material requirements for steel and concrete structuresmaterial requirements for steel and concrete...

Material Requirements for Steel and

Concrete Structures

Chiew Sing-Ping

School of Civil and Environmental Engineering

Nanyang Technological University, Singapore

2

Scope

Materials Concrete

Reinforcing steel

Structural steel

Seismic Requirements (BC3: 2013) Materials for seismic design

Detailing for seismic design

3

Structural Eurocodes

SS EN 1990 (EC0):

SS EN 1991 (EC1): Basis of structural design

Actions on structures

Design of concrete structures

Design of steel structures

Design of composite steel and concrete structures

Design of timber structures

Design of masonry structures

Design of aluminium structures

Geotechnical design

Design of structures for earthquake resistance

SS EN 1992 (EC2):

SS EN 1993 (EC3):

SS EN 1994 (EC4):

BS EN 1995 (EC5):

BS EN 1996 (EC6):

BS EN 1999 (EC9):

SS EN 1997 (EC7):

SS EN 1998 (EC8):

4

SS EN 1992

Design of concrete structures BS EN 206-1

Specifying

concrete

BS EN 10080

Reinforcing

steel

BS EN 13670

Execution of

structures

BS EN 10138

Prestressing

steel

National Annex

BS 8500

Specifying

concrete

BS 4449

Reinforcing

steel

BS 8666

Reinforcing

scheduling

Concrete structures (EC2)

5

Concrete

Six density classes of lightweight concrete are defined in EN206-1.

Density class 1.0 1.2 1.4 1.6 1.8 2.0

Density (kg/m3) 801-

1000

1001-

1200

1201-

1400

1401-

1600

1601-

1800

1801-

2000

Density

(kg/m3)

Plain concrete 1050 1250 1450 1650 1850 2050

Reinforced concrete 1150 1350 1550 1750 1950 2150

Normal concrete • Strength class C12/15 – C90/105

• Density 2400 kg/m3

Lightweight concrete

• Strength class LC12/13 – LC80/88

• Density ≤ 2200 kg/m3

used in design to calculate self-weight

6

fck (MPa) 12 16 20 25 30 35 40 45 50 55 60 70 80 90

fck,cube (MPa) 15 20 25 30 37 45 50 55 60 67 75 85 95 105

fcm (MPa) 20 24 28 33 38 43 48 53 58 63 68 78 88 98

fctm (MPa) 1.6 1.9 2.2 2.6 2.9 3.2 3.5 3.8 4.1 4.2 4.4 4.6 4.8 5.0

fctk, 0.05 (MPa) 1.1 1.3 1.5 1.8 2.0 2.2 2.5 2.7 2.9 3.0 3.1 3.2 3.4 3.5

fctk, 0.95 (MPa) 2.0 2.5 2.9 3.3 3.8 4.2 4.6 4.9 5.3 5.5 5.7 6.0 6.3 6.6

Ecm (GPa) 27 29 30 31 33 34 35 36 37 38 39 41 42 44

εc1 (‰) 1.8 1.9 2.0 2.1 2.2 2.25 2.3 2.4 2.45 2.5 2.6 2.7 2.8 2.8

εcu1 (‰) 3.5 3.2 3.0 2.8 2.8 2.8

εc2 (‰) 2.0 2.2 2.3 2.4 2.5 2.6

εcu2 (‰) 3.5 3.1 2.9 2.7 2.6 2.6

n 2.0 1.75 1.6 1.45 1.4 1.4

εc3 (‰) 1.75 1.8 1.9 2.0 2.2 2.3

εcu3 (‰) 3.5 3.1 2.9 2.7 2.6 2.6

Strength and deformation characteristic for normal concrete

Concrete

7

flck (MPa) 12 16 20 25 30 35 40 45 50 55 60 70 80

flck,cube (MPa) 13 18 22 28 33 38 44 50 55 60 66 77 88

flcm (MPa) 17 22 28 33 38 43 48 53 58 63 68 78 88

flctm (MPa) flctm = fctm η1

flctk, 0.05 (MPa) flctk, 0.05 = fctk, 0.05 η1

flctk, 0.95 (MPa) flctk, 0.95 = fctk, 0.95 η1

Elcm (GPa) Elcm = Ecm ηE

εlc1 (‰) kflcm (Ecm ηE)

εlcu1 (‰) εlc1

εlc2 (‰) 2.0 2.2 2.3 2.4 2.5

εlcu2 (‰) 3.5 η1 3.1 η1 2.9 η1 2.7 η1 2.6 η1

n 2.0 1.75 1.6 1.45 1.4

εlc3 (‰) 1.75 1.8 1.9 2.0 2.2

εlcu3 (‰) 3.5 η1 3.1 η1 2.9 η1 2.7 η1 2.6 η1

Strength and deformation characteristic for lightweight concrete

Concrete

η1 = 0.40+0.60ρ/2200 ηE = (ρ/2200)2

8

Modulus of elasticity Ecm

The modulus of elasticity of a concrete is controlled by the

moduli of elasticity of its components. Approximate values

for the modulus of elasticity Ecm, for concrete with quartzite

aggregates are given in Table 3.1 (EC2).

For limestone and sandstone aggregates the values should

be reduced by 10% and 30% respectively. For basalt

aggregates the values should be increased by 20%

9

Creep and Shrinkage Creep coefficient is determined by the following factors:

• Relative humidity

• Element geometry

• Strength class

• Age at loading

• Cement class

• Stress/strength ratio at loading

10

Creep and Shrinkage The total shrinkage is taken as the sum of the autogenous shrinkage and

drying shrinkage:

εcs = εca + εcd

The autogenous shrinkage is related to concrete class.

The drying shrinkage is estimated by the following factors:

• Relative humidity

• Element geometry

• Strength class

• Cement class

0

50

100

150

200

250

0 100 200 300 400

C50/60

C45/55

C40/50

C35/45

C30/37

C25/30

C20/25

C55/67 C60/75

C70/85

C80/95

C90/105

Time (days)

Auto

genous s

hrinkage

11

Stress-strain relations

Parabolic-Rectangular Bi-Linear

4

0.53

1 1 0

2.0 50

1.4 2.34 90 /100 50

2.0 50

2.0 0.085 50 50

3.5

n

cc cd c c2

c2

c cd c2 c cu2

ck

ck ck

c2 ck

c2 ck ck

cu2 ck

for

for

for MPa

for MPa

for MPa

for MPa

for

f

f

n f

n f f

f

f f

f

(?

)

(?

)

(?

)

4

50

2.6 35 90 /100 50cu2 ck ck

MPa

for MPaf f (?

)

4

1.75 50

1.75 0.55 50 / 40 50

3.5 50

2.6 35 90 /100 50

c3 ck

c3 ck ck

cu3 ck

cu3 ck ck

for MPa

for MPa

for MPa

for MPa

f

f f

f

f f

(?)

(?)

(?)

(?

)

12

Stress-strain relations

Higher strength concrete shows more brittle behavior.

Concrete stress-strain relations

0

10

20

30

40

50

60

70

0 0.0005 0.001 0.0015 0.002 0.0025 0.003 0.0035 0.004

C50/60

C45/55

C40/50

C35/45

C30/37

C25/30

C20/25

C55/67 C60/75

C70/85

C80/95

C90/105

σc (MPa)

ε

13

EC2 permits a rectangular stress block to be used for section design

Rectangular stress distribution

λ = 0.8 for fck ≤ 50 MPa

λ = 0.8 – (fck – 50)/400 for 50 < fck ≤ 90 MPa

η = 1.0 for fck ≤ 50 MPa

η = 1.0 – (fck – 50)/200 for 50 < fck ≤ 90 MPa

fck (MPa) λ η

≤ 50 0.800 1.00

60 0.775 0.95

70 0.750 0.90

80 0.725 0.85

90 0.700 0.80

Stress-strain relations

λ: defining the effective height of the compression zone

η: defining the effective strength.

14

Reinforcing steel

Reinforcing bars Coils

Welded fabric Lattice girders

Cold-reduced steel wires

Hot-rolled Wire Rod Dia. 5.5mm to 14mm YS : 300 N/mm2

Profiling Rollers - Dia. Reduction e.g. 8mm > 7mm Finished Wire Coils

Dia. 5mm to 13mm, YS : 500 N/mm2

15

Welded fabric

Resistance Welding

Welded Mesh

Cold

Rolled

Wire

Straightening & Cutting

Computerised Machine

Wires in coil / pre-cut

form

16

17

Reinforcing steel

EC2 does not cover the use of plain or mild steel reinforcement.

Principles and rules are given for deformed bars, de-coiled rods, welded

fabric and lattice girders.

There is no technical reason why other types of reinforcement should not

be used. Relevant authoritative publications should be consulted when

other types reinforcement are used.

EN 10080 provides the performance characteristic and testing methods but

does not specify the material properties. These are given in Annex C of

EC2.

18

Reinforcing steel

Performance requirements

• Strength (fyk or f0.2k, ft)

• Ductility (εuk and ft/fyk)

• Weldability

• Bendability

• Bond characteristics (fR)

19

Reinforcing steel

Stress-strain relations for reinforcing steel

Strength

Yield strength fyk or f0.2k and tensile strength ft.

Ductility Ratio of tensile strength to yield strength ft/fyk

Elongation at maximum force εuk.

Tensile test

Universal Testing Machine Tensile Test Coupon Extensometer

Computer and Datalogger Analog Datalogger Analog Datalogger

21

Weldability

Weldability is usually defined by two parameters:

Carbon equivalent value (CEV)

Limitations on the content of certain elements

The maximum values of individual elements and the carbon equivalent

value are given below.

Table Chemical composition (% by mass)

Carbon

Max.

Sulphur

Max.

Phosphorus

Max.

Nitrogen

Max.

Copper

Max.

CEV

Max.

Cast analysis 0.22 0.050 0.050 0.012 0.80 0.50

Product analysis 0.24 0.055 0.055 0.014 0.85 0.52

22

Properties of reinforcement

Product form Bars and De-coiled rods Wire fabrics

Class A B C A B C

Characteristic yield strength

fyk or f0.2k (MPa) 400 to 600

k = (ft/fy)k ≥1.05 ≥1.08 ≥1.15

<1.35 ≥1.05 ≥1.08

≥1.15

<1.35

Characteristic strain at

maximum force εuk(%) ≥2.5 ≥5.0 ≥7.5 ≥2.5 ≥5.0 ≥7.5

Bendability Bend/Re-bend test -

Maximum bar size

deviation from ≤ 8mm

normal mass (%) > 8mm

± 6.0

± 4.5

Properties of reinforcement (Annex C – EC2)

The UK has chosen a maximum value of characteristic yield strength, fyk= 600 MPa,

But 500 MPa is the value assumed in BS4449 for normal supply.

23

Reduces congestion

• Fewer bars needed

• Increases bar spacing

• Reduces bar diameter

Faster construction

• Placing/tying bars (labor)

• Less weight (crane)

Concrete placement is easier

Higher strength reinforcing steel

Advantage of higher strength reinforcing steel:

There is a push to use reinforcing steel with higher yield

strength of 600 MPa because EC2 permits it.

24

Structural steel (EC3)

Performance requirements

• Strength — able to carry load

• Ductility — able to sustain permanent deformation

• Weldability — able to transfer load

• Toughness — able to absorb damage without fracture

25

High strength steel (HSS)

Normal strength steel: Steel grades S235 to S460

High strength steel: Steel grades greater than S460 up to S700

Compared to normal strength steel, high strength steel has lower

ductility.

26

Why use HSS

When strength-to-weight is important, for example, in

bridges to facilitate construction and crane structures.

Studies show that the ratio of the tensile residual stress

to yield stress of the member seems to decrease with

increasing yield strength in hot-rolled sections.

More favorable buckling curves may be used for high

strength steel for S460.

Higher buckling resistance due to favorable buckling

curves.

27

Buckling curves

28

Buckling curves

29

EC3 has additional ductility requirements compared to

BS5950 in terms of stress ratio, elongation and strain ratio.

Ductility requirements

Normal strength steel

(fy ≤ 460 N/mm2)

• fu/fy ≥ 1.10

• Elongation at failure not less

than 15%

• εu ≥ 15εy εy is the yield stain

high strength steel

(460 N/mm2 <fy ≤ 700 N/mm2)

• fu/fy ≥ 1.05 (EC3-1-12)

• fu/fy ≥ 1.10 ( UK NA to EC3-1-12)

• Elongation at failure not less than

10%

• εu ≥ 15 εy

30

Some product standards only have requirements on nominal yield and

tensile strength, or their minimum values. The stress ratio calculated

according to these nominal values cannot comply with EC3.

Problem

Standard Grade Nominal yield strength (MPa) Nominal tensile strength (MPa) Stress ratio

AS 1397

G450 450 480 1.07

G500 500 520 1.04

G550 550 550 1.00

AS 1595 CA 500 500 510 1.02

EN 10149

S 550MC 550 600 1.09

S 600MC 600 650 1.08

S 650MC 650 700 1.08

S 700MC 700 750 1.07

EN 10326 S550GD 550 560 1.02

ISO 4997 CH550 550 550 1.00

31

Reinforcement Structural steel

A B C Normal strength High strength

Yield strength

(MPa) 400 to 600 ≤ 460

> 460

≤ 700

Modulus of

elasticity (GPa) 200 210

ft/fy or fu/fy ≥ 1.05 ≥ 1.08 ≥ 1.15

< 1.35 ≥ 1.10

≥ 1.05

≥ 1.10 (NA)

Elongation (%) ≥ 2.5 ≥ 5.0 ≥ 7.5 ≥ 15 ≥ 10

Ultimate strain εu ≥ 15εy

Comparison of structural steel and reinforcing steel

Structural steel and reinforcing steel

32

EC2 EC3 EC4

Concrete

Normal C12/15- C90/105

_

C20/25 - C60/75

Light

weight LC12/13 – LC80/88 LC20/22 - LC60/66

Reinforcing steel 400 - 600 N/mm2 _ 400 - 600 N/mm2

Structural steel _ ≤ 700 N/mm2 ≤ 460 N/mm2

Material comparison

These ranges in EC4 are narrower than those given in EC2 ( C12/15 –

C90/105) and EC3 ( ≤ 700 N/mm2) because there is limited knowledge

and experimental data on composite members with very high strength

concrete and high strength steel.

33

Material for seismic design

Ductility Class DCL

(Low)

DCM

(Medium)

DCH

(High)

Concrete grade No limit ≥ C16/20 ≥ C20/25

Steel Class (EC2,

Table C1) B or C B or C Only C

Longitudinal bars only ribbed only ribbed

Material limitations for ’primary seismic members’

DCL - ductility class ‘low’

DCM - ductility class ‘medium’

DCH - ductility class ‘high’

For ‘secondary seismic members’, they do not need to conform to

these requirements.

34

Detailing for seismic design

In addition, for seismic detailing, there are stringent

requirements for reinforcing steel mainly focusing on:

Bar diameter

Bar spacing

Minimum bar numbers

Minimum reinforcement area

Maximum reinforcement area

35

DCH DCM DCL

Longitudinal bars

ρmin 0.5 fctm/fyk (EC2)

ρmax ρ'+0.0018fcd/(μφεsy,dfyd) 0.04 (EC2)

dbl/hc bar crossing

interior joint

-

dbl/hc bar anchored at

exterior joint

-

Transverse reinforcement

Out critical

regions

spacing Min {0.75d; 15Φ; 600} (EC2)

ρmin (EC2)

In critical

regions

dbw,min 6mm -

spacing Min{hw/4;24dbw;175;6dbl} Min{hw/4;24dbw;225;8dbl} -

ctm ykMax 0.26 f f ; 0.13%

d ctm

yd

max

6.25 1+0.8v f

fρ1+0.75

ρ

7.5 d ctm

yd

max

1+0.8v f

fρ1+0.5

ρ

ctmd

yd

f6.25 1+0.8v

f 7.5 ctm

d

yd

f1+0.8v

f

Detailing for primary seismic beams

ck yk0.08 f f

36

Detailing for primary seismic columns

DCH DCM DCL

Cross-section hc,bc,min 250 mm - -

Longitudinal bars

ρmin 1% (EC2)

ρmax 4% 4% (EC2)

dbl,min 8 mm

Bars per column side 3 2 (EC2)

Transverse reinforcement

Out critical regions

spacing Min {20dbl;bc; hc; 400} (EC2)

dbw Max {0.25dbl; 6} (EC2)

Within critical regions

dbw,min Max {0.25dbl; 6} (EC2)

spacing Min{b0/3;125;6dbl} Min{b0/2;175;8dbl} -

Volumetric ratio ωwd 0.08 -

αωwd -

In critical region at

column base:

ωwd 0.12 0.08 -

αωwd -

Ed yd cMax 0.1N f ; 0.002A

bl yd ywdMax 6;0.4d f f

φ d sy,d c 030μ ν ε b b -0.05

φ d sy,d c 030μ ν ε b b -0.05

37

Detailing for primary seismic walls

DCH DCM DCL

Boundary elements: In critical region:

Longitudinal bars

ρmin 0.5% 0.2% (EC2)

ρmax 4% (EC2)

Transverse bars

dbw,min 6 mm Max {0.25dbl; 6} (EC2)

spacing Min{b0/3;125;6dbl} Min{b0/2;175;8dbl} Min {20dbl;bc; hc; 400} (EC2)

Volumetric ratio ωwd 0.12 0.08 -

αωwd -

Web:

Vertical bars

ρv,min Wherever εc >0.2%: 0.5%; elsewhere 0.2% 0.2% (EC2)

ρv,max 4% (EC2)

dbv,min 8mm - dbv,max bwo/8 -

spacing Min (25dbv; 250mm) Min (3bwo; 400mm) (EC2)

Horizontal bars

ρh,min 0.2% Max (0.2%; 0.25ρv) (EC2)

dbv,min 8mm -

dbv,max bwo/8 -

spacing Min (25dbh; 250mm) 400mm (EC2)

bl yd ywdMax 6;0.4d f f

φ d sy,d c 030μ ν ε b b -0.05

38

Conclusions

There is a push to use higher strength concrete, higher

strength reinforcing steel and structural steel in

Structural Eurocodes.

Be careful with steel products, some product standards

may not comply with more stringent Eurocodes ductility

requirements, for e.g. AS1397, SS2 vs. SS560, etc.

For seismic design, there are more stringent

requirements for ductility in reinforcing steel in terms of

higher steel class (B or C only).

In addition, there are more stringent requirements for

seismic detailing for reinforcing steel in terms of bar

diameter and bar spacing, and minimum and maximum

reinforcement.