Steel_Ch1 - Introduction

8/17/2019 Steel_Ch1 - Introduction

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68402: Structural Design of Buildings II

61420: Design of Steel Structures62323: Architectural Structures II

Monther Dwaiat

Assistant !rofessor

De"art#ent of Building $ngineering

An%&a'ah &ational (ni)ersit*

Introduction to tructural

Design of teel


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+ontents

Structural Design

Design Loads

Structural Steel - Properties Design philosophies

Determining load and resistance factors

Load and resistance factors


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Introduction to Design of teel

tructures

General Introduction

• Structural design is a systematic & iterative process that involves:

• Identification of intended use & occupancy of a structure – by oner

•Development of architectural plans & layout – by architect

• Identification of structural frameor! – by engineer

• "stimation of structural loads depending on use & occupancy

• #nalysis of the structure to determine member & connection designforces

• Design of structural members & connections

• $erification of design

• %abrication & "rection – by steel fabricator & contractor

•Inspection & #pproval – by state building official


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!ri#ar* ,es"onsi-ilities

he primary responsibilities are:

• 'ner - primary responsibility is deciding the use &occupancy( & approving the arch) plans of the building)

• #rchitect - primary responsibility is ensuring that thearchitectural plan of the building interior is appropriate

for the intended use & the overall building is

aesthetically pleasing)

• "ngineer – primary responsibility is ensuring thesafety & serviceability of the structure( i)e)( designing

the building to carry the loads safely)


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• %abricator – primary responsibility is ensuring that thedesigned members & connections are fabricatedeconomically in the shop or field as re*uired)

•+ontractor,"rector - primary responsibility is ensuringthat the members & connections are economicallyassembled in the field to build the structure)

• State uilding 'fficial – primary responsibility isensuring that the built structure satisfies the

appropriate building codes accepted by the .ovt)

!ri#ar* ,es"onsi-ilities


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tructural Design

+onceptually( from an engineering standpoint( theparameters that can be varied /somehat0 are:• he material of construction• he structural framing plan)

he choices for material include:

• Steel • 1einforced concrete• Steel-concrete composite construction)

he choices for structural framing plan include:

•2oment resisting frames)

• raced frames)• Dual frames• Shear all frames( and so on)

he engineer can also innovate a ne structural framing

plan for a particular structure if re*uired)


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tructural Design

#ll viable material 3 framing plan alternatives

must be considered & designed to compare the

individual material 3 fabrication , erection costs to

identify the most efficient & economical design forthe structure)

%or each material 3 framing plan alternative

considered( designing the structure consists ofdesigning the individual structural components(

i)e)( the members & the connections( of the

framing plan)


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Determination of dimensions and selection of cross sections) he design process is a loop:

Structural Design

Assume dimensions, structural conditions and cross sections

Structural Analysis

Selection of cross sections to satisfy structural requirements

Does the design violate the initial assumptions?

Final Design

YES NO


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Structural Design

'ptimal structural design shall achieve balance beteenthe folloing re*uirements:

Strength

Economy

Serviceaility

Optimal design


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,oles and res"onsi-ilities of the

structural steel designer

#rrange and proportion the members of the structures(using engineer4s intuition and sound engineeringprinciples( so that they can be practically erected( havesufficient strength /safe0( and are economical )

• Practicality: "nsure structures can be fabricated and erectedithout problems

• Safety: "nsure structures can safely support the loads)"nsure deflections and vibrations are

controlled for occupants comfort)

• +ost: 2inimi5e costs ithout sacrifice of strength/consider labor costs in fabrication anderection( not 6ust material costs0


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Basic Structural Sha"es

russes

%rames / eam-+olumn0

•eams

• .irders• +olumns

Space trusses,frames


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Steel Structures

eams-%rames

racing

!olumns

Purlins


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Steel Structures

Industrial,Par!ingstructures 7%rames8


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Steel Structures

9oists,russes


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Steel Structures

igh rise buildings


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Steel Structures

.irder bridges


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Steel Structures

russ bridges


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Steel Structures

+able stayed & suspended bridges


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Structural members are categori5ed based up on theinternal forces in them) %or e;ample:

• ension member –sub6ected to tensile a;ial force only

• +olumn or compression member –sub6ected to compressive a;ialforce only

• ension,+ompression member –sub6ected to tensile,compressivea;ial forces

• eam member –sub6ected to fle;ural loads( i)e)( shear force &bending moment only) he

• a;ial force in a beam member is negligible)

• eam-column member – member sub6ected to combined a;ialforce & fle;ural loads /shear

• force( & bending moments0

tructural Me#-ers


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• In trusses:• #ll the members are connected using pin,hinge connections)• #ll e;ternal forces are applied at the pins,hinges)• #ll truss members are sub6ected to a;ial forces /tension or

compression0 only)

• In frames:• he hori5ontal members /beams0 are sub6ected to fle;ural loadsonly)

• In braced frames:• he vertical members /columns0 are sub6ected to compressive a;ial

forces only)• he diagonal members /braces0 are sub6ected totension,compression a;ial forces only)

• In moment frames• he vertical members /beam-columns0 are sub6ected to combined

a;ial & fle;ural loads)

tructural Me#-ers


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tructural +onnections

2embers of a structural frame are connected togetherusing connections) Prominent connection types include:

• russ , bracing member connections are used to connect to ormore truss members together) 'nly the axial forces in the

members have to be transferred through the connection forcontinuity)

• Simple shear connections are the pin connections used toconnect beam to column members) 'nly the shear forces aretransferred through the connection for continuity) he bending

moments are not transferred through the connection)• 2oment connections are fix connections used to connect beam to

column members) oth the shear forces & bending moments aretransferred through the connections ith very small deformations/full restraint 0)


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tructural +onnections

russ connectionruss connection

Simple ShearSimple Shearconnectionconnection

2oment resisting2oment resisting

connectionconnection


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tructural .oads

he building structure must be designed to carry or resistthe loads that are applied to it over its design-life) hebuilding structure ill be sub6ected to loads that have beencategori5ed as follos:

• Dead Loads /D0: are permanent loads acting on the structure)hese include the self-eight of structural & non-structuralcomponents) hey are usually gravity loads)

• Live Loads /L0: are non-permanent loads acting on the structuredue to its use & occupancy) he magnitude & location of live loads

changes fre*uently over the design life) ence( they cannot beestimated ith the same accuracy as dead loads)

•


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• Sno Loads /S0: are vertical gravity loads due to sno(hich are sub6ected to variability due to seasons &

drift)

• 1oof Live Load /Lr

0: are live loads on the roof causedduring the design life by planters( people( or by

or!ers( e*uipment( & materials during maintenance)

• $alues of structural loads can be computed based onthe design code)

tructural .oads


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Dead .oads /D0

Dead loads consist of the eight of all materials ofconstruction incorporated into the building including but notlimited to alls( floors( roofs( ceilings( stairays( built-inpartitions( finishes( cladding & other similarly incorporatedarchitectural & structural items( & fi;ed service e*uipment

such as plumbing stac!s & risers( electrical feeders( &heating( ventilating( & air conditioning systems)

In some cases( the structural dead load can be estimated

satisfactorily from simple formulas based in the eights &si5es of similar structures) %or e;ample( the averageeight of steel framed buildings is = - =)> !Pa( & theaverage eight for reinforced concrete buildings is ? - >!Pa)


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Dead .oads /D0

%rom an engineering standpoint( once the materials andsi5es of the various components of the structure aredetermined( their eights can be found from tables thatlist their densities) See ables @)A & @)=( hich are ta!en

from ibbeler( 1)+) /@BBB0, Structural Analysis, Cth"dition)


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Dead .oads /D0


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.i)e .oads u##ar* 2a-le

uilding floors are usually sub6ected to uniform live loads orconcentrated live loads) hey have to be designed to safely support

these loads)

Type of occupancy kPa

'ffices A)? - ?

+orridors ?

1esidential A

Stairs and e;it ays ?

Stadiums ?

Sideal!s @A


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3ind .oads

Design ind loads for buildings can be based on: /a0 simplifiedprocedure /b0 analytical procedure & /c0 ind tunnel or small-scale procedure)

1efer to #S+" E-F? for the simplified procedure) his simplified

procedure is applicable only to buildings ith mean roof heightless than @G m or the least dimension of the building)

he ind tunnel procedure consists of developing a small-scalemodel of the building & testing it in a ind tunnel to determinethe e;pected ind pressures etc) It is e;pensive & may be

utili5ed for difficult or special situations)

he analytical procedure is used in most design offices) It isfairly systematic but somehat complicated to account for thevarious situations that can occur:


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@= K z K zt K d ! " /,m! 0

3ind .oads


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q z – Static wind pressure

V - the wind velocity in m/s

K d - a directionality factor (= 0!" see #a$le %& pa'e !0

K zt - a topo'raphic factor (= )0 * - the importance factor (=)0

K z - varies with hei'ht z a$ove the 'round level ( see #a$le %+ pa'e , e.posure structure surrounded $y $uildin's/forests/ at least %m

hei'ht

e.posure 1 open terrain

3ind .oads


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# significant portion of Palestine has H @FF !m,h) #t theselocation

qz H CFA K z /,m! 0

he velocity pressure qz is used to calculate the designind pressure / p0 for the building structure conservatively

as follos:

p H q #$ p /,m! 0

3ind .oads


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AS+$ %05 "g 7

J5 - varies ith height 5 above the ground level

# – large city centers

– urban, suburban area

+ – open terrain ith scattered obstructions

D – %lat unobstructed surface


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G - gust effect factor (= 0.85)

C p - external pressure coefficient from Figure 6-6 page 48-49

in !C" #-05 or

C p = 0.8 $in%$ar%

C p = -0.5 lee$ar%

C p = -0.# si%e$alls

C p = -0.# slope&0.#5

3ind .oads

/@)?0

• Note that:

• # positive sign indicates pressure acting toards a surface)

• egative sign indicates pressure aay from the surface


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$8a#"le 161 3ind .oad

+onsider the building structure ith the structural floor plan & elevationshon belo) "stimate the ind loads acting on the structure hen the

ind blos in the east-est direction) he structure is located in

ablus)

15 m 15 m

15 m

15 m

Plan


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6 @ 3 m

6 @ 3 m


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$elocity pressure /qz 0

• K d - directionality factor H F)G?• K zt - topographic factor H @)F

•" - importance factor H @)F

• H @FF !ph in ablus

q z = 402 K z (N/m2)

• K 5 - varies ith height 5 above the ground level• K z values for %xposure &, $ase !



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AE=)C J5

AF=)=

@C?)A

AF=)=


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3 m

3 m

3 m

3 m

3 m

3 m

155.8

16.518!."

11."

#!$.8

##1.5

#3#."

1"5.#


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Bacground of Structural Steel

"conomical production in large volume not available until mid @Bth century and the introduction of the essemer process) Steel becamethe principal metallic structural material by @GBF)

Steels consists almost entirely of iron /over BGK0 and small *uantitiesof carbon( silicon( manganese( sulfur( phosphorus( and other

elements) he *uantities of carbon affect properties of steel the most)

Increase of carbon content increases hardness and strength

Alloy steel – has additional amounts of alloy elements such chronium(

vanadium( nic!el( manganese( copper( or 5irconium) *he American Society for *esting of +aterials AS*+- specifies e;act

ma;imum percentages of carbon content and other additions for anumber of structural steels) +onsult 2anual( Part A( able A-@ to A-=for availability of steel in structural shapes( plate products( and

structural fasteners)


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ASM classifications of structural

steels

%ar&on 'teel' – #=>( #?=( #?FF( #?F@( #?AB( #?EF) aveell-defined yield point) Divided into four categories:

• Lo-carbon steel / F)@?K0• 2ild steel /F)@? to F)ABK( structural carbon steels0

•2edium-carbon steel /F)= to F)?BK0

• igh-carbon steel /F)> to @)EK0 (i)h*+tren)th ,o-*lloy 'teel' – #ACA( #?EA( #?GG(

#>F>( #>FE( #>@G( #EFB

•


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Ad)antages and disad)antages of

steel as a structural #aterial

#dvantages

• igh strength per unit of eight → smaller eight of structures• Nniformity• "lasticity

• Long lasting• Ductility• oughness• "asy connection

• Speed of erection• #bility to be rolled into various si5es and shapes• Possible reuse and recyclable

f


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Ad)antages and disad)antages of

steel as a structural #aterial

Disadvantages

• 2aintenance costs• %ire protection,%ireproofing costs• Susceptibility to buc!ling failure

• %atigue• rittle fracture


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*"es of Steel

hree basic types of steel used for structural steel• Plain +arbon Steel• Lo-alloy steel• igh-alloy 7specialty steel8

he most commonly used is mild steel - #S2 #=>

ypical high strength steel:

he higher the steel strength( the higher the carbon content and

the less ductile it is)

248 (36 )

400 (58 )

y

u

F MPa ksi

F MPa ksi

=

=

290 344 (42 50 )

444 482 (63 70 )

y

u

F MPa ksi

F MPa ksi

= − −

= − −

2S#3 24&4344 (50 )

448 (65 )

y

u

F MPa ksi

F MPa ksi

=

=

2S#3 24


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Stress%strain cur)e Standard Plain +arbon Steel

)(

)(

5en'th6ri'inal 5

n 7eformatio 5

o

∆=ε

)(

)(

2rea 2

5oad 8 f =

Yield plateauYield plateauFFyy

Strain "Strain "

##

Stress "Stress " f f ##

FFuu

EE

Nec$ing % FractureNec$ing % FractureStrain &ardeningStrain &ardening

ElasticElastic


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hat is a .i#it State


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.i#it States

Strength Limit Statesa0 Loss of "*uilibrium

b0 Loss of load bearing capacity

c0 Spread of local failure

d0 $ery large deformations

Serviceability Limit States

a0 ";cessive deflection

b0 ";cessive local damage

c0 Nnanted vibration


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Design !hiloso"hies

#lloable Stress Design /#SD0

Plastic Design /PD0

Load and 1esistance %actor Design /L1%D0


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Allowa-le Stress Design

Service loads are calculated as e;pected during servicelife)

Linear elastic analysis is performed)

# factor of safety /%'S0 of the material strength is assumed

/usually =-C0

Design is satisfactory if /maximum stress . allo/able

stress0

Limitations

• +ase specific( no guarantee that our design covers all cases• #rbitrary choice of %'SO

FOS

StrengthMaterialStressAll!a"le =


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!lastic Design

Service loads are factored by a 7load factor8) he structure is assumed to fail under these loads( thus(

plastic hinges ill form under these loads 7Plastic #nalysis8)

he cross section is designed to resist bending moments

and shear forces from the plastic analysis) 2embers are safe as they are designed to fail under these

factored loads hile they ill only e;perience service loads)

Limitations

• o %'S of the material is considered( neglecting the uncertainty inmaterial strength

• #rbitrary choice of overall %'SO

. d d , i t 9 t D i


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.oad and ,esistance 9actor Design

/.,9D

L1%D is similar to plastic design in that it performs designith the assumption of failure - 1eliability ased Design

Service loads are multiplied by load factors /γ 0 and linearelastic analysis is performed)

2aterial strength is reduced by multiplying the nominalmaterial strength by a resistance factor /φ0

he design rule is: Load "ffect 1esistance

•


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.oad and ,esistance 9actor Design

/.,9D

1esistance: Shear( ending( #;ial %orces #dvantages of L1%D

• on-case specific( statistical calculations guarantee populationbehavior)

• Nniform factor of safety as both load and material factors are tiedby reliability analysis


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!ro-a-ilistic Basis for .,9D

If e have the probability distribution of the load effect /Q0 and the materialresistance /10 then:

• he probability of failure can be represented by observing the probability of the function /1-Q0• he probability of failure P% can be represented as the probability that Q R 1:

)roaility

of failure


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AIS+ .oad co#-inations #IS+ considers the folloing load combinations in design

ni i i 9: φ γ ≤∑

)(5%06%&2%&2 9or S or 5 5 7r

++−

74%&&−

)8%0(5%0)(6%&2%&3 W or L Ror S or L Dr

++−

S 5 ; 7 2%05%00%&2%&5 ++±−

6 0%9 (&%6 &%0 ) D Wor E − ±

)(5%05%06%&2%&4 Ror S or L LW D r +++−

00%&75%0 −=φ

i i :∑γ

ni 9φ

Dead loads /D0 Live loads /LL0

• 'ccupancy load/L0

• 1oof load /Lr0• Sno load /S0• 1ain loads /10• ruc!s and

pedestrians

Steel_Ch1 - Introduction

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