Design of Steel , limit state method to find steeldesign
1. 1
Limit State Method of Design
Name : Jatin Majhi Roll No: 34701322007 Subject Name : Design of Steel Structures Subject Code : CE(PC)604
2. 2
Designer has to ensure the structures, he
designs are:
• Fit for their purpose
• Safe
• Economical and durable
INTRODUCTION
Name : Jatin Majhi Roll No: 34701322007 Subject Name : Design of Steel Structures Subject Code : CE(PC)604
3. 3
• Uncertainties affecting the safety of a
structure are due to:
uncertainty about loading
uncertainty about material strength and
uncertainty about structural dimensions
and behaviour
INTRODUCTION-2
Name : Jatin Majhi Roll No: 34701322007 Subject Name : Design of Steel Structures Subject Code : CE(PC)604
4. 4
Frequency
Curve (a)
Risk of failure
Variation of resistance
(R.M.) between nominally
identical materials
Load used in calculation Strength (or
resistance)
used in
calculations
Curve (b)
Variation of maximum life
time load effects (B.M.)
Statistical Meaning of Safety
Load effects divided
by
Resistance moment
INTRODUCTION-3
Name : Jatin Majhi Roll No: 34701322007 Subject Name : Design of Steel Structures Subject Code : CE(PC)604
5. 5
• Uncertainties in service life are due to:
Variability of the loads
Variability of the load distribution through the structure
• Characteristic resistance:
Value of resistance below which not more than a prescribed
percentage of test results may be expected to fall
• Characteristic load:
Value of the load, which has an accepted probability of not
being exceeded during the life span of the structure.
INTRODUCTION - 4
Name : Jatin Majhi Roll No: 34701322007 Subject Name : Design of Steel Structures Subject Code : CE(PC)604
6. Actions
• Permanent Actions Qp
• Variable Actions Qv
• Accidental Actions Qa
6
Name : Jatin Majhi Roll No: 34701322007 Subject Name : Design of Steel Structures Subject Code : CE(PC)604
7. Characteristic Actions Qc
• Are those values of different
actions that are not expected to be
exceeded with more than 5 %
probability during the life of the
structure
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Name : Jatin Majhi Roll No: 34701322007 Subject Name : Design of Steel Structures Subject Code : CE(PC)604
8. Design Action Qd
• Qd = ∑ fk Qck
k
8
Name : Jatin Majhi Roll No: 34701322007 Subject Name : Design of Steel Structures Subject Code : CE(PC)604
9. Design Strength Sd
•Sd = Su / m
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Name : Jatin Majhi Roll No: 34701322007 Subject Name : Design of Steel Structures Subject Code : CE(PC)604
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• Stresses caused by the characteristic loads must be
less than an “allowable stress”, which is a fraction of
the yield stress.
• Allowable stress may be defined in terms of a “factor
of safety" which represents a margin for overload and
other unknown factors which could be tolerated by the
structure.
ALLOWABLE STRESS DESIGN (ASD)
Name : Jatin Majhi Roll No: 34701322007 Subject Name : Design of Steel Structures Subject Code : CE(PC)604
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Allowable stress = (Yield stress) / (Factor of safety)
Limitations
•material non-linearity
•non-linear behaviour in the post buckled state and
the property of steel to tolerate high stresses by
yielding locally and redistributing the loads not
accounted for.
•no allowance for redistribution of loads in statically
indeterminate members
ALLOWABLE SRESS DESIGN (ASD) - 2
Name : Jatin Majhi Roll No: 34701322007 Subject Name : Design of Steel Structures Subject Code : CE(PC)604
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A typical example of a set of load combinations is
given below, which accounts for the fact that the
dead load, live load and wind load are all unlikely to
act on the structure simultaneously at their
maximum values:
(Stress due to dead load + live load) < allowable
stress
(Stress due to dead load + wind load) < allowable
stress
(Stress due to dead load + live load + wind) <
1.33 times allowable stress.
Name : Jatin Majhi Roll No: 34701322007 Subject Name : Design of Steel Structures Subject Code : CE(PC)604
13. Summary of WSD
• In practice there are severe limitations to this
approach. These are the consequences of material
non-linearity, non-linear behaviour of elements in
the post-buckled state and the ability of the steel
components to tolerate high theoretical elastic
stresses by yielding locally and redistributing the
loads. Moreover the elastic theory does not readily
allow for redistribution of loads from one member
to another in a statically indeterminate structures.
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Name : Jatin Majhi Roll No: 34701322007 Subject Name : Design of Steel Structures Subject Code : CE(PC)604
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LIMIT STATE DESIGN
•“Limit States" are the various conditions in which a
structure would be considered to have failed to fulfil the
purpose for which it was built.
•“Ultimate Limit States” are those catastrophic states,which
require a larger reliability in order to reduce the probability of
its occurrence to a very low level.
•“Serviceability Limit State" refers to the limits on acceptable
performance of the structure.
Name : Jatin Majhi Roll No: 34701322007 Subject Name : Design of Steel Structures Subject Code : CE(PC)604
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General Principles of Limit State Design
» Structure to be designed for the Limit States at which
they would become unfit for their intended purpose by
choosing, appropriate partial safety factors, based on
probabilistic methods.
» Two partial safety factors, one applied to loading (f) and
another to the material strength (m) shall be employed.
Name : Jatin Majhi Roll No: 34701322007 Subject Name : Design of Steel Structures Subject Code : CE(PC)604
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f allows for;
– the possible deviation of the actual behaviour
of the structure from the analysis model
– deviation of loads from specified values and
– reduced probability that the various loads
acting together will simultaneously reach the
characteristic value.
Name : Jatin Majhi Roll No: 34701322007 Subject Name : Design of Steel Structures Subject Code : CE(PC)604
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m takes account;
– the possible deviation of the material in the
structure from that assumed in design
– the possible reduction in the strength of the
material from its characteristic value and
– manufacturing tolerances.
– Mode of failure (ductile or brittle).
Name : Jatin Majhi Roll No: 34701322007 Subject Name : Design of Steel Structures Subject Code : CE(PC)604
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Limit State of Strength Serviceability Limit State
Strength (yield, buckling)
Stability against overturning and sway
Fracture due to fatigue
Brittle Fracture
Deflection
Vibration
Repairable damage due to fatigue
Corrosion
Fire
Table 1: Limit States
Name : Jatin Majhi Roll No: 34701322007 Subject Name : Design of Steel Structures Subject Code : CE(PC)604
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PARTIAL SAFETY FACTOR
S*
R*
S*
- factored load effect
R*
- factored resistance of the element being
checked, and is a function of the nominal value of
the material yield strength.
Name : Jatin Majhi Roll No: 34701322007 Subject Name : Design of Steel Structures Subject Code : CE(PC)604
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Combination
Limit State of Strength Limit state of Serviceability
DL
LL’ WL
/
EL
AL
DL
LL’ WL
/EL
Leadin
g
Accompan
ying
Leading Accompan
ying
DL+LL+CL 1.5 1.5 1.05 1.0 1.0 1.0
DL+LL+CL+
WL/EL
1.2
1.2
1.2
1.2
1.05
0.53
0.6
1.2
1.0 0.8 0.8 0.8
DL+WL/EL 1.5
(0.9)*
1.5 1.0 1.0
DL+ER 1.2
(0.9)
1.2
DL+LL+AL 1.0 0.35 0.35 1.0
*
This value is to be considered when the dead load contributes to stability against overturning is critical or the dead load causes reduction in stress due to other loads.
‘
When action of different live loads is simultaneously considered, the leading live load is whichever one causes the higher load effects in the member/section.
Abbreviations: DL= Dead Load,LL= Imposed Load (Live Loads), WL= Wind Load, CL= Crane Load (Vertical/horizontal), AL=Accidental Load,
ER= Erection Load, EL= Earthquake Load.
Partial Safety Factor for Loads
Name : Jatin Majhi Roll No: 34701322007 Subject Name : Design of Steel Structures Subject Code : CE(PC)604
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Sl. No. Definition Partial Safety Factor
1 Resistance, governed by yielding m0 1.10
2 Resistance of member to buckling m0 1.10
3 Resistance, governed by ultimate stress m1 1.25
4 Resistance of connection m1
(i) Bolts-Friction Type, mf
(ii) Bolts-Bearing Type, mb
(iii) Rivets, mr
(iv) Welds, mw
Shop
Fabrications
Field
Fabrications
1.25
1.25
1.25
1.25
1.25
1.25
1.25
1.50
Partial Safety Factor for Material
Name : Jatin Majhi Roll No: 34701322007 Subject Name : Design of Steel Structures Subject Code : CE(PC)604
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LIMIT STATES FOR DESIGN
PURPOSES
•Ultimate Limit State is related to the maximum design
load capacity under extreme conditions. The partial
load factors are chosen to reflect the probability of
extreme conditions, when loads act alone or in
combination.
•Serviceability Limit State is related to the criteria
governing normal use. Unfactored loads are used to
check the adequacy of the structure.
•Fatigue Limit State is important where distress to the
structure by repeated loading is a possibility.
Name : Jatin Majhi Roll No: 34701322007 Subject Name : Design of Steel Structures Subject Code : CE(PC)604
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Type of
Building
Deflection Design Load Member Supporting Maximum Deflect
Industrial
building
Vertical
Live load/Wind load
Purlins and Girts
Purlins and Girts
Elastic cladding
Brittle cladding
Span / 150
Span / 180
Live load Simple span Elastic cladding Span / 240
Live load Simple span Brittle cladding Span / 300
Live load Cantilever span Elastic cladding Span / 120
Live load Cantilever span Brittle cladding Span / 150
Live load or Wind
load
Rafter supporting
Profiled Metal Sheeting Span / 180
Plastered Sheeting Span / 240
Crane load
(Manual operation) Gantry Crane Span / 500
Crane load
(Electric operation up to 50 t)
Gantry Crane Span / 750
Crane load
(Electric operation over 50 t)
Gantry Crane Span / 1000
Lateral
No cranes Column Elastic cladding Height / 150
No cranes Column Masonry/Brittle cladding Height / 240
Crane + wind
Gantry (lateral) Crane(absolute) Span / 400
Relative displacement between rails 10 mm
Crane+ wind
Column/frame Gantry (Elastic cladding; pendent
operated)
Height / 200
Column/frame Gantry (Brittle cladding; cab operated) Height / 400
Limit State of serviceability
Name : Jatin Majhi Roll No: 34701322007 Subject Name : Design of Steel Structures Subject Code : CE(PC)604
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Other
Buildin
gs
Vertical
Live load Floor & Roof
Elements not
susceptible to cracking
Span / 300
Live load Floor & Roof
Elements susceptible
to cracking
Span / 360
Live load
Cantilever
Elements not
susceptible to cracking
Span / 150
Live load
Elements susceptible
to cracking
Span / 180
Lateral
Wind Building
Elastic cladding Height / 300
Height / 500
Brittle cladding
Wind Inter storey drift ---
Storey
height / 300
Name : Jatin Majhi Roll No: 34701322007 Subject Name : Design of Steel Structures Subject Code : CE(PC)604
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TOPICS COVERED AND CONCLUSIONS
•Review of the provisions of safety,
consequent on uncertainties in loading and
material properties.
•Allowable Stress Design
•Limit State Design
Name : Jatin Majhi Roll No: 34701322007 Subject Name : Design of Steel Structures Subject Code : CE(PC)604
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THANK YOU
Name : Jatin Majhi Roll No: 34701322007 Subject Name : Design of Steel Structures Subject Code : CE(PC)604
