Cap 3 2015

download Cap 3 2015

of 33

description

Steel, Bogdan Stefanescu

Transcript of Cap 3 2015

  • 3. STRUCTURAL STEEL

    59

    Chapter 3

    STRUCTURAL STEEL

    3.1. MATERIALS

    Figure 3.1 shows, in term of diagram (stressstrain), the behaviour of various materials in the process of loading and unloading.

    Fig. 3.1. Models of behaviour diagrams

    Figure 3.1a shows a typical linear elastic behaviour. Loading and unloading follow the same straight line O A O. The elastic deformation disappears just after unloading. The diagram in figure 3.1b is a non-linear elastic behaviour. The elastic deformation disappears just after unloading, but the loading and unloading line is no longer straight, even it remains the same for both processes. Figure 3.1c shows a viscous behaviour. Loading follows a curve (O A), while unloading goes on a different path (A B O). The plastic deformation disappears in time (B O). The

    O O O

    O O

    O

    A A A

    A A A A A

    A A

    B

    B B

    B B

    ( a ) ( b ) ( c )

    ( d ) ( e ) ( f )

  • 3. STRUCTURAL STEEL

    60

    diagram in figure 3.1d is a typical elasto-plastic behaviour. Loading follows a path (O A) which is different from the unloading one (A B). Unloading goes on a straight line, the elastic deformation is removed, but the plastic deformation is kept. Figure 3.1e shows a bilinear elasto-plastic behaviour. Loading (O A A) and unloading (A B) follow different straight lines. The elastic deformation is removed, but the plastic deformation is kept. Structural steel is generally modelled as bilinear elasto-plastic symmetrical material, having a large yielding plateau (Fig. 3.1f).

    3.2. STEEL MAKING

    Steel making is a hearth-process (vatr, cuptor), based on the refining principle (afinare). The molten blast-furnace iron is saturated with carbon, about 4% by weight and cast iron (font) is obtained. By heating, this molten metal (the melt temperature of steel is superior to that of iron) and, by introducing oxygen (Fig. 3.2), carbon in excess is reduced in the refining process and furnace iron is transformed in steel. In most of the steel-making procedures, the primary reaction is the combination of carbon and oxygen to form a gas. If oxygen in excess is not removed (by adding ferrosilicon, aluminium, etc.) the gaseous product FeO continues to evolve during solidification. This oxide (FeO) is very dangerous, because it makes steel fragile. In the old Romanian codes there are three qualities: not killed steel poorly deoxidized (n); killed steel deoxidized (k); strongly killed steel strongly deoxidized (with aluminium or silicon) (kf). Killed steels (deoxidized steels) are characterised by a relatively high degree of uniformity in composition and properties. Low alloy steels are always killed.

    Fig. 3.2. Simplified scheme of the steel making process

    Burned gases Oxygen lance

    Burner Gas or liquid fuel

    Molten furnace iron

  • 3. STRUCTURAL STEEL

    61

    Silicon (Si) increases the strength of steel and favours the formation of a fine grain structure. Aluminium (Al) is a good deoxidiser. Sulphur (S) and phosphorus (P), impurities that result in the steelmaking process, must be limited in structural steel at about 0,05% each, since they unfavourably affect steel fragility.

    The following nomenclature is used by the metallurgist:

    Ferrite or -Fe The bcc form of iron in which up to 0,02%C by weight may be dissolved.

    Cementite Iron carbide Fe3C (which contains about 6,67%C). Pearlite The laminar mixture of ferrite and cementite described

    earlier. The overall carbon content of the mixture is 0,8% by weight.

    Austenite or -Fe The fcc form of iron which exists at high temperatures and which can contain up to approximately 2%C by weight.

    Steel Alloys containing less than 2% carbon by weight. Cast Iron Alloys containing more than 2% carbon by weight.

    Steel used in structures such as bridges, buildings and ships, usually contains between 0,1% and 0,25% carbon by weight. [16]

    3.3. CHEMICAL COMPOSITION. CRYSTALLINE STRUCTURE

    More than 70 elements in the Mendeleyev Periodic System are metals. Metals form, together with their alloys, a large multitude of substances, having very diverse properties. In spite of this diversity, the energy spectrum of electrons in metals represents the common characteristic that allows all metals to be described from a single standpoint. Metals and their alloys possess a crystalline structure formed by a crystalline lattice (Fig. 3.3), where the atoms are placed in the knots of the lattice.

    Fig. 3.3. Types of crystalline lattice of steel

    Body-centred cubic crystal (bcc)

    Face-centred cubic crystal (fcc)

    ,

  • 3. STRUCTURAL STEEL

    62

    Fig. 3.4 The iron-carbon phase diagram [17]

    The crystal structure of a metal is not strictly periodical in each given instant of time, because of oscillating motion of some quasi-free valence electrons (Paulus model) in the lattice. The polarization of the surrounding atoms occurs in the electric field of these electrons. Atoms become dipoles (i.e. a system with two electric charges +q, q) and the interaction of dipoles creates the metallic bond, expressed by attraction forces between sufficiently distant atoms in the lattice. The greater the number of valence electrons is, the greater the strength of the metallic bond is. When molten metal solidifies, the crystallization process begins around some centres of crystallization (Fig. 3.5). Every centre grows up and so, when the solidifying process comes to the end, a lot of crystallites called grains appear.

    beginning advanced close to final Fig. 3.5. Scheme of the crystallization process

  • 3. STRUCTURAL STEEL

    63

    The crystal structure of iron, as a theoretic pure metal, consists of ferrite grains (Fe). Iron is a mild and very plastic material (fy = 120 N/mm2; fu = 250 330 N/mm2, u = 50%), with a non-linear diagram (Fig. 3.6).

    Fig. 3.6. Behaviour diagrams

    Carbon steel is an iron-carbon alloy, with a carbon percentage limited at 0,15 0,25%. Remarks concerning the chemical composition: 1. Each percentage of 0,1% increases the ultimate stress (fu) by 80 90 N/mm2 and

    the yield stress (fy) by 40 50 N/mm2. 2. The carbon percentage must be limited at 0,25%, since a percentage superior to

    that dramatically diminishes the strain at rupture u (= r) and unfavourably affects weldability.

    3. The carbon steel grade (0,15% < %C < 0,25%) called mild steel, which is a very largely used structural steel, is characterised by the following:

    the crystalline structure is composed of ferrite-pearlite grains; the ferrite grains give a very good plasticity u (= r) 25%; the pearlite grains (pearlite = a mechanical mixture of six parts of ferrite

    and one part of cementite Fe3C), containing an average percentage of 0,90%C, give a good resistance;

    U

    high alloy steel

    low alloy steel

    non-alloy steel

    iron

    F (failure)

    pearlite ferrite

    Fe

    Y E

    P

    arctg E

    y 0,2% ~18% ~30% ~50%

    fp e fy

    fu

  • 3. STRUCTURAL STEEL

    64

    the behaviour is perfectly linear elastic up to the limit of proportionality fp (point P on diagrams) and it is defined by the linear law of Hooke

    = E ( 3.1 ) pearlite grains restrain the tendency of ferrite grains to deform plastically;

    the behaviour is still elastic in the range P E on diagrams up to the elastic limit (point P) but it is no longer linear;

    in the elastic range, the form of the lattice is modified under loading but the distances between atoms remain in the limit of the full active metallic bond and thus, after unloading, the lattice regains its initial form (Fig. 3.7);

    Fig. 3.7. Elastic deformation of the crystalline structure

    at point Y, mild steel undergoes yielding i.e. a large elongation horizontal plateau, without any increase of the stress;

    plastic deformation is a shear irreversible one (Fig. 3.8); a complete tensile test puts in evidence these phenomena (Fig. 3.9);

    Fig. 3.8. Shear deformation of the crystalline structure

    a

    a a 2a

    a + 1a

    P P

  • 3. STRUCTURAL STEEL

    65

    Fig. 3.9. Tensile test of a standard specimen

    A0 = initial cross-section area of the specimen; d0 = initial diameter of the cross-section of the specimen; L0 = initial distance between gauge points; Au = minimum (ultimate) cross-section area of the specimen after failure; Lu = final distance between gauge points;

    %100L

    L100L

    LL00

    0uur

    =

    == ( 3.2 )

    after yielding, a new internal equilibrium is to be made up in a typical strain hardening (line Y U) i.e. for a deformation 2 > 1 a force F2 > F1 is necessary;

    at point U, the diagram falls away and stops at the elongation value u (= r), called the ultimate elongation at rupture, which corresponds to the specimen breaking;

    4. The diagram outlines the typical qualities of a structural steel: Plasticity = property to keep a total or partial plastic deformation;

    Ductility = plastic quality appreciated by the ratio plastic

    ultimate

    ;

    Tenacity = property to keep large deformation before failure, under great forces;

    A0

    Au

    d0

    L0

    Lu = L0 + L

    F

    F

  • 3. STRUCTURAL STEEL

    66

    5. The Prandtl model is usually accepted for the stressstrain diagram () in order to simplify calculation (Fig. 3.10).

    Fig. 3.10. The Prandtl model for steel behaviour

    Low alloy structural steel is steel alloyed with manganese, which has the quality to increase the ultimate stress and the yield stress, without an important diminution of plasticity. The percentage of Mn is limited for structural steel to 1,71,8%, because it favours fragility. Remarks 1. A percentage of 0,220,25%C increases the yield stress from fy 120 N/mm2 (of

    iron) to fy 235 N/mm2 and it diminishes the strain at rupture u from about 50% to about 25%.

    2. A percentage of 0,200,22%C together with a percentage of 1,31,5Mn increase the yield stress of a high strength low alloy steel to fy = 360400 N/mm2 and they diminish the strain at rupture u to the limit allowed for structural steel, i.e. 1520%.

    3. Generally, an increase of steel strength is associated with decrease of ductility. 4. Quite recently, the Luxemburg steel manufacturer ARBED succeeded in

    producing a structural steel that increases the values of the yielding limit fy. It is a new technology, called QST (Quenching and Self Tempering): after the last rolling pass, an intense water cooling is applied to the whole

    surface of the shape, so that the skin is quenched (Fig. 3.11);

    real

    Prandtl fy

  • 3. STRUCTURAL STEEL

    67

    Entry QST Bank Quenching Self Tempering 1600F 871C 1100F 593C Fig. 3.11. The idea of the QST process

    cooling is interrupted before the core is affected by quenching and the outer layers are tempered by the flow of heat from the core to the surface, during the temperature homogenization phase.

    Due to its metallurgical principle, without increasing the percentage of alloying elements, it results: a high yield strength (fy 500 N/mm2) without a decrease of u strain; an excellent weldability.

    For some necessities, as well as for high strength bolts, high alloy steels are produced with great ultimate (tensile) strength (fu = 8001000 N/mm2) and high yield stress (f0,2 = 640900 N/mm2) but with a poor ductility.

    3.4. ROLLING PROCESS INFLUENCE

    Most of the products for steel construction are obtained by hot rolling process. Basically, the hot rolling process consists of passing, in several steps, of a steel bar through a certain space determined by two shaped rolling cylinders which rotate in contrary directions (Fig. 3.12) realizing both the advance of the bar and the changing of its shape from one step to another (Fig. 3.13).

    water

  • 3. STRUCTURAL STEEL

    68

    Fig. 3.12. Simplified scheme of the hot-rolling process

    1st step 2nd step 3rd step final step Fig. 3.13. Simplified scheme of the steps to realise a hot-rolled product

    The temperature and the timing of the process are controlled. Every passing between rolling cylinders produces the self-strain hardening (H) of steel (grains are broken and pressed) and every period between two passings allows the tendency (R) of re-crystallization (grains tend to return to their initial form). Finally, a much more compact structure with fine grains is obtained and, as a result, both the yield stress and the ultimate strength increase (Fig. 3.14).

    Fig. 3.14. Simplified scheme of grains evolution during hot-rolling

    t1 t2 < t1

    T 10001200C H1 (hardening) R1(re-crystallization) 1st step

    hardening

    Hi Ri ith step

    Hfinal Rfinal

    Gfinal Ginitial G (magnitude of grains)

    600800C

  • 3. STRUCTURAL STEEL

    69

    It is important to note that the increase in strength depends on the thickness of the material, i.e. the less the thickness is the greater the yield stress and the ultimate strength are. The rolling process is very important in increasing both the ultimate and the yield strengths. Two steels with the same chemical composition have different strengths, depending on the metallurgical process.

    3.5. RESISTANCE AGAINST BRITTLE FRACTURE

    Under some circumstances, the plastic qualities of a structural steel can be unfavourably affected. That is especially the case of: dynamic loading; low temperature. Under such circumstances, plastic deformations do not develop and brittle fractures occur. A brittle fracture is characterised by a sharp unexpected fracture, without previous plastic deformation. The tendency to brittle fracture, called toughness, is appreciated by a pendulum test (Fig. 3.15). A special hammer breaks a typical specimen under a mechanical work L equal to: L = G(h1 h2) ( 3.3 ) where G is the weight of the hammer which drops from the height h1 and, after producing the rupture of the specimen, rises to the height h2. The mechanical work L (3.3), measured in Joules, characterizes the resistance to brittle fracture. Codes require a minimum value of 27J in order to provide a good behaviour.

    Fig. 3.15. Simplified scheme of the toughness test

    55

    2 45

    Charpy specimen (V notch)

    10

    10

    h2

    h1

  • 3. STRUCTURAL STEEL

    70

    Toughness is strongly unfavourably influenced by low temperatures (Fig. 3.16).

    0

    20

    40

    60

    80

    100

    120

    140

    -

    25-

    23-

    21-

    19-

    17-

    15-

    13-

    11 -9 -7 -5 -3 -1 1 3 5 7 9 11 13 15 17 19 21 23 25 27 29

    Fig. 3.16. Variation of the mechanical work L with temperature Codes define the transition temperature (T) (Fig. 3.16) as the temperature below which toughness decays to an unacceptable value.

    3.6. INFLUENCES OF TEMPERATURE ON THE PROPERTIES OF STEEL

    Mechanical properties (E, fy (c), fu (u), u) vary with temperature (Fig.3.17) (EN 1993-1-2 [18]). It is to notice that: up to T = 100C there are no significant changes;

    0.000

    0.200

    0.400

    0.600

    0.800

    1.000

    20 100 200 300 400 500 600 700 800 900 1000 1100 1200

    Fig. 3.17. Variation of some mechanical properties of steel with temperature

    Temperature (C)

    Mechanical work (J)

    27J Transition temperature

    y

    ,y,fy f

    fk =

    EEk

    ,E

    =

    p

    ,p,pk

    =

    (C)

  • 3. STRUCTURAL STEEL

    71

    for T > 200C fu (u) and fy (c) decrease dramatically while u increases; Youngs modulus (E) also decreases; as a result, the resistance of steel structures to fire is an important problem and special measures are necessary;

    for T < 0C fu (u) and fy (c) increase but, as it was showed before, the fragility of steel increases.

    3.7. FATIGUE BEHAVIOUR

    Fatigue tests show the lowering of mechanical strength after a cycle of stress having an oscillating intensity in time (Fig. 3.18). In EN 1990 [10], fatigue (def. 1.3.1.1) is defined as the process of initiation and propagation of cracks through a structural part due to action of fluctuating stress.

    Fig. 3.18. Types of loading cycles

    In the elastic range, the variation of fatigue strength, 0, depends on (Fig. 3.19): the type of loading cycle (Fig. 3.18); the designed details.

    Fig. 3.19. Whler diagram

    constant

    alternate

    time

    max max

    max

    max

    min min

    min min

    n (number of cycles)

    O

    6 106 3 106 106

  • 3. STRUCTURAL STEEL

    72

    In modern codes, the fatigue verification is defined with regard to (Fig.3.20): the range of stresses: = max min, where compression is (); the designed details.

    Mf

    R

    ( 3.4 )

    where: R = fatigue strength; Mf = 1,25,..., 1,35 partial safety factor.

    Fig. 3.20. n diagrams depending on the designed details

    EN 1993-1-9 [15] defines two methods for fatigue assessment: damage tolerant method or (metoda degradrilor acceptabile) safe life method. (metoda duratei sigure de via)

    The damage tolerant method should provide an acceptable reliability that a structure will perform satisfactorily for its design life, provided that a prescribed inspection and maintenance regime for detecting and correcting fatigue damage is implemented throughout the design life of the structure. The safe life method should provide an acceptable level of reliability that a structure will perform satisfactorily for its design life without the need for regular in-service inspection for fatigue damage. The safe life method should be applied in cases where local formation of cracks in one component could rapidly lead to failure of the structural element or structure. The general form of the system of check equations recommended by EN 1993-1-9 [15] chapter 8 is as follows:

    parent metal butt weld

    (log())

    n

    n

    n (log(n)) 104 105 106 107 108

  • 3. STRUCTURAL STEEL

    73

    (1) Nominal, modified nominal or geometric stress ranges due to frequent loads 1 Qk (see EN 1990) should not exceed

    rangesstressshearfor3/f5,1rangesstressdirectforf5,1

    y

    y

    (8.1)

    (2) It should be verified that under fatigue loading 0,1

    / MfC2,EFf

    and (8.2)

    0,1/ MfC

    2,EFf

    (3) Unless otherwise stated , in the case of combined stress ranges E,2 and E,2 it should be verified that:

    0,1//

    5

    MfC

    2,EFf3

    MfC

    2,EFf

    +

    (8.3)

    In the previous relations, stresses should be calculated at the serviceability limit state

    nominal stresses should be calculated at the site of potential fatigue initiation

    the design value of stress range to be used for the fatigue assessment should be the stress ranges Ff E,2 corresponding to NC = 2106 cycles

    the fatigue strength for nominal stress ranges is represented by a series of (log R) (log N) curves and (log R) (log N) curves (S-N-curves), which correspond to typical detail categories. Each detail category is designated by a number which represents, in N/mm2, the reference value C and C for the fatigue strength at 2 million cycles

    Table 3.1 in EN 1993-1-9 [15]: Recommended values for partial factors for fatigue strength

    Assessment method Consequence of failure Low consequence High consequence Damage tolerant 1,00 1,15

    Safe life 1,15 1,35

    The detail categories C and C are given in tables like in the example below:

    Table 8.2 in EN 1993-1-9 [15]: Welded built-up sections (extract) Detail

    category Constructional detail Description Requirements

    125

    Continuous longitudinal welds:

    1) Automatic butt welds carried out from both sides.

    2) Automatic fillet welds. Cover plate ends to be checked using detail 6) or 7) []

    Details 1) and 2):

    No stop/start position is permitted except when the repair is performed by a specialist and inspection is carried out to verify the proper execution of the repair.

  • 3. STRUCTURAL STEEL

    74

    112

    3) Automatic fillet or butt weld carried out from both sides but containing stop/start positions.

    4) Automatic butt welds made from one side only, with a continuous backing bar, but without stop/start positions.

    4) When this detail contains stop/start positions category 100 to be used.

    The values for the reference values C and C are given in the following diagrams:

    Dire

    ct st

    ress

    ra

    nge

    R [N

    /mm

    ]

    10

    100

    1000

    1,0E+04 1,0E+05 1,0E+06 1,0E+07 1,0E+08 1,0E+09

    3m = 3

    1

    m = 5

    140125112

    1

    364045505663718090

    100

    160

    2

    2 5

    1 Detail category C 2 Constant amplitude

    fatigue limit D 3 Cut-off limit L

    Endurance, number of cycles N

    Figure 7.1 in EN 1993-1-9 [15]: Fatigue strength curves for direct stress ranges

  • 3. STRUCTURAL STEEL

    75

    Sh

    ear

    stre

    ss ra

    nge

    R [N

    /mm

    ]

    10

    100

    1000

    1,0E+04 1,0E+05 1,0E+06 1,0E+07 1,0E+08 1,0E+09

    2

    m = 5

    1

    100

    80

    1

    2

    1 Detail category C 2 Cut-off limit L

    Endurance, number of cycles N

    Figure 7.2 in EN 1993-1-9 [15]: Fatigue strength curves for shear stress ranges

    In the plastic range the fatigue strength strongly depends on the magnitude of the plastic deformations. The phenomenon is called low cycle fatigue. The number n of critical cycles may be determined according to: ( ) ( ) 83,1log4,2nlog =+ ( 3.4 ) It results: = 5% n = 20

    = 1% n = 933

    = 0,5% n = 4926

    = 0,3% n = 16785 The following experimental values were obtained: fracture at 650 cycles for = 1%; fracture at 16 cycles for = 2,5%. The fatigue behaviour in the plastic range is a very important phenomenon for buildings in seismic zones.

  • 3. STRUCTURAL STEEL

    76

    3.8. CORROSION

    In contact with atmosphere, structural steel corrodes. Rust FeO (iron-oxide) is formed, whose volume is twice grater than the corroded pattern material, and so the material deteriorates. Protection against corrosion is typical for steel structural members and may be realised: 1. by painting, containing:

    one or more primer coatings; two or more coloured paint coatings.

    Painting is the most common rust-preventing method. Here are some provisions about painting included in Annex F of the code EN

    1090-2 [20], which covers execution of steel structures. F.6 Coating methods F.6.1 Painting The surface condition of the component shall be checked immediately prior to painting to ensure that it complies with the required specifications, EN ISO 12944-4, EN ISO 8501 and EN ISO 8503-2 and the manufacturer's recommendations for the product about to be applied. Painting shall be undertaken in accordance with EN ISO 12944-7. If two or more coats are to be applied, a different colour shade shall be used for each coat. Structures with an expected life of the corrosion protection above 5 years with a C3 (and above) corrosivity category should have additional edge protection, by a stripe coat, extending across approximately 25 mm on both side of the edge and applied to a nominal thickness appropriate to the coating system. Work shall not proceed if:

    the ambient temperature is below that recommended in the manufacturer's recommendations for the product to be applied;

    the surfaces to be coated are wet; the temperature of the surfaces to be coated is less than 3 C above the dew point unless

    otherwise specified in the product datasheet. Painted surfaces shall be protected against the accumulation of water for a period after application as required by the product data sheet. The packing of painted components into bundles shall not commence until the paint manufacturer's declared hardening time has expired. Adequate well ventilated space, protected against the influence of weather, shall be provided to allow the coating to harden sufficiently. Appropriate measures shall be taken to prevent damage to the coating during packing and handling. NOTE: Cold formed components are often produced as nesting profiles. Tightly packing components into nested bundles before the paint treatment is sufficiently hardened may result in damage. 2. by galvanizing and electro-zinkage (zinc plating), typical for corrugated sheets; Here are some provisions about metal spraying and galvanizing included in

    Annex F of the code EN 1090-2 [20], that covers execution of steel structures. F.6 Coating methods

    F.6.2 Metal spraying Thermal metal spraying shall be of zinc, aluminium or zinc/aluminium 85/15 alloy and be undertaken in accordance with EN ISO 2063.

  • 3. STRUCTURAL STEEL

    77

    Thermal metal sprayed surfaces shall be treated with a suitable sealer before overcoating with paint in accordance with F.6.1. This sealer shall be compatible with the overcoating paint and shall be applied immediately after metal spraying cooling so as to avoid oxidation or moisture trapping. F.6.3 Galvanizing Galvanizing shall be undertaken in accordance with EN ISO 1461. Galvanized surfaces of cold-formed components shall be provided by using precoated steel strip or by hot dip galvanizing after manufacturing. NOTE 1: Coating masses, finishes and surface qualities are specified in EN 10326 and EN 10327. If hot dip galvanizing after manufacturing is specified, it shall be undertaken in accordance with EN ISO 1461 and requirements for procedure qualification of the dipping process shall be specified. NOTE 2: Light gauge cold-formed components often lack inherent stiffness. Long components composed of thin material can be susceptible to twisting due to stress relieving at the elevated temperature of the zinc bath. Requirements for the inspection, checking or qualification of the preparation to be carried out before subsequent overcoating shall be specified. 3. using weathering steel (oel patinabil, oel rezistent la coroziune); 4. using stainless steel (EN 1993-1-4 [19]).

    3.9. SHAPES

    3.9.1. General

    Steel structures design is based on the use of: 1. standard profiles, obtained by hot rolling process (Fig. 3.21):

    Fig. 3.21. Standard profiles

    2. built-up sections, obtained by welding hot rolled plates (Fig. 3.22):

    IPE, W, UB HEA, HEB, HEM, UC IPN UPN UAP

    L, LNP L, LNP IPET, HEAT, HEBT

    ROR RHS, MSH, TPS, RAUTA, VHP

    hollow sections

  • 3. STRUCTURAL STEEL

    78

    Fig. 3.22. Built-up cross-sections

    3. thin-walled cold-formed shapes, obtained by bending cold rolled thin plates or sheets (Fig. 3.23):

    Fig. 3.23. Cold-formed shapes

    The most recent works tend to consider in calculation the actual structural elements, with their structural and geometrical imperfections, that is a tendency to abandon the ideal perfect bars. The following aspects are to be considered: structural imperfections of the material; geometrical imperfections of the structure.

    3.9.2. Structural imperfections

    The most important structural imperfections of steel structural members are: the presence of residual stresses; the non-homogeneity of mechanical properties over the cross-section.

    3.9.2.1. Residual stresses

  • 3. STRUCTURAL STEEL

    79

    At the end of the rolling process the temperature is equal to approximately 600C. The exposed parts (the flange edges and mid web) tend to cool faster than the area around the flange to web joints. As a result, the complete cooling of the most exposed parts precedes the cooling of the flange to web joint areas, which remain hot for a longer period. This produces self-balanced residual stresses, whose distribution on the cross-section depends on the shape of the section (Fig. 3.24). Tests showed that residual stresses can be great enough, especially in jumbo sections (res = 0,5fy ... fy).

    Fig. 3.24. Example of distribution of residual stresses

    Remarks 1. Great residual stresses also appear in welded built-up sections. 2. Residual stresses are less important in cold-formed sections.

    3.9.2.2. Non-homogeneity of mechanical properties

    Non-homogeneity of mechanical properties is a result of many factors but the most important one is the difference in thickness between web and flange, in hot rolled shapes as well as in welded ones. In cold-formed sections an important increase of the yielding limit (fy) and of the ultimate strength (fu) occurs in the corner areas as a result of the hardening due to the bending (or rolling) process (Fig. 3.25).

    ( )

    ( ) ( ) ( + )

    ( + )

    ( + )

  • 3. STRUCTURAL STEEL

    80

    Fig. 3.25. Increase of mechanical properties in the corner areas

    3.9.3. Geometrical imperfections

    The most important problems to be considered are: variation of cross-section properties (A, I, W) along the member; mid-span initial deflection (f0); load eccentricity (e0). Tests on different shapes proved that the variation of cross-section properties along the member is very poor, so they may practically be considered as deterministic variables. Generally, the mid-span deflection f0 is described by the ratio f0/L (L being the span). The statistical distribution of f0/L looks like the one in figure 3.26.

    Fig. 3.26. Statistical distribution of the mid-span deflection f0

    1

    1

    1

    2 3 4 5 6 7 8

    2

    3 4 5 6

    7 8

    2 3 4 5 6 7 8

    fu

    fy

    rolling bending initial plate

    increase due to cut down

    strength

    fibre

    L f0

    actual n

    f0/L 10 -3

    0,2 0,7 1,0

  • 3. STRUCTURAL STEEL

    81

    Codes usually accept a value f0 equal to:

    1000Lf0 = ( 3.5 )

    The load eccentricity e0 can be designed or accidental and, depending on the values, it may be neglected or not in calculation.

    3.10. STRUCTURAL STEEL REQUIREMENTS

    In order to provide good elasto-plastic behaviour and for technological reasons, structural steels shall comply with some special requirements (Fig. 3.27): a high strength, appreciated by a yield stress fy 235 N/mm2 and fu/fy > 1,2; a quite large yielding plateau; an adequate tenacity ( = property to keep large plastic deformation before failure

    under great forces), appreciated by u 15% and u 20y; a good resistance against brittle fracture, appreciated by the fracture energy

    KV(+20C) 27J; a good weldability, i.e. the property of steel to be welded in normal conditions. For ductility purposes, EN 1993-1-1 [13] expresses some limits, given in the National Annexes, for which the recommended values are as follows: fu/fy 1,10; elongation at failure not less than 15%; the elongation at failure on a gauge length of

    5,65 oA (where A0 is the original cross-sectional area);

    u 15y , where y is the yield strain (y = fy / E). EN 1993-1-1 [13] recommends the following values for material coefficients: modulus of elasticity 2mm/N000210E =

    shear modulus mm/N00081)1(2EG

    +=

    Poissons ratio in elastic stage 3,0=

    coefficient of linear thermal expansion perK61012 = (for T 100 C)

  • 3. STRUCTURAL STEEL

    82

    Fig. 3.27. Typical behaviour diagram of structural steel

    Remarks 1. The high strength is the major advantage of steel, relative to the strength of other

    common structural materials: wood, masonry, concrete, etc. Unlike masonry and concrete, which are weak in tension, steel is strong both in tension and compression.

    An image of the strength of steel is given by the ratio:

    =

    yL

    fc ( 3.6 )

    A physical meaning of this factor is the greatest length of a bar whose cross-section is able to support its self-weight.

    yyyy

    fLfLf

    AVf

    AN

    ===

    ==

    In these relations: fy yielding limit of steel; weight per unit volume of steel; N axial force generated by the self-weight of the bar; A area of the cross-section of the bar; V the volume of the bar; normal stress on the most loaded cross-section of the bar. Here are some values of this factor (cL) for different materials:

    ordinary structural steel: 2800 4000 m;

    fu

    fy

    u y

  • 3. STRUCTURAL STEEL

    83

    aluminium alloys: 2500 10000 m; reinforced concrete in compression: 100 1300 m; glass fibre: 96000 184000 m; carbon fibre: 150000 210000 m.

    2. The capacity of steel to yield is a fundamental requirement for structural steel: a) Residual stresses result from the fabrication process both in rolled profiles

    and in welded built-up structural members (Fig. 3.28). The tendency to shorten of the warmer flange to web joint is braked by the other fibres of the cross-section. Consequently, once cooling completed, the flange to web joint remains in tension, while the other parts are in compression. Residual stresses, in tension and in compression, are self-balanced on the cross-section.

    Fig. 3.28. Example of residual stress distribution

    b) Figures 3.29 and 3.30 show the effects of residual stresses on a loaded structural member in tension and in bending, respectively.

  • 3. STRUCTURAL STEEL

    84

    Fig. 3.29. Influence of residual stresses on a tensioned member

    Fig. 3.30. Influence of residual stresses on a member in bending

    It is to note that the stress distribution according to the theory is strongly affected by the existing residual stresses.

    c) For the members in figures 3.29 and 3.30 the evolution of the total stress = res + N (or M) ( 3.7 ) in the bottom flange is examined in figure 3.31 (the phenomena in the bottom

    flange are the same for the member in tension or in bending). It is to observe (F = N or M):

    when F fy res all fibres behave in the elastic range (Fig. 3.31a, b); when fy res < F fy some fibres behave elastically and some plastically

    (Fig. 3.31c); when F = fy all fibres behave plastically (Fig. 3.31d).

    = res + N

    = res + M

  • 3. STRUCTURAL STEEL

    85

    Fig. 3.31. Evolution of the total stress in the bottom flange

    As a result:

    the uniform stress distribution in the bottom flange, usually accepted in current design, is a conventional one (it is correct only in the final state, when F = fy);

    the elastic behaviour in the meaning no plastic deformation after unloading is real only for the entire section as a whole, or when F is insignificant;

    in order to allow all fibres to reach the yield stress, steel used for structural members must possess a large plastic yielding plateau. When this fundamental requirement is not satisfied, the material has a fragile behaviour and a fragile rupture is to be expected in the moment when in a single fibre, the most stressed one, the stress reaches its ultimate strength value (like cast iron, glass, etc.).

    d) The good plastic behaviour is also fundamental in the situations of a structural member in tension with an important hole (Fig.3.32) or in the case of a structural member in tension with a non-homogeneous cross-section.

    = res + F F

    res

    a)

    b)

    c)

    d)

  • 3. STRUCTURAL STEEL

    86

    Fig. 3.32. Stress distribution around a hole in a member in tension

    3. An adequate tenacity and the avoidance of brittle fracture are fundamental requirements for a structural steel in order to create the possibility to transform the structure into a plastic mechanism (Fig.3.33), able to dissipate energy. The capacity to dissipate energy is a very important requirement for a structure situated in seismic regions when it is subjected to severe earthquake motions.

    Fig. 3.33. Example of plastic mechanism for a structure during earthquake

    4. A good weldability in normal conditions is an important requirement in order to avoid an uneconomical cost of fabrication.

    The above qualities of a structural steel result from: chemical composition and crystalline structure; the process of steel making; the rolling process influence.

    3.11. STRUCTURAL STEEL GRADES

    max

    min

    average

  • 3. STRUCTURAL STEEL

    87

    Table 3.3 shows the main characteristics for the most common Romanian structural steel grades, according to STAS 500/280. Table 3.3. Main characteristics for some Romanian steel grades

    Steel type Nominal steel grade

    Ultimate tensile

    strength r

    (N/mm2)

    Ultimate elongation

    %

    Yield strength c Material factor

    m = c/R Design strength R

    (N/mm2) thickness t (mm)

    t 16 16 < t 40 Carbon

    steel OL37 360 440 25...26 240 230 1,09

    220 210 High

    strength low

    alloyed

    OL44 430 540 22...25 280 270 1,08 260 250

    OL52 510 630 21...22 350 340 1,11 315 300

    Remarks 1. Steel grades with fine grains (OCS) are fabricated for special welded structures. 2. Corrosion resistant structural steels (RCA, RCB) and stainless steels (ORC) are

    fabricated for special use. The unified European pre-standard EUROCODE 3 [2] uses the steel grades defined in the European standard EN 10025:1990+A1:1993 [5]. This standard was adopted in Romania as SR EN 10025:1990+A1:1994 [6]. Although there is a good correspondence between the steel grades defined by STAS 500/280 and the ones described in [5] and [6], an exact equivalence is not possible. EN 1993-1-1 [13] uses the steel grades defined in the family of standards EN 10025. These standards were adopted in Romania as SR EN 10025-1 SR EN 10025-6+A1 [21], [22], [23], [24], [25], [26]. Although there is a good correspondence between the steel grades defined by STAS 500/280 and the ones described in [21] [26], an exact equivalence is not possible. Steels used in EN 1993-1-1 [13] are designated as follows (EN 100271, 2) [27], [28]. Table 3.4. Example of steel grade designation according to EN 100271 [27]

    Letter Figure Symbol 1 Symbol 2

    example S 355 J2

  • 3. STRUCTURAL STEEL

    88

    Letter S for structural steel Figure Minimum value of the yielding limit in N/mm2 for the lowest range of

    thickness Symbol 1 Fracture energy in Joules for a given temperature, defined as follows: Table 3.5. Definition of the fracture energies

    C 20 0 20 30 40 50 60

    27 J JR J0 J2 J3 J4 J5 J6

    40 J KR K0 K2 K3 K4 K5 K6

    60 J LR L0 L2 L3 L4 L5 L6

    Symbol 2 Obtaining mean of steel: M : thermo mechanical N : normalised or by normalising rolling Q : quenched and tempered

    G : other characteristics followed by 1 or 2 digits if necessary. Table 3.6. shows examples of values of some mechanical characteristics for the most common structural steel grades given in EN 1993-1-1 [13]; fy is the yielding limit, whilst fu is the ultimate strength.

    Table 3.6. Mechanical characteristics for some steel grades given in EN 1993-1-1 [13] (EN 1993-1-1 Tab. 3.1. Nominal values of yield strength fy and ultimate tensile

    strength fu for hot rolled structural steel) Standard and steel grade

    Nominal thickness of the element t [mm] t 40 mm 40 mm < t 80 mm

    fy [N/mm2] fu [N/mm2] fy [N/mm2] fu [N/mm2] EN 10025-2 S 235 235 360 215 360 S 275 275 430 255 410 S 355 355 510 335 470 S 450 440 550 410 550 EN 10025-3 S 275 N/NL 275 390 255 370 S 355 N/NL 355 490 335 470

  • 3. STRUCTURAL STEEL

    89

    S 420 N/NL 420 520 390 520 S 460 N/NL 460 540 430 540 EN 10025-4 S 275 M/ML 275 370 255 360 S 355 M/ML 355 470 335 450 S 420 M/ML 420 520 390 500 S 460 M/ML 460 540 430 530 EN 10025-5 S 235 W 235 360 215 340 S 355 W 355 510 335 490 EN 10025-6 S 460 Q/QL/QL1 460 570 440 550

    Table 3.7. Mechanical characteristics for some steel grades given in EN 1993-1-1 [13] (EN 1993-1-1 Tab. 3.1. (continued): Nominal values of yield strength fy and

    ultimate tensile strength fu for structural hollow sections) Standard and steel grade

    Nominal thickness of the element t [mm] t 40 mm 40 mm < t 80 mm

    fy [N/mm2] fu [N/mm2] fy [N/mm2] fu [N/mm2] EN 10210-1 S 235 H 235 360 215 340 S 275 H 275 430 255 410 S 355 H 355 510 335 490 S 275 NH/NLH 275 390 255 370 S 355 NH/NLH 355 490 335 470 S 420 NH/NHL 420 540 390 520 S 460 NH/NLH 460 560 430 550 EN 10219-1 S 235 H 235 360 S 275 H 275 430 S 355 H 355 510 S 275 NH/NLH 275 370 S 355 NH/NLH 355 470

  • 3. STRUCTURAL STEEL

    90

    S 460 NH/NLH 460 550 S 275 MH/MLH 275 360 S 355 MH/MLH 355 470 S 420 MH/MLH 420 500 S 460 MH/MLH 460 530

    The partial factors M are applied to the various characteristic values of resistance as follows: resistance of cross-sections to excessive yielding, including local buckling M0; resistance of members to instability assessed by member checks M1; resistance of cross-sections in tension to fracture M2;

    resistance of joints special provisions. The values of the partial factors M may be defined in the National Annex. The following numerical values are recommended for buildings and they are adopted in the Romanian National Annex:

    M0 = 1,0

    M1 = 1,0

    M2 = 1,25 Table 3.8. Partial safety factors for joints given in EN 1993-1-8 [18] (EN 1993-1-8

    Tab. 2.1) Resistance of members and cross-sections M0 , M1 and M2 see EN 1993-1-1 Resistance of bolts Resistance of rivets

    Resistance of pins M2

    Resistance of welds

    Resistance of plates in bearing Slip resistance - for hybrid connections or connections under fatigue loading - for other design situations

    M3 M3

    Bearing resistance of an injection bolt M4 Resistance of joints in hollow section lattice girder M5 Resistance of pins at serviceability limit state M6,ser

    Preload of high strength bolts M7

  • 3. STRUCTURAL STEEL

    91

    Resistance of concrete c see EN 1992

    NOTE: Numerical values for M may be defined in the National Annex. Recommended values are as follows: M2 = 1,25 ; M3 = 1,25 for hybrid connections or connections under fatigue loading and M3 = 1,1 for other design situations; M4 = 1,0 ; M5 = 1,0 ; M6,ser = 1,0 ; M7 = 1,1.