This section is from the "Architectural Iron And Steel, And Its Application In The Construction Of Buildings" book, by WM. H. Birkmire.. Also see Amazon: Architectural Iron And Steel, And Its Application In The Construction Of Buildings.
The strength of the girder in this form is generally taken as equal to the sum of two beams used singly.
Bolts And Rivets as used for beams and girders must be proportioned to resist shearing, and the area of their bearing must be such that the metal against which they bear shall not be crushed. The strains allowed per square inch on these members are: shearing, 7500 pounds; crushing, 12,500 pounds. The shearing strain is measured on the area of the cross-section of the bolt or rivet; the crushing strain, on the area obtained by the product of the diameter of the rivet by the thickness of the plate or web on which it bears. It is customary to disregard the friction between the parts joined, as being too uncertain an element to be relied on to any great extent.
Take, for example, two plates of wrought iron 8 inches wide by 1/2 inch thick which overlap each other for a joint, with 45,000 pounds strain on the plates. What number of rivets will be required to resist the strain on the joint?
The area of a rivet J of an inch in diameter is 0.4417 square inch; this multiplied by 7500 pounds, the safe shearing, = 3312.75 pounds, the safe amount of strain each rivet can sustain without shearing. Dividing 45,000 by this, we get 13.6, - say 14 rivets.
The bearing area of each rivet is 3/4 inch by 1/2 inch = 3/8 square inch; this multiplied by 12,500 pounds for crushing would equal 4688 pounds, the safe compression for each rivet. 45,000 ö 4688 = 9.6, - say 10 rivets.
This latter condition is very otten overlooked in riveted work. Its observance in most cases of riveted girders with single webs gives the size and number of rivets to be used, and in thin webs the bearing value may be small, necessitating a thicker web than would otherwise be required.
For the above calculation the plates as overlapped would be in single shear. If the plates butted and a small plate were placed each side of the joint, or as an angle knee each side of a framed or coped beam, the rivets would be in double shear and have twice the value. If there are two cover plates, experience shows that each had better be equal to two thirds the thickness of the butted plates, although theory requires each to be but half their thickness. Plates butting require twice as many rivets as laps, because in the lap each rivet passes through both joined plates; and in the butt through only one, in case one cover plate is used. If the plates joined are sufficiently wide, the rivets should be placed zigzag, this being a much better system of riveting than the chain, that is, rivets following each other in one or more parallel lines.
Pins must be calculated for shearing, bending and bearing strains, but one of the latter two in almost every case determines the size to be used.
The pressure on the pins, multiplied by the leverage with which it acts on the pin supports, is the bending moment. The strain allowed per square inch of the cross-section is 15,000 pounds for iron and 20,000 pounds for steel.