fabrication design

7/24/2019 Fabrication Design

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PCI 6thEdition

Fabrication Design


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Presentation Outline

Planning Discussion

Stripping Process Design and Analysis

Prestress / Post Tension Effects

Handling Devices Stripping Stress Examples

Storage Discussion

Transportation Discussion

Erection Discussion


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Introduction

Te loads and forces on precast and prestressed

concrete members during production!

transportation or erection "ill fre#uently re#uire a

separate analysis $oncrete strengts are lo"er

Support points and orientation are usually

different from members in teir final position


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Pre-Planning Piece Size

Te most economical piece si%e for a pro&ect isusually te largest! considering te follo"ingfactors'

Stability and stresses on te element duringandling

Transportation si%e and "eigt regulations ande#uipment restrictions


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Pre-Planning Piece Size

Available crane capacity at bot te plant andte pro&ect site(

Position of te crane must be considered! since

capacity is a function of reac Storage space! truc) turning radius! and oter

site restrictions


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Planning and Setup

*nce a piece as been fabricated! it is necessary to remove itfrom te mold "itout being damaged(

Positive drafts or brea)a"ay forms sould be used to allo" amember to lift a"ay from te casting bed "itout becoming

"edged "itin te form Ade#uate draft also serves to reduce trapped air bubbles(


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Planning and Setup

+ifting points must be located to )eep member stresses "itin limits andto ensure proper alignment of te piece as it is being lifted

,embers "it unsymmetrical geometry or pro&ecting sections mayre#uire supplemental lifting points and auxiliary lifting lines to acieveeven support during andling

-$ome.alongs or -cain.falls are fre#uently used for tese auxiliarylines


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Planning and Setup

0en te member as areas of small crosssection or large cantilevers! it may be necessaryto add a structural steel -strongbac) to te piece

to provide added strengt


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Planning and Setup

,embers tat re#uire a secondary process prior tosipment! suc as sandblasting or attacment ofaunces! may need to be rotated at te productionfacility( 1n tese cases! it may be necessary to cast in

extra lifting devices to facilitate tese maneuvers


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Planning and Setup

0en developing member sapes! te designersould consider te extra costs associated "itspecial rigging or forming! and pieces re#uiringmultiple andling


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Stripping: General

*rientation of members during storage! sipping and final in.place position iscritical in determining stripping re#uirements

Tey can be ori%ontal! vertical or some angle in bet"een

Te number and location of lifting devices are cosen to )eep stresses "itin teallo"able limits! "ic depends on "eter te -no crac)ing or -controlledcrac)ing criteria is to be used


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Stripping: General

1t is desirable to use te same lifting devices forbot stripping and erection2 o"ever! additionaldevices may be re#uired to rotate te member toits final position


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Stripping: General

Panels tat are strippedby rotating about oneedge "it lifting devices

at te opposite edge "illdevelop moments asso"n


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Stripping: General

0en panels are stripped tis "ay! care

sould be ta)en to prevent spalling of

te edge along "ic te rotationoccurs

A compressible material or sand bed

"ill elp protect tis edge


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Stripping: General

,embers tat arestripped flat fromte mold "ill

develop temoments so"n


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Stripping: General

1n some plants!tilt tables orturning rigs areused to reduce

strippingstresses


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Stripping: General

Since te section modulus "it respect to tetop and bottom faces may not be te same! tedesigner must select te controlling designlimitation'

Tensile stresses on bot faces to be less tan tat "ic"ould cause crac)ing

Tensile stress on one face to be less tan tat "ic "ouldcause crac)ing! "it controlled crac)ing permitted on teunexposed face

$ontrolled crac)ing permitted on bot faces


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Stripping: General

1f only one of te faces is exposed to

vie"! te exposed face "ill generally

control te stripping metod


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Rigging Configurations

Stresses and forces occurring duringandling are also influenced by te type

of rigging used to oo) up to te

member


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+ift line forces for a t"o.point lift using

inclined lines are so"n


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0en te sling angle is small! te components offorce parallel to te longitudinal axis of temember may generate a significant moment dueto secondary effects


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0ile tis effect can and sould be accountedfor! it is not recommended tat it be allo"ed todominate design moments


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$onsideration sould begiven to using spreaderbeams! t"o cranes oroter mecanisms toincrease te sling angle

Any suc special andlingre#uired by te design

sould be clearly so"non dra"ings


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3sing a spreader beam can also eliminate te use ofrolling bloc)s

4ote tat te spreader beam must be sufficientlystiffer tan te concrete panel to limit panel

deflections and crac)ing +ifting oo) locations! oo) eigts! and slinglengts are critical to ensure even lifting of temember

For analysis! te panel acts as a continuous beam

over multiple supports


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Stripping Design

To account for te forces on te member

caused by form suction and impact! it is

common practice to apply a multiplier to temember "eigt and treat te resulting force

as an e#uivalent static service load(

Te multipliers cannot be #uantitatively

derived! so tey are based on experience


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Stripping Design

P$1 provides a table of typical values


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Factor of Safety

0en designing for stripping and andling! tefollo"ing safety factors are recommended'

5 3se embedded inserts and erection devices "it a

pullout strengt at least e#ual to four 678 times te

calculated load on te device(5 For members designed -"itout crac)ing! te

modulus of rupture 6,*98 ! is divided by a safety

factor of :(;(

7.5

1.5= 5


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Stress Limits & Crack Control

Stress limits for prestressed members during

production are discussed in Section 7(


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Stress Limits & Crack Control

,embers "ic are exposed to vie" "ill generally bedesigned for te -no discernible crac)ing criteria 6seeE#( 7(.?


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Benefits of Prestressing

Panels can be prestressed! using eiter

pretensioning or post.tensioning(

Design is based on $apter :> of A$1 =:>.

?

restricted to less tan ! must be

follo"ed(5


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Benefits of Prestressing

1t is recommended tat te average stress due toprestressing! after losses! be "itin a range of :?? psi

Te prestressing force sould be concentric "it teeffective cross section in order to minimi%e camber!

altoug some manufacturers prefer to ave a sligtin"ard bo" in te in.place position to counteracttermal bo"

1t sould be noted tat concentrically prestressedmembers do not camber! ence te form adesion

may be larger tan "it members tat do camber


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Strand Recomendation

1n order to minimi%e te possibility of splittingcrac)s in tin pretensioned members! testrand diameter sould not exceed tatso"n in te table belo"

Additional ligt transverse reinforcement maybe re#uired to control longitudinal crac)ing


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Strand Recommendations

0en "all panels are post.tensioned! care must beta)en to ensure proper transfer of force at te

ancorage and protection of ancors and tendons

against corrosion

Straigt strands or bars may be used! or! to reduce

te number of ancors! te metod so"n may be

used


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Strand Recommendation

1t sould be noted tat if an unbonded tendon

is cut! te prestress is lost( Tis can

sometimes appen if an unplanned openingis cut in at a later date


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Handling Devices

Since lifting devices are sub&ect to dynamicloads! ductility of te material is a re#uirement

Deformed reinforcing bars sould not be used

as te deformations result in stressconcentrations from te sac)le pin

Also! reinforcing bars may be ard grade orre.rolled rail steel "it little ductility and lo"

impact strengt at cold temperatures

i i


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Handling Devices

Strain ardening from bending may causeembrittlement

Smoot bars of a )no"n steel grade may be

used if ade#uate embedment or mecanicalancorage is provided

Te diameter must be suc tat locali%edfailure "ill not occur by bearing on te

sac)le pin

Ai f C bl L


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Aircraft Cable Loops

For smaller precast members! aircraft cable can beused for stripping and erection purposes

Aircraft cable comes in several si%es "it differentcapacities

Te flexible cable is easier to andle and "ill not leaverust stains on precast concrete

Ai f C bl L


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Aircraft Cable Loops

For some small precast members suc as coping! teflexible loops can be cast in ends of members andtuc)ed bac) in te &oints after erection

Aircraft cable loops sould not be used as multipleloops in a single location! as even pull on multiple

cables in a single oo) is extremely difficult to acieve 3ser sould ensure tat te cable is clean and tat

eac leg of te loop is embedded a minimum of 7> in(

P i S d L


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Prestressing Strand Loops

Prestressing strand! bot ne" andused! may be used for lifting loops

Te capacity of a lifting loop

embedded in concrete is dependentupon te strengt of te strand! lengtof embedment! te condition of testrand! te diameter of te loop! and

te strengt of te concrete

P t i St d L


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Prestressing Strand Loops

As a result of observations of lift loop beaviorduring te past fe" years! it is important tatcertain procedures be follo"ed to preventbot strand slippage and strand failure

Precast producers tests and/or experienceoffer te best guidelines for te load capacityto use

A safety factor of 7 against slippage orbrea)age sould be used

St d L R d ti


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Strand Loops Recommendations

1n lieu of test data! te recommendationslisted belo" sould be considered "en usingstrand as lifting loops(

5 ,inimum embedment for eac leg of te loopsould be


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$ontinued'5 Te diameter of te oo) or fitting around

"ic te strand lifting eye "ill be placedsould be at least four times te diameterof te strand being used

5 Do not use eavily corroded strand orstrand of un)no"n si%e and strengt(

St d L R d ti


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1n te absence of test or experience! it isrecommended tat te safe load on a

single :/< in( diameter )ips

Te safe "or)ing load of multiple loops

may be conservatively obtained by

multiplying te safe load for one loop by:(B for double loops and


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To avoid overstress in one loop "enusing multiple loops! care sould beta)en in te fabrication to ensure tat

all strands are bent te same Tin "all conduit over te strands in

te region of te bend as been usedto reduce te potential for overstress



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0en using double or tripleloops! te embedded endsmay need to be spreadapart for concrete

consolidation aroundembedded ends "itoutvoids being formed bybundled strand

Th d d I t


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Threaded Inserts

Treaded inserts can ave 4$

64ational $ourse8 or coil treads Ancorage is provided by loop! strut

or reinforcing bar

1nserts must be placed accurately

because teir safe "or)ing loaddecreases sarply if tey are notperpendicular to te bearing surface!or if tey are not in a straigt line"it te applied force

Th d d I t


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Threaded Inserts

Embedment of inserts close to an edge "ill greatlyreduce te effective area of te resisting concretesear cone and tus reduce te tension safe "or)ingload of te embedded insert

0en properly designed for bot insert and concretecapacities! treaded inserts ave many advantages

Ho"ever! correct usage is sometimes difficult toinspect during andling operations

Threaded Inserts


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Threaded Inserts

1n order to ensure tat an embedded insert acts

primarily in tension! a s"ivel plate as indicated in

sould be used

1t is extremely

important tat

sufficient treads

be engaged to

develop testrengt of te bolt

Threaded Inserts


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Threaded Inserts

For straigt tension loads only! eye bolts or "irerope loops provide a fast metod for andlingprecast members(

Do not use eiter device if sear loading conditions

exist(

Proprietary Devices


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Proprietary Devices

A variety of castings

or stoc) steel

devices! macined

to acceptspeciali%ed lifting

assemblies are used

in te precast

industry

Proprietary Devices


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Proprietary Devices

Tese proprietary devices are usually recessed 6using a -poc)et former8to provide access to te lifting unit( Te recess allo"s one panel to beplaced against anoter "itout cutting off te lifting device! and alsoelps prevent spalling around te device

+onger devices are used for edge lifting or deep precast concretemembers

Sallo" devices are available for tin precast concrete members(

Proprietary Devices


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Proprietary Devices

Te longer devices

usually engage a

reinforcing bar to

provide greater pulloutcapacity! and often ave

oles for te bar to pass

troug as so"n to te

left

Proprietary Devices


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Proprietary Devices

Tese units ave a rated capacity as

ig as


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Proprietary Devices

9einforcing bars are re#uired in t"o directions over te base to fully develop te lifting unit! as

so"n in Figure belo"

Tese inserts are

rated up to > tons

Proprietary Devices


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Proprietary Devices

Some lifting eyes do not s"ivel! so rotation may bea concern

1n all cases manufacturer recommendations

sould be rigorously follo"ed "en using any oftese devices

Wall Panel Example


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Wall Panel Example

Tis example and oters in $apter ; illustrate teuse of many of te recommendations in tis capter

Tey are intended to be illustrative and general only

Eac manufacturer "ill ave its o"n preferredmetods of andling

Wall Panel Example


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Wall Panel Example

Given:

A flat panel used as a loadbearing "all on a t"o.storystructure! as so"n on next slide

Section properties 6nominal dimensions are used fordesign8'

Solid panel Panel "it openingsA = C? in? in.?< suggests te total deformation yt! at any time can be estimated as'

yt

=

1 + ( )

Storage


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amplification due to creep and srin)age as afunction of @6reinforcement ratio for non.prestressedcompressionreinforcement!

As/bt8

Transportation


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Te metod used for transport can affect testructural design because of si%e and "eigtlimitations and te dynamic

Except for long prestressed dec) members! mostproducts are transported on eiter flatbed or lo".

boy trailers Trailers deform during auling Si%e and "eigt limitations vary from one state to

state

+oads are furter restricted on secondary roads Te common payload for standard trailers

"itout special permits is


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+o".boy trailers permit te eigt to be increased to about :? to :< ft(

Ho"ever tey ave a ave a sorter bed lengt(

Tis eigt may re#uire special routing to avoid lo" overpasses and overead "ires

Transportation


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Erection is simplified "en members are transported in te same orientation tey "ill ave in testructure

For example! single.story "all panels can be transported on A.frames "it te panels uprigt

A.frames also provide good lateral support and te desired t"o points of vertical support

Transportation


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+onger units can be transportedon teir sides to ta)e advantage

of te increased stiffness

compared "it flat sipment

Transportation


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1n all cases! te panel support locations sould be consistent "it te panel design

Panels "it large openings sometimes re#uire strongbac)s! braces or ties to )eepstresses "itin te design values

Transportation


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For members not symmetrical "it respect to te bending axis! te follo"ingexpressions can be used for determining te location of supports to give e#ualtensile stresses for positive and negative bending moments

Transportation


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For one end cantileveredK

0ereyb distance from te bending axis to te bottom fiber

yt distance from te bending axis to te top fiber

x =1

21 +

1 +

?

???

?

???

Transportation


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For t"o ends cantileveredK

0ereyb distance from te bending axis to te bottom fiber

yt distance from te bending axis to te top fiber

x =1

2 1 + 1 +

?

?

??

?

?

??

Erection


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Precast concrete members fre#uently must bereoriented from te position used to transportto its final construction position

Te analysis for tis -tripping 6rotating8

operation is similar to tat used during oterandling stages

1n capter ; in te P$1 andboo)! maximummoments for several commonly used tripping

tecni#ues are illustrated

Tripping Design Guide


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Erection


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0en using t"o crane lines! te center of

gravity must be bet"een tem in order toprevent a sudden sifting of te load "ile itis being rotated

To ensure tat tis is avoided! te stabilitycondition so"n must be met'

e >

2 2 2

2

Erection


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Te capacities of lifting devices must be cec)edfor te forces imposed during te trippingoperation! since te directions vary

0en rotating a panel "it t"o crane lines! te

pic) points sould be located to prevent te panelfrom an uncontrolled roll on te roller bloc)s canbe done by sligtly offsetting te pic) pointlocations to sift te "eigt to"ard te uppercrane line lift points! or by using cain drags on

te rolling bloc)

Erecting Wall Panels Example


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Given'Te "all panels "itopenings used onprevious examples

Problem'

Determine appropriateprocedures for erectingte "all panels "itopenings! panel "ill besipped flat



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Assumptions +imit stresses to 6?(=;7 )si8(

$rane as main and auxiliary lines(

A telescoping man lift is available on site(

Solution:

Try tree.point rotation up using stripping inserts androlling bloc)' To simplify! conservatively use solid panel6no openings8 to determine moments(

5 f !



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" = 1.0/

=

35.17( ) 1( )35.17

2

#

$%&

'(

35.17 0.604 +0.292

2#$%

&'(

= 23.4

1 = 2 = 11.7



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1n Hori%ontal Position

Terefore! = point pic) not ade#uate

)* = 35.17( ) 1( ) 23.4 = 11.8

MMA+ = 69.6 = 835.2

=1.2(835.2 )

635= 1.58 > 0.354



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no"ing from te stripping analysistat a four.point pic) can be used!te configurations so"n ere maybe used

Ho"ever! tis rigging may become

unstable at some point duringtripping! i(e(! continued rotation"itout tension in +ine A

Terefore! te lo"er end of tepanel must stay "itin inces of teground to maintain control(



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Jecause te previous configuration

re#uires six rolling bloc)s and can be

cumbersome! te metod so"n on tefollo"ing slide may be an alternative



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Erection Bracing Introduction


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Tis section deals "it te temporary bracing "icmay be necessary to maintain structural stability of aprecast structure during construction

0en possible! te final connections sould be used

to provide at least part of te erection bracing! butadditional bracing apparatus is sometimes re#uiredto resist all of te temporary loads

Erection Bracing Introduction


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Tese temporary loads may include "ind! seismic!eccentric dead loads including construction loads!unbalanced conditions due to erection se#uenceand incomplete connections Due to te lo"

probability of design loads occurring duringerection! engineering &udgment sould be used toestablis a reasonable design load

Erection Bracing Responsibilities


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Proper planning of te construction process is essential forefficient and safe erection

Se#uence of erection must be establised early! and teeffects accounted for in te bracing analysis and tepreparation of sop dra"ings

Te responsibility for te erection of precast concrete mayvary as follo"s'5 6see also A$1 =:>.?< Section :?(=8



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Te precast concrete manufacturer supplies teproduct erected! eiter "it is o"n forces! or by anindependent erector

Te manufacturer is responsible only for supplying teproduct! F(*(J( plant or &obsite

Erection is done eiter by te general contractor or byan independent erector under a separate agreement



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Te products are purcased by an independent erector "o

as a contract to furnis te complete precast concretepac)age(

9esponsibility for stability during erection must be clearlyunderstood(

Design for erection conditions must be in accordance "it all

local! state and federal regulations( 1t is desirable tat tisdesign be directed or approved by a Professional Engineer



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1t is desirable tat tis design be directed or approved by aProfessional Engineer

Erection dra"ings define te procedure

on o" to assemble te components into te final structure

Te erection dra"ings sould also address te stability of

te structure during construction and include temporaryconnections



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0en necessary! special dra"ings may be re#uiredto include soring! guying! bracing and specificerection se#uences

1t is desirable tat tis design be directed or

approved by a Professional Engineer Erection dra"ings define te procedure

on o" to assemble te components into te finalstructure



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Te erection dra"ings sould also address te stability of te structureduring construction and include temporary connections

0en necessary! special dra"ings may be re#uired to include soring!guying! bracing and specific erection se#uences



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For large and/or complex pro&ects! a pre.&ob conference prior tote preparation of erection dra"ings may be "arranted! in orderto discuss erection metods and to coordinate "it oter trades

Handling Equipment


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Te type of &obsite andling e#uipmentselected may influence te erection se#uence!and ence affect te temporary bracingre#uirements

Several types of erection e#uipment areavailable! including truc).mounted and cra"lermobile cranes! ydraulic cranes! to"er cranes!monorail systems! derric)s and oters

Te PCI Recommended Practice for Erectionof Precast Concrete provides more informationon te uses of eac(

Surveying and Layout


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Jefore products are sipped to te &obsite! a field cec) of te pro&ect is recommended to ensure tat priorconstruction is suitable to accept te precast units

Tis cec) sould include location! line and grade of bearing surfaces! notces! bloc)outs! ancor bolts!cast.in ard"are! and dimensional deviations

Site conditions suc as access ramps! overead electrical lines! truc) access! etc(! sould also be cec)ed

Surveying and Layout


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Any discrepancies bet"een actual conditions and tose so"n on dra"ings sould be addressed before erection isstarted

Surveys sould be re#uired before! during and after erection'5 Jefore! so tat te starting point is clearly establised and any potential difficulties "it te support structure are determined early(

5 During! to maintain alignment(

5After! to ensure tat te products ave been erected "itin tolerances(

Loads on Structure


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Te publication Design Loads on Structures During Construction 6SE1/AS$E =B.?


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$olumns "it eccentric loads from oter framing

members produce sides"ay "ic means tecolumns lean out of plumb

A similar condition can exist "encladding panels are erected on oneside of a multistory structure

Temporary Loading Examples


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3nbalanced loads due to partially complete

erection may result in beam rotation

Te erection dra"ings sould address tese

$onditions



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Some solutions are'5 1nstall "ood "edges

bet"een flange of teeand top of beam

5 3se connection tocolumns tat preventrotation

5 Erect tees on botsides of beam

5 Prop tees to levelbelo"



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9otations and deflections of framing members may

be caused by cladding panels( Tis may result inalignment problems and re#uire connections tatallo" for alignment ad&ustment after all panels areerected



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1f construction e#uipment suc as concrete buggies! man.lifts! etc(! areto be used! information suc as "eel loads and spacing sould beconveyed to te designer of te precast members and te designer ofte erection bracing

Factors of Safety


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Suggested safety factors are so"n

Jracing inserts cast into precast members =

9eusable ard"are ;+ifting inserts 7

Bracing Equipment and Materials


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For most one.story andt"o.story ig

components tat re#uire

bracing! steel pipe

braces similar to toseso"n are used



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Proper ancoring of te braces to te precastmembers and deadmen must be considered

0en te pipe braces are in tension! tere maybe significant sear and tension loads applied

to te deadmen Properly designed deadmen are a re#uirement

for safe bracing

$able guys "it turnbuc)les are normally used

for taller structures



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Since "ire rope used in cable guys can resist onlytension! tey are usually used in combination "itoter cable guys in an opposite direction

$ompression struts! "ic may be te precastconcrete components! are needed to complete

truss action of te bracing system A number of "ire rope types are available

4ote tat capacity of tese systems is oftengoverned by te turnbuc)le capacity

General Considerations


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$areful planning of te erection se#uence is important

Tis plan is usually developed by a coordinated effortinvolving te general contractor! precast erector!precaster production and sipping departments and astructural engineer

A properly planned erection se#uence can reducebracing re#uirements

For example! "it "all panel systems a corner can firstbe erected so tat immediate stability can be acieved



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Similar considerations for sear "all structures canalso reduce bracing re#uirements

All parties sould be made a"are of te necessity ofclosely follo"ing erection "it te "elded diapragmconnections

Tis includes te diapragm to sear "all connections



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1n order for precast erection to flo" smootly'

5 Te site access and preparation must be ready

5 Te to.be.erected products must be ready

5 Precast sipping must be planned

5 Te erection e#uipment must be ready

5 Jracing e#uipment and deadmen must be ready


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