CE4103 Precast Concrete Technology

(1)

CE4103

Design

Project

Precast

Concrete

Technology

Professor

Professor TTanan KiangKiang HweeHwee Dept of

Dept of CivilCivil && EnvironmentalEnvironmental EngineeringEngineering National

National UniversityUniversity of of SingaporeSingapore

What is

Precast

Concrete?

••

Concrete

Concrete that

that has

has been

been prepared

prepared for

for casting,

casting, cast

cast and

and cured

cured in

in a

a

location

location which

which is

is not

not its

its final

final destination

destination

•

• Precast

Precast

concrete

structure

–

– an

an assemblage

assemblage of

of precast

precast elements

elements

which,

which, when

when suitably

suitably connected

connected together,

together, form

form a

a 3D

3D framework

framework

capable

(2)

KH

KHTTaann 33

Why

Precast

Concrete?

••

Less

Less labour

labour

••

High

High quality

quality &

& more

more flexible

flexible design

design

••

Faster

Faster construction

construction

••

Low

Low maintenance

maintenance

••

Pricing

Pricing known

known

(3)

KH

KHTTaann 33

Why

Precast

Concrete?

••

Less

Less labour

labour

••

High

High quality

quality &

& more

more flexible

flexible design

design

••

Faster

Faster construction

construction

••

Low

Low maintenance

maintenance

••

Pricing

Pricing known

known

••

May

May be

be dismantled

dismantled &

& re

re

_‐‐

used

18

18 wkwk constructionconstruction Wausau,

Wausau, WI,WI, JeffersonJefferson St.St. RampRamp retail

retail floorfloor withwith 44 levelslevels of of parkingparking aboveabove thin

(4)

KH

KHTTaann 55

BOX

BOX SYSTEMSYSTEM Hilton

Hilton GardenGarden HotelHotel Detroit

Detroit 10 10 storystory floors

floors useuse 8’8’ hollowhollow corecore

28’

28’widewidewallwallsegments,segments,

each

eachsegmentsegmentsetsetandandanchoredanchored~1hr~1hr

“simple”

“simple”systemsystem

contractor

contractor suggestedsuggestedreplacement:replacement:

originally

originallyconcreteconcretemasonrymasonryunits,units,

8”

8” wallwallsamesamethicknessthicknessasasCMUCMU

(12”

(12” CMUCMUononfirst,first,8”8”wallwallused)used)

2

(5)

KH

KHTTaann 77

Applications

•

• Building

Building

structures

•• Residential Residential buildingsbuildings

•• Office Office buildingsbuildings

•• Warehouses Warehouses && industrialindustrial buildingsbuildings

•• Others Others

•

• Parking

Parking

structures

••

Stadiums

Stadiums // Arenas

Arenas

••

Bridge

Bridge structures

structures

••

Others

References

•• Elliott, K.S., Elliott,K.S., PrecastPrecast ConcreteConcrete Structures,Structures, ButterworthButterworth_‐‐Heinemann, 2002,Heinemann,2002, 375p375p

•• Elliott, K.S. Elliott,K.S. MultiMulti_‐‐storeystorey PrecastPrecast ConcreteConcrete FramedFramed Structures,Structures, Backwell ScienceBackwell Science Ltd,Ltd, 1996,1996, 601p.

601p.

•• Bljuger, F Bljuger,F.. DesignDesign of of PrecastPrecast ConcreteConcrete Structures.Structures. EllisEllis Horwood,Horwood, Chichester,Chichester, UK,UK, 1988.1988.

•• British Standards BritishStandards Institution,Institution, TheThe StructuralStructural UseUse of of Concrete:Concrete: BSBS 81108110_‐‐1997,1997, London,London, 1997.

1997.

•• Construction Industry ConstructionIndustry DevelopmentDevelopment Board,Board, StructuralStructural PrecastPrecast ConcreteConcrete Handbook,Handbook, Singapore,

Singapore, 22ndnd_ed,_{ed, 2001.}_2001.

•• Haas, A.M. Haas,A.M. PrecastPrecast ConcreteConcrete DesignDesign andand Applications,Applications, AppliedApplied ScienceScience Publishers,Publishers, London,London, 1983.

(6)

L1

Materials

CE4103

CE4103PrecastPrecast ConcreteConcrete TechnologyTechnology

• Concrete • Concrete • Steel

• Steel reinforcementreinforcement • Pre

• Pre

_‐‐

tensioningtensioning steelsteel • Structural

• Structural steelsteel && boltsbolts • Non

• Non

_‐‐

cementitiouscementitious materialsmaterials

by

by ProfessorProfessorTTANANKiangKiangHweeHwee

Dept of

Dept of CivilCivil&&EnvironmentalEnvironmentalEngrgEngrg

National

NationalUniversityUniversityof of SingaporeSingapore

Concrete

Concrete used

used in

in Precast

Precast Elements

Elements

C Coommppoonneenntt TyTyppee GGrraaddee f f cucuat 28at 28 days days (MPa) (MPa) Demould Demould concrete concrete strength strength (MPa) (MPa) Design Design strength strength (MPa) (MPa) Tensile Tensile strength strength (MPa) (MPa) E Eccat 28at 28 days days (GPa) (GPa) E Ecici (GPa) (GPa) Beams, Beams, shear walls, shear walls, staircases, staircases, wet-cast wet-cast R RCC GG4400 4400 2200--2255 1188..00 NN//AA 2288 NN//AA

(7)

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Concrete used in Composite Construction

Component f _cu (MPa) f _t(MPa) E_c(GPa)

In-situ 25 - 25 In-situ 30 - 26 Precast reinforced 40 - 28 Prestressed 50 3.2 30 Prestressed 60 3.5 32

Steel Reinforcement

• Hot

_‐

rolled ribbed bars for main/flexural reinforcement

• 16, 20, 25, 32, 40 mm

• Mild

_‐

steel bars for shear links, projecting loops, etc

• Column stirrups: 8, 10 mm

• Beam stirrups & distribution/anti_‐crack bars: 10, 12 mm

• Welded fabric/mesh

• Flat panels, walls, etc.: A142 (6 mm bars @ 200 mm centres bw) & A193 (7 mm bars @ 200 mm centres bw)

• One_‐way spanning units: C283 (6 mm bars @ 100 mm centres x 5 mm bars @ 400 mm centres)

• Characteristic strength & Young’s modulus

• f _y = 500 MPa

(8)

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Pre

_‐

tensioning Steel (1)

• 2 main types used:

• Plain or indented/crimped wire

• 7

_‐

wire helical strand

• Class 2 – 5% low relaxation

• Stress after 1000 hr = 0.95 x original

Pre

_‐

tensioning Steel (2)

Type Diameter (mm) Cross-section area (mm2₎ Characteristic load (kN) Nominal characteristic strength (MPa) Elastic modulus (GPa) Wire 5.0 19.6 30.8 1570 205 7.0 38.5 60.4 1570

(9)

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Structural Steel & Bolts

• Used at connections, in particular

• Include rolled hollow sections (RHS, SHS), channels & angles, plates & welded

_‐

Ts, etc

• Steel grade: 43 (mostly) and 50

• Hot

_‐

dipped galvanized steel for exposed connections

• Grade 43 & 50 (more highly stressed plate) plates

• Black bolts 4:6 and 8:8

• High

_‐

strength friction grip bolts

Non

_‐

Cementitious Materials

• Epoxy

_‐

based mortars for connections (partial or complete) where

rapid gain in strength is required (e.g. 40 MPa in 2

_‐

3 hrs)

• Thermal expansion of epoxy materials (7x that of concrete) should be

accounted for.

• Occasionally used as pressure injections for crack filling or to restore

tensile strength

• Neoprene, rubbers & mastics used for soft bearing, back strips, etc.

(refer to PCI Manual on Architectural Precast Cladding)

(10)

L2 Design Theories

CE4103 Precast Concrete Technology

• Basis for design • Shear

_‐

friction theory • Horizontal interface shear • Strut

_‐

and

_‐

tie model

by Professor TAN Kiang Hwee Dept of Civil & Environmental Engrg National University of Singapore

Basis for Design (1)

• Basis for analysis & design involve recommended methods of design and detailing for RC & PC

• Main difference between precast buildings & cast in

_‐

situ buildings

(11)

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Basis for Design (2)

• Design of connections

• Of fundamental importance & must be carefully considered

• Connections must respond to:

• Resistance to all design forces

• Ductility to deformations

• Volume changes

• Durability & fire resistance

• Production & construction considerations

Some examples (1)

• Movement between precast members

• Volumetric changes due to shrinkage, thermal or load induced strains

(12)

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Some examples (2)

• Flexural rotation of member ends

Some examples (3)

• Lateral splitting

(13)

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Some examples (4)

• Loss of bearing

• Due to accidental loading

H >R

Some examples (5)

• Loss of bearing

(14)

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Codes/Guides on

PrecastConcrete

• Singapore Standard SS EN 1992

_‐

1 _‐

1 : 2008, Eurocode 2: Design of

concrete structures – Part 1

_‐

1: General rules and rules for buildings,

Ch. 10, Additional rules for precast concrete elements & structures

• ACI318

_‐

05, ACI Building Code, Chaps. 16 & 17

• ACI 550R

_‐

96, Design Recommendations for Precast Concrete

Structures

• PCI Design Handbook, PCI, 6

th

_Edition

• BCA Publications

Shear Friction Theory (1)

• Exterior edges of precast members are acted upon by large concentrated loads.

• They are subject to a type of failure called shear friction.

• These large forces cause the vulnerable part of the member to shear off along a plane on which high shear stresses act.

(15)

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Shear Friction Theory (2)

• For failure to be classified as shear friction, the bending moment on the failure surface must be small

• Design against shear friction failure is based on positioning steel reinforcement across the potential failure surface

Block of concrete anchored to a concrete surface by a steel dowel of area A_vf

Failure surface

Shear Friction Theory (3)

F _r =  N =  A_vf f _y

where  is the coefficient of friction. V _u=  F _r=  (  A_vf f _y )

where  is a reduction factor. For shear  = 0.75 (ACI). Amount of steel needed for a particular V _u is

A_vf = V _u / (  f _y )

Failure surface

(16)

KH Tan 29

Typical values of



(ACI 11.7.4.3)

• Concrete cast monolithically

1.4 

• Concrete placed against

hardened roughened concrete

1.0 

• Concrete placed against

unroughened hardened concrete

0.6 

• Concrete anchored to structural steel

0.7 

where



= 1.0 for normal weight concrete, 0.85 for sand

_‐

lightweight concrete, and 0.75 for all lightweight concrete

Inclined shear friction reinforcement

(17)

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Typical examples (1)

(18)

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Typical examples (3)

Horizontal Interface Shear (1)

Two independent beams No horizontal shear

(19)

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Horizontal Interface Shear (2)

• Horizontal shear stresses on the contact surface between an uncracked elastic precast beam and slab can be computed from

V = shear force acting on the section in question

Q = first moment of the area of the slab or flange about the neutral axis of the composite section

I_c = Moment of inertia of the composite section

b_v= width of the interface between the precast beam and cast_‐in_‐place slab

v c h b I Q V V



Horizontal Interface Shear (3)

• ACI defines horizontal shear force V _nh to be transferred as

 V _nh



V _u

which gives

• This is based on the observation that in an element directly over the beam web, v_nh = v_nand v_n = V _n /b_v d d b V v v u nh



/



(20)

KH Tan 37

Horizontal Interface Shear (4)

• Alternative method

• At midspan, the force in the_{compression zone is C. All of}

this force acts above the interface. At the end of the beam, the force in the flange is zero. Thus the horizontal shear force to be transferred across the interface between the midspan and the support is

 V _nh = C

Horizontal Interface Shear (5)

• A similar derivation could be made if the flange were in the tension.

(21)

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Horizontal Interface Shear (6)

• For the simply

_‐

supported beam shown:

K = 2 at end K = 0 at midspan

• Limit values v_nh (ACI318)

Contact Surfaces Ties Limiting v_nh(MPa)

Intentionally roughened

None

Not roughened Minimum A_v (Cl. 16.7) Intentionally roughened A_v f _y 55 . 0 55 . 0         s b f A v y v 6 . 0 79 . 1

Horizontal Interface Shear (7)

• In all cases the contact surfaces must be clean and free of laitance.

• The words “intentionally roughened” imply that the surface has been roughened with a “full amplitude” of 6 mm, where “full amplitude” refers to the total height (twice the amplitude) of the roughness.

• The “wave length” of the roughness is intended to be of the same magnitude as the height, say 6 to 19 mm.

• When the stress due to factored shear force at the section exceeds 3.45 MPa, ACI requires design using shear friction.

(22)

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Horizontal Interface Shear (8)

• ACI requires that ties be provided for horizontal shear be not less

than the minimum stirrups required for safety.

• The tie spacing shall not exceed 4 times the least dimension of the

supported element which is usually the thickness of the slab, but not

more than 600 mm.

• The ties must be fully anchored.

(23)

(24)

(25)

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Strut

_‐

and

_‐

Tie Models

• Half

_‐

joints

Strut

_‐

and

_‐

Tie Models

(26)

L3 Frames, Components & Connections

• Identification of building frames • Selection of Components • Roof and floor slabs • Staircases • Beams • Columns • Bracing walls • Slab

_‐

to

_‐

beam connections • Beam

_‐

to

_‐

column connections

Identification of Building Frames

• Building design optimization

• Maximize repetitive & modular dimensions for plan layout & member dimensions

(27)

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Structural Systems

floor load

continuous load path

Collector for Lateral Loads:

Resist Lateral Load roof or floors acting as diaphragms

Wind or EQ load collected by diaphragm

Brought to columns

The roof/floor should work as a diaphragm to transfer horizontal forces from that level to the vertical system.

(28)

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Vertical System:

Bracing to Resist Lateral Drift wall lateral loads

Lateral loads are transferred to ground by vertical system:

columns walls

frames bracing

System Components

frame systems

(29)

KH Tan 55

Frame Variations (1)

All frame system Frame collects and transfers vertical loads; columns act as cantilevers in resisting lateral loads,

Components:

• precast floor panels • precast beams and

spandrels

• precast columns

Frame Variations (2)

All frame with interior shear wall/core

Frame carries vertical loads;

shear walls resist lateral loads,

Components:

• precast floor panels • precast wall panels • precast beams and

spandrels

(30)

KH Tan 57

Frame Variations (3)

Exterior shear walls with interior frame Perforated exterior wall elements resist lateral load + vertical, interior frame carries vertical loads

Components:

• precast floor panels • precast beams and

spandrels

• precast load bearing walls • precast columns

Frame Components

top layer floor = hollow core solid slabs double Tee collector = inverted Tee “L” or spandrel beam

(31)

KH Tan 59

columns with corbels

Simple system: stacked components

2. inverted Tee sits on column corbels 3. floor slab sits

on ledgers

The “stacked” nature of the system leaves it unstable in some conditions, we need to provide “positive connections” to ensure stability (integrity requirements) 2

1. columns set first, held with temporary bracing

Bearing Wall / Box Components

top layer

bottom layer

floor = hollow core solid slabs

collector = _walls

transfer =

(32)

KH Tan 61

Box components

1. set a bearing wall into place, with temporary bracing

2. stack floor slabs on top of the bearing wall, then place another wall above and repeat the

sequence _{Again “stacked” nature of the system:}

need to provide “positive connections” to ensure stability (integrity requirements) 1

• “up and out” construction system

• stacked components conditionally stable until positive connections made

(33)

KH Tan 63

columns and middle bay placed

temporary bracing is

essential until shear walls are

placed, or connections are

(34)

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Precast Components & Systems

(35)

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Selection of Components

• Standard components

• Slabs, beams & columns

• Dimensions & load bearing capacity (available in catalogues, handbooks)

• Non

_‐

standard components

• Architectural concrete, e.g. façade elements

• Designed by architects

Roof & Floor Slabs

• 4 main types

• Prestressed hollow core floor

• Reinforced & prestressed double

_‐

T floor

• Composite prestressed plank

_‐

floor or composite beam and plank

• Beam & block floors

with or w/o structural topping screed with structural topping screed

(36)

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Hollow Core _{• Most widely used}

• Highly efficient design & production • Hollow cores used for air heating/

cooling

(37)

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Composite Planks

• Precast slabs (and rectangular beams) used as permanent formwork for an in_‐situ concrete topping

• Robustness equal to that of cast in_‐situ construction

• Floor slab has smooth finish on soffits

• 2.4 m wide, rapid fixing

Beam & Block Floors

(38)

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Basic properties & performance characteristic of precast flooring

Staircases

• Plan configuration and

compatibility with the structure

(39)

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2_‐flight staircase

3_‐flight staircase

• “External”

• where floor loading is predominately non_‐symmetrical

• “Internal”

• where floor loading is approximately symmetrical

(40)

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Internal beams

• Exterior (edge &

spandrel) beams

(41)

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Columns

• Min 300 x 300 mm

• Up to 20 m in length

(usual 12 – 13 m)

• Braced structures:

• A_c = N / 28

• Unbraced structures:

• 2

_‐

storey: 300 mm sq. • 3

_‐

storey: 350 mm sq.

Bracing Walls

• Provides stability and as surrounding walls or boxes for staircases/lift shafts

• Classification:

• Infill walls

• 150_‐300 mm thick

• Acts as diagonal strut

• Cantilever walls

• Designed as deep beams

• Shear cores or boxes

• Location & distribution

• Centre of resistance to coincide with centre of

mass & geometric centroid of completed building

(42)

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Slab

_‐

to

_‐

Beam Connections

Beam

_‐

to

_‐

Column Connections

(43)

L4 Precast Frame Analysis

• Types of precast frames • Simplified frame analysis • Stabilizing methods

Types of Precast Frames (1)

• Skeletal frame

• Flexibility of placement of internal partitions

• Office & retail development

• Wall frame

• Less architectural freedom

• More economical & faster to build

• Hotels, schools, offices & domestic housing

• Portal frame

(44)

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Types of Precast Frames (2)

Types of connections (1)

Pinned – dowel / bolts / tie_‐

bars / welded plates Rigid – continuity steel / couplers / bolts / steel shoes

(45)

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Types of connections (2)

Load transfer (1)

(46)

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Load transfer (2)

Horizontal loads – Unbraced structure

H < H_crit

(47)

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Simplified Frame Analysis (1)

• Global analysis

• A 2

_‐

D in

_‐

plane simplification is appropriate

• Identify positions for connections

Simplified Frame Analysis (2)

• Continuous frame (unbraced)

Pts of contraflexure in:

• Beams: near beam

_‐

column joints (gravity loads predominant) & at mid

_‐

span (horizontal loads

predominant)

(48)

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Simplified Frame Analysis (3)

• Pin

_‐

jointed frame (unbraced)

• Beams transfer no mt

• Columns alone achieve stability

• Not practical for > 10 m or 3

_‐

storey height

• Need to brace for taller structures

Simplified Frame Analysis (4)

• Portal U

_‐

frames

(49)

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Simplified Frame Analysis (5)

• H

_‐

frame

Substructuring Methods

• Sub

_‐

frames can be used to determine M, V and N throughout the structure

• No moment re

_‐

distribution is permitted at pinned connections

• Horizontal wind loads are not considered in sub

_‐

frames, but are added subsequently to columns.

• Elastic analysis is used to determine moments, forces and deflection

• Plastic (ultimate) section analysis is used for component design

• Critical load combinations

• All spans with maximum ultimate load

(50)

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_‐

jointed frame

• Beam subframe

h 2 / 2 h 3 / 2

(51)

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• Upper column subframe

h 1 / 2 h 3 / 2 h 2 L₄ /2 L₅ /2            3 3 2 2 2 2 5 5 4 4 , h EI h EI h EI e R e R M _col_upper            2 2 1 1 2 2 5 5 4 4 , h EI h EI h EI e R e R M _col_lower

• Ground floor column subframe

h 2 / 2 h 1 L₃ /2 L₄ /2            2 2 1 1 1 1 4 4 3 3 , 75 . 0 75 . 0 h EI h EI h EI e R e R M _col_upper            2 2 1 1 1 1 4 4 3 3 , 2 h EI h EI h EI e R e R M _col_upper            2 2 1 1 1 1 4 4 3 3 , h EI h EI h EI e R e R M _col_upper

(52)

KH Tan 101

Example 1

Bending moments in beam X & columns Y & Z = ?

Given: beam_‐column connections are pinned & foundation is rigid

–Distance from edge of column to centre of beam end reaction = 100 mm –g_k = 40 kN/m; q_k = 30 kN/m

–Load factors: 1.35 for DL and 1.5 for LL

Solution

Beam X: e = 450/2 + 100 = 325 mm M₁ = [99 x (8 – 2 x 0.325)2_{/ 8 =} 668.5 kNm Column Z: R₁ = 396 kN ; R₂ = 120 kN Column Y: R₁_{= 99 x 8 / 2 = 396 kN} w _max = 1.35 x 40 + 1.5 x 30 = 99 kN w _min = 40 kN

(53)

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Stabilizing Methods

• Structural components will not

form a stabilizing system until

connections are completed

• A stabilizing system must

comprise

• A horizontal system (floor diaphragm) • A vertical system

• Horizontal system

• Floor diaphragm

analysed as a deep beam supported by shear

walls/cores, columns, and bracings

(54)

KH Tan 105

Example 2

• Determine the shear wall reactions and maximum

moment and shear force for the floor diaphragm

shown.

Solution

Ultimate horizontal load = 1.5 x 3 = 4.5 kN/m Support reactions:

R₁ = 84.4 kN R₂ = 50.6 kN

(55)

KH Tan 107

• Vertical system

• Skeletal, wall or portal frame

• Classification based on bracing:

• Unbraced frame –

horizontal force resisted by moment_‐resisting frame action, or cantilever action of columns

• Braced frame – horizontal force resisted by

cantilever action of walls/cores, in_‐plane panel action of

walls/cores, infill walls, cross bracing, etc.

• Partially braced frame

• Type of stabilizing system may be different in other directions

• Do not use different stabilizing systems in the same direction

• Centroid of stabilizing system should be close to centre of external pressure so as to avoid large torsional effects