Material Requirements for Steel and Concrete Structures

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1 Material Requirements for Steel and Concrete Structures Chiew Sing-Ping School of Civil and Environmental Engineering Nanyang Technological University, Singapore

2 Materials Concrete Reinforcing steel Structural steel Scope Seismic Requirements (BC3: 2013) Materials for seismic design Detailing for seismic design 2

3 Structural Eurocodes SS EN 1990 (EC0): SS EN 1991 (EC1): Basis of structural design Actions on structures SS EN 1992 (EC2): SS EN 1993 (EC3): SS EN 1994 (EC4): BS EN 1995 (EC5): BS EN 1996 (EC6): BS EN 1999 (EC9): 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 SS EN 1997 (EC7): SS EN 1998 (EC8): Geotechnical design Design of structures for earthquake resistance 3

4 Concrete structures (EC2) BS EN Execution of structures BS EN Prestressing steel BS EN Specifying concrete BS 8500 Specifying concrete SS EN 1992 Design of concrete structures National Annex BS EN Reinforcing steel BS 4449 Reinforcing steel BS 8666 Reinforcing scheduling 4

5 Normal concrete Concrete Strength class C12/15 C90/105 Density 2400 kg/m 3 Lightweight concrete Strength class LC12/13 LC80/88 Density 2200 kg/m 3 Six density classes of lightweight concrete are defined in EN Density class Density (kg/m 3 ) Density (kg/m 3 ) Plain concrete Reinforced concrete used in design to calculate self-weight 5

6 Concrete Strength and deformation characteristic for normal concrete f ck (MPa) f ck,cube (MPa) f cm (MPa) f ctm (MPa) f ctk, 0.05 (MPa) f ctk, 0.95 (MPa) E cm (GPa) ε c1 ( ) ε cu1 ( ) ε c2 ( ) ε cu2 ( ) n ε c3 ( ) ε cu3 ( )

7 Concrete Strength and deformation characteristic for lightweight concrete f lck (MPa) f lck,cube (MPa) f lcm (MPa) f lctm (MPa) f lctm = f ctm η 1 f lctk, 0.05 (MPa) f lctk, 0.05 = f ctk, 0.05 η 1 f lctk, 0.95 (MPa) f lctk, 0.95 = f ctk, 0.95 η 1 E lcm (GPa) E lcm = E cm η E ε lc1 ( ) kf lcm (E cm η E ) ε lcu1 ( ) ε lc1 ε lc2 ( ) ε lcu2 ( ) 3.5 η η η η η 1 n ε lc3 ( ) ε lcu3 ( ) 3.5 η η η η η 1 η 1 = ρ/2200 η E = (ρ/2200) 2 7

8 Modulus of elasticity E cm The modulus of elasticity of a concrete is controlled by the moduli of elasticity of its components. Approximate values for the modulus of elasticity E cm, 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% 8

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 9

10 Autogenous shrinkage Creep and Shrinkage The total shrinkage is taken as the sum of the autogenous shrinkage and drying shrinkage: ε cs = ε ca + ε cd 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 C90/105 C80/95 C70/ C60/75 C55/67 C50/60 C45/55 C40/50 C35/45 C30/37 C25/30 C20/ Time (days)

11 Stress-strain relations Parabolic-Rectangular Bi-Linear n c f 1 1 for 0 c2 f for c cd c c2 c cd c2 c cu2 n 2.0 for f 50MPa ck n fck / 100 for fck 50MPa (? 2.0 for f 50MPa c2 (? ck f 50 for f 50MPa c2 ck ck cu2 (? 3.5 for f ck 50MPa cu2(? fck / 100 for fck 50MPa 4 (? c3 (? 1.75 for f 50MPa ck f 50 / 40 for f 50MPa 3.5 for f 50MPa c3 ck ck cu3 (? (? ck f /100 for f 50MPa cu3 ck ck 4 11

12 Stress-strain relations Higher strength concrete shows more brittle behavior. 70 σ c (MPa) C90/105 C80/95 C70/85 C60/75 C55/67 C50/60 C45/55 C40/50 C35/45 C30/37 C25/30 C20/ Concrete stress-strain relations ε 12

13 Stress-strain relations EC2 permits a rectangular stress block to be used for section design λ = 0.8 λ = 0.8 (f ck 50)/400 for f ck 50 MPa for 50 < f ck 90 MPa η = 1.0 for f ck 50 MPa η = 1.0 (f ck 50)/200 for 50 < f ck 90 MPa λ: defining the effective height of the compression zone η: defining the effective strength. f ck (MPa) λ η Rectangular stress distribution 13

14 Reinforcing steel Reinforcing bars Coils Welded fabric Lattice girders 14

15 Cold-reduced steel wires Hot-rolled Wire Rod Dia. 5.5mm to 14mm YS : 300 N/mm 2 Finished Wire Coils Dia. 5mm to 13mm, YS : 500 N/mm 2 Profiling Rollers - Dia. Reduction e.g. 8mm > 7mm 15

16 Welded fabric Computerised Machine Wires in coil / pre-cut form Straightening & Cutting Resistance Welding Cold Rolled Wire Welded Mesh 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 provides the performance characteristic and testing methods but does not specify the material properties. These are given in Annex C of EC2. 17

18 Reinforcing steel Performance requirements Strength (f yk or f 0.2k, f t ) Ductility (ε uk and f t /f yk ) Weldability Bendability Bond characteristics (f R ) 18

19 Strength Reinforcing steel Yield strength f yk or f 0.2k and tensile strength f t. Ductility Ratio of tensile strength to yield strength f t /f yk Elongation at maximum force ε uk. Stress-strain relations for reinforcing steel 19

20 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 Sulphur Phosphorus Nitrogen Copper CEV Max. Max. Max. Max. Max. Max. Cast analysis Product analysis

22 Properties of reinforcement Properties of reinforcement (Annex C EC2) Product form Bars and De-coiled rods Wire fabrics Class A B C A B C Characteristic yield strength f yk or f 0.2k (MPa) 400 to 600 k = (f t /f y ) k < <1.35 Characteristic strain at maximum force ε uk (%) Bendability Bend/Re-bend test - Maximum deviation from normal mass (%) bar size 8mm > 8mm ± 6.0 ± 4.5 The UK has chosen a maximum value of characteristic yield strength, f yk = 600 MPa, But 500 MPa is the value assumed in BS4449 for normal supply. 22

23 Higher strength reinforcing steel There is a push to use reinforcing steel with higher yield strength of 600 MPa because EC2 permits it. Advantage of higher strength reinforcing steel: Reduces congestion Fewer bars needed Increases bar spacing Reduces bar diameter Faster construction Placing/tying bars (labor) Less weight (crane) Concrete placement is easier 23

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 24

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. 25

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. 26

27 Buckling curves 27

28 Buckling curves 28

29 Ductility requirements EC3 has additional ductility requirements compared to BS5950 in terms of stress ratio, elongation and strain ratio. Normal strength steel (f y 460 N/mm 2 ) high strength steel (460 N/mm 2 <f y 700 N/mm 2 ) f u /f y 1.10 Elongation at failure not less than 15% ε u 15ε y ε y is the yield stain f u /f y 1.05 (EC3-1-12) f u /f y 1.10 ( UK NA to EC3-1-12) Elongation at failure not less than 10% ε u 15 ε y 29

Standard Grade Nominal yield strength (MPa) Nominal tensile strength (MPa) Stress ratio AS 1397 G450 450 480 1.07 G500 500 520 1.

30 Problem 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. Standard Grade Nominal yield strength (MPa) Nominal tensile strength (MPa) Stress ratio AS 1397 G G G AS 1595 CA EN S 550MC S 600MC S 650MC S 700MC EN S550GD ISO 4997 CH

31 Structural steel and reinforcing steel Yield strength (MPa) Comparison of structural steel and reinforcing steel Modulus of elasticity (GPa) Reinforcement f t /f y or f u /f y Structural steel A B C Normal strength High strength 400 to < > (NA) Elongation (%) Ultimate strain ε u 15ε y 31

32 Material comparison EC2 EC3 EC4 Concrete Normal C12/15- C90/105 Light weight LC12/13 LC80/88 _ C20/25 - C60/75 LC20/22 - LC60/66 Reinforcing steel N/mm 2 _ N/mm 2 Structural steel _ 700 N/mm N/mm 2 These ranges in EC4 are narrower than those given in EC2 ( C12/15 C90/105) and EC3 ( 700 N/mm 2 ) because there is limited knowledge and experimental data on composite members with very high strength concrete and high strength steel. 32

33 Material for seismic design Material limitations for primary seismic members 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 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. 33

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 34

35 Detailing for primary seismic beams DCH DCM DCL Longitudinal bars ρ min 0.5 f ctm /f yk Max 0.26 f f ; 0.13% (EC2) ρ max ρ' f cd /(μ φ ε sy,d f yd ) 0.04 (EC2) d bl /h c bar crossing interior joint d bl /h c bar anchored at exterior joint Transverse reinforcement Out critical regions In critical regions spacing ρ min vd f ρ f ρ max ctm yd Min {0.75d; 15Φ; 600} (EC2) (EC2) d bw,min 6mm - spacing Min{h w /4;24d bw ;175;6d bl } Min{h w /4;24d bw ;225;8d bl } v d fctm ρ ρ max v ctm v d f f yd d 0.08 f f ck yk f f yd ctm f yd ctm yk

36 Detailing for primary seismic columns DCH DCM DCL Cross-section h c,b c,min 250 mm - - Longitudinal bars ρ min 1% Max 0.1N f ; 0.002A (EC2) ρ max 4% 4% (EC2) d bl,min 8 mm Bars per column side 3 2 (EC2) Transverse reinforcement Out critical regions spacing d bw Within critical regions d bw,min bl yd ywd Max 6;0.4d f f Min {20d bl ;b c ; h c ; 400} (EC2) Max {0.25d bl ; 6} (EC2) Max {0.25d bl ; 6} (EC2) spacing Min{b 0 /3;125;6d bl } Min{b 0 /2;175;8d bl } - Volumetric ratio ω wd αω wd 30μφν dεsy,d bc b In critical region at column base: ω wd αω wd 30μ ν ε b b φ d sy,d c 0 Ed yd c 36

37 Detailing for primary seismic walls DCH DCM DCL Boundary elements: In critical region: Longitudinal bars ρ min 0.5% 0.2% (EC2) ρ max 4% (EC2) Transverse bars d bw,min Max6;0.4d bl fyd fywd 6 mm Max {0.25d bl ; 6} (EC2) spacing Min{b 0 /3;125;6d bl } Min{b 0 /2;175;8d bl } Min {20d bl ;b c ; h c ; 400} (EC2) Volumetric ratio ω wd αω wd 30μ ν ε b b φ d sy,d c 0 Web: Vertical bars ρ v, min Wherever ε c >0.2%: 0.5%; elsewhere 0.2% 0.2% (EC2) ρ v,max 4% (EC2) d bv,min 8mm - d bv,max b wo /8 - spacing Min (25d bv ; 250mm) Min (3b wo ; 400mm) (EC2) Horizontal bars ρ h, min 0.2% Max (0.2%; 0.25ρ v ) (EC2) d bv,min 8mm - d bv,max b wo /8 - spacing Min (25d bh ; 250mm) 400mm (EC2) - 37

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. 38

Material Requirements for Steel and Composite Structures

Material Requirements for Steel and Composite Structures Chiew Sing-Ping School of Civil and Environmental Engineering Nanyang Technological University, SINGAPORE 22 January 2015 Scope Higher Strength