N-W.F.P. University of Engineering and
Technology Peshawar
Steel Structures
CE-409
By: Prof Dr. Akhtar Naeem Khan
chairciv@nwfpuet.edu.pk
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Course Content
 Design philosophies
 Introduction to Steel Structures
 Design of Welded connections
 Design of Bolted connections
 Design of Tension Members
 Design of Compression Members
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Prof. Dr Akhtar Naeem Khan
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Course Content
 Design of Column Bases
 Design of Beams
 Design of Composite Beams
 Design of Plate Girders
CE-411: Lecture 01
Prof. Dr Akhtar Naeem Khan
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N-W.F.P. University of Engineering and
Technology Peshawar
Lecture 01: Design Philosophies
By: Prof Dr. Akhtar Naeem Khan
chairciv@nwfpuet.edu.pk
4
Topics to be covered
 Design philosophies
 Limit States
 Design Considerations
 Allowable Stress Design (ASD)
 Load and Resistance Factor Design (LRFD)
 Design process
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Design Philosophies
 A general statement assuming safety in
engineering design is:
 Resistance ≥ Effect of applied loads ---(1)
 In eq(1) it is essential that both sides are
evaluated for same conditions and units e.g.
compressive stress on soil should be
compared with bearing capacity of soil
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Design Philosophies
 Resistance of structures is composed of
its members which comes from
materials & X-section
 Resistance, Capacity, and Strength are
somewhat synonym terms.
 Terms like Demand, Stresses, and
Loads are used to express Effect of
applied loads.
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Limit States
 When particular loading reaches its
limit, failure is the assumed result, i.e.
the loading condition become failure
modes, such a condition is referred to
as limit state and it can be defined as
 “A limit state is a condition beyond
which a structural system or a structural
component ceases to fulfill the function
for which it is designed.”
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Limit States
 There are three broad classification of
limit states:
1. Strength limit states
2. Serviceability limit states
3. Special limit states
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Limit States
Strength Limit States:
• Flexure
• Torsion
• Shear
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Fatigue
• Settlement
• Bearing
•
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Limit States
Serviceability Limit States:
• Cracking
• Excessive Deflection
• Buckling
• Stability
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Limit States
Special Limit States:
 Damage or collapse in extreme
earthquakes.
 Structural effects of fire, explosions, or
vehicular collisions.
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Limit States
 Design Approach used must ensure that
the probability of a Limit State being
reached in the Design/Service Life of a
structure is within acceptable limits;
 However, complete elimination of
probability of a Limit State being
achieved in the service life of a structure
is impractical as it would result in
uneconomical designs.
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Design Considerations
• Structure and Structural Members should
have adequate strength, stiffness and
toughness to ensure proper functioning
during service life
• Reserve Strength should be available to
cater for:
–
Occasional overloads and underestimation of loads
–
Variability of strength of materials from those specified
–
Variation in strength arising from quality of
workmanship and construction practices
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Design Considerations
 Structural Design must provide adequate
margin of safety irrespective of Design
Method
 Design Approach should take into account
the probability of occurrence of failure in
the design process
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Design Considerations
 An important goal in design is to prevent
limit state from being reached.
 It is not economical to design a structure
so that none of its members or
components could ever fail. Thus, it is
necessary to establish an acceptable level
of risk or probability of failure.
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Design Considerations
 Brittle behavior is to be avoided as it will
imply a sudden loss of load carrying
capacity when elastic limit is exceeded.
 Reinforced concrete can be made
ductile by limiting the steel
reinforcement.
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Design Considerations
 To determine the acceptable margin of
safety, opinion should be sought from
experience and qualified group of
engineers.
 In steel design AISC manuals for ASD &
LRFD guidelines can be accepted as
reflection of such opinions.
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Design Considerations
 Any design procedure require the
confidence of Engineer on the analysis of
load effects and strength of the materials.
 The two distinct procedures employed by
designers are Allowable Stress Design
(ASD) & Load & Resistance Factor
Design (LRFD).
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Allowable Stress Design
(ASD)
 Safety in the design is obtained by
specifying, that the effect of the loads
should produce stresses that is a fraction
of the yield stress fy, say one half.
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Allowable Stress Design
(ASD)
• This is equivalent to:
FOS = Resistance, R/ Effect of load, Q
= fy/0.5fy
=2
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Allowable Stress Design
(ASD)
 Since the specifications set limit on the
stresses, it became allowable stress
design (ASD).
 It is mostly reasonable where stresses
are uniformly distributed over X-section
(such on determinate trusses, arches,
cables etc.)
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Allowable Stress Design
(ASD)
Mathematical Description of A S D
 Rn


Q
i
Rn = Resistance or Strength of the component being designed
Φ = Resistance Factor or Strength Reduction Factor

= Overload or Load Factors

 = Factor of Safety FS
Qi = Effect of applied loads
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Allowable Stress Design
(ASD)
Mathematical Description of Allowable Stress Design
In ASD we check the adequacy of a design in terms of stresses
therefore design checks are cast in terms of stresses for
example if:
Mn = Nominal Flexural Strength of a Beam
M = Moment resulting from applied unfactored loads
FS = Factor of Safety
Mn
FS
Fy I / c
FS I / c
 M
fb
Fy

  Fb 
FS

CE-411:Lecture No. 1
or

M
I /c
Fcr 
Fb 

FS 
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Section Modulus

Section Modulus:
S ≥ effect of load/Allowable stress
= M/fb ------(ii)
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ASD Drawbacks
 Implied in the ASD method is the
assumption that the stress in the
member is zero before any loads are
applied, i.e., no residual stresses exist
from forming the members.
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Variation of Residual Stress
with Geometry
Material A has more Residual Stresses due to:
1. Non uniform cooling
2. Cutting a plate into smaller
pieces reveals the stresses
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ASD Drawbacks
• ASD does not give reasonable measure
of strength, which is more fundamental
measure of resistance than is allowable
stress.
• Another drawback in ASD is that safety
is applied only to stress level. Loads are
considered to be deterministic (without
variation).
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Load and Resistance Factor
Design (LRFD)
 To overcome the deficiencies of ASD,
the LRFD method is based on:
Strength of Materials
 It consider the variability not only in
resistance but also in the effects of load.
 It provides measure of safety related to
probability of failure.
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Load and Resistance Factor
Design (LRFD)

Safety in the design is obtained by specifying that the reduced
Nominal Strength of a designed structure is less than the effect
of factored loads acting on the structure
 Rn  n Qi
Rn = Resistance or Strength of the component being designed
Qi = Effect of Applied Loads
n = Takes into account ductility, redundancy and operational imp.
Φ = Resistance Factor or Strength Reduction Factor
 = Overload or Load Factors


= Factor of Safety
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The role of ‘n’
Ductility: It implies a large capacity for inelastic
deformation without rupture

Ductility will ensure
redistribution of load through
inelastic deformation.
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The role of ‘n’
Redundancy:
1. A simply supported beam is a
determinate structure so it has no
redundant actions.
2. A fixed beam is indeterminate by 2
degrees so it has two redundant actions.
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Redundancy
Yielding will initiate at mid span due to maximum moment at mid span
with no Redistribution of load
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Redundancy
Yielding will initiate at supports due to maximum moment at supports
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Redundancy
Redistribution of load to mid span after yielding of section at supports
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The role of ‘n’
Operational Importance:
A hospital and a school require more
conservative design than an ordinary
residential building.
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Operational Importance
→ hospital
→ park
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LRFD Advantages
 LRFD accounts for both variability in
resistance and load.
 It achieves fairly uniform levels of
safety for different limit states.
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LRFD Disadvantages
 It’s disadvantage is change in design
philosophy from previous method.
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Comparison of ASD and
LRFD Design Approaches
 ASD combines Dead and Live Loads
and treats them in the same way
 In LRFD different load factors are
assigned to Dead Loads and Live Loads
which is appealing
 Changes in load factors and resistance
factors are much easier to make in
LRFD compared to changing the
allowable stress in ASD
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Comparison of ASD and
LRFD Design Approaches
 LRFD is intrinsically appealing as it
requires better understanding of behavior
of the structure in its limit states
 Design approach similar to LRFD is being
followed in Design of concrete structures
in form of Ultimate Strength Design -- why
not use similar approach design of steel
structures?
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Comparison of ASD and
LRFD Design Approaches
 ASD indirectly incorporates the Factors
of Safety by limiting the stress whereas
LRFD aims to specify Factors of Safety
directly by specifying Resistance
Factors and Load Factors
 LRFD is more rational as different
Factors of Safety can be assigned to
different loadings such as Dead Loads,
Live Loads, Earthquake Loads and
Impact Loads
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Comparison of ASD and
LRFD Design Approaches
 LRFD considers variability not only in
resistance but also in the effects of load
which provides measure of safety related
to probability of failure
 It achieves fairly uniform levels of safety
for different limit states.
 ASD still remains as a valid Design
Method
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Comparison of ASD and
LRFD Design Approaches
In LRFD For Tension Members:
1.2D + 1.6 L = 0.90 Rn  1.33D + 1.78 L = Rn (LRFD)
In ASD Factor of Safety FS = 1.67, Therefore:
1.0D + 1.0 L = Rn / 1.67  1.67D + 1.67D L = Rn (ASD)
LRFD 1.33D  1.78L

ASD 1.67 D  1.67 L

0.8  1.07 ( L / D)
1  ( L / D)
…. (A)
In LRFD For Dead Load Case:
1.4D = 0.90 Rn  1.56D = Rn (LRFD)
LRFD
1.56 D

ASD 1.67 D  1.67 L
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
0.93
1  ( L / D)
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…. (B)
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Comparison of ASD and
LRFD Design Approaches
3%
1.0
LRFD
ASD
0.93
0.9
1.2D + 1.6L
0.83
0.8
1.4D
0.7
0.12
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1
2
4
3
Live Load
Dead Load
5
6
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AREA Code for Design of
Railway Structures
 AREA Stands for American Railway
Engineers Association (AREA)
 Railway Bridges and Structures are usually
designed using provisions of the AREA
Code
 AREA Code uses only the Allowable Stress
Design Method. However, the allowable
stresses and design requirements may differ
from AISC/ASD method
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AASHTO Code for Design of
Highway Bridges
• AASHTO Stands for Association of American
State and Highway Transportation Officials
(AASHTO)
• Highway Bridges are usually designed using
provisions of the AASHTO Code
• AASHTO Code uses both ASD and LRFD
Design Methods
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The role of various Codes
 It is very difficult to devise a design code that is
applicable to all uses and all types of
structures such as buildings, highway bridges,
railway bridges and transmission towers
 The responsibility of infrastructure on roads,
bridges and electrical transmission towers
rests with the organization responsible for
approving, operating and maintaining these
facilities
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The role of various Codes
 Uses and critical loads may be different in
different types of structures and no one
code can cater to all the different important
considerations
 For above reasons different codes prevail
and will continue to do so
 AISC ASD Code and LRFD Code primarily
is pertinent to Building Structures.
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Overview of LRFD Manual
 Part 1: Dimensions and properties
 Part 2: General Design considerations
 Part 3: Design of flexural members
 Part 4: Design of compression members
 Part 5: Design of Tension members
 Part 6: Design of members subject to
combined loading
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Overview of LRFD Manual
 Part 7: Design considerations for bolts
 Part 8: Design considerations for welds
 Part 9: Design of connecting elements
 Part 10: Design of simple shear connections
 Part 11: Design of flexible moment
connections
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Overview of LRFD Manual
 Part 12: Design of fully restrained (FR)
moment connections
 Part 13: Design of Bracing connections and
truss connections
 Part 14: Design of Beam bearing plates,
Column base plates, anchor rods,
and column splices.
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Overview of LRFD Manual
 Part 15: Design of Hanger connections,
Bracket plates, and Crane-rail
connections
 ANSI/LRFD Specifications for structural
steel Buildings.
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Design Process
1. Functional planning
• Development of a plan that will enable the structure to
fulfill effectively the purpose for which it is to be built
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Design Process
The involvement of Structural engineer in the functional planning is very imp
because an Architect can suggest a plane which is practically not possible.
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Design Process
2. Structural scheme
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Design Process
2. Structural scheme (Contd.)
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Design Process
3. Preliminary Member Sizing of Beams
 Deflection Considerations
 ASD Commentary L3.1 suggests following
Limits:
CE-411:Lecture No. 1
L
D

L
D
L
D
 20

800
Fy ( Ksi )
800
Fy ( Ksi )
For fully stressed Beams & Girders
For Beams & Girders subject to
vibrations
For Roof Purlins
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Design Process
3. Preliminary Member Sizing of Beams
Design Moment
 Strength/Capacity Considerations
Beam
Unbraced Length
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Design Process
3. Preliminary Member Sizing of Columns
 Strength/Capacity Considerations
Tributary Area
• Use of Tributary Areas and
Column Tables
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Tributary Area
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Design Process
4. Structural Analysis - Modeling
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Design Process
4. Structural Analysis - Analysis
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Design Process
5. Design Review/ Member Modification
• Must be chosen so that they will be able to resist,
within appropriate margin of safety, the forces
which the structural analysis has disclosed.
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Design Process
6. Cost Estimation
• Make a tentative cost estimates for several
preliminary structural layouts.
• Selection of constructional material based on:
• Availability of specific material
• Corresponding skilled labor
• Relative costs
• Wage scales
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Design Process
7. Preparation of Structural Drawings & Specifications
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Thanks
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