Test-Special Steel Structures_2012

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SPECIAL STEEL STRUCTURES - TEST

1. Cold formed sections

1.1 Typical shapes of cold formed sections

1.2 Effects of cold forming

The manufacturing process modify the mechanical properties of the profiles alteration of the stress-strain

curve of the steel.

Strain hardening provides an increase of the yield strength and sometimes, of the ultimate strength that isimportant in the corners and still appreciable in the flanges, while press braking lets these characteristics

unchanged in these zones.

fyb, fu - yield strength, respectively ultimate tensile strength, N/mm2;

t- thickness of the steel plate;

( )ybuybya ff

A

Cntff

+=

2


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A - gross area of the cross section (mm2);

C=7 for cold rolling and 5 for other methods of cold forming;

n - number of folders at 900 having the internal radius r


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- for end stiffeners:

1.4. Sectional characteristics of the cross of cold formed shapes involved in resistance and stability

verifications; position of the shear center C for different types of sections (symmetric or non- symmetric)

42411

min 2,9266000

)(83,1 tRt

btI

p


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1.5. Section in axial compression: show the dimensions of the effective area and the shift of neutral axis

1.6. Section in bending: show the dimensions of the effective area and the shift of neutral axis

1.7 Strength verifications of class 4 sections

1. Members in Tension

NEdtensile force due to design load

Nt,Rd the minimum of

- net section resistance depending on fastener type according to chapter 4, Joints andConnections

2. Members in Compression

NEd compressive force due to design load;

The shift of the neutral axis, eNis the cause of an additional bending moment

3. Members in Bending

MEd the bending moment due to the design load

1.8 Stability verifications of class 4 sections

2. Towers and masts

2.1 Towers: destination, types, geometry, internal members, steel sections used in the design of towers

High slender structures used for:

radio-aerial transmission;

headgear for the extraction of coal and petroleum;

derricks for drilling;

chimneys for evacuation of gases in the industrial areas or for air intake etc.

They can have different types of structures:

spatial lattice structure with a simple, regular cross sections, square or triangular;

circular constant hollow section or in the shape of a conic frustum; (cylindrical or conic shells

-membranes are a flexible solution).

The shape of the tower follows the shape of the equal strength solid -shape of frustum of a pyramid. The width

of the base 1/8 and 1/15 of the total height; the bottom side of 1...2 m and the slope of the faces from 2.5 to 5%.

RdtEdNN

,

);( ,0

Rdt

M

yaF

Af

RdtF

,

RdcEdNN

,

1

,

M

effy

Rdc

AfN

=

NEdEdeNM =

1M

effy Wf

RdcEdMM

,


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Tower with triangular section from circular hollow sections- general view, cross section and details of joints

between the legs, struts and diagonals and details of connections between two diagonals, directly and with

gussets


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2.2. Actions and their combination on towers; deflections of towers

Actions

Towers are spatial structures, fixed in the foundations and free at the top and all the loading cases are

submitted to the static scheme of cantilevered beam.

The actions are determined by:

dead loads; live loads - the weight of the equipment and personnel, also loads coming from different functional

necessities;

variable actions - wind action taken with the maximum intensity, ice and their combinations at

different temperatures, in which the wind is taken with the reduced value;

seismic loads must also be taken into account when checking the structure.

Combinations of Actions: usually, the combinations of the variable actions considered for design are:

A. Limit states of strength and stability:

Wind with maximum intensity, for a temperature of10150 C;

Ice and reduced intensity of the wind speed, for a temperature of50 C;

No wind, no ice, extreme temperatures according to specific climatic conditions; in our country:

t max,summer=+400C and t min, winter=30

0C.

All the actions are affected by their partial safety factor and also, the factor of grouping the variable

actions is imposed, according to STAS 10101/0A-78

B.Serviceability limit states: the same combinations, only the partial safety factors either are missing, or

are altered according to this state

Verifications in the limit state of serviceability - maximum deflections of towers

M o , m - moments determined in the Mohr-Maxwell method;

K1.2, taking into account the capacity of deformation of the ties.

2.3 Wind action on towers :

- determination of the internal forces in the legs and diagonals of square and triangular towers for

different wind attack angle;

- effect of eccentricity of the wind action upon the internal forces in the members of towers

100/0

0max Hdx

EI

mMkH

=


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The efforts in the diagonals of the tower with four faces are determined with the relationships:

Determination of the efforts in the legs of the tower:

P - the weight of the tower, equipment and devices; loads on the platforms etc.;

MP - moments coming from vertical loads acting eccentric as against the vertical axis of the tower;

MH - the bending moments determined by the horizontal loads (wind, earthquake, etc.) at a certain level where

we check;

n - number of faces of the polygon at the tower base;

ri - distance between the gravity centre of the tower and the strut i which has to be checked;

- the angle in the horizontal plan determined by the line that goes through the centre of the cross section of the

tower and the strut the most compressed, on the direction of the wind action;

- the angle between the axis of the strut and the vertical line.

l

abhr

h

MV

r

MD o

o

o

2)(;

2;

2

0 ===

cos

cos)(2

cos +

=i

PH

rn

MM

n

PN


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2.4 Guyed masts: destinations, design criteria, types of masts considering the position and number of guying

wires

= Lattice spatial systems obtained from hot rolled open sections or cold formed hollow sections supported

(articulated) on foundations and guyed at different levels with wires.

Destination: Masts are structures exclusively used for radio aerial transmission.

Design details:

C Masts are in carbon steel or low alloy steel; guying system is made of steel wires with diameters


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C The connections are with or without gussets. Horizontal elements are made of circular hollow sections,

directly connected to legs or, with flanges or I sections in the case of the connections in the platform area.

C The tubular sections are inter-connected in factory with welded seams and joints at the building site are

provided with bolted connections.

C The details of anchorage systems and of foundations support allow the rotation. The legs may also be

separately anchored in the foundation (as in the case of towers) and in this case the mast is encased in the

foundation.

The number of guying levels may be from 16, depending on the technological necessities, the height of the

mast and the dimensions of the transversal cross section.

2.5 Wind action on masts and on the guying wires

Wind is the most important action, 90% of the maximum efforts; on the mast and on the cables in the most

unfavorable situation and combination of actions.

Wind on the cables - a constant value and equal with the pressure at 2/3 of the height of top edge of the wire.

Wind pressure on the mast is considered with a constant value on the distance between two anchorage pointsand it is determined as the moderated value of mean pressures acting on this distance; the most unfavorable

situation for the mast is when wind pressure is considered at the maximum value, the outside temperature being

+20oC.

The dynamic design must take into account the period of vibration of the anchored masts is:

( sec), (H in m). This relationship is sufficiently correct provided the mast is not charged with

important concentrated masses that may alter the static scheme.

a) Wind normal to the wall of a square section mast

b) Wind on the diagonal of a square section mast

c) Wind normal to the wall of a triangular section mast

d) Wind along one guying wire of a triangular section mast

e) Wind parallel to one wall of a triangular section mast (normal to one wire)

HT 01.00 =


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2.6 Foundations for guyed masts (simplified concept)

3 Electric Supply Transmission Lines:

3.3 Types of ESTL lines, components, geometry of a current tower

= S.E.T.L. is an independent system that transmits and distributes the electric energy in the territory. This

system is made of: conductor wires, insulation units, clamps, fittings, towers and their foundations, also various

devices necessary to tie to the earth.

The electric transmission lines (E.T.L.) are classified into three categories according to their importance:

I category - electric transmission lines forhigh voltage that transport high electric power to cities or

important industrial area, with continuous services;

II category - electric transmission lines forhigh voltage that transport low electric powerto small towns

or ordinary factories and also they may be distribution lines for high voltage lines;

III category - electric transmission lines forlow voltage.

The elements of these systems are:

conductors;

insulating units and fittings;

towers with cross-arms;

foundations.


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3.4 Actions and combinations on the towers: normal and accidental states

Self-weight of the tower, including the equipment (cross-arms, insulators etc. )- the deposit of ice increases the

weight of the tower with 10%:- towers without ice, Gst;- towers with ice, 1.1Gst where: -Gst- sum of the masses of all the elements of the tower.

Self -weight of the conductor(mono-threaded):

Conductor without ice:Conductor with ice:

where: ag- the span for vertical actions, in m;

Weight of the chains of insulators - in the absence of ice, Gizand in the presence, 1.1Giz

gcagG =

1

gcchagG =

3


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The effect of wind pressure on conductors: forces normal to the axis of the line:In the case when wind acts along the line:

The effect of wind pressure on the chains of insulators is a force acting on the wind direction - either parallel tothe line or normal to the line:\

Sagging forces from the cables

Sagging forces in the conductors - they are horizontal forces, p0, from the stresses in the cables due totemperature, self-weight and other external actions, according to a specific group of actions applied in the fixing

point of the chain of insulators to the tower, acting horizontally, in the direction of the conductors.

If loading is different in the two spans, for ex., the ice is acting only on one span, then apart the vertical

reaction there will be a horizontal reaction also along the alignment, its value being:

Sagging in cables transmitted to the insulators and to the towers: according to the regulations of P104:

- normal exploitation conditions:

where p0 is determined according to previous versions of groups of actions.

- accidental (damaged) conditions:

Wind on the tower as it follows:

Wind action on the whole surface of the tower is taken into account with its direction (pressure or suction). It is

determined with the relationship:

Normal exploitation conditions:

- Limit state of strength and stability:

- Limit state of deformations:

Accidental conditions (damage):

-Limit state of strength and stability:

-Limit state of deformations:

where:

-Pi- permanent actions on the whole line in the limit state considered;- Vi- variable actions simultaneously compatible;

-Ei- accidental actions: damage due to braking of one or more conductors, seismic actions etc.

2sin= vvcc agF

fgF vcc =2

izviz AgF iz =

lglgV == 33 22

1

21 000HTT = ( )

+++=

2

2

1

1022113 coscos

2

1

a

h

a

hTaagV

cspT = 00

cspT = 00

pcg stsvst =

inin VP +

+ ii VP

++ iaiaia EVP

++iii

EVP

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Test-Special Steel Structures_2012