Area of
boiler
end.		 Working
pressure.
1017.87×60 = 2765 lbs. per cross-sectional square inch of rivet.
50×.4417
Number
of
rivets.		 Area of
each rivet.

By multiplying the area of the boiler end by the working pressure, we get the total steam pressure acting to shear the rivets, and by multiplying the number of rivets by the area of one rivet, we get the total area resisting the steam pressure, and then by dividing the one quantity into the other, we get the shearing stress per square inch of rivet section.

In the case of longitudinal seams, we have as follows, the pitch being say 21⁄8 and the rivets 3⁄4.

Diameter
of boiler
in inches.		 Steam
pressure.		 Pitch.
36×60×2.125 = 5196 lbs. per square inch of rivet area.
2×.4417
Rivets
in one
pitch.		 Area
of
rivet.

It is seen, therefore, that the stress placed by the steam pressure on the transverse seam is about one-half of that it places on the longitudinal seam. But, as before remarked, the transverse seam is subject to racking strains, from which the longitudinal seams are exempt; thus, for example, the expansion of the boiler diameter, whether uniform or not, does not strain the longitudinal seam, whereas it may severely strain the transverse seam.

The English Board of Trade rules, in assigning values to the various constructions and qualities of workmanship, assign a certain value, in the form of an addition to the factor of safety, which takes into account the difference in the stress upon the transverse and longitudinal seams, the quantities in each case having been determined both from experiment and from experience. A comparison of the different values may be made as follows:

The rules take a boiler shell made of the best material, with all the rivet holes drilled after the strakes are rolled into shape and put together, with all the seams (both longitudinal and transverse) fitted with double butt straps each at least five-eighths of the thickness of the shell plates they cover, and with all the seams at least double riveted, with rivets having an allowance of not more than 75 per cent. over the single shear, and provided that the boilers have been open to the inspection of their surveyors during the whole period of construction, and say that such a boiler shell shall be allowed a factor of safety (divisor of seam strength) of 5.

But for every departure from this, which they deem the best mode of construction, a penalty in the shape of an addition to the factor of safety is made. These additions to the factors of safety with reference to the longitudinal as compared to the transverse seams, are given in the following table:

Nature of the deviation in the construction or workmanship.Addition to
the factor 5
if the
deviation
is in the
longitudinal
seam.		Addition to
the factor 5
if the
deviation
is in the
transverse
seam.
The holes not fair and good .75.2
Holes drilled out of place after bending .15.1
Holes drilled before bending .3.15
Holes punched after bending .3.15
Holes punched before bending .5.2
Joints lapped and double riveted instead of having double butt straps .2.1
Joints double riveted but have single butt straps .3.1
Joints single riveted and have a single butt strap1.0.2
Joints lapped and single riveted1.0.2