In many cases a row of rivets must be driven below another row and in material which is perpendicular to the material in which the first row is driven. Such a case is in the cover plate of a plate girder, or for that matter in most cases of cover plates. In such cases it is desirable to know what spacing must be used in order that the dolly will not be interfered with by the rivet already driven in the other row. Table IX gives such information. It is to be noted that the value 1' is the distance from the inner side of the leg of the angle, and is not the gauge. For example, let it be required to determine the minimum stagger for finch rivets in a 31-inch leg of a 31"X 3"X 1" angle. The distance Y is then equal to the gauge of a 3-inch leg less the thickness of the angle, or Looking along the top row the value l8 inches is found and going downward to the -1-inch line of values, •?,, inch is found to be the least distance that the rivet under consideration may be driven from the one in the other leg of the angle.
In some cases it is possible to drive rivets opposite if the proper row is driven first. Thus, in the 5"X31">q" angle of Fig. 44, if rivets in the 5-inch leg were driven first, those in the 3-inch leg must stagger by :inch, as figured above, Fig. 44a, but if the rivets in the 3-inch leg were driven first, the distance F=--- 3" --*" whiCh, being outside the values in Table IX, show that the rivets in the 5-inch leg may be driven with a zero stagger, or just opposite.
Certain clauses in most specifications call attention to the fact that rivets must not lie used in tension. While it is desirable not to have rivets in tension, and their use to resist tensile stresses should not be encouraged, yet a rivet has a distinct value when used in tension. Also, tests of a confidential nature have come under the author's observation, and they tend to prove that rivets so used show as great an efficiency as a turned bolt of the same diameter.
However, the strength of such rivets must not be assumed as being equal to a bolt of equal diameter, but must be computed.
The head of the rivet must be drawn out to full size, and the distance "h," Fig. 45, determined. The value of the rivet in tension is then given by the formula 3.14 where 8,=the unit shearing stress; d=the diameter of the rivet; and ti=the value as determined above.
For a finch rivet 1i=0.45 inch and, therefore, this value of the rivet in tension, being taken at 10,000 pounds per square inch, is which is seen to be considerable, and which is equal to the body of the rivet being strained up to 20,050 pounds per square inch.
It is thus seen that the head more than develops the strength of the body of the rivet. Therefore, in figuring the amount a rivet should take in tension, one should multiply the area of the cross section by the allowable unit stress decided upon. Since the speci fications do not give this, it will be safe to use the ultimate strength for rivet steel with a factor of safety of 4. Since the ultimate strength
of rivet steel should be about 50,000 pounds per square inch, this would make the allowable 12,500 pounds, and a finch rivet would have a value of • 12,500X 0.6013 = 7,510 pounds which is less than the amount required to strain its head up to the maximum allowable.
On account of the fact that riveted heads are not driven sym metrically the value of the rivet in tension is not certain, and their use in tension is not to be advised. Use turned bolts.
The bearing and shearing values of rivets may be found in the handbooks of the various manufacturers. The values for all values of allowable stresses are not usually given, but by a little trouble almost any values may be obtained by dividing those values there given, or by taking a multiple of them. For example, the bearing value of a Finch rivet in a plate, unit allowable bearing stress 18,000 pounds, may be obtained by taking n- times the value given in the 12,000-pound table, giving 6,S85 pounds.
In cases of the webs of channels or I-beams, or other thicknesses of metal which are not in even sixteenths of an inch, but are given in decimal fractions, the values may, with the help of the slide rule, be obtained from the tables. For example, let it be required to find the value of a Finch rivet in bearing in the web of a 15"X 33# channel, the unit-bearing stress allowed being 15,000 pounds. From Cambria, the thickness is seen to be 0.4, and the bearing of a -Finch rivet in a --inch plate is found to be 6,563 pounds. Therefore, the value sought will be 6,563 V — 0.5 X 0.4= 5,250 pounds For convenience in rivet spacing, Table X will be found con venient, the value of any number of spaces of a given length being determined at a glance.
Bolts, Nuts, and Washers. Bolts are made by forming a head on one end and cutting a thread on the other end of an iron rod. In such cases the body of the bolt does not represent the strength, but the area at the root of the threads. In the handbooks is given the diameter of the screw thread for any bar or bolt of given diameter, and from this the strength of a bolt may be calculated, once the allowable unit tensile stress is determined. The diameter given for the rod or bolt is the diameter of the upset screw end. The strength of the bolt is then obtained by multiplying the diameter of the screw at root of thread by itself, by 0.7854,* and by the allowable unit stress, thus Strength of Bolt= 0.7854 die X S For example, let it be required to determine the strength of a 1-1-inch bolt, the unit allowable stress in tension being 18,000 pounds per square inch. It is while if the area of the body of the bolt was used the strength would be 31,800, from which it is seen that in determining the strength of bolts care must be taken to use the diameter at the root of the thread.