IS800-5LSM_642

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Dr S R Satish Kumar, IIT Madras 2

Designer has to ensure the structures, hedesigns are:

Fit for their purpose

Safe

Economical and durable

INTRODUCTION

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Following Uncertainties affect the safetyof a structure

about loading

about material strength and

about structural dimensions

about behaviour under load

INTRODUCTION

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LIMIT STATE DESIGN

Limit State: State at which one of the conditions pertainingto the structure has reached a limiting value

Limit States

Limit States of Strength Limit States of Serviceability

Strength as governed by material Deflection

Buckling strength Vibration

Stability against overturning, sway Fatigue cracks (reparable damage)

Fatigue Fracture Corrosion

Brittle Fracture Fire resistance

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RANDOM VARIATIONS

Resistance, S

Load effect, Q

f(S)f(Q)

Qm

Frequency

Probabil i ty density functions for strength and load effect

Sm

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LIMIT STATES DESIGN

Basis of Limit States

Design

Fig. 1 Probabil ity distr ibution of the safety margin R-Q

R-QR-Q0

(R-Q)m

f(R-Q)(R-Q)

2Q

2s

mm QS

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PROBABILITY OF FAILURE

2Q

2R

mm

QRmf

QR

QRP

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SAFETY INDEX

2Q

2S

mm QS

Pf= [- ] 2.32 3.09 3.72 4.27 4.75 5.2 5.61

Pf= (-) 10-2 10-3 10-4 10-5 10-6 10-7 10-8

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PARTIAL SAFETY FACTOR

mukfk SQ /

)V1(S)V1(Q 2ssqm2qqsm

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ALLOWABLE SRESS DESIGN (ASD)

Allowable stress = (Yield stress) / (Factor of

safety)

Limitations

Material non-linearity

Non-linear behaviour in the postbuckled state

and the property of steel to tolerate high

stresses by yielding locally and redistributingthe loads not accounted for.

No allowance for redistribution of loads in

statically indeterminate members

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LIMIT STATES DESIGN

Limit States" are various conditions in which astructure would be considered to have failed to fulfilthe purpose for which it was built.

Ultimate Limit States are those catastrophic

states,which require a larger reliability in order toreduce the probability of its occurrence to a verylow level.

Serviceability Limit State" refers to the limits on

acceptable performance of the structure duringservice.

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General Principles of

Limit States Design

Structure to be designed for the Limit States atwhich they would become unfit for their intendedpurpose by choosing, appropriate partial safetyfactors, based on probabilistic methods.

Two partial safety factors, one applied to loading(f) and another to the material strength (m)shallbe employed.

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f allows for;

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.

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LIMIT STATES DESIGN

(Load * Load Factor) (Resistance )(Resistance Factor)

mtakes account;

Possible deviation of the material in thestructure from that assumed in design

Possible reduction in the strength of the

material from its characteristic value

Manufacturing tolerances.

Mode of failure (ductile or brittle)

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IS800 SECTION 5 LIMIT STATE DESIGN

5.1 Basis for Design

5.2 Limit State Design 5.3 Actions

5.4 Strength

5.5 Factors Governing the Ultimate Strength 5.5.1 Stability

5.5.2 Fatigue

5.5.3 Plastic Collapse

5.6 Limit State of Serviceability 5.6.1 Deflection

5.6.2 Vibration

5.6.3 Durability

5.6.4 Fire Resistance

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5.1.3

The potential for catastrophic damage shall be limited or

avoided by appropriate choice of one or more of thefollowing:

i) avoiding, eliminating or reducing exposure to hazards,

which the structure is likely to sustain.

ii) choosing structural forms, layouts and details anddesigning such that

the structure has low sensitivity to hazardous conditions.

the structure survives with only local damage even after serious

damage to any one individual element by the hazard.

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Conditions to be satisfied to avoid a

disproportionate collapse

building should be effectively tied together at

each principal floor level and each column shouldbe effectively held in position by means of

continuous ties (beams) nearly orthogonal

each storey of the building should be checked toensure disproportionate collapse would not

precipitate by the notional removal, one at a time,

of each column.

check should be made at each storey byremoving one lateral support system at a time to

ensure disproportionate collapse would not

occur.

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Actions

5.3.1 Classification of Actions

by their variation with time as given below: a) Permanent Actions (Qp): Actions due to self-

weight of structural and non-structural components,

fittings, ancillaries, and fixed equipment etc.

b) Variable Actions (Qv): Actions due to constructionand service stage loads such as imposed (live) loads

(crane loads, snow loads etc.), wind loads, and

earthquake loads etc.

c) Accidental Actions (Qa):Actionsdue to

explosions, impact of vehicles, and fires etc.

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Combination

Limit State of Strength Limit state of Serviceability

DLLL WL

/

EL

AL DLLL WL

/ELLead

ing

Accompa

Nying

Leadi

ng

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+ER1.2

(0.9)1.2

DL+LL+AL 1.0 0.35 0.35 1.0

Partial Safety Factors (Actions)

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PARTIAL SAFETY FACTORS (Strength)

Sl.

No

Definition Partial Safety Factor

1 Resistance, governed by

yielding mo

1.1

2 Resistance of member to

buckling mo

1.1

3 Resistance, governed by

ultimate stress m1

1.25

4 Resistance of connection m1

Bolts-Friction TypeBolts-Bearing Type

Rivets

Welds

Shop

Fabrication

s

Field

Fabricatio

ns

1.25

1.25

1.25

1.25

1.25

1.25

1.25

1.50

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5.5 Factors Governing the Ultimate Strength

frame stability against overturning and sway

Fatigue design shall be as per Section 13 of this

code. When designing for fatigue, the load factor

for action,f, equal to unity shall be used for the

load causing stress fluctuation and stress range. Plastic Collapse Plastic analysis and design may

be used if the requirement specified under the

plastic method of analysis (Section 4.5) are

satisfied.

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5.6 Limit State of Serviceability

Deflectionsare to be checked for the most

adverse but realistic combination of service loadsand their arrangement, by elastic analysis, using a

load factor of 1.0

Suitable provisions in the design shall be made forthe dynamic effects of live loads, impact loads and

vibration/fatiguedue to machinery operating loads.

The durabilityof steel structures shall be ensured

by following recommendations of Section 15. Design provisions to resistfireare briefly

discussed in Section 16.

LIMITING DEFLECTIONS under LL Only

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LIMITING DEFLECTIONS under LL Only

Type of

building

Deflectio

nDesign Load Member Supporting

Maximum

Deflection

Indus

trial

building

Vertical

Live

load/Wind

load

Purlins and

GirtsPurlins and

Girts

Elastic claddingBrittle cladding

Span / 150Span / 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

SheetingSpan / 180

Plastered Sheeting Span / 240

Crane load

(Manualoperation)

Gantry Crane Span / 500

Crane load

(Electric

operation

over 50 t)

Gantry Crane Span / 1000

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DEFLECTION LIMITS under LL Only

DeflectionDesign Load Member Supporting

Maximum

DeflectionLateral

Crane+

wind

No cranes ColumnElastic

claddingHeight / 150

No cranes ColumnMasonry/brittle

claddingHeight / 240

Crane Gantry(lateral) Crane Span / 400

Vertical

Live load Floors & roofs

Not

susceptible

to cracking

Span / 300

Live load Floor & RoofSusceptible to

crackingSpan / 360

Lateral Wind Building --- Height / 500

WindInter storey

drift

---Storey height /

300

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Documents

IS800-5LSM_642