LAP SEAM OR BUTT-STRAP?


| May/June 1995



A rivet in single shear

Box 10264, Burbank, CA 91510

1 don't respond much to articles I have read in magazines and I am not sure why I am doing this one. I guess I just thought I had something to say. In the March-April issue of IMA, there was an article written in 'Soot in the Flues' about Lap Seam and Butt-Strap joints. First, I don't claim to be an expert! Working on Steam Tractors is a hobby to me. This hobby comes with some special rules so what I have learned I have had to learn by necessity, mostly to please the state so I could go on working and running steam tractors!

In the article it was stated that a rivet in single shear was only half as strong as a rivet in double shear. True enough. A rivet in single shear must be sheared through in one place, a rivet in double shear must be sheared through in two places, see fig #1. In practice the strength of the rivet in double shear is taken to be 1.75% stronger. With this in mind, a butt-strap joint should be twice as strong as a lap seam joint because a rivet in a butt-strap joint is in double shear as compared to a lap seam joint, right? Try as I might, I just could not make this work! Not with twice as many, half as many, or the same number of rivets! The only way I could make this happen would be to design a really poor lap-seam and compare it against a properly designed Butt-Strap joint. (I will show you some math in a moment.) Let me define some terms, the seam we are talking about is the seam on the barrel of a boiler that runs the length of the barrel, that is left to right in most steam tractors. A lap joint is where one plate overlaps the other, a butt-strap joint is where the ends are butted up against each other and straps are used to hold them together (see fig #2). Joints are also classified by the number of rows of rivets used to hold them together. The two joints we see most often on our steam tractors are the double riveted lap joint, and the double riveted, double butt-strap joint. Otherwise called a Butt and double Strap joint, because it has two straps holding the butt together. Figure 2 shows an example of both of these. For the rest of this article, a 'lap joint' will refer to a double riveted lap joint and a 'butt-strap joint' will refer to a double riveted butt and double strap joint. When we talk about joint efficiency, we are talking about the strength of the joint compared to the strength of the same metal if there were no joint at all. The strength of a joint is always less that the strength of the metal with no joint. On our steam tractors, a well designed lap joint will show an efficiency around 73%, a butt-strap joint will be around 82%. It is interesting to note that in the earlier boilers they used a lap joint with only one row of rivets holding it together; this had an efficiency of about 57% and modern boilers that are fusion welded together can have an efficiency of 100%, if the weld is ground flat to the plate, stressed relived and certain other criteria are met. OK, here are the math definitions. If you're not interested in this part just skip it and I will sum up what happen after its all done.

TS=tensile strength of the boiler plate (psi)
t=thickness of the boiler shell (inches)
b=thickness of the butt straps in inches (where used)
P=pitch of rivets (inches). See figure #2
d=diameter of rivet after driving (inches)
a=cross sectional area of rivet after driving (inches)
s=shear strength of rivet in single shear (psi)
S=shear strength of rivet in double shear (psi)
c=crushing strength of boiler plate (psi)
n=number of rivets in single shear
N=number of rivets in double shear

Note that the diameter of a rivet is the diameter after driving it in. This is because a rivet will expand when it is red hot, that is to say an 11/16 diameter rivet when heated red hot will not go into an 11/16 hole, so you would use a inch hole and the calculation will use '. Here is the math for a lap joint.

A = P*t*TS
B = (P-d)*t*TS
C = n*s*a (n = 2 for our lap joint)
D = n*d*t*c