In the case of columns, girders and trusses, this practice would not do, as the details are too complicated and it is necessary to show all details exactly in their true relation in order to make them clear.

In the case of columns this can be done on a standard size sheet, generally 12 X 30 in. or 18 X 30 in. Girder sheets and truss sheets generally vary in size with the particular conditions of each case.

The first operation necessary is to draw out the outlines of the member to be detailed, showing a side elevation and plan, or end view and sections where necessary to clearly show all the work to be done. Make no unnecessary drawing; as, for instance, if a side elevation and plan will clearly express all the work to be done, do not spend any time making an end view or sections. If, on the other hand, an elevation and a cross section will enable you to show everything, then do not make any plan, as, in general, it is less work to make a cross section than a plan.

The above should be followed with caution, as it is necessary to be very sure that all the views required to give a clear understanding of the details are given.

Rivet Holes, Etc. Holes for rivets are either simply punched, or punched to a smaller size than that actually required and reamed out to the full size, or else the holes are drilled. Rivet holes are seldom drilled, except under special specifications, owing to the increased expense. On almost all work at present the holes are simply punched. In case reaming or drilling is required the shop drawing must indicate it clearly.

Where the holes are simply punched the usual specification is that the diameter of the punch shall not exceed the diameter of the rivet, nor the diameter of the die exceed the diameter of the punch by more than one-sixteenth of an inch.

Where the holes are punched and afterward reamed, the usual specification is "All rivet holes in medium steel shall be punched with a punch 1/8 in. (sometimes 1/8 in.) less in diameter than the diameter of the rivet to be used, and reamed to a diameter 1/16 in. greater, or they may be drilled out entire ".

The effective diameter of the driven rivet shall be assumed the same as before driving, and in making deductions for rivet holes in tension members, the hole will be assumed one eighth of an inch larger than the driven rivet.

The pitch of rivets is generally specified about as follows: "The pitch of rivets shall not exceed sixteen times the thickness of the plate in the line of strain, nor forty times the thickness at right angles to the line of strain. The rivet pitch shall never be less than three diameters of the rivet. At the ends of compression members it shall not exceed four diameters of the rivet for a length equal to the width of the members."

Rivets and Riveting. Rivets are spoken of as "shop rivets" or "field rivets" according to whether they are to be driven in the shop or in the field during the erection of the work. It is sometimes impossible to drive rivets by machine in the shop, owing to their location being inaccessible for the riveter. In such cases they must be driven by hand and are referred to as hand-driven rivets. Driving rivets by hand is necessarily more expensive than if done by machinery, and it is part of the duties of a competent structural draftsman to so design the details as to require the least possible driving of rivets by hand, whether in the shop or field. In erecting large jobs the field riveting is often done by machine riveters. There are numerous types of machine riveters, the principal power used being either compressed air or hydraulic power.

In order that rivets may be driven by the riveting machine it is necessary to have a certain amount of clearance from the heads of other rivets which project from the other leg of an angle if the two rivets are opposite or nearly opposite each other. This is shown in Fig. 189, together with a table giving sizes of rivet heads and clearances for machine driving. At the bottom of this table please note that a must not be less than 1/4 in. + 1/2h. Suppose we wish to drive two rivets, each 7/8 in. diameter, and both to have full heads exactly in the same line in the two legs of an angle. Now, if we desire to know how close we can drive the rivet in the horizontal leg to the back of the angle, we first find the value of h for a 7/8 in. rivet, which is 1 7/16 in. Then a = 1/4 in. + 1/2 (l 7/16 in.) = 31/32 in. Add this to the height of the rivet, which, for a 7/8- in. rivet is 21/32 in., and we have 1 5/8 in. as the distance from the center of the rivet in the horizontal leg of the angle to the side of the vertical leg of angle nearest to this rivet. But all measurements to locate the position of rivets are given from the backs of angles; hence we must add the thickness of the angle in order to find where the rivet in the horizontal leg should be spaced. Suppose the angle to be | in. thick, then 1 5/8 in. + 3/8 in. = 2 in. would be the least distance from the back of the angle that we could drive either rivet in order to have the riveting machine clear the other.

Rivets could, however, be spaced nearer to the back of the angle if the rivets are "staggered", i.e., if those in the vertical leg were spaced so as to come in between the two adjacent ones in the horizontal 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 (sometimes 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 red 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. 189 shows conventional signs for representing the different kinds of rivet heads desired, and this code is in general use in the United States.