Rivets could, however, be spaced nearer to the back of the angle if the rivets arc "staggered", i.e., if those in the vertical leg were spaced so as to come in between the two adjacent ones in the hori zontal leg. An example of staggered rivets is shown in Fig. 233.
Conventional Signs. In erecting some classes of structural steel work, especially in light highway bridges and small roof truss jobs, the connections are often made with bolts instead of rivets. The rivets used for structural steel work are round headed (some times called "button head") rivets. It is necessary sometimes to flatten the heads of rivets after the rivet is driven, and before it has had time to cool. This is done by simply striking the reel hot head of the rivet and flattening it to the extent desired. Wherever a flat tened head would interfere with some connecting part of a structure it is necessary to countersink the heads, sometimes on one end of the rivet and sometimes on both ends. Fig. 1S9 shows conventional signs for representing the different kinds of rivet heads desired, and this code is in general use in the United States.
It is very important to show on all shop drawings the diameter of rivets to be used in the work, and if different sizes of rivets or rivet noles for field rivets occur in the same member, then these must be indicated on the drawing by a note prominently displayed so that the shop men may readily find it and avoid error. The sizes of rivets generally used for structural steel and bridge work arcs in., in., or in. in diameter, although special work may require smaller sizes, and occasionally rivets 1 in. in diameter are used for very heavy work.
Rivets are made with one head formed, and the shank of the rivet must be long enough to project through the parts to be joined, and far enough out on the other side to form a full perfect head when subjected to the pressure of the machine. After the rivet has been heated to a cherry red it is inserted in the rivet hole and the riveter is placed so that the cap fits over the head aiready formed, and the other jaw of the machine presses against the protruding shank of the rivet and forms the head. It is desirable that riveting machines be made to hold on to the two ends of the rivet with the full pressure until the rivet partially cools.
The terms "rivet pitch" and "rivet spacing" refer to the dis tances center to center between rivets. For example, if the rivets are spaced 3 in. apart for a certain distance along a member of a structure, we refer to the rivets for this portion of the member as being of three-inch pitch. Fig. 190 gives the lengths of rivets re
quired for a given "grip".
1. Given an 18-in., 55-lb. I-beam with a 4 X 4 X 3-in. shelf angle riveted on one side; what length of rivet should be ordered for riveting this angle on in the field ? 2. In Fig. 187 of Part II, is shown a 12-in. beam girder bolted to a cap angle on a column; what length of bolts should be ordered for this connection? 3. If the beams shown in Fig. 187 are 61 in. center to center, and are bolted up, using standard cast iron separators, what lengths should be ordered for these separator bolts ? 4. Suppose a 12-in., 40-lb. beam and a 7-in., 15-lb. beam are framed opposite each other on a 13-in., 60-lb. girder; if standard con nection angles are used, what length of field rivets should be ordered for the connection of the beams to the girder? 5. If it is necessary to drive two rivets of t in. diameter exactly opposite in the two legs of an angle 3 X 3 X in.; how close to the back of the angle can the rivets be spaced ? Strength of Joints. The student should now become famil iar with the method of calculating the strength of joints and connec tions. We will take first the connection of one beam framed to another. The rivets in the connection, of course, are the only means of transmitting the load from the beam to the girder. There are two sets of these rivets, one set through angles on the end of the beam to be carried and the other set through the outstanding legs of these angles and through the web of the girder. The load must go from the beam through the first set of rivets into the connection angles, and then from the angles through the second set of rivets into the girder.
The rivets through the angles securing them to the web of the beam are subject to failure in two ways. (1) The rivet might break along the two planes coincident with the faces of the web of the beam, thus allowing the beam to drop between the two angles—this method of failure is called "shearing" of the rivets. (2) The rivets might crush the metal of the web of the beam on the upper semi-circumfer ence of the rivets; this is called failure by "bearing." In designing a connection, the number of rivets is determined by whichever provision against these two methods of failure gives the greatest required number. The strength of a rivet as regards shearing and bearing is called its value, and in order to determine the number of rivets to carry a given load in connections of this character, it is only necessary to determine the value to be used for one rivet. This value is determined in the following way: