Connections in steel structures

CONNECTIONS IN STEEL
STRUCTURES
By:-
ANAND BHATNAGAR

TYPES OF CONNECTIONS
• Welded connections
• Riveted connections
• Bolted connections

Welded Connections
• Structural welding is a process by which the parts that are to be
connected are heated and fused, with supplementary molten metal
at the joint.
• A relatively small depth of material will become molten, and upon
cooling, the structural steel and weld metal will act as one continuous
part where they are joined.
• The additional metal is deposited from a special electrode, which is
part of the electric circuit that includes the connected part.

TYPES OF WELDS
• Fillet (Mostly used, Weaker than groove and others)
• Slot (expensive - poor transmission of tensile forces)
• Groove ( More reliable than others)

Advantages
• Economical – Cost of materials and labors.
• Efficiency is 100% as compared to rivets (75- 90%)
• Fabrication of Complex Structures – Easy – like
Circular Steel pipes.
• Provides Rigid Joints – Modern Practice is of Rigid Joints.

Disadvantage
• No provision for expansion or contraction therefore greater chances of
cracking.
• Uneven heating and cooling - member may distort - may result in
additional stresses.
• Inspection is difficult and more costlier than rivets.

Riveted connection
• Used for very long time.
• The length of the rivet should sufficient to form the second head.
• Design - very similar to bearing type of
bolted connection.

INSTALLING A REVITED CONNECTION
• Heating of the rivet
• Inserting it to size hole
pressure to the head.
• Squeezing the plain End
by Pneumatic driver
Round head.
• On Cooling Reduces in
Length–Clamping Force

Design of Welded Connections
• Fillet welds are most common and used in all structures.
• A fillet weld can be loaded in any direction in shear, compression, or
tension. However, it always fails in shear.
• The shear failure of the fillet weld occurs along a plane through the
throat of the weld, as shown in the Figure below.

• Strength of fillet weld
P = 𝑃𝑞x L x t
where
P = strength of the joint
𝑃𝑞= permissible stress ( 108 Mpa N/mm2 )
L = effective length
t = throat thickness = K x s
s = weld size
K = constant
• For the most common case i.e. welded surface meeting at 60° to 90°
t = 0.7 x s
P = 0.7 x 𝑃𝑞 x L x s

Design of Riveted Connections
• The perfect theoretical analysis for stress distribution in riveted
connections cannot be established. Hence a large factor of safety is
employed in the design of riveted connections. The riveted
connections should be as strong as the structural members. No part
in the riveted connections should be so overstressed. The riveted
connections should be so designed that there is neither any
permanent distortion nor any wear. These should be elastic. In
general, the work of fabrication is completed in the workshops where
the steel is fabricated.

ARRANGEMENT OF RIVETS
• Chain Riveting • Diamond Riveting

STRENGTH OF RIVETED JOINT
• Shear strength of rivet
Ps =
π
4
𝑑2 τ 𝑣 in single shear
Ps = 2 x
π
4
𝑑2
τ 𝑣 in double shear
• Bearing strength of rivet
Pb = d x t x σb
• Tearing strength of plate
Pt = (p-d) x t x σt
• Rivet value: smaller of the bearing strength and shearing strength of the
rivet.
Where
τ 𝑣 = allowable shear stress in the rivets
d = dia of the rivet
σb = allowable bearing stress
t = thickness of thinner plate
σt = allowable tension stress
p = pitch

Chain Riveting
• Strength of plate in tearing = (b-3D).t.pt
• Where b= width of the plate; D=Gross diameter of the rivet and
t=Thickness of the plate.
• When safe load carried by the joint (P) is known, width of the plate
can be found as follows;

Diamond Riveting
• Strength of the plate in tearing in diamond riveting section 1-1
• When the safe load carried by the joint (P) is known, width of the
plate can be found as follows
• At section 2-2: All the rivets are stressed uniformly, hence strength of
the plate at section 2-2 is
• At section 3-3,

SPECIFICATION FOR DESIGN OF RIVETED
JOINT
• Members meeting at Joint: The centroid axes of the members meeting at a joint should intersect
at one point, and if there is any eccentricity, adequate resistance should be provided in the
connection.
• Centre of Gravity: The centre of gravity of group of rivets should be on the line of action of load
whenever practicable.
• Pitch:
a. Minimum pitch: The distance between centres of adjacent rivets should not be less than
2.5 times the gross diameter of the rivet.
b. Maximum pitch: Maximum pitch should not exceed 12t or 200 mm whichever is less in
compression member and 16t or 200 mm whichever is less in case of tension members,
when the line of rivets lies along the line of action of force. If the line of rivets does not lie
along the line of action of force, its maximum pitch should not exceed 32t or 300 mm
whichever is less, where t is the thickness of the outside plate.
• Edge Distance: A minimum edge distance of approximately 1.5 times the gross diameter of the
rivet measured from the centre of the rivet hole is provided in the rivet joint. Table 6.1 gives the
minimum edge distance as per recommendations of BIS in IS : 800-1984.

DESIGN PROCEDURE FOR RIVETED JOINT
• For the design of a lap joint or butt joint, the thickness of plates to be joined is known
and the joints are designed for the full strength of the plate. For the design of a structural
steel work, force (pull or push) to be transmitted by the joint is known and riveted joints
can be designed. Following are the usual steps for the design of the riveted joint:
• Step 1:
• The size of the rivet is determined by the Unwin’s formula
• Where d= nominal diameter of rivet in mm and t= thickness of plate in mm.
• The diameter of the rivet computed is rounded off to available size of rivets. Rivets are
manufactured in nominal diameters of 12, 14, 16, 18, 20, 22, 24, 27, 30, 33, 36, 39, 42
and 48 mm

Step 2:
The strength of rivets in shearing and bearing are computed. Working stresses in rivets and plates are
adopted as per ISI. Rivet value R is found. For designing lap joint or butt joint tearing strength of plate is
determined as follows
Pt=(p-D).t.pt
Where p=pitch of rivets adopted, t=thickness of plate and pt = working stress in direct tension for plate.
Tearing strength of plate should not exceed the rivet value R (Ps or Pb whichever is less) or
From this relation pitch of the rivets is determined.
Step 3:
In structural steel work, force to be transmitted by the riveted joint and the rivet value are known. Hence
number of rivets required can be computed as follows
The number of rivets thus obtained is provided on one side of the joint and an equal number of rivets is
provided on the other side of joint also.

Step 4:
For the design of joint in a tie member consisting of a flat, width/thickness of the flat is known. The section is
assumed to be reduced by rivet holes depending upon the arrangements of the rivets to be provided, strength
of flat at the weakest section is equated to the pull transmitted by the joint. For example, assuming the section
to be weakened by one rivet and also assuming that the thickness of the flat is known we have
Where b= width of flat, t=thickness of flat, pt=working stress in tension in plate and P=pull to be transmitted
by the joint. From this equation, width of the flat can be determined.

Connections in steel structures

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