Material Requirements for Steel and Concrete StructuresMaterial Requirements for Steel and Concrete Structures Chiew Sing-Ping ... Design of composite steel and concrete structures
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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.
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