LECTURE 1 INTRODUCTION – PART PART 1 January 30, 2012
Design of steel and prestressed
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Introduction •Introduction •Conceptual design of building- studies earlier •Design Codes 1)BS EN 1993-1-12!!" Eurocode 3 Design of Steel Structures #art 1-1 $eneral rules and rules for buildings% Britis& Standards' 2)BS EN 1993-1-(2!!"% Eurocode 3 Design of Steel structures #art 1-( Design of *oints% Britis& Standards' • +ctions +ctions •,ributar areas . studied earlier •/aterial be&a0iour #roperties of aterials . studied earlier
Design of steel and prestressed
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Introduction •Introduction •Conceptual design of building- studies earlier •Design Codes 1)BS EN 1993-1-12!!" Eurocode 3 Design of Steel Structures #art 1-1 $eneral rules and rules for buildings% Britis& Standards' 2)BS EN 1993-1-(2!!"% Eurocode 3 Design of Steel structures #art 1-( Design of *oints% Britis& Standards' • +ctions +ctions •,ributar areas . studied earlier •/aterial be&a0iour #roperties of aterials . studied earlier
Design of steel and prestressed
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Introduction • • •
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Engineering Design consiss o! T"o T"o sages sages #easi$i%iy &u'y( Conce)ua% 'esign In0ol0es 'esign In0ol0es coparison of t&e alternati0e fors of structure and selection of ost suitable tpe ,&e success of t&is stage relies to a large etent on t&e engineering *udgeent and instinct% bot& of 4&ic& are t&e outcoe of an ear5s eperience of designing structures' Deai%e' 'esign 'esign in0ol0es detailed design of t&e c&osen structure ,&e detailed also re6uires t&ese attributes but is usuall ore dependent upon a t&oroug& understanding of t&e codes of practice for structural design nael EC2 and EC3 ,&ese docuents are based on t&e eperience of an generations of engineers% and t&e results of researc&' ,&e &elp to ensure safet and econo of construction% and t&at ista7es are not repeated'
Design of steel and prestressed
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Introduction • &at is Structural steel: • Steel - an ade etal . containing 9(; or ore iron . sall aounts of eleents deri0ed fro ra4 aterials and also eleents added to ipro0e certain properties''C% Si% /n% #%S% Niobiu%
Design of steel and prestressed
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+d0antages of Steel • • • • • • • • •
• • •
@ig& strengt& to lo4 4eig&t - good for long span bridges% tall buildings Aniforit-properties do not c&ange 4it& tie unli7e concrete Elasticit .be&a0es closer to design assuptions t&an ost aterials . follo4s @oo7e5s la4 to fairl &ig& stress Ductilit . 4it&stand etensi0e deforation 4it&out failure under &ig& tensile stress . free fro sudden failure +dditions to eisting structures ,ie sa0ing leibilit in fabrication =euse on deolition Disad0antages of Steel
/aintenance cost . corrosion . re6uires periodic treatent ire proofing . strengt& treendousl reduced at &ig& teperature . &ig& cost of fire proofing Susceptibilit to buc7ling for long slender ebers atigue . strengt& reduced if large nuber of stress re0ersals
Design of steel and prestressed
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$eneral Steel #roperties • ,&e iportant c&aracteristics of steel for design purposes are . ield stress -) . ultiate stress u)'tensile strengt& . odulus of elasticit E) . percent elongation ε) . coefficient of t&eral epansion α)
Design of steel and prestressed
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,&e ,ension test
Design of steel and prestressed
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Design of steel and prestressed
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Design of steel and prestressed
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Design of steel and prestressed
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$rade of Steel and Design Strengt& table 3'1 EN 1993-1-1)page 2> $rade of Steel
ield Strengt& Altiate strengt& or Design fu Strengt&N 2)
$rade ""
S8"!
88!
""!
$rade "!
S3""
3""
"1!
$rade 83
S2?"
2?"
83!
$rade 3>
S 23"
23"
3>!
Design of steel and prestressed
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Design of steel and prestressed
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• ,&e four grades are S23"%S2?"% S3""% S8>! • S8>! is t&e strongest% but t&e lo4er grades are ost coonl used in structural applications' • S stands for FStructuralG • ,&e nuber indicates t&e ield strengt& of t&e aterial in N 2' Design of steel and prestressed
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Conceptual Design of building • • •
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Design . process b 4&ic& an optiu solution is obtained' In an design% certain criteria ust be establis&ed to e0aluate 4&et&er or not an optiu &as been ac&ie0ed Design Deterination of o0erall proportions and diensions of t&e supporting frae4or7 and t&e selection of indi0idual ebers' Ai* o! &rucura% Design – ,o pro0ide 4it& due regard to econo a structure capable of fulfilling its intended function and sustaining t&e specified loads for its intended life' ,&e design s&ould facilitate safe fabrication% transport% &andling and erection- account future aintenance% final deolition% reccling and reuse of aterials =esponsibilit ,&e structural engineer% 4it&in t&e constraints iposed b t&e arc&itect nuber of stories% floor plan%'') is responsible for structural design' P+i%oso)+ies( T+eories use' !or 'esign Elastic design% #lastic design and Hiit State Design
Design of steel and prestressed
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O$ec o! &rucura% Design • Safet t&e structure doesn5t fall do4n during lifetie) • Ser0iceabilit &o4 4ell t&e structure perfors in ter of appearance and deflection) • ulfill re6uireents of client • Econo an efficient use of aterials and labor) A%ernai-es • Se0eral alternati0e designs s&ould be prepared and t&eir costs copared
Design of steel and prestressed
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#lastic Design • Atilises strengt& of steel beond ield point • ,&e structure a be loaded beond t&e ield point if • ,&e tendenc of t&e fibre at t&e ield point stress to4ard plastic deforation is resisted b t&e ad*acent fibres • ,&ose parts of t&e structure t&at reain in t&e elasticstress range are capable of supporting t&is increental load • ,&e ultiate load is reac&ed 4&en t&ese conditions cease to eist and t&us t&e structure collapses • #lastic design is concerned 4it& an allo4able load% 4&ic& e6uals t&e ultiate load di0ided b an appropriate factor called t&e load factor' Design of steel and prestressed
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Hiit State Concept in Design • Stated in cl 2'2 EN 1993-1-1 2!!" Eurocode 3 Design of Steel Structures #art 1-1 $eneral rules and rules for buildings% Britis& Standards • ,&e standard gi0es recoendations for t&e design of structural steel 4or7 using &ot rolled sections% flats% plates% &ot finis&ed structural &ollo4 sections and cold fored structural &ollo4 sections% in buildings and allied structures • Structures s&ould be designed b considering t&e liit states beond 4&ic& t&e 4ould becoe unfit for t&eir intended use
Design of steel and prestressed
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Hiit states
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Eaples of liit states rele0ant to steel structures are gi0en in ,able 1'
U%i*ae %i*i saes .UL&/ &er-icea$i%iy %i*i saes.&L&/ Strengt& cl>'1
Deflection
Stabilit against o0erturning and s4a stabilit
8 O Standard de0iation Design of steel and prestressed
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Classification of +ctions •
#E=/+NEN, +C,INS ) are due to 4eig&t of t&e structure i'e' 4alls% peranent partitions% floors% roofs%
finis&es and ser0ices ,&e actual 4eig&ts of aterials $7) s&ould be used in design calculationsP but if not 7no4n use densit in 7N3 fro EN 1991-12!!2' +lso included in t&is group are 4ater and soil pressures% forces due to settleent etc
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<+=I+BHE +C,INS L) Iposed floor Hoads 4) are 0ariable actionsP gi0en for 0arious d4ellings in EN 1991-1-12!!2' ,&ese loads include a sall allo4ance for ipact and ot&er dnaic effects t&at a occur in noral occupanc' Do not include forces resulting fro t&e acceleration and bra7ing of 0e&icles or o0eent of cro4ds' ,&e loads are usuall gi0en as distributed loads or an alternati0e concentrated load ind +ctions 54) +re 0ariable but for con0enience are epressed as static pressures in EN 1991-1-82!!2)' ,&eral effects need to be considered for c&ines% cooling to4ers% tan7s and cold storage ser0ices' Classified as indirect 0ariable actions' Design of steel and prestressed
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• +ctions to be ta7en for ade6uate perforance in fire • +CCIDEN,+H +C,INS+) • +ccidental actions during eecution include scaffolding% props and bracing EN 1991-1->2!!2)' ,&ese a in0ol0e consideration of construction loads% instabilit and collapse prior to copletion of t&e pro*ect • Eart&6ua7e Hoads t&e effects of ground otion are siulated b a sste of &oriQontal forces)EN199(-(2!!8) • +ctions induced b cranes and ac&iner EN 199132!!8) • Ipact and Eplosions co0ered in EN 1991-1-?2!!8)'
Design of steel and prestressed
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C+aracerisic an' Design Loa' •
&en c&ec7ing t&e safet of a eber% t&e designer cannot be certain about t&e load t&e eber ust carr because a) of t&e 0ariabilit of t&e occupanc or en0ironental loading% and b) because of unforeseen circustances 4&ic& a lead to an increase in t&e general le0el of loading% errors in analsis% errors during construction etc
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Ite a) is allo4ed b using t&e c&aracteristic 0alue'
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,&e c&aracteristic load is t&e 0alue abo0e 4&ic& t&e load lies in onl sall percentage of cases'
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Statistical principles cannot be used at present to deterine c&aracteristic loads because sufficient data is not a0ailable'
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,&erefore t&e c&aracteristic loads are norall ta7en to be t&e design loads fro ot&er codes of practice BS >8( and BS >399'
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Design Hoad is t&e 0alue used in design calculations . product of c&aracteristic load and partial safet factors in order to increase reliabilit
Design of steel and prestressed
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Co*$inaions o! Design Acions •
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= ,@E AH,I/+,E HI/I, S,+,E% t&ree alternati0e cobinations of actions% odified b appropriate partial safet factors J)% ust be in0estigated a) undaental a cobination of all peranent actions including self 4eig&t$7)% t&e doinant 0ariable action L7) and cobination 0alues of all ot&er 0ariable actionsR!L7) b) + cobination of t&e doinant 0ariable actionsR!L7)' ,&is cobination assues t&at accidents of s&ort duration &a0e a lo4 probabilit of occurrence c)Seisicreduces t&e peranent action partial safet factorJ$)4it& a reduction factor )bet4een !'(" and 1 = SE=
abricated sections Built-up sections
abricated sections can be 4elded or bolted
Design of steel and prestressed
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Cold rolled sections Cold fored =ectangular @ollo4 sections
Design of steel and prestressed
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Differences bet4een cold fored and &ot rolled sections • • • • • •
Cold-fored steel &as been 4idel used in building construction% fro residential &ouses to industrial buildings' Cold-fored steel offers 0ersatilit in building because of its lig&t4eig&t and ease of &andling and use' Cold-fored steel represents o0er 8" percent of t&e steel construction ar7et in AS% and t&is s&are is increasing ,&e &ot-rolled steel s&apes are fored at ele0ated teperatures 4&ile t&e cold-fored steel s&apes are fored at roo teperature' Cold-fored steel structural ebers are s&apes coonl anufactured fro steel plate% s&eet or strip aterial' ,&e anufacturing process in0ol0es foring t&e aterial b eit&er press-bra7ing or cold roll-foring to ac&ie0e t&e desired s&ape' Eaples of t&e cold-fored steel are corrugated steel roof and floor dec7s% steel 4all panels% storage rac7s and steel 4all studs' Design of steel and prestressed
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#ress-bra7ing is often used for production of sall 6uantit of siple s&apes'
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Cold roll-foring is t&e ost 4idel used et&od for production of roof% floor and 4all panels' It is also used for t&e production of structural coponents suc& as Cees% Tees% and &at sections' Sections can usuall be ade fro s&eet up to >! inc&es 1'") 4ide and fro coils ore t&an 3%!!! feet 1%!!!) long'
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During cold roll-foring% s&eet stoc7 is fed longitudinall t&roug& a series of rolls% eac& of 4&ic& 4or7s t&e s&eet progressi0el until it reac&es t&e desired s&ape' + siple section a re6uire as fe4 as si pairs of roll% but a cople s&ape can re6uire as an as 28 to 3!' ,&e t&ic7ness of aterial t&at can be fored generall ranges bet4een !'!!8 !'1!) up to !'312 inc&es ?'?)% alt&oug& &ea0 dut cold foring ills can &andle steel up to U of an inc& 19) t&ic7'
Design of steel and prestressed
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Cold rolling /ill Design of steel and prestressed
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Cold rolled s&apes
Design of steel and prestressed
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Design of steel and prestressed
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Differences bet4een cold fored and &ot rolled steel • • • • •
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t&ic7ness s&apes' Since cold-fored steel ebers are fored at roo teperature% t&e aterial becoes &arder and stronger' Its lig&t4eig&t a7es it easier and ore econoical to ass-produce% transport and install' ne of t&e ain differences bet4een designing 4it& cold-fored steel s&apes and 4it& &ot-rolled structural s&apes is t&at 4it& t&e &ot-rolled% one is priaril concerned about t4o tpes of instabilit colun buc7ling and lateral buc7ling of unbraced beas' ,&e diensions of &ot-rolled s&apes are suc& t&at local buc7ling of indi0idual constituent eleents generall 4ill not occur before ielding' ,&is is not t&e case 4it& cold-fored ebers' @ere local buc7ling ust also be considered because% in ost cases% t&e aterial used is t&in relati0e to its 4idt&' ,&is eans t&at t&e indi0idual flat% or plate% eleents of t&e section often &a0e 4idt& to t&ic7ness ratios t&at 4ill perit buc7ling at stresses 4ell belo4 t&e ield point'
Design of steel and prestressed
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E6APLE 1 Deterine t&e properties I% T% S of >1! 229 AB 12" section 4it& a 3!! 2! plate 4elded to eac& flange
• Because of setr of t&e section t&e centroid of t&e plated AB is at t&e 4eb centre
Design of steel and prestressed
8"
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M VI$$+r 2) M 9("!!2 3!! 2!OW>11'92!)2X21!!!!
Z x =
M 21(29! C/8
218290
( ( 611.9 + 2 × 20) /(2 ×10 )
= 6697cm4
Design of steel and prestressed
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S&ape factor • S&ape factor is defined as ν
=
plastic mod ulus Elastic mod ulus
=
S xx Z xx
E6APLE 3 Deer*ine +e s+a)e !acor !or a recangu%ar secion o! "i'+ 10 ** an' 'e)+ 700 **8 T M bd2>M1! "!!2> S M bd28 M 1! "!!28 ,&erefore s&ape factor M ST M >8 M 1'" Design of steel and prestressed
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