Fig. 45.
If the spaces between the stays are too great, or the plate too thin, there is a danger of the structure yielding through the plate bulging outwards between the points of attachment of the stays, thus allowing the latter to draw through the screwed holes made in the plates.
In designing boilers with stayed surfaces, care should be taken that the opposite plates connected by any system of stays should, as far as possible, be of equal area, otherwise there is sure to be an unequal distribution of load in the stays, some receiving more than their proper share, and moreover, the least supported plate is exposed to the danger of buckling.
Rule for Finding Pressure or Strain on Bolts.
The absolute stress or strain on a flat surface of a steam boiler, which is carried by the stays, can be easily determined by a simple rule:
Choose 3 stays as A B C in [Fig. 46], measure from A to B in inches, and from A to C. Multiply these two numbers together and the result is the number of square inches of surface depending upon one bolt for supporting strength.
Example.
Suppose the stays measure from center to center 5 inches each way with steam at 80 lbs., then