Topic 6.5: Rivets & Welds - Riveted Joints
In this topic we will discuss riveted and bolted joints. We will generally treat riveted and bolt joints in the same way and use the same method of analysis. Basically, in a riveted joint a heated rivet is forced into a hole connecting two "plates" (or beams). As the rivet cools, a tension develops in the rivet and the plates are forced together. In bolted joints, high strength bolts are inserted into connecting holes between plates (or beams) and then tightened to a percentage (initially approximately 70 percent) of the allowable bolt tensile strength. Generally for bolts the hole is slightly larger than the bolt, and this is taken into account in a careful analysis. In our discussion, for both rivets and bolts, we will assume the hole diameter is the same as the rivet or bolt diameter.
Before we examine the specific modes of failure, we will list some assumptions used in our discussion.
- That the rivets and bolts completely filled the connecting holes.
- That the applied loads are carried equally by the rivets (bolts).
- That the rivet (bolt) shear stress is distributed uniformly over the cross sectional rivet (bolt) area.
- That the tensile load carried by the plate is also distributed equally across the plate material.
We will also ignore the effect of friction in carrying the load. That is, there is significant friction between plates riveted or bolted together. This friction may play a significant part is the amount of load a joint can carry. Because the friction effect can vary substantially, we will not try to include the contribution of friction to supporting the load. This is a conservative approach in the sense that by ignoring the effect of friction, the joint in actuality will normally carry more than the calculated strength of the joint.
There are two basic types of riveted (bolted) joints - Lap Joints and Butt Joints. A lap joint is shown in Diagram 1, and a butt joint is shown in Diagram 2.
In a Lap Joint, two plates are overlapped and rivets or bolts penetrate the two plates connecting them together.
In a Butt Joint, the two main plates are butted up against each other and then covered with one or two cover plates. Then rivets or bolts penetrate the cover plates and the two main plates connecting them together. The load is transferred through one main plate to the cover plates by the rivets and then transferred back to the second main plate. There is a symmetric rivet pattern above each main plate as shown in Diagram 2.
The distance between rivets in a row in riveted (bolted) joint pattern is known as the Pitch. The distance between rows in a riveted (bolted) joint pattern is known as the Back Pitch (or Transverse Pitch, or Gauge). The first row (row 1) of a pattern is the row which is closest to the applied load. As a general guideline for steel or aluminum plates, the minimum pitch is three times the diameter of the rivet (bolt), and the edge pitch (distance from the nearest rivet to the edge) is 1.5 times the rivet (bolt) diameter.
Joint Failure: There are a number of ways in which a riveted (bolted) joint may fail.
1. Rivet Shear: As shown in Diagram 3, a side view of a lap joint, the rivet area between the two main plates is in shear. We obtain the formula for the Strength of the Joint in Rivet Shear by simply using the definition of the shear stress - which is the force parallel to the area in shear divided by area. Thus if we take the allowable shear stress for the rivet material times the cross sectional area of the rivet this gives us the load one rivet area could carry in shear before it failed (By failed, we mean having exceeded the allowable stress.)
So we can write:
Privet shear = N (pi * d2/4) 
all. Where:
N = Number of areas in shear. This equals the number of rivets in a lap joint or in a butt joint with one cover plate, and twice the number of rivets in a butt joint with double cover plates.
A = pi * d2/4 (or pi * r2) is the cross sectional area of the rivet in shear.all = The allowable shear stress for the rivet material.
2. Rivet/Plate Bearing Failure: This is compression failure of either the rivet or the plate material behind the rivet.
As shown in middle diagram in Diagram 4, when we consider the top main plate, the top main plate is being pulled into the fixed rivet. This puts the plate material behind the rivet into compression, and if the load is large enough the plate material may fail in compression. From the rivet's perspective, the plate is being pulled into it, and this puts the rivet into compression.
Again if the load is large enough, the rivet material may fail in compression. Which will fail first in compression depends, of course, on the maximum allowable compressive stress for the rivet and plate material - the lowest allowable compressive stress material will fail first.
To determine the load which will cause failure, we again multiply the stress by the area. In this case, it is common practice to take the area in compression as the vertical cross sectional area of the rivet (Diagram 5), for both the area of the rivet in compression and the area of plate in compression.
So we can write:
Pbearing = N (d*t) 
all , where
N = Number of rivets in compression
d = Diameter of rivet
t = Thickness of the main plateall = Allowable compressive stress of the rivet or plate material
3. Plate Tearing: This is a tensile failure of the plate material normally at the rivet row positions, that is, the plate will tear first where the holes are in the plate, just as paper towels tear where the perforations are located.
As is shown in Diagram 6, if we cut the plate material at rivet row 1 and look at the left end section, we see that for equilibrium the plate material is in tension. To determine the applied load, P, the plate can carry before it would fail in tension, we multiply the allowable tensile stress by the area in tension. This area is the cross sectional area of the plate which, if solid, would be the width of the plate times the thickness of the plate (A = w*t). However, we have cut the plate at rivet row 1, and we have to subtract the diameter of the rivet from the width of the plate (since the area of the plate is reduce due to the rivet hole).
Then we can write:
Prow1 = (w - n d)t all., where
w = Width of the main plate
n = number of rivets in row (in this example, row 1, 1 rivet)
d = Diameter of rivet
t = Thickness of the main plate all = Maximum allowable tensile stress for the plate material
The formula for plate tearing are rivet rows beyond row 1 have to be modified somewhat, due to the fact that rows beyond row 1 are no longer carrying the entire load, P, since some of the load has already be transferred to the second plate. We will go into this in more detail in a later example.
There are several additional ways the joint may fail, including plate shear - which may occur if a rivet is placed to close to the end of the plate, and the plate material behind the rivet fails in shear. If proper placing of rivets is maintained, this mode is not normally a problem.
We will consider only the three main modes of failure discussed above.
We will now look at several examples of Riveted Joints,
Please select :
Topic 6.5a: Riveted Joints - Example 1
Topic 6.5b: Riveted Joints - Example 2
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