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