Design Philosophies: -
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Design of Steel
Structure consists of design of Steel members and their Connections. So that
they can safely and economically resist and transfer the applied load to the
ground.The design process
begins with Selection of trail section and Checking its Safety. The design of
Structural Steel elements is based on attendance on initial yielding.
i)
Attainment of full yielding
ii)
Tensile Strength
iii)
Critical buckling
iv)
Maximum Deflection permitted
v)
Stress Concentration
vi)
Fatigue
Vii)
Brittle Fracture
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The Design Philosophy
are used and listed below in order of their evolution: -
1)
Working
Stress Method
2)
Ultimate
Load Method
3) Limit State Method
Ultimate Tensile Strength:
-
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It is the max.
Stress that the material Can with Stand while being Stretched or pulled before failure\
breaking.
Yield Strength: -
Ø
It is the Stress at which the Stress-Strain
Curve for Axial Loading deviates the Strain of 0.2% from the linear elastic
line on the Stress Strain Curve become non-linear.
1) Working Stress Methods: -
Ø
It is the elastic
method of design.According to this
method, the members are designed on basis of working Stress & those will
never exceed permissible Stress according to Code.
Ø
“A permissible Stress" is defined as the ratio
of yield Stress according to factor of safety.
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Permissible Stress < Yield Stress
FOS
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LIMITATIONS OF WORKING STRESS METHOD: -
Ø According to this
method, Failure load is factor of safety times working load; which is not true.
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Factored Load = Working Load x FOS < Ultimate
Load
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Ø Actually, it is more because a material can resist the load after yield apparatus at a fibre.In Structures, just formation of plastic hinge is not the Collapse criteria. Since, it can resist load till some more hinges formed resulting into Collapse mechanism.It gives An Economical section.It deals only with elastic behaviour of matter.The Strength of the section at the working Load is estimated from the yield Strength of the section.
ADVANTAGES OF WORKING STRESS METHOD: -
Ø
The Method is
simple and reliable.
2) Ultimate Load Method: -
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From the Stress Strain
curve, it is observed that higher loads than elastic method Can be applied in
the structure. This is due to the fact that a major portion of the Curve lies
beyond the elastic limit. This Strength is
Called “Deserved Curve” and based upon this strength plastic design is made. This method is based
on failure Conditions rather than working load condition.The Strength of the
section is estimated from ultimate Strength of the section. In plastic design
method, the working loads are multiplied by a load factor, get the collapse
load and the members are designed on the basis of collapse Strength.Since, actual load
should be less than the Collapse Strength by a factor of safety; the members
designed should be Safe.
ADVANTAGES ULTIMATE LOAD METHOD: -
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Re- distribution of
internal forces is accounted and considered.
DISADVANTAGES ULTIMATE LOAD METHOD: -
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It does not
guarantee Serviceability performances like deflection, instability, Crack-width
and fatigue etc.
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So, to take care of
design requirements from Strength and Serviceability criteria, limit state method
is developed.
3) Limit State Method: -
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The acceptable
limit for the safety & serviceability requirements before failure occurs is
called “Limit State”.It is Similar to
plastic designs which consider most critical limit state of Strength and
Serviceability.The section design Should
Satisfy serviceability requirements Such as limitations of deflection,
vibration and Shouldn't Collapse under Accidental loads.The Limit State are
Classified as:
1)
Limit
State of Strength
2)
Limit
State of serviceability
1) Limit State of Strength:
-
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For Checking the
Strength and Stability of Structure the loads are multiplied by relevant load
factor (ɣf)
given in IS 800:2007 tale no.04.The modified loads
are called “factor loads” account for the uncertainties involved in estimating
the magnitude of dead and live loads.The design Strength of members or its
Connections are determined by dividing Ultimate Strength with respect to
partial Safety factor (ɣm)
for material given in IS 800:2007 tables.
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It includes:
i)
Plastic Collapse.
ii)
Stability against sway, overturning & sliding
iii)
fatigue
2) Limit State of serviceability:
-
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It is limit State
beyond which the service criteria such as defection, vibration, repairable
damage due to fatigue, corrosion, fire resistance is no longer made. Load factor (ɣf)
is used for all loads to check Serviceability requirements.
Limit State
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Load Factor
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Strength
Serviceability
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>1.0
1.0
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Ø
It includes:
i)
Deflection
ii) Durability
iii) Vibration
iv) Fire resistance