CONNCETIONS: -
Ø Various elements on a Steel Structure like tension and compression & flexural of members are connected fasteners Connectors.
Ø The need of designing Connections are: -
1) To connect plates, angles, channel, I-Sections etc;
2) To connect different members at the ends.
Ø The connections are generally provided in the following cases:
1)Unless the requirement to account for the heavy load and long span is met, then the built-up parts must be given. In this case, to get a good section this section should be connected together.
2) The length of the standard section needs to be connected to another section in case of longer duration. In this case it is necessary to attach the proper design of multiple parts of the connections.
3) At the end the different members need to be linked (e.g. secondary beams to be linked to primary beams, columns, footings, etc.)
Ø Design of connections are very important because the failure of joints is Sudden and catastrophic. Normally, a connection failure is not as ductile as that of a Steel member failure. Connections may be classified according to the following:
a) Method of Fastening (rivets, bolts, welding)
b) Connection Rigidity (rigid, Simple, Semi-rigid)
Ø According to IS code, based on rigidity connection, the joint can be defined as:
1) Rigid
2) Simple
3) Semi-rigid
Semi-Rigid: -
Ø Semi-rigid connections have sufficient rigidity to hold the original angles between members.
Ø In reality all the connections will be Semi-Rigid.
Types of Connections: -
The following three types of connections may be made in steel connections:
1) Riveted Connections
2) Bolted Connections
3) Welded Connections
WELDED CONNECTIONS:
Ø Welding is the
process of the joining two metal pieces by establishing metallurgical bond b/w
them.When two structure
members are joined by means of weld, the Connection is called “welded
Connections”.
Types of welding process: -
Assumptions in the Analysis of welded joints: -
Ø
The welds
connecting to various plates are homogeneous, isotropic and elastic elements.
Ø
The parts connected
by the weld are rigid and their deformation is therefore neglected.
Ø
Only Stresses due
to external loads are considered. Effects of residual Stresses, Stress Concentration and Shape of a weld are
neglected.
Advantages of welded connection: -
Ø
As no hole is
required for welding, hence no reduction of area. So, structural members are
more effective in taking the load.
Ø
In welding
connection, filler plates, gusseted plated, connecting angles etc; are rot used
which leads to reduce overall weight of the structure.
Ø
Welded joints are
more economical as less material is required.
Ø
The efficiency of
welded joints is more than that of the riveted/ bolted joints.
Ø
The welded joints
are better than bulky riveted/ bolted joints.
Ø
Welded joints are
better for impacts loads and vibration.
Ø
The speed of
fabrication helps Compress production schedules.
Ø
Complete rigid
joints can be provided with wedding process.
Ø
No noise is
produced during the welding process.
Ø
The welding process requires less work Space
in Comparison to riveting.
Ø
Any Shape of joint can be made with easy.
Disadvantages of Welded joints: -
Ø
Welded joints are
brittle therefore, their fatigue Strength is less than the members joined.
Ø
Due to uneven
heating & Cooling of the members during the welding, the members may
distort resulting in additional Stresses.
Ø
Skilled labour and
electricity are required for welding.
Ø
No provision for
expansion & Contraction is kept in welded Connection & therefore, there
is possibility of cracks.
Ø
The inspection of
welding work is more difficult and costlier than the riveting work.
Ø
Defects like
internal air protect slag inclusion & incomplete penetration is difficult
to defect.
Types of Welds: -
Classified according to their formations:
-
There are three types of welded joints: -
1)
Fillet welds (Lap weld)
2) Butt Welds (groove weld)
3) Slot welds
4) Plug welds
Basic Types of Welds and their Symbols: -
1) FILLET WELD: -
Ø
When two lapped
plates are to be joined, fillet welding is use. Thus, these welds are called"
Lap welds".Fillet weld is
approximately triangular Cross-section joining two surfaces approximately at
right angles to each other in lap joint, tee-point or corner point.When the C/s of
fillet weld is isosceles triangle with face at 45°, it is known as "Standard
fillet weld" In Special circumstances 60°, 30° are also used.A fillet weld is
known as" Concave fillet” weld or “fillet weld" or “MITER fillet
weld” depending upon the Shape of the weld face.
Few terms used while designing the fillet welds: -
Ø Size of fillet weld.
Ø
Throat of fillet weld.
Ø
Effective length of
fillet weld.
Ø
End return.
Ø
Overlap.
Ø
Side fillet.
Ø
Intermittent fillet
weld.
Ø
Single fillet weld.
Ø
Permissible Stress
& Strength of fillet weld.
Size of Fillet Weld: - (Clause 10.5.2.3; T-21; P-78)
Ø
The sides
containing the right angle of the fillet weld are called “legs”. The Size of weld is
specified by the minimum leg length. The length is the
distance from the root of the weld to the toe of the weld measured along the
fusion face.
Notes: -
1) When the min. Size of the fillet wed is
greater than the thickness of the thinner part. The minimum size of the word
should be taken as the thickness of the thinner part.
2) When the thicker part is > 50 mm thick.
Species precautions like pre-heating etc; will have to be taken.
Specification: -
Ø
As per Clause 10.5.2.1 of IS 800:2007, for deep
penetration welds, where depth of penetration beyond the root run min. of 2.4
mm. Therefore, size of fillet weld is (min. leg site + 2.4mm).
Ø
The max size of
fillet weld= Thickness of the number plate -1.5mm
=
3/4 of nominal thickness of angle
Effective Throat Thickness: -
(Clause 10.5.3.1 of IS 800 T-22, P-16 )
Ø
” Effective throat
Thickness of a fillet weld is the perpendicular distance from the root to the
hypotenuse joining the two ends of the legs, while reinforcement is neglected.
Ø Effective throat thickness shall not be less than 3mm
and generally not exceeding 0.7t.(or 1.0t under Specific circumstances), where
't' is thickness of the thinner plate. (Clause 10.5.3.1, T-22, P-78).
Effective length of fillet: -
|
Effective
length (le) = Overall length – 2S
|
Specifications: -
Ø
The effective length should not be less than
four times the Size of weld. Otherwise Size of the weld must be taken as the
one-fourth of the effective length.
|
i.e. Lmin >
4S
|
End Return: - (10.5.1.1, P-78)
Specification: -
Ø
The fillet weld
terminating at the end or Side of the member should be returned around the
Corner whenever practicable for a distance not less than twice the weld size.
Ø
End returns of
length equal to twice the size of the weld are provided at each end of
longitudinal fillet weld.
Overlap: - (10.5.1.1, P-78)
Ø
The
overlap of a lap joint should be less than four times the thickness of the
thinner plate or 40 mm whichever is more
Intermittent Welds: - (10.5.5, P-79)
Ø
Length shall not be
less than 4 times the thickness of weld or 40mm whichever is more.
Ø
The min. Clear
Spacing of intermittent weird shall be 12t for Compression joints and 16 t for
tensile joints.
Where,
t = thickness of thinner plate joined.
Ø
The intermittent
welds shall not be used in positions Subjected to dynamic repetitive and
alternating Stresses.
2) BUTT WELD: -
Ø
Butt welds are also
known as "Groove welds”. Depending Upon the e shape of the groove made for
welding butt welds.A Butt weld is used
when the plates to be joined are in the Same plane or when T-Joint is desired. Butt Weld is
usually designed for direct tension, but if shear is also there then due
provision Shad be made.Reinforcement gives
efficiency to the joint and it ensure the strength.
Few terms used while designing the butt welds: -
1)
Size of the weld.
2) Effective length of the weld.
3) Effective area of the butt weld.
4) Reinforcements.
Size of the Weld: -
Size
of the weld is Specified by the effective throat thickness as follows:
(a)
The Size of weld is the thickness of thinner plate.
(b)The
effective throat thickness in case of complete penetration is taken as the
thickness of the thinner part.
Ex: - Double -V, Double-u, Double-j & Double level
butt joints are the examples of completely
penetrated butt weld.
(c)
In case of incomplete penetration of butt weld the effective throat thickness
is taken as the 7/8th of the thickness of the thinner part. But for the purpose
of stress calculation the effective throat thickness Should not exceed 5/8th of
the thickness of the thinner part.
(d) The
difference in thickness b/w the two plates should not be more than 25% of the
thickness (or) 3mm whichever is more.
(e) If
the difference is more than 25% tapering less than 1 in 5 is to be done.
Effective length of the Butt Weld: -
Ø
It is the area of
the butt web for which the specified size (i.e. the throat thickness) of the weld
exists.
Specification:
-
Ø
The effective
length should not be less than four times the size of weld. Otherwise Size of
the weld must be taken as the one fourth of the effective use length.
|
i.e. Lmin >
4S
|
Note: -
Ø
In
drawing only effective lengths are shown the welder must provide an additional
length of 2S to get the overall depth.
Effective Areas: -
Ø
The
effective area of butt weld is the product of the effective throat Thickness
and the effective length of the butt weld.
Reinforcements: -
Ø
Reinforcement is
extra weld metals which make the throat dimension at least 10% greater than the
thickness of the well material. The extra deposit
of metals above the thinner plate b/w 1mm to 3mm is not considered for stress
calculation or design. The Reinforcement
is provided to Increase the efficiency of the joint.
Stress in the butt weld: -
Ø
The stresses of the
butt weld should be taken equal to the stresses of the parent metal in Case of
shape weld. These values are reduced to 80% in Case of field weld.
Intermittent butt welding: -
Ø
If intermittent
butt welding is used, it shall have an effective length of not less than four
times the weld Size and space between two welds shall not be more than 16 times
the thickness of thinner part.
3) SLOT AND PLUG WELD
a) Slot Welds: -
Ø
A typical Slot used
in which a plane with circular hole is kept with another plate to be joined and
the fillet Welding is made along the periphery of the hole.
(b)Plug Welds: -
Ø
A typical plug weld
in which Small holes are in one pale and it kept over another parent to be
connected and then an entire hole is filled with filler material.
Ø
Slot and plug welds
are may be used along with fillet weld when Sufficient length of weld not available
to have more shear strength and tension given tension for supporting given
load.Plug Weld and Slot
Welds will reduce unsupported dimension b/w fillet weld so that load buckling
failure maybe checked.
Ø
“A Slot” is cut in
one of the overlapping members and the welding metal is filled in the slot.If the slot is
small and completely filled with Weld metal, it is called "Plug Weld”.If the periphery of
the slot is filled with weld metal, it is called “Slot weld”.
Specifications for design of slot plug welds as per (IS 8l6- 1969): -
Ø
The width of
diameter of the slot should not be less than three times the thickness of the
part in which the Slots formed or 25mm,whichever is greater.The distance
between edge of the part and edge of the Slot or plug or b/w adjacent slot or
plug should not less than 3 times the thickness of thinner member or 25mm
whichever is greater.
Ø
Corners at the
enclosed ends should be rounded to a radius not less than one and a half times
The Thickness of the upper plate (or) 12mm whichever is greater.The effective area
of a plug weld shall be considered as the nominal area of the hole.The design stress
on a plug/Slot weld will be as Same as than in fillet weld and is specified in (IS
800: 2001 Clause No 10.5.9.1.1).
Notes: -
Ø
The
design procedure for plug and slot welds are same as for in case of fillet
welds.