cape3 actualizat

7/28/2019 CAPE3 actualizat

1/33

3. STRUCTURAL STEEL

57

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

AA

B

B B

BB

( a ) ( b ) ( c )

( d ) ( e )

( f )


2/33

3. STRUCTURAL STEEL

58

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).

3.3. 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. 3.10),

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. 3.10. Simplified scheme of the steel making process

Burned gases

Oxygen lance

Burner Gas or liquid fuel

Molten furnace iron


3/33

3. STRUCTURAL STEEL

59

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 maybe 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 andwhich 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]

3.2. 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. 3.2), where the atoms are placed in the knots of the lattice.

Fig. 3.2. Types of crystalline lattice of steel

Body-centred cubic

crystal (bcc)

Face-centred cubic

crystal (fcc)

,


4/33

3. STRUCTURAL STEEL

60

Fig. 3.2.bis 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 ofthese 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. 3.3). 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. 3.3. Scheme of the crystallization process


5/33

3. STRUCTURAL STEEL

61

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. 3.4).

Fig. 3.4. 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

YE

P

arctg E

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

fpe

fyfu


6/33

3. STRUCTURAL STEEL

62

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. 3.5);

Fig. 3.5. 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. 3.6); a complete tensile

test puts in evidence these phenomena (Fig. 3.7);

Fig. 3.6. Shear deformation of the crystalline structure

a

a a 2a

a + 1a

P P


7/33

3. STRUCTURAL STEEL

63

Fig. 3.7. 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

L100

L

LL

00

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 ratioplastic

ultimate

;

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

forces;

A0

Au

d0

L0

Lu = L0 + L

F F


8/33

3. STRUCTURAL STEEL

64

5. The Prandtl model is usually accepted for the stressstrain diagram () in order

to simplify calculation (Fig. 3.8).

Fig. 3.8. 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 SelfTempering):

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. 3.9);

real

Prandtlfy


9/33

3. STRUCTURAL STEEL

65

Entry QST Bank Quenching Self Tempering

1600F 871C 1100F 593C

Fig. 3.9. 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 ofu 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. 3.11) realizing both the advance of the bar and the changing

of its shape from one step to another (Fig. 3.12).

water


10/33

3. STRUCTURAL STEEL

66

Fig. 3.11. Simplified scheme of the hot-rolling process

1st step 2nd step 3rd step final step

Fig. 3.12. 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 yieldstress and the ultimate strength increase (Fig. 3.13).

Fig. 3.13. Simplified scheme of grains evolution during hot-rolling

t1 t2 < t1

T10001200C H1 (hardening)

R1(re-crystallization) 1st

step

hardening

Hi

Ri i step

Hfinal

Rfinal

Gfinal Ginitial G (magnitude of grains)

600800C


11/33

3. STRUCTURAL STEEL

67

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. 3.14). 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. 3.14. Simplified scheme of the toughness test

55

2

45

Charpy specimen

(V notch)

10

10

h2

h1


12/33

3. STRUCTURAL STEEL

68

Toughness is strongly unfavourably influenced by low temperatures (Fig. 3.15).

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. 3.15. Variation of the mechanical work L with temperature

Codes define the transition temperature (T) (Fig. 3.15) 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.3.16)(EN 1993-1-2 [18]). It is to notice that:

up to T = 100C there are no significant changes;

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.

Temperature (C)

Mechanical work (J)

27J

Transition temperature


13/33

3. STRUCTURAL STEEL

69

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. 3.16. Variation of some mechanical properties of steel with temperature

3.7. FATIGUE BEHAVIOUR

Fatigue tests show the lowering of mechanical strength after a cycle of stresshaving an oscillating intensity in time (Fig. 3.17). In EN 1990 [10], fatigue (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. 3.17. Types of loading cycles

In the elastic range, the variation of fatigue strength, 0, depends on (Fig. 3.18): the type of loading cycle (Fig. 3.17);

y

,y,fyf

fk =

E

Ek ,E

=

p

,p

,pk

=

(C)

constant

alternate

time

maxmax

max max

min min

minmin


14/33

3. STRUCTURAL STEEL

70

the designed details.

Fig. 3.18. Whler diagram

In modern codes, the fatigue verification is defined with regard to (Fig.3.19):

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. 3.19.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)

n (number of cycles)

O

6 106

3 106106

parent metal butt weld

(log())

n

n

n (log(n))

104 105 106 107 108


15/33

3. STRUCTURAL STEEL

71

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 inspectionfor fatigue damage. The safe life method should be applied in cases where localformation

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:

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

rangesstressshearfor3/f5,1

rangesstressdirectforf5,1

y

y

(8.1)

(2) It should be verified that under fatigue loading

0,1/ MfC

2,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 itshould be verified that:

0,1//

5

MfC

2,EFf

3

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 stressranges FfE,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 typicaldetail 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 forfatigue strength

Consequence of failureAssessment method

Low consequence High consequenceDamage tolerant 1,00 1,15


16/33

3. STRUCTURAL STEEL

72

Safe life 1,15 1,35

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

Table Error! No text of specified style in document..2 in EN 1993-1-9 [15]:Welded built-up sections (extract)

Detail

categoryConstructional 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 usingdetail 6) or 7) in Error!

Reference source not found..

Details 1) and 2):

No stop/start position is permitted

except when the repair isperformed by a specialist and

inspection is carried out to verify

the proper execution of the repair.

112

3) Automatic fillet or butt weldcarried out from both sides but

containing stop/start positions.

4) Automatic butt welds made

from one side only, with a

continuous backing bar, butwithout stop/start positions.

4) When this detail contains

stop/start positions category 100

to be used.

The values for the reference values CandC are given in the following diagrams:

DirectstressrangeR

[N/mm]

10

100

1000

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

3

m = 3

1

m = 5

140125

112

1

36

4045

5056

63718090

100

160

2

2 5

1 Detail category C

2 Constant amplitude

fatigue limitD

3 Cut-off limitL

Endurance, number of cycles N

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


17/33

3. STRUCTURAL STEEL

73

ShearstressrangeR

[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 limitL

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 ofthe plastic deformations. The phenomenon is called low cycle fatigue.

The numbern of critical cycles may be determined according to:

( ) ( ) 83,1log4,2nlog =+ ( 3.5 )

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.


18/33

3. STRUCTURAL STEEL

74

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], that covers execution of steel structures.

F.6 Coating methodsF.6.1 PaintingThe surface condition of the component shall be checked immediately prior to painting to ensure thatit complies with the required specifications, EN ISO 12944-4, EN ISO 8501 and EN ISO 8503-2 andthe 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 acrossapproximately 25 mm on both side of the edge and applied to a nominal thickness appropriate to thecoating system.Work shall not proceed if:

the ambient temperature is below that recommended in the manufacturer's recommendationsfor 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 asrequired by the product data sheet.The packing of painted components into bundles shall not commence until the paint manufacturer'sdeclared 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 shallbe taken to prevent damage to the coating during packing and handling.NOTE: Cold formed components are often produced as nesting profiles. Tightly packing componentsinto 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 methodsF.6.2 Metal spraying

Thermal metal spraying shall be of zinc, aluminium or zinc/aluminium 85/15 alloy and be undertakenin accordance with EN ISO 2063.


19/33

3. STRUCTURAL STEEL

75

Thermal metal sprayed surfaces shall be treated with a suitable sealer before overcoating with paint inaccordance with F.6.1. This sealer shall be compatible with the overcoating paint and shall be appliedimmediately after metal spraying cooling so as to avoid oxidation or moisture trapping.

F.6.3 GalvanizingGalvanizing 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 withEN 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 thinmaterial 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 beforesubsequent 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. 3.20):

Fig. 3.20. Standard profiles

2. built-up sections, obtained by welding hot rolled plates (Fig. 3.21):

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


20/33

3. STRUCTURAL STEEL

76

Fig. 3.21. Built-up cross-sections

3. thin-walled cold-formed shapes, obtained by bending cold rolled thin plates or

sheets (Fig. 3.22):

Fig. 3.22. 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


21/33

3. STRUCTURAL STEEL

77

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, whichremain 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. 3.23).

Tests showed that residual stresses can be great enough, especially in jumbo

sections (res = 0,5fy ... fy).

Fig. 3.23. 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. 3.24).

( )

( ) ( )

( + )

( + )

( + )


22/33

3. STRUCTURAL STEEL

78

Fig. 3.24. 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 propertiesalong 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 off0/L looks like the one in figure 3.25.

Fig. 3.25. 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

increasedue tocut down

strength

fibre

L

f0

actual

n

f0/L 10-3

0,2 0,7 1,0


23/33

3. STRUCTURAL STEEL

79

Codes usually accept a value f0 equal to:

1000

Lf0 = ( 3.6 )

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. 3.26):

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 formaterial coefficients:

modulus of elasticity2mm/N000210E =

shear modulus mm/N00081)1(2

EG

+=

Poissons ratio in elastic stage 3,0=

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


24/33

3. STRUCTURAL STEEL

80

Fig. 3.26. 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 ( 3.7 )

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

A

Vf

A

N===

==

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

uy


25/33

3. STRUCTURAL STEEL

81

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. 3.27). 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. 3.27. Example of residual stress distribution

b) Figures 3.28 and 3.29 show the effects of residual stresses on a loaded

structural member in tension and in bending, respectively.


26/33

3. STRUCTURAL STEEL

82

Fig. 3.28. Influence of residual stresses on a tensioned member

Fig. 3.29. 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 3.28 and 3.29 the evolution of the total stress

= res + N (orM) ( 3.8 )in the bottom flange is examined in figure 3.30 (the phenomena in the bottom

flange are the same for the member in tension or in bending).

It is to observe (F = N orM):

when F fy res all fibres behave in the elastic range (Fig. 3.30a, b);

when fy res < F fy some fibres behave elastically and some plastically

(Fig. 3.30c);

when F = fy all fibres behave plastically (Fig. 3.30d).

= res + N

= res + M


27/33

3. STRUCTURAL STEEL

83

Fig. 3.30. 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.3.31) or in the case of a

structural member in tension with a non-homogeneous cross-section.

= res + FF

res

a)

b)

c)

d)


28/33

3. STRUCTURAL STEEL

84

Fig. 3.31. 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 transformthe structure into a plastic mechanism (Fig.3.32), 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. 3.32. 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


29/33

3. STRUCTURAL STEEL

85

Table 3.1 shows the main characteristics for the most common Romanian

structural steel grades, according to STAS 500/280.

Table 3.1. Main characteristics for some Romanian steel grades

Yield strength cDesign strength R

(N/mm2)

thickness t (mm)

Steel type Nominalsteelgrade

Ultimatetensile

strength

r(N/mm2)

Ultimateelongation

%

t 16 16 < t 40

Materialfactor

m = c/R

240 230Carbonsteel

OL37 360 440 25...26

220 210

1,09

280 270OL44 430 540 22...25260 250

1,08

350 340

Highstrength

lowalloyed OL52 510 630 21...22

315 300

1,11

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 [13] are designated as follows (EN 100271, 2) [27], [28].

Table 3.2. Example of steel grade designation according to EN 100271 [27]

Letter Figure Symbol 1 Symbol 2

example S 355 J2

Letter S for structural steel


30/33

3. STRUCTURAL STEEL

86

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 3.3. 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 3.4. 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 3.4. 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)

Nominal thickness of the element t [mm]

t 40 mm 40 mm < t 80 mm

Standard and

steel grade

fy [N/mm2] fu [N/mm

2] fy [N/mm

2] fu [N/mm

2]

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

S 420 N/NL 420 520 390 520


31/33

3. STRUCTURAL STEEL

87

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 3.4. 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)

Nominal thickness of the element t [mm]

t 40 mm 40 mm < t 80 mm

Standard and

steel grade

fy [N/mm2] fu [N/mm

2] fy [N/mm

2] fu [N/mm

2]

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

S 460 NH/NLH 460 550


32/33

3. STRUCTURAL STEEL

88

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 asfollows:

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. Thefollowing 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 3.5. Partial safety factors for joints given in EN 1993-1-8 [18] (EN 1993-1-8Tab. 2.1)

Resistance of members and cross-sectionsM0 , 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 bearingSlip 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

Resistance of concrete c see EN 1992


33/33

3. STRUCTURAL STEEL

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 orconnections under fatigue loading and M3 = 1,1 for other design situations; M4 = 1,0 ;

M5 = 1,0 ; M6,ser= 1,0 ; M7 = 1,1.

cape3 actualizat

Documents