1.Advantages of steel structures:
a) case erection-no formwork
b)speed of erection-much of struct may be prefabr. away from the site.
-largely self supporting during erection.
c)modification at later date-extension/strengthening relatively easy
d)long selfweight- permits large clear spans
e)good dimensional control-prefabrication in the shop ensures accurate work
2.Basic materials used in blast furnice to prod steel (basic components in iron making)
-iron ore |
-coke | =>melted=>remove volatile mater.
-limestone |
-sinter
3.Short description of basic oxygen(BOS)steel making process
Three main processes exist for the production of steel. The oldest of these is the open heart process. Because it is slow and therefore relatively uneconomic it is steadily being replaced by the basic oxygen (BOS) process and the electric arc method which was originally devised to produce high quality steels, requiring precise control over their composition. Today most structural steel is made using the BOS process shown in Fig. 2.1. This beginns with the operation known as charging, in which a mixture of molten iron and up to 30% scrap is poured into the top of the BOS vessel (with a volume of 75-100 t). High purity oxygen is then blown in at great speed using a water -cooled lance. This combines with excess carbon and other unwanted impurities which then fl 24124o146y oat off as slag.
During
this time, the temperature and chemical composition are carefully monitored and when both are adjudged
correct, the steel is tapped into a ladle. At this stage, a sample is taken for chemical analysis
and subsequent examination of its
physical properties. The results of these appear on the mill certificate which must be provided to the eventual purchaser of the steel. From the ladle,
the still molten metal is cast into moulds where it solidifies into the ingots
which will be taken to the mill to be
rolled into plates, structural
sections, bars and strip. This takes about 40 minutes (compared with 10 hours
in the open - heart method) and may involve an initial charge of more than 350
tonnes.
4.Section types for steel members
-standard items comprising joists, universal beams, universal columns, channels, angles and hollow sections, which are rolled by the various mills at regular intervals
-more specialized items comprising
tees, bulb flats, sheet piling, rails and special application sections, which
are rolled to suit individual customer demand
5.Steel
contains less than 1% carbon
6.Structural steel has a carbon content of 0,25% carbon
7.Cast iron contains about 4% carbon
8.Face centred cubic crystal- 
9.Body centred cubic crystal-desen
10.Draw
stress-strain diagr for structural steel
E-Young modulus
E= / ; Et=d /d
P-upper limit of elastic behaviour (lim of proportionality)
ft-tang modulus
fe-elastic limit stress
fe< < -no elastic behaviour
11.Formula for percentage elongation in tensile stress
Percentage elongation=
12.Percentage reduction in area=
13.Huber-Hencky von Mises yielding criterion
14.Steel grades used in Ro for gen constr purpose
OL371n;OL372n;OL373k;OL374k=>subgrades for the same steel OL
OL=rolled steel;37=ultimate tensile strenght-370N/mm(mm la a doua)
1=subgrade
14.Cathegories of imperfections for a steel member
Structural or mechanical imperfections steel profiles consist of
-presence of residual stresses
-non-homog distribution of mechanical charact over the CS
Residual or locked in stresses represent the balanced internal stress state in steel bars as a consequence of the industrial manufacturing procedures. They occur in bodies undergoing non-uniform plastic deformations. In absence of external forces, residual stresses are always elastic. The non- homogenous deformation condition, which creates residual stresses in steel sections, is due to thermal (cooling, welding, flame cutting) and mechanical (cold rolling straightening) industrial procedures.
The non - homogenous distribution of mechanical characteristic over the cross sections of steel bars is also due to their production technique. Among the various mechanical characteristics, the structural behaviour of a steel bar is most deeply influenced by the variation in yield stress
15.Scheme with the main layers of a paint system
Total thickness of protection = 120 mm
-25 mm-finishing coat-resist environ
-30 mm,30 mm-undercoat (s)-builds thickness
-35 mm-priming coat (s)-whets surface, adheres
CLEANLINESS-SURFACE
STEEL
Resistance randomness(6.5.1.)
Def.of the charact values of R (Xk) dep on average value Xm and S
Types of variable loads
-loads due to fabrication, erection and testing.
-loads due to the use of the structure (live loads, thermal effects, lateral thrust of materials against walls, etc).
-loads due to natural phenomena (snow, wind, earthquake) or to unaccountable events (impacts, blast, accidents).
Ultimate limit of a structure-maximum carrying capacity of the structure
Ultimate limit states of a structure include:
-loss of equilibrium of a part or of the whole of the structure considered as a rigid body
-transformation of the structure, or of one of its parts into a mechanism due to plastic hinges being formed
-rupture of critical sections of a structure or excessive deformation before a mechanism is formed due to lack of ductility
-general or local instability due to second order effects
Serviceability limit states include
-deformation of the structure or of any part of it which could adversely affect the appearance or efficiency of the structure.
-local damage such as plasticization, local buckling, bolted joint slipping, cracks in welded joints, which might imply excessive maintenance or lead to corrosion.
-vibrations due to wind or to machinery that can render the structure unusable or cause an increase in loading state due to resonance phenomena or may lead to disconfort, alarm or harm its proper function.
Max M that a plasticised section can bear
Definition of the shape factor of a section
Types of welded joints
Two types of weld are in common use, butt and fillet welds.
-in the former the weld metal is generally contained within the profiles of the welded elements.
-in the latter deposited weld metal is external to the profile of the welded elements. Obviously, the complete length cannot be melted simultaneously.
Edge preparation in butt welds
The depth the arc melts into the plastic is called depth of penetration
Welding syst. (dep on the technology )
MMA-manual metal arc
This is most widely used arc welding process. It is manually operated and requires considerable skill to produce good - quality welds. The electrode consists of a steel core wire (3.2 - 6.0 mm diameter) and the covering flux contains alloying elements (for example, manganese and silicon). The arc melts the parent metal and electrode. As metal is transfered from the end of the weld pool, the welder moves the electrode to keep the arc length constant. This is esential, as the width of the weld run is largely governed by the arc length. The flux melts with the core wire and flows over the surface of the pool to form a slag, which must be removed after solidification
MAG-Metal active welding
This process sometimes referred to as "metal-inert gas" (MIG) welding, although strictly the term MIG should be limited to the use of pure argon as a shielding gas, which is not used for steel.
MAG is also manually operated. The arc and pool are shielded by a gas which does not react with molten steel; in current practice the shielding gas is carbon dioxide or a (80%; 20%) mixture of argon and carbon dioxide. No flux is necessary to shield the pool (the alloying elements are in the electrode wire), but sometimes a flux-cored electrode is used to produce a slag which controls the
weld (for example, in large fillet welds in the horizontal - vertical position). The arc length is controlled by the power supply unit.
SAW-submerged arc welding
This a fully mechanized process, in which the welding head is moved along the joint by a tractor or lead-screw. The electrode is a bare wire which is advanced by a governed motor. The voltage and current are selected at the begining of weld and are maintained at the preselected values by feedback systems which, in practice, vary in sophistication. The flux is in the form of granules and is placed on the surface of the joint. The arc operates below the surface of the flux, melting a proportion of it to form a slag. Unfused flux is collected and re-used for the next weld.(Figure 8.15.).
Stud welding
Effective throat thickness for fillet weld profiles(drawing)
Bolts are connecting elements consisting of
(main elements of bolt)
A metal pin having a head (usually hexagonal) and a partially threaded shank 9.1-a)
A nut (uaually hexagonal, Fig. 9.1-b)
Washers
(usually round, Fig. 9.1 -c).
Where vibrations occur and the nut might loosen, lock nuts (Fig 9.1-d) or spring washers (Fig. 9.1 -e) should be used.
Joints with bolts in shear only. Failure mechanism which appear at ultimate load
Collaps due to bolt failure(fig9.7a)
Collapse duet o hole failure (.b,c)
Collapse for tension failure of plates(d)
Formula
for the design shear strenght of a bolt is
-shear in non-threaded part
-shear in thread part
n=no of bolt shear sections
A= Ares=
R =shear strenght of the bolt
Types of compound tension members, acc to the type of connection between the chords
-Laced members (fig. 10.3-a)
-Batten plates members (fig. 10.3-b).
-Buttoned members = closely spaced built-up members (fig. 10.3-c).
Compression members: effect of imperfect geometry
ECCS-buckling curves-a,b,c,d
Types of CS for compression members
The cross-section of axially compressed members should be built depending on:
- loading level (magnitude of load);
- the type of the structure to which they belong;
-end connections of the member
Scheme of laced compression members
Frequent types of CS used for bent elements
Resistance ultimate limit state of a bent member
Formula of biaxial bending
where:
-Mx, My - are the bending moments about major (x-x) and minor (y-y) inertia axis, obtained in the same cross-section.
-Wx, Wy - are the section moduli about major and minor inertia axis.
The increasing factor 1.1 was applied to steel strength R considering the fact that maximum stress, is reached in isolated points of cross- section only.
As a suplementary condition in case of biaxial bending, the resistance checking relations from monoaxial bending should be separately fulfilledd for both inertia axis.
Serviceability limit state of a bent member-condition &formula of mid span deflection in case of a simply supp member
Types of tensioned members
Name of the zone affected by the welding heat
-heat affected zone-HAZ
Resistance ultimate limit state in case of tensioned members
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