of practice. For example. IBC (2006).
Remember that in executing your design
you should consider how the system would
be constructed.
 Any value derived from computations
should not be regarded as an absolute value.
Any computed value, such as settlement,
should instead be treated as an
approximated expected value. You should
investigate what-if scenarios, paying
specific attention to the effects of soil
characteristics and groundwater conditions.
It is best to examine a range of predicted
expected values based on prior experience.
Consider the consequences of the
conceivable range of variations in input soil
characteristics instead of experience. For
instance, if the friction angle is uncertain,
how would the design alter (system type,
geometry, constructability, and costs) if the
inaccuracy is ± 5°?
SUMMARY AND CONCLUSION
The foundation design process is always
a unique task as every geotechnical problem is
unique; hence versatile solution for the adoption
of the design approach is still difficult.
Optimization in the design process makes it
further complicated. A framework for the

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selection of the design approach has been
established. All the design approaches should be
incorporated separately and never include two
different approaches into a single geosystem
design. The output values coming from different
design approaches should be analyzed with
conservativeness or non-conservativeness. For
the LRFD method, the factors incorporated must
be studied and analyzed with reliability analysis.
The compatibility of resistance factors versus
load factors must be studied before implication
in the design procedure.
ABBREVIATIONS
LS : Limit State
ASD : Allowable Stress Design
ULS : Ultimate Limit State
LRFD : Load and Resistance Factor
Design
CFEM : Canadian Foundation
Engineering Manual
AASHTO : American Association of State
Highway and Transportation
Officials
IBC : International Building Code
FSM : Forced Strength Method
PFM : Partial Factor Method
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