EC Seminar March 2015

Design and detailing of Prestressed Building Floors in

Singapore using Euro Code

March 18th 2015

Max Meyer Group Technical Officer

VSL International

1. Prestressing

a. Structural effects of prestressing

b. Partial prestressing concept

c. Corrosion protection of prestressing steel

2. Why to prestress building floors

3. Design specifics when designing and detailing prestressed floors in buildings

4. PT hardware for prestressing floors in buildings

5. Design/detailing of a warehouse floor using BS8110 and EC2

Concrete in Compression

Concrete in Tension

Compression and tension due to normal force

Concrete in Compression

Concrete in Tension

Concrete in Tension

PRECOMPRESSION

By PRESTRESSING

Compressive and tensile normal stresses due to bending

Tension

Tension Tension

Load Load

Tension

Tension Tension

Steel to take tension forces

Passive steel to take tension in concrete

BALANCING of external loading by prestressing

Stress-strain diagram RC and pre-stress steel

Flexural strains over cross section: i) Non stressed pre-stressing steel ii) stressed pre-stressing steel iii) RC High strength steel must be stressed to fully utilize its ultimate strength

(Graphs are not to scale)

Prestressing tendons need to be stressed not Just for SLS

Stress-Strain Diagrams

for prestressing strands and reinforcement

Main effects of prestressing

1. Precompression

Less cracks higher stiffness

2. Balancing of external loads

Less elastic and creep deformation

Slender members

3. Use of high strength steel

Less congestions

Less material to handle

1. Prestressing








Partial Prestressing Concept

Magnitude of prestressing force P can vary from P=0 (passively reinforced concrete) to a P large enough to balance with a suitable tendon profile fully bending moments due to a given external loading.

What is a sensible amount of prestressing force P depends on type of structure and on loading.

Pretensioning Posttensioning

Internal External

TYPES OF PRESTRESSING

1. Prestressing








Corrosion protection of prestressing steel (internal and bonded prestressing)

Initial approach

Pretensioning strands: by alkaline characteristics of concrete, strands are embedded in

with structural/restraint perimeter cracks (if any) small enough not to comprise corrosion confinement function of concrete.

Posttensioning strands: by alkaline characteristics of grout around strands in tendon

duct and of concrete, tendons are embedded in

with structural/restraint perimeter cracks (if any) small enough not to comprise corrosion confinement function of concrete and grout.

Corrosion protection of prestressing steel (internal and bonded prestressing)

Todays approach

Pretensioning strands: no improvements. Of particular concern are still the end zones of pretensioned girders, where cracking is a structural necessity during load transfer.

Posttensioning strands: Provision of different hardware configuration for three protection levels (PL 1 to PL 3)

PL1 (EC): alkaline characteristics of grout around strands in tendon duct and of concrete tendons are embedded in (with structural/restraint perimeter cracks if any small enough not to comprise corrosion confinement function of concrete and grout).

PL2 and PL3 (state of the art PT technology for higher exposure classes and lower structural protection layers): conventional metallic tendon duct is replaced by a leak tight HDPE duct (PT Plus duct) and provision of permanent grout cap for the anchorages.

Increase in concrete cover or provision of compression around tendons is not considered to be an effective and reliable method to improve corrosion protection for internal bonded prestressing strands.

TYPE OF PRESTRESSING

pretensioning Strand embedded in crack free concrete

PL1

Conventional Posttensioning (EC2)

Grout around strands confined by metallic duct embedded in crack free concrete

PL1

Posttensioning With unbonded monostrands

Grease around strand confined by leak Tight PE tube

PL2

State of the art Posttensioning PL2

Grout around strands confined by leak tight PT Plus duct

PL2

1. Prestressing








Prestressed Floor Slabs in Buildings A Singapore Success Story

Orchard Tower, 1978 Marina Bay Sands, 2011

DESIGN ADVANTAGES More for less

Shallow structural depth: flat plates, drop panel slabs and banded beam/1-way slab instead of beam/slab systems:

more number of floors for given height

less enclosed space for given number of floors (less aircon running costs)

Less obstruction for M+E ducting/wiring

Bigger column-free spans without need for excessive structural depth less limitations for future potential change of use

Better crack & deflection control

CONSTRUCTION ADVANTAGES More for less

Use of high strength steel with 4 x UTS of ordinary reinforcement (less material to be handled)

Simple geometry, which allows to cast entire floor efficiently in situ (precasting of only part of floor does not really improve productivity similar to bridge decks, which are only partially precast)

Less on-site labour

Quicker turn-around of formwork

Faster construction

Labour, time and material savings

Improved productivity

Savings in costs

Taikooshing Cityplaza 3 & 4

66 m 3

2 m

Pour 1 Pour 2 Pour 3 Pour 4

300

1800 x 500 Edge Beam

Construction sequences/cycles

Activities

Stressing

Flying forms

Reinforcement

Tendons

Concreting

Curing

Columns

Day 5Day 1 Day 2 Day 3 Day 4

4-Day Construction Cycle

Day 1 AM: Stressing of PT tendons

Day 1 PM: Stripping of forms

Day 1 PM: Flying of table forms

Day 2 AM: Installation of rebar & tendons

Day 3 AM: Concreting

Day 4: Curing of concrete & Casting columns

Fast-Track Construction Traditional Construction

Earlier access for follow-on trades

Prestressing of building floors is not a technical necessity as prestressing of concrete bridge girders but a choice for economical reasons.

1. Prestressing








Structural aspects of prestressed floor slabs in buildings

Made from reinforced and prestressed concrete, which is a non linear composite material.

Part of the overall building frame.

Highly statically in determined.

Activated in numerous stages staggered over long period of time.

Have always significant restraints against free shortening of floors due to prestressing and associated creep.

Structural aspects of prestressed floor slabs in buildings

Not possible to accurately model concrete floor slabs in buildings.

Need for different models to address different aspects of design and detailing:

Gravity loading

Lateral loading

Shortening due to prestressing

Most important principles:

Equilibrium

Ductility

Prestressed floor slabs in buildings

Deviation forces more important than compression since difficult to predict precisely (restraint effects).

Different to concrete bridges applied load similar or bigger than dead load.

Prestressing is a choice for economical reasons.

Partial Prestressing is a must!

Partial prestressing is most economical, if drape for tendon profiles is maximized

minimum permissible concrete cover (magnitude of cover has big impact on

achievable drape for tendon profiles in thin members)

Design aspects Pre-stressing losses

Friction losses

Elastic shortening, if more than 1 cable is stressed

Relaxation of prestressing steel

Creep

Shrinkage

External load (self weight)

Prestressing layout

Prestressing load case modelled with externally applied anchor and deviation forces

Deformation of individual spans

Secondary moment due to prestressing moment

Design aspects: Secondary Moment

1. Prestressing








VSL Slab P-T systems for thin members slabs of building floors

unbonded bonded bonded

VSL monostrand system VSLab system Vslab system

HDPE duct Metallic galvanized duct HDPE PT Plus duct

Permanent grout cap Permanent grout cap

PL2 PL1 PL2

VSL multistrand P-T system for deep members beams of floors in buildings

bonded bonded

VSL Gc System VSL Gc System

Metallic galvanized duct HDPE PT Plus duct

Permanent grout cap

PL1 PL2

Design and detailing of reinforcement in D region at

tendon anchorages

BS8110 EC2

Local zone reinforcement

Designer PT supplier

General zone reinforcement

Designer Designer

Local zone reinforcement

General zone reinforcement

1. Prestressing








Design/detailing of a warehouse floor using BS8110 and EC2

Surya Kusuma

Fabian Graber

Design Report prepared by

Presented example is not an optimized design.

For all items comprehensive sets of formula are given with direct references to the code.

The Structural Eurocode Programme

EN 1990 Eurocode 0 Basis of structural design

EN 1991 Eurocode 1 Actions on structures

EN 1992 Eurocode 2 Design of concrete structures

EN 1993 Eurocode 3 Design of steel structures

EN 1994 Eurocode 4 Design of composite steel and concrete structures

EN 1995 Eurocode 5 Design of timber structures

EN 1996 Eurocode 6 Design of masonry structures

EN 1997 Eurocode 7 Geotechnical design

EN 1998 Eurocode 8 Design of structures for earthquake resistance

EN 1999 Eurocode 9 Design of aluminium structures

Nationally determined parameters defined in National Annex


1. Floor systems in buildings and PT layouts

2. Design philosophy

3. Partial factors

4. Material properties

5. Durability

6. Design input

7. Selection of floor system and PT hardware, preliminary sizing of beams and slab

8. Basic rebar and PT layout at support and midspan

9. Definition of PT profile, calculation of PT losses; selection of uplift forces to be achieved by PT

10. Structural analysis

11. Discussion of flexural fibre stress envelopes at SLS, deformations and static forces at ULS

12. ULS checks

Floor Systems in Buildings

Structural system

Loading in kN/m2

Light LL=2.5 to 7.5

Medium LL=7.5 to 15

Heavy LL=15 to 30

Flat Slabs L/38 to L/42

Flat Slabs with drop panels

L/37 to L/43 L/30 to L/39 L/28 to L/34

beam slab beam slab beam slab

1-way slab/beam L/25 to L/31

L/32 to L/37

L/15 to L/19

L/27 to L/36

L/12 to L/17

L/16 to L/20

2-way slab/beam L/15 to l/17

L/29 to L/36

2-way flat slab (RC middle strip)

2-way flat slab

PT layout

Floor System PT Layout

2-way flat slab with drop panels (RC middle strip)

2-way flat slab with drop panels

PT layout

Floor System PT Layout

Ribbed beams & slab 1-way slab/beam

PT layout




3. Partial factors


5. Durability

6. Design input






12. ULS checks

Design philosophy for design of floors in buildings

BS8110 EC2

resistance ULS: bending, beam and punching shear, torsion

ULS: Bending, beam and punching shear, torsion (fatigue)

serviceability SLS Crack control Deformation Not covered: Vibration (Fatigue) Durability Fire resistance

SLS Stress limitations in concrete and steel Crack control Deformation Not covered: Vibration

durability Corrosion protection of embedded steel Resistance of concrete to attack

Fire resistance X

Design philosophy ULS

Effects due to design values of actions=

Effects of (partial load factor) x (characteristic value of actions)

Effects of the corresponding resistance, which is a function of design value of the material

property ({characteristic value of the material property}/{partial factor for material})

Design philosophy SLS

Effects due to design values of actions

Limiting design values of the relevant serviceability criterion




3. Partial factors


5. Durability

6. Design input






12. ULS checks

Partial factors for actions (ULS) BS8110 EC2

adverse beneficial LC with wind

Adverse (unfavourable)

Beneficial (favourable)

Accompanying variable action

Self weight 1.4 1.0 1.4/1.2/1.0

1.35 or =.925*1.35 = 1.26

1.0 1.35

Superimposed dead load

1.4 1.0 1.4/1.2/1.0

1.35 1.0 1.35

Shrinkage 1.0 1.0 1.0

Prestressing (1.2) 0.9 0.9

Live load 1.6 0 1.2/0 1.5 or o*1.5 = 1.0*1.5 = 1.5

0 1.5*o= 1.5*1.0 / 0

Partial factors for materials (ULS)

BS8110 EC2

ULS ULS

Persistent/ transient

accidental Persistent/ transient

accidental

Concrete 1.5 (bending/ normal force)

1.3 1.5 1.2

Reinforcement 1.15 1.0 1.15 1.0

Prestressing 1.15 1.0 1.15 1.0




3. Partial factors


5. Durability

6. Design input






12. ULS checks

BS8110

EC2

BS8110 (cube)

EC2 (cylinder)

Design compressive stress ULS

17.9Mpa 18.1Mpa

Permissible fibre compressive stress SLS

transfer Min {0.5*25=12.5;0.4*40 = 16} = 12.5Mpa

0.6*20 = 12.0Mpa

service General: 0.33*40 = 13.2Mpa At support: 0.4*40=16Mpa

0.45*32 = 14.4Mpa

Permissible flexural fibre tensile stress SLS

transfer Class 2: 1.8Mpa 2.1Mpa

service Class 2: 2.3Mpa Class 3: 4.0Mpa

3.0Mpa

Hypothetical flexural fibre tensile stress

Class 3, 0.2mm: Slab (275mm): Beam (800mm):

Shear stress (to control compressive stress in inclined compression strut)

5.1Mpa 1.11*0.52*18.1/~2 = 5.2Mpa

BS8110

EC2

BS8110 EC2

Minimum reinforcement for crack control

For 16 at 200mm and 0.3mm crack width: 240MPa

Maximum stress level in passive reinforcement in cracked zone

(2.3+4)*0.5/1%= 315MPa 0.8*500 = 400MPa

Stress limitation in reinforcement (SLS)

BS8110

EC2

BS8110 EC2

Stressing force 75%*1860 = 1395Pa 90%*1636 = 1472MPa =79%*1860

Maximum force after transfer

70%x1860 = 1302Mpa 85%*1636 = 75%*1860 = 1391MPa

Maximum stress level SLS in cracked zone

Not defined 0.75*1860 = 1395Mpa




3. Partial factors


5. Durability

6. Design input






12. ULS checks

Durability Environment Exposure

conditions

Mild Sheltered from severe rain

Moderate Sheltered from severe rain, exposed to condensation

Severe Severe rain, alternate wetting and drying

Very severe Sea water

Extreme Abrasive actions

Class designation

No risk of corrosion or attack

Xo

carbonation XC1 to XC4

chlorides XD1 to XD3

Chlorides from sea water

XS1 to XS3

Freeze/thaw attack

XF1 to XF4

Chemical attack XA1 to XA3

Nominal cover Minimum cover

Maximum crack width

Table 7.1N gives recommended values for maximum crack width for different exposure classes

With regard to prestressing recommended values shall be use in absence of more detailed requirements

FIB has defined and published such more detailed requirements, which ensure, that internal prestressing tendons exposed to higher exposure classes are well protected against corrosion without need for increased concrete cover and need for compression (of questionable effect in buildings due to restraint effects).

Exposure class

Reinforced members

Prestressing

Plastic ducts providing leak tight encapsulation

Steel ducts

Unbonded in plastic ducts

Bonded in PT Plus ducts

Quasi permanent LC Quasi permanent LC

Frequent LC

X0, XC1 0.4mm 0.4mm 0.2mm 0.2mm

XC2, XC3, XC4

0.3mm 0.3mm Compression check

0.2mm

XD1, XD2, XS1, XS2, XS3

Compression check

Compression check

Modified table 7.1N (EC2 1992-1-1 page 119)




3. Partial factors


5. Durability

6. Design input






12. ULS checks

Four storey warehouse building floors do not have to be designed for transfer of horizontal

loading

Design working life: 50 years

Exposure:

Location: Singapore, inland (tropical, but not coastal)

Non air conditioned (average relative humidity in Singapore 85%)

Exposure class XC3 (concrete inside building with high air humidity sheltered from rain)

Loading:

Selfweight: 25kN/m3

SDL: 2kN/m2

LL (warehouse): 15kN/m2

Fire rating: 2 hours




3. Partial factors


5. Durability

6. Design input






12. ULS checks

BS8110 EC2

Beam Type of PT GC 6-12, PL1 GC 6-12, PL2

Duct Corrugated metal duct, circular 80/87

PT Plus plastic duct, circular 76/81

Friction coefficient 0.18rad-1 0.12rad-1

Wobble factor 0.005m-1 0.005m-1

Slab Type of PT VSLab 6-4 and 6-5, PL1

VSLab 6-4 and 6-5, PL2

Duct Corrugated metal duct, flat 20x90

PT Plus plastic duct, Flat 25x90

Friction coefficient 0.18rad-1 0.12rad-1

Wobble factor 0.005m-1 0.005m-1




3. Partial factors


5. Durability

6. Design input






12. ULS checks

Typical cross-section BS8110

Typical cross-section EC2




3. Partial factors


5. Durability

6. Design input






12. ULS checks

tendon profile beam

tendon profile slab

BS8110 EC2

Beam Slab Beam Slab

Drape 615mm 180mm 510mm 153mm

Short term losses 13.9% 15.8% 11.4% 12.3%

Shrinkage strain 220 280 260 290

Creep coefficient 2.5 2.5 2.6 2.3

Relaxation 49MPa 49MPa 67MPa 67MPa

Long term losses 14.9% 12.9% 8.9% 8.7%

Uplift/DL 105% 170% 110% 150%




3. Partial factors


5. Durability

6. Design input






12. ULS checks

Equivalent frame analysis

This model is only for the calculation of static forces from vertical loading and for calculation of secondary effects due to primary moment from load case

prestressing.

beam effective flange width BS8110

beam effective flange width EC2

Selfweight

Prestressing

T=0

T=inf

Superimposed dead load

Live load (pattern loading)

Loading

For concrete strength at 20Mpa (fck)

For concrete strength at t=28 days

Concrete fibre stresses

BS8110 EC2

SLS SLS-1 T0

transfer Fibre stresses

SLS-1 T0

transfer Fibre stresses

SLS-2 Tinf

Pattern loading

Fibre stresses

SLS-2 /SLS-5 Tinf

Charac-teristic LC

Fibre stresses; Check, where sections are crackedreinf. to limit crack width

0.75*LL Deflection SLS-3 Tinf

Quasi-permanent LC

min reinforcement due to crack control; Deflection

SLS-4 Tinf

Quasi-permanent LC

Deflection

ULS-bending Maximum bending

ULS-shear Maximum shear

ULS-torsion Maximum torsion

ULS-support reactions (column loading)

Moment and normal force




3. Partial factors


5. Durability

6. Design input






12. ULS checks

SLS top fibre stresses for beam BS8110

SLS bottom fibre stresses for beam BS8110

SLS top fibre stresses for beam EC2

SLS bottom fibre stresses for beam EC2

SLS top fibre stresses for slab BS8110

SLS bottom fibre stresses for slab BS8110

SLS top fibre stresses for slab EC2

SLS bottom fibre stresses for slab EC2

beam BS8110

slab BS8110

beam EC2

slab EC2

MULS for beam BS8110

VULS for beam BS8110

MULS for beam EC2

VULS for beam EC2

MULS for slab BS8110

VULS for slab BS8110

MULS for slab EC2

VULS for slab EC2




3. Partial factors


5. Durability

6. Design input






12. ULS checks

Applied Capacity

BS8100 EC2 BS8100 EC2

Bending [kNm]

Beam support face

-3458 -3572 -4080 -4072

midspan 4347 4055 4825 4623

Slab support face

-1957 -2051 -2459 -2416

midspan 1488 1422 1833 1792

Shear [kN]

Beam support face

2270 2177 Stirrups: 1.89mm

Stirrups: 7.17mm

Slab support face

1860 1827

THE END

1. Transfer plates

2. Vertical elements

3. Foundations

Content

Transfer Plate - Function

Transfer of high concentrated forces Thick plates or beams with high rebar content Shear Controlled

Layered Construction

Stage 1: Casting 1st layer (~ 1/3 d) Stressing bottom tendons

Stage 2: Casting remaining plate (supported by 1st layer) Stage 3: Stressing middle and top tendons

Stage 4: Construction of floors above can proceed

020

40

60

80

100

120

Concrete Rebars Fmwk Cost

RC

PT

MATERIAL & COST COMPARISON PT VS RC

TRANSFER PLATE

Pacific Place

Plate area: 1,400m2

Plate thickness: 4.5m thick

Concrete volume: 6,300m3

Layered construction: 3 x 1.5m thk

Reinforcement ratio: RC @ 480 kg/m3 to 180 kg/m3

PT ratio: 27kg/m3

Technical Paper:

Design of Concrete Slabs for Transverse Shear, Peter Marti, ACI Journal 87-S19

Arrangement of Load Bearing Wall & Columns

Pacific Place Transfer Plate

Pacific Place

Completed During Construction

Design Advantages

Thinner Plate reduced selfweight

Better crack control

Better deflection control

Enhanced shear strength

Reduced shear at support

Construction Advantages

Thinner Plate - less concrete

Less reinforcement, less congestion

Layered construction

Lighter supporting false work

Faster construction

1. Transfer plates


3. Foundations

Content

Special Applications:

ICC: PT Out-Rigger VSL AF6-31

ELEVATION PLAN

International Commerce Center Hong Kong (ICC)

Integrated Resort Sands

Singapore

Prestressed shear walls

Temporary Props Max Cap. 8000kN Prop Length up to ~40m

Temporary Post Tensioning AF 6-19


Singapore


Singapore

1. Transfer plates


3. Foundations

Content

PT foundation rafts assure load transfer in soft ground and water tightness below water table

Warehouse, Switzerland Raffle City, Singapore

PT provides material and labour savings, and reduces congestion in 5 m deep raft

10,000 m2 Bur Juman raft, Dubai

PT raft beats RC raft in competitive bid in Dubai and provides improved serviceability

Note: Conventional raft had up to 5 layers of 50 mm reinforcing bars each way. The PT raft had typically 2 layers of 40 mm, and used 21 kg/m3 PT bonded PT tendons.

Documents

EC Seminar March 2015