Welded and Riveted Connections
WELDED CONNECTIONS
Types of Welding
Chapter 7: I. Welded Connections
Heat only Heat and Pressure (Fusion Welding)
Chapter 7: I. Welded Connections
Heat only: Thermit Welding - is an exothermic welding process
that uses thermite to melt metal, which is poured between two workpieces to form a welded joint.
Gas Welding – is a metal joining process in which the ends of pieces to be joined are heated at their interface by producing coalescence with one or more gas flames.
Arc Welding - is a type of welding that uses a welding power supply to create an electric arc between an electrode and the base material to melt the metals at the welding point.
Thermit Welding
A thermite mixture using iron (III) oxide
Thermite is a pyrotechnic composition of a metal powder and a metal oxide, which produces an exothermic oxidation-reduction reaction known as a thermite reaction.
Gas Welding
Arc Welding
Arc Stick-Welding Rods
Chapter 7: I. Welded Connections
Heat and Pressure (Fusion Welding): Forge Welding - is a solid-state welding process that
joins two pieces of metal by heating them to a high temperature and then hammering them together.
Spot Welding - is a process in which contacting metal surfaces are joined by the heat obtained from resistance to electric current flow. Work-pieces are held together under pressure exerted by electrodes.
Flash Welding - is a type of resistance welding that involves pressing two ends together, while simultaneously running a current between them.
Forge Welding
Spot Welding
Flash Welding
Chapter 7: I. Welded Connections
Seam Welding - is an adaptation of resistance spot welding and involves making a series of overlapping spot welds by means of rotating copper alloy wheel electrodes to form a continuous leak tight joint.
Projection Welding - is a variation of spot welding. Projections are designed in one part. These act as current concentrators for the welding process. When the two parts are mated together, these projections are the high points that first make contact.
Upset Welding - is a special way of welding, in which two pieces of material are forged together at elevated temperatures.
Seam Welding
Projection Welding
Upset Welding
Welding Properties of Materials
Chapter 7: I. Welded Connections
Gaseous oxides may cause blowholes Soluble oxides reduce the strength and toughness of the
weld Insoluble oxides cause slag inclusion in the weld Plain carbon steels except spring steel and tool steel can
be satisfactorily welded Lower carbon steels are the most easily welded Nickel, chromium, and vanadium improve the welding
qualities slightly
Strength of Welds
Chapter 7: I. Welded Connections
Weld metal has better properties with the shielded arc than with bare weld material
Butt welds may be assumed to have approximately 80 percent of the strength of the base metal if the welds are flush
Plug welds are as strong as the weld metal in shear
Chapter 7: I. Welded Connections
Items which affect the strength of the weld: Soundness of the weld Whether or not the loading is static or fatigue loading Whether shielded or unshielded Type of joint Type of stress existing within the joint
Strength of Fillet Welds
Chapter 7: I. Welded Connections
5 most common types of weld: Fillet welds – normal and parallel welds Lap welds Edge welds Butt welds Plug welds
Fillet Weld
Lap Weld
Edge Weld
Butt Weld
Plug Weld
Eccentric Loads
Chapter 7: I. Welded Connections
Proportioning of parallel Fillet welds:
∑Mcg;
aLas = bLbsand
Lb = L – La
from which
La = bL/(a+b)and
Lb = aL/(a+b)
Chapter 7: I. Welded Connections
Eccentrically loaded Fillet welds:
Ss max = √(s12 + s2
2 + 2s1s2cosθ)
where
cosθ = b/√(a2 + b2)
s1 = F/tL
s2 = (Fe √(a2 + b2))/2J
Welded Pressure Vessels
Chapter 7: I. Welded Connections
The ASME Boiler Code contains rigid rules for the welding, inspection, and testing of vessels to be used as containers of gases and liquids under pressure.
Some Practical Welding Considerations
Chapter 7: I. Welded Connections
Welds should be placed symmetrically about the axis of the welded member unless the loading is unsymmetrical
For unsymmetrical members like angles, the welded lengths are determined by the method previously outlined
When the strength of the weld is computed, ½ in. should be subtracted from the length to allow for starting and stopping the weld
When plates of unequal thickness are butt-welded, the edge of the thicker plate should be reduced so that it is of approximately the same thickness as the thinner plate
RIVETED CONNECTIONS
Chapter 7: II. Riveted Connections
USES OF RIVETED JOINTS
Tanks Pressure Vessels Bridges Building Structures
Rivets on Bridges
Rivets on a Tank
Rivets on a Pressure Vessel
Rivets on Building Structures
Chapter 7: II. Riveted Connections
Three Classes of Riveted Joints: Strength and Rigidity are the chief requirement Both Strength and Rigidity are required Sealing against fluid leakage as well as strength
and rigidity are required
Chapter 7: II. Riveted Connections
RIVETS
Proportions of rivet heads: Button head Cone head Steeple head Flat head Pan head Countersunk head French or Oval Countersunk head
Chapter 7: II. Riveted Connections
Boiler and Structural Rivets consideration: The holes for the rivets should be approximately
1/16 in. larger in diameter than the rivet
Chapter 7: II. Riveted Connections
TYPES OF RIVET JOINTS
Single-riveted lap joint Single-riveted butt joint with single strap Triple-riveted butt and double strap joint
with straps of equal width
Chapter 7: II. Riveted Connections
ASSUMPTIONS ON THE CONVENTIONAL DESIGN OF RIVETED JOINTS
Common Failure in Riveted Joints: Double shear of rivet Tearing in plate Crushing of the plate Bending of plate
Chapter 7: II. Riveted Connections
Common Assumptions in Riveted Joints:1. The load is distributed among the rivets according
to the shear areas.2. There is no bending stress in the rivets.3. The tensile stress is equally distributed over the
section of metal between the rivets.4. The crushing pressure is equally distributed over
the projected area of the rivets.
Chapter 7: II. Riveted Connections
5. In a rivet subjected to a double shear, the shear is equally distributed between the two areas in shear.
6. The hole into which the rivets are driven do not weaken the member if it is in compression.
7. After they have been driven, the rivets completely fill the hole.
8. Friction between adjacent surfaces does not affect the strength of the joint.
Chapter 7: II. Riveted Connections
NOTATION USED WITH RIVETED JOINTSF = total load carried by any repeating group of rivets. lbFt , Fs , Fc = total load that may be carried in tension, shear, or crushing by a
repeating group. lbst , ss , sc = unit stress in tension, shear, or crushing. psi
t = main-plate thickness. intc = cover-plate thickness. in
d = rivet-hole diameter. in
p = pitch or center distance of rivet holes. in. In joints of more than one row of rivets, the pitch is measured in the outer row. Subscript refer to the row, beginning with the inner row.
Chapter 7: II. Riveted Connections
pb = back pitch or distance between rows of rivets. in
pc = pitch on calking edge or outer row of cover plate. in
n = number of rivet areas in any repeating group; when used with a subscript n refers to the row indicated by the subscript.
a = edge distance or distance from plate edge to center of nearest rivet. in
e = joint efficiency; when used with a subscript t, s, or c it refers to the efficiency in tension, shear, or crushing only.
Chapter 7: II. Riveted Connections
EFFICIENCY OF RIVETED JOINTS
The efficiency of the joint is defined as the ratio of the load that will produce the allowable stress in any part of the joint to the load that will produce the allowable tension stress in the unpunched plate
Chapter 7: II. Riveted Connections
RIVET DIAMETERS
In boilers and high pressure vessels, where temperature are high, the rivets expand and completely fill the holes. Hence, in all strength calculations, the diameter of the hole is used and not the undriven diameter of the rivet
Chapter 7: II. Riveted Connections
SOME PRACTICAL RIVET CONSIDERATIONS The ASME Boiler Code requires that the edge
distance must be not less than 1½d or more than 1¾d.
In order to ensure a reasonable rigidity, and allow for corrosion and unknown handling stresses, certain minimum thickness must be maintained.
Chapter 7: II. Riveted Connections
DESIGN OF A TYPICAL BOILER JOINT
Chapter 7: II. Riveted Connections
Sample Design:A boiler is to be designed for a steam pressure of 350 psi.The diameter of the largest course of the drum is 54 in.
The working stress to be used are st equal to 11,000, ss equal to 8,800, and sc equal to 19,000 psi.
Solution:1. t = PD/2 st e = (350)(54)/(2)(11,000)(e) = 0.859/e in.
2. t = 0.859/0.85 = 1.011, say 1 1/16 in.
Chapter 7: II. Riveted Connections
3. pc = d + 4√(tc2/P) = d + 4√(0.752/350) = d + 3.98 in.
p = 2 pc = 2d + 7.96
Ft = (p – d)(t)(st) = (2d + 7.96 – d)(1.0625)(11,000)
= (d + 7.96)(11,688) lb
Fs = (9)(d2/4)(ss) = (9)(d2/4)(8,800) = 62,190d2 lb
Chapter 7: II. Riveted Connections
Ft = Fs
(d + 7.96)(11,688) = 62,190d2
d = 1.31, say 1 5/16 in.
The rivet diameter will be 1/16 in. less, or 1 ¼ in.
4. pc = d + 3.98 = 1.3125 + 3.98 = 5.2925 in., say 5 ¼
andp = 2 x 5.25 = 10 ½
Chapter 7: II. Riveted Connections
5. F = ptst = 10.5 x 1.0625 x 11,000 = 122,720 lb
Chapter 7: II. Riveted Connections
TANK AND STRUCTURAL JOINT
The permissible working stresses for the design of the riveted connections in bridges, building structures, and machine frames are somewhat higher than those used in pressure-vessel design.
Chapter 7: II. Riveted Connections
ECCENTRIC LOADS ON STRUCTURAL CONNECTIONS The line of application of the load generally
should pass through the center of gravity of the rivet areas.
RIVETS