Design of Steel Structures
Department of Civil EngineeringUniversity of Engineering & Technology, Taxila
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Chapter – 1
Fundamentals of Steel Design
Design of Steel Structures
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Basic Design Equation
• In design, the applied forces and moments due to external loads are equated to the maximum resistive forces and moments with a FOS which is always greater than or equal to one.
• The concept may be summarized by the
following design equation:
Load Effects X Factor of safety (F.O.S)
Max. Internal Resistance offered by Material of the Structure
=
3
Basic Design Equation
• Load effects are defined as the forces,
stresses and deformations produced in a
structural component by the applied loads.
• A simply supported beam of span L subjected
to a point load P can be analyzed to get the
maximum bending moment of PL/4.
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Basic Design Equation
• However, this bending moment will only be
produced if the material of the beam is strong
enough to develop the required strength.
• This means that the answer of analysis may
be true for bigger steel girder but may not be
true for small wooden batten.
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Factor of Safety
• Factor of safety is required to bring the
structure from the state of collapse to a
usable state. It additionally covers the
following aspects:
1. Uncertainties in applied forces or loads.
2. The deflections should be small at service
load conditions.
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Factor of Safety
3. To cover uncertainties in material strength.
4. To cover, in part, poor workmanship.
5. To cover unexpected behavior in case the
theory is not fully developed.
6. To cover natural disasters.
7. The stresses produced during fabrication and
erection.
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Factor of Safety
8. Presence of residual stresses and stress
concentrations.
In case of allowable stress design, the factor
of safety is applied in the form of safety factor
(Ω), while in case of LRFD, it is applied in the
form of overload factors and the resistance
factor (Φ).
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Nominal Strength
• Nominal strength (Rn) is defined as the
strength of a structure or component to resist
load effects determined by using formulas
given in the specifications.
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In name only
Types of Design
• Load and Resistance Factor Design (LRFD),
Strength Design or Limit State Design
• Allowable Stress Design (ASD)
• Plastic Design
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1. Load & Resistance Factor Design (LRFD)
• Major part of FOS is applied on load actions
called overload factor.
• Minor part of FOS is taken on RS of design
equation called resistance factor or capacity
reduction factor (ø).
• Resistance factor (ø) is lesser than or equal
to 1.0 and is applied on material strength.
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1. Load & Resistance Factor Design (LRFD)
• The design equation is checked for each
strength and serviceability limit states one-
by-one.
• Limit state is defined as the limiting stage in
the loading after which the structure
cannot fulfill its intended function due to
strength or serviceability considerations.
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1. Load & Resistance Factor Design (LRFD)
• Analysis of structures for loads is performed
considering the structure to be within elastic
range.
• However, inelastic behavior, ultimate failure
modes and redistribution of forces after
elastic range are considered in this method.
• This is more realistic design as compared
with the old Allowable Stress Design.13
1. Load & Resistance Factor Design (LRFD)
• Nominal strength (Rn) is defined as the
strength of the structure or its component
determined by using formulas given in
specifications.
• Any particular load effect increased by the
load factors is called the Required Strength
(Ru).
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1. Load & Resistance Factor Design (LRFD)
• The nominal strength reduced by the
resistance factor (ΦRn) is called the Design
Strength.
• The design equation in case of LRFD
becomes:
Ru ≤ (ø)Rn
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Advantages of Using LRFD
• LRFD is another tool for steel design, which
provides a flexibility of options to the designer
in selecting the design methodology.
• Economical in case dead loads are larger,
compared with live loads.
• Every type of load may be given a different
FOS depending upon its probability of
overload, number of severe occurrences and
changes in point of application.16
Advantages of Using LRFD
• Behavior at collapse including ductility,
warning before failure and strain hardening
etc.
This is not directly possible in ASD because
here the structure is considered at service
stage and not approaching close to collapse.
• More safe structures result due to better
awareness of behavior near collapse.17
Advantages of Using LRFD
• Plastic design concepts may conveniently be
employed in LRFD Method.
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Disadvantages of Using LRFD
• Elastic behavior considered for load analysis
and ultimate plastic behavior taken for
material strengths are not compatible,
however, percentage difference is less.
• Engineers experienced in ASD have to
become familiar with this technique.
• Old books and design aids become
ineffective.19
Disadvantages of Using LRFD
• Validity of previous designs is still to be
checked according to ASD.
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2. Allowable Stress Design (ASD)• F.O.S is taken on right side of the basic
design equation. This is denoted by Ω.
• Allowable strength (Rn/Ω) is defined as the
nominal strength divided by the safety factor.
Loads Effects Material Resistive Forces
FOS=
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2. Allowable Stress Design (ASD)
• Required ASD Strength (Ra) is the load
effect obtained from the service loads without
any additional factor.
• The design equation for ASD becomes:
• This method is now gradually replaced by
LRFD for the structures, where behavior near
collapse is fully understood.
Ra ≤ Rn/Ω
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2. Allowable Stress Design (ASD)• It is still preferred by some engineers for
important structures like atomic reactors and
pre-stressed concrete.
• It is included in the specifications as an
alternate method of design.
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Advantages of ASD• Elastic analysis for loads and elastic material
behavior compatible for the design.
• Senior engineers are used to this method.
• Old famous books are according to this
method.
• Was the only design method in past.
• Is included as alternate design method in
AISC-05 Specifications.
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Disadvantage of ASD• Latest research and literature is very much
limited.
• Same factor of safety is used for different
loads.
• The failure mode is not directly predicted.
• With some overloading, the material stresses
increases but do not go to collapse. (The
failure mode cannot be observed).
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Disadvantage of ASD• The ductility and warning before failure
cannot be studied precisely.
• Results cannot be compared with
experimental tests up to collapse.
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3. Plastic Design• It is somewhat similar to the LRFD but here
the analysis for loads is performed
considering the collapse mechanism of the
structure.
• Full reserve strength due to indeterminacy of
the structure and inner elastic portion of the
structure is utilized.
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3. Plastic Design• Inelastic material behavior is considered in
the analysis and design.
• Deflections and other serviceability
conditions become more important along with
the strength requirements.
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DESIGN STRENGTH
• In LRFD, design strength of all elements is
obtained as resistance factor multiplied with
maximum stress that can be developed
multiplied with sectional area or section
modulus.
• The design strength is also called the load
capacity, or sometimes only capacity, of a
member.
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DESIGN STRENGTH
• An example to explain the difference between
the member capacity and the applied load is
that of a bottle.
• This bottle may have a fixed liquid retaining
capacity of suppose 1 litre.
• However, it may be empty at times meaning
that the amount of liquid retained in it is zero
litres but the capacity of the bottle still
remains the same. 30
DESIGN STRENGTH
• Any amount of liquid may be poured in this
bottle that is not exceeding 1 litre.
• Similarly, load capacity of a member exists
with a fixed value.
• The applied load may have a different value
with only one condition that the applied load
must be lesser than or equal to the member
capacity for stability.
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CAPACITY ANALYSIS OF
STRUCTURES
• Knowing the material properties and
dimensions of the member, finding the
maximum loads that can be applied on the
member using the design equation is called
Capacity Analysis or Analysis of
Structures.
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DESIGN OF STRUCTURES
• Knowing the expected loads and span
lengths of the members in the basic design
equation, finding the required material
properties and cross-sectional dimensions is
called Design of Structures.
• In steel structures, the design mainly consists
of a selection out of already available
sections in the market.
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DESIGN OF STRUCTURES
• Structural Design may be defined as “a
mixture of art and science, combining the
experience and intuitive feeling for the
behavior of the structure with a sound
knowledge of the principles of statics,
dynamics, mechanics of materials, and
structural analysis, to produce a safe
economical structure which will serve its
intended purpose.”
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Objectives of Structural Designer
• Design is a process by which an optimum
solution is obtained satisfying certain criteria.
• Minimum cost
• Minimum weight
• Minimum construction time
• Minimum labour
• Maximum efficiency of operation
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Objectives of Structural Designer
• The structural designer must learn to arrange
and proportion the parts of his structures so
that they can be practically erected and will
have sufficient strength and reasonable
economy.
• These important items, called safety, cost
and practicability are discussed next:
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Objectives of Structural Designer
1. The structure must safely support the loads
to which it is subjected.
The deflections and vibrations should not be
so excessive as to frighten the occupants.
2. The designer must keep the construction,
operation and maintenance costs at the
lowest levels without sacrificing the strength.
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Objectives of Structural Designer
3. Designers need to understand fabrication
methods and should try to fit their work to the
available fabrication facilities, available
materials and the general construction
practices.
Some designers lack in this very important
aspect and their designs cause problems
during fabrication and erection.38
Objectives of Structural Designer
• Designer should learn everything possible
about the detailing, the fabrication and the
field erection of steel besides the loads,
mechanics, and the expected material
strengths.
• The designer must have information
concerning the transportation of the
materials to site, labor conditions,
equipment for erection 39
Objectives of Structural Designer
problems at site, field tolerances and the
required clearances at the site.
• This knowledge helps to produce reasonable,
practical and economical designs.
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Procedure of the Structural Design
• The structural framework design is the
selection of the arrangement and sizes of
structural elements so that service loads
may be safely carried.
• Structural designer has to complete the
following steps to get a successful design:
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Procedure of the Structural Design
• The general layout of the structures.
• Studies of the possible structural forms that
can be used.
• Consideration of loading conditions.
• Analysis of stresses and deflections, etc.
• Design of parts.
• Design of assembly and connections.
• Preparation of design drawings.42
• The above design procedure for a whole structure requires iterations and the main steps are listed below:
1. The functions to be performed by the structure and the criteria for optimum solution of the resulting design must be established. This is referred to as the planning stage.
2. The general layout of the structure is decided.
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3. Different arrangements of various elements to serve the functions in step 1 are considered.
The possible structural forms that can be used are studied and an arrangement appearing to be best is selected for the first trial, called preliminary structural configuration. Only in very rare cases, it has to be revised later on.
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4. Loading conditions are considered and the
loads to be carried by the structure are
estimated.
5. Based on the decisions of the earlier steps,
trial selection of member sizes is carried
out depending on thumb rules or assumed
calculations to satisfy an objective criterion,
such as least weight and cost.
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6. Structural analysis involving modeling the loads and the structural framework to obtain internal forces, stresses and deflections is carried out.
7. All strength and serviceability requirements along with the predetermined criteria for optimum are checked. If any check is not satisfied, the member sizes are revised. This stage is called evaluation of the trial member sizes.
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8. Repetition of any part of the above sequence
found necessary or desirable as a result of
evaluation is performed in this stage called
redesign.
9. The rivets, bolts and welds along with other
joining plates and elements are designed.
The process is termed as the design of
assembly and connections.
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10.It is determined whether or not an optimum
design has been achieved, and the final
decision is made.
11.Drawings are prepared to show all design
details. An estimate for the required
quantities is also made. This stage of design
is called preparation of design documents.
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Procedure of the Structural Design
• The important sub-steps in the design of
parts (step 7 above) are shown in the form of
a flow chart in Fig 1.1
• Objectives of the design must always be kept
in mind while using this flow chart.
• The selection of trial section in step 2
depends on the main objectives, availability
of material, construction requirements and
compatibility with other members. 49
Collect and list all the known data
Select trial sectionbased on assumed stresses/
effectiveness of cross-sectionalalternatively, selection tables may be used
Apply all stability checks
Perform strength checks
Perform serviceability checks
Accept section if all checks aresatisfied, other-wise revise
Write Final Selection
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Limit State
• Limit state is defined as the stage in the
loading after which the structure cannot
fulfills its intended function due to strength
or serviceability considerations.
• The term limit state is preferred compared
with failure because in most cases of limit
states, the actual failure or collapse does not
occur.
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Limit State
• Limit states are generally divided into two
categories, strength and serviceability.
• Strength or safety limit states means
conditions of loading corresponding to
maximum ductile flexural strength (i.e.,
plastic strength), ultimate ductile shear
strength, buckling, fatigue, fracture,
overturning and sliding, etc.
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Limit State
• Serviceability limit states are those
concerned with occupancy of the building,
such as the deflection, vibration, permanent
deformation and cracking.
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Limit State
• The structure should not cross any strength
or serviceability limit for a perfect design. All
the applicable limits are to be checked by
using the available procedures.
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End of File
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