steel th 2
TRANSCRIPT
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Design of Steel Structures
Department of Civil Engineering
University of Engineering & Technology, Taxila
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Chapter1
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
=
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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 ofPL/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
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
(), while in case of LRFD, it is applied in theform of overload factors and the resistance
factor ().
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Nominal Strength
Nominal strength (Rn) is defined as the
strength of a structure or component to resistload effects determined by using formulas
given in the specifications.
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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 designequation called resistance factor or capacity
reduction factor ().
Resistance factor () 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 tostrength 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
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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 theload 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 (Rn) is called the Design
Strength.
The design equation in case of LRFD
becomes:
Ru ()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
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Advantages of Using LRFD
Behavior at collapse including ductility,
warning before failure and strain hardening
etc.
This is not directly possible in ASDbecause
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.
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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.
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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 basicdesign equation. This is denoted by .
Allowable strength (Rn/) is defined as thenominal strength divided by the safety factor.
Loads Effects Material Resistive ForcesFOS
=
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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/
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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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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 stressesincreases 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 loadcapacity, 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
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DESIGN STRENGTH
Any amount of liquid may be poured in this
bottle that is not exceeding 1 litre.
Similarly, load capacity of a member existswith 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
Capaci ty Analys is or Analys is 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 Structu res.
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 practicabilityare 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 levelswithout 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.
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Objectives of Structural Designer
Designer should learn everything possible
about the detailing, the fabricationand 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
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Objectives of Structural Designer
problems at site, field tolerances and the
required clearancesat 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 thefollowing steps to get a successful design:
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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 notsatisfied, the member sizes are revised. This
stage is called evaluationof the trial member
sizes.46
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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
decisionis 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 2depends on the main objectives, availability
of material, construction requirements and
compatibility with other members. 49
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Collect and list all the known data
Select trial section
based on assumed stresses/effectiveness of cross-sectional
alternatively, 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 functiondue to strength
or serviceability considerations.
The term limit state is preferred compared
with failure because in most cases of limitstates, 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 meansconditions of loading corresponding to
maximum ductile flexural strength (i.e.,
plastic strength), ultimate ductile shearstrength, 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