Steel Structures CE-409

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Steel Structures CE-409

• By Prof Dr. Akhtar Naeem Khan
• chairciv_at_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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Course Content

• Design of Column Bases
• Design of Beams
• Design of Composite Beams
• Design of Plate Girders

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Lecture 01 Design Philosophies

• By Prof Dr. Akhtar Naeem Khan
• chairciv_at_nwfpuet.edu.pk

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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
• Strength limit states
• Serviceability limit states
• Special limit states

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Limit States

• Strength Limit States
• Flexure
• Torsion
• Shear

• 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 Resistance or Strength of the component
being designed F Resistance Factor or
Strength Reduction Factor Overload or
Load Factors
Factor of Safety FS
Qi Effect of applied loads
Prof. Dr. Akhtar Naeem Khan
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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
Prof. Dr. Akhtar Naeem Khan
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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 Resistance or Strength of the component
being designed Qi Effect of Applied Loads n
Takes into account ductility, redundancy and
operational imp. F 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
• A simply supported beam is a determinate
  structure so it has no redundant actions.
• 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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? hospital
Operational Importance
? 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

• Its 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)
. (A)
In LRFD For Dead Load Case 1.4D 0.90 Rn ?
1.56D Rn (LRFD)
. (B)
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Comparison of ASD and LRFD Design Approaches
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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

For fully stressed Beams Girders
For Beams Girders subject to vibrations
For Roof Purlins
Prof. Dr. Akhtar Naeem Khan
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Design Process
3. Preliminary Member Sizing of Beams

• Strength/Capacity Considerations

Prof. Dr. Akhtar Naeem Khan
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Design Process
3. Preliminary Member Sizing of Columns

• Strength/Capacity Considerations

Tributary Area

• Use of Tributary Areas and Column Tables

Prof. Dr. Akhtar Naeem Khan
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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