Material Requirements for Steel and Composite Structures

Chiew Sing-PingSchool of Civil and Environmental EngineeringNanyang Technological University, SINGAPORE

22 January 2015

2

Scope

Higher Strength Materials Concrete (fck ≥ 50 MPa) Reinforcing steel (fsk ≥ 500 MPa) Structural steel (fyk ≥ 460 MPa)

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 designActions on structures

Design of concrete structuresDesign of steel structuresDesign of composite steel and concrete structuresDesign of timber structuresDesign of masonry structuresDesign of aluminium structures

Geotechnical designDesign 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 1992Design of concrete structures

BS EN 206-1Specifying concrete

BS EN 10080Reinforcing

steel

BS EN 13670Execution of structures

BS EN 10138Prestressing

steel

National Annex BS 8500

Specifying concrete

BS 4449Reinforcing

steel

BS 8666Reinforcing scheduling

Concrete structures (EC2)

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Concrete

Six density classes of lightweight concrete are defined in EN206-1.Density class 1.0 1.2 1.4 1.6 1.8 2.0Density (kg/m3) 801-

10001001-1200

1201-1400

1401-1600

1601-1800

1801-2000

Density (kg/m3)

Plain concrete 1050 1250 1450 1650 1850 2050Reinforced 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

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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

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

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

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

fctm (MPa) flctm = fctm η1

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

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

Ecm (GPa) Elcm = Ecm ηE

εc1 (%) kflcm (Ecm ηE)

εcu1 (%) εlc1

εc2 (%) 2.0 2.2 2.3 2.4 2.5

εcu2 (%) 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

εc3 (%) 1.75 1.8 1.9 2.0 2.2

εcu3 (%) 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

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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%

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Creep and ShrinkageCreep coefficient is determined by the following factors:

• Relative humidity • Element geometry• Strength class• Age at loading• Cement class• Stress/strength ratio at loading

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Creep and ShrinkageTotal shrinkage strain is taken as the sum of the autogenous shrinkage and drying shrinkage strains:

εcs = εca + εcd

Autogenous shrinkage strain is related to concrete class.Drying shrinkage strain is affected by the following factors:

• Relative humidity • Element geometry• Strength class• Cement class

0

50

100

150

200

250

0 100 200 300 400

C50/60C45/55C40/50C35/45C30/37C25/30C20/25

C55/67C60/75

C70/85

C80/95

C90/105

Time (days)

Aut

ogen

ous

shrin

kage

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Stress-strain relationsParabolic-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 50 503.5

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 MPafor

f

fn f

n f f

f

f ff

(?)

(?)

(?)

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

(?)

(?)

(?)

(?)

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Stress-strain relations- higher strength concrete shows more brittle behavior.

concrete stress-strain relations under ambient temperature

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

C50/60C45/55C40/50C35/45C30/37C25/30C20/25

C55/67C60/75

C70/85

C80/95

C90/105

σc (MPa)

ε

Note: under elevated temperature (fire situation), no design recommendations beyond Class 1 Concrete Grade C60/75 in EC2 Part 1-2

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

Rectangular stress distribution

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

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

fck (MPa) λ η

≤ 50 0.800 1.0060 0.775 0.9570 0.750 0.9080 0.725 0.8590 0.700 0.80

Stress-strain relations

λ: defining the effective height of the compression zone η: defining the effective strength.

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Reinforcing steel

Reinforcing barsCoils

Welded fabric Lattice girders

Cold-reduced steel wires

Hot-rolled Wire RodDia. 5.5mm to 14mmYS : 300 N/mm2

Profiling Rollers- Dia. Reductione.g. 8mm > 7mmFinished Wire Coils

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

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Welded fabric

Resistance Welding

Welded Mesh

Cold Rolled Wire

Straightening & Cutting

Computerised Machine

Wires in coil / pre-cut form

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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.

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Reinforcing steel Performance requirements

• Strength (fyk or f0.2k, ft)• Ductility (εuk and ft/fyk)• Weldability• Bendability • Bond characteristics (fR)

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Reinforcing steel

Stress-strain relations for reinforcing steel

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

Ductility Ratio of tensile strength to yield strength ft/fykElongation at maximum force εuk.

Tensile test

Universal Testing Machine Tensile Test Coupon Extensometer

Computer and Datalogger Analog Datalogger Analog Datalogger

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WeldabilityWeldability 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

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Properties of reinforcement

Product form Bars and De-coiled rods Wire fabrics

Class A B C A B C

Characteristic yield strengthfyk 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 sizedeviation from ≤ 8mmnormal 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.

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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 but there is a lack of experiment data to calibrate/support its use.

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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

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High strength steel (HSS) Normal strength steel: Steel grades S235 to S460High strength steel: Steel grades greater than S460 up to S690

Compared to normal strength steel, high strength steel has lower ductility.

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Why use HSS

When strength-to-weight is important, for example, inbridges to facilitate construction, retractable roofs andlifting crane structures.

Studies show that the ratio of the tensile residual stressto yield stress of the member seems to decrease withincreasing yield strength in hot-rolled sections.

More favorable buckling curves may be used for highstrength steel for S460.

Higher buckling resistance due to favorable bucklingcurves.

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Buckling curves

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Buckling curves

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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

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Some product standards have requirements on nominal yield and tensile strength, or their minimum values only. The stress ratio calculated according to these nominal values cannot comply with EC3, for e.g. profiled sheet sheeting..

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

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Reinforcement Structural steelA B C Normal strength High strength

Yield strength (MPa) 400 to 600 ≤ 460 > 460

≤ 700Modulus 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 ≥ 10Ultimate strain εu ≥ 15εy

Comparison of structural steel and reinforcing steel

Structural steel and reinforcing steel

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EC2 EC3 EC4

ConcreteNormal 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 knowledgeand experimental data on composite members with very high strengthconcrete and high strength steel.

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Material for seismic design

Ductility Class DCL (Low)

DCM (Medium)

DCH (High)

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

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Detailing for seismic designIn 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

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Detailing of primary seismic beams

For DCL following EC2 For DCM&DCH critical regions (detailing to EC8)

out of critical regions (detailing to EC2)

Critical region lcr = hw (depth of beam) for DCMlcr = 1.5hw for DCH

< 50 mm

lcr

Standard Detailing to EC2

s

h w

lcr

critical region critical region

Beam-column Joint “special” confinement toclause 5.4.3.3 (EC8)

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DCH DCM DCLLongitudinal barsρmin 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 0.26f f ; 0.13%

d ctm

yd

max

6.25 1+0.8v ffρ1+0.75

ρ

7.5 d ctm

yd

max

1+0.8v ffρ1+0.5

ρ

ctmd

yd

f6.25 1+0.8vf

7.5 ctmd

yd

f1+0.8vf

Detailing of primary seismic beams

ck yk0.08 f f

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Detailing of primary seismic columns

l crs

l cr

criticalregion

criticalregion

horizontal confinement reinforcementin beam-column joint not less than

that in critical region of column

For DCL detailing to EC2

For DCM&DCH critical regions (detailing to EC8)out of critical regions (detailing to EC2)

Critical region

for DCMfor DCH

hc is the largest cross-sectional dimension of columnlcl is the clear length of the column

max ; 6;0.45cr c cll h l

max 1.5 ; 6;0.6cr c cll h l

Beam-column joint “special” confinement toclause 5.4.3.3 (EC8)

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Detailing of primary seismic columnsDCH DCM DCL

Cross-section hc,bc,min 250 mm - -Longitudinal barsρmin 1% (EC2)ρmax 4% 4% (EC2)dbl,min 8 mmBars per column side 3 2 (EC2)Transverse reinforcementOut critical regionsspacing Min {20dbl;bc; hc; 400} (EC2)dbw Max {0.25dbl; 6} (EC2)Within critical regionsdbw,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

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Detailing for primary seismic wallsDCH DCM DCL

Boundary elements:In critical region:Longitudinal barsρmin 0.5% 0.2% (EC2)ρmax 4% (EC2)Transverse barsdbw,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 barsρv,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 barsρh,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

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Conclusions There is clear advantage in using higher strength grade

concrete, reinforcing steel and structural steel but there are still some work to be done.

Be careful with some products; they may not comply with more stringent Eurocode 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 stringent requirements for seismic detailing for reinforcing steel in terms of bar diameter and bar spacing, and minimum and maximum reinforcement.