In some cases tracing paper is used in making small unimpor tant drawings. This paper should be of good quality in order that it may stand erasing, since mistakes are liable to occur and these must necessarily be corrected. The best tracing paper is brittle and will not stand much handling. For this reason its use for expensive drawings is not to be recommended.
Stress Sheet. The stress sheets for various structures are usually not made in the drafting room, but are made in the designing room of the company. Much data and many computations are made by the designer which would be of use to the draftsman in detailing. All of this information should be placed upon the stress sheets. The making of a stress sheet should be and usually is done by men of considerable experience. Plates I, II, V, and VI show stress sheets of a truss bridge, roof truss, and a plate girder, respectively, and while these can not be said to be perfect, yet they indicate the engineering practice of our larger bridge corporations and may be taken for examples. (For Plates, see pages G3, 82, SS, and S9.) Since the ordering of material is of great importance it will be discussed here somewhat at length. Although this is usually done by men of considerable experience, yet it is advisable that the draftsman should know the method of procedure in order that he may be able to make the detail drawings more advisedly.
Layout. Typical Case. As has been mentioned before, the checker makes details to a large size scale from which he determines the size and amount of material required for certain members. In order to illustrate this, let it be required to determine the size of the plates and the length of the angles used in the cross frame of the plate girder shown on Plate VI. Here the checker first lays off the center to center of the girder to a small scale, say, 3" to 1'. These lines are marked 1 in Fig. 6. He next draws a web to the proper thickness and then follows in turn the flange angles and the bracing angles as indicated by the numbers 2, 3, etc., on the figure. The number of rivets in the top and bottom angle and in the diagonal should be given on the stress sheet. These should be laid off on the "layout," which is the name for the drawing that has just been made, any spacing, preferably 3 inches, being used so that the plate may be kept as small as possible. It is as a usual thing not possible to have the rivet spacing in both the diagonal and the top and the side angles of equal spacing. The number of rivets is usually put in the diagonal at about a 3-inch spacing, and the spacing of the rivets in the top and the side angles is so varied as to fill out the plate as indicated. No rivets should come closer to the edge of the
plate than 11 inches nor further from the edge than 2 inches, and no plate should be less than an even number of inches in width although its length may be in feet, or in inches to an eighth of an inch. It is not policy to place the length of the plate in sixteenths of an inch, since the shopmen are unable to cut that close. Therefore, in deter mining the size of the plate the rivets should be so placed that a suf ficient number should go in and the size of the plate be kept an even number of inches in width. If the rivets alone governed the size of the plate, it would be as indicated by the dotted lines in Fig. 6, and the dimensions would then be as indicated by the dimensions with a line drawn around it. The correct size of the plate is as indicated by the full line.
The length of the line from intei-section to intersection point is S'-9*" as indicated upon the drawing. In order to have the length of the diagonal to come out the nearest sixteenth of an inch, the distance of the first rivet from the intersection is taken arbi trarily and is as indicated here, 7 inches. It is not necessary to give this dimension to a thirty-second of an inch, since if the diagonal varies that much from the computed length, it can be drawn up into place by using a drift pin and can be riveted up without injuring the material.
Use by Checker and Draftsmen. The checker has now deter mined the size of the plate and the length of the diagonal angle and he records them upon the material bill which is to be sent to the mills as an order for material. This layout together with a copy of the material bill should be given to the draftsman when he starts to detail the girder. He will then have the size of a plate and the size of an angle for that particular girder so that the material which has been ordered, probably months before, and has arrived before the draftsman starts the detail, can be used and will be used in that girder. In case the draftsman details the cross frame without con sulting the layouts and bills of material, he is liable to draw up a detail which will demand a plate larger or smaller than that ordered for that particular plate; in the first case a new plate will be required, the ordered plate being placed in the stock pile until some other job conies up in which it can be used; and in the second case the ordered will have to be cut down to the size of plate the draftsman has used, thus necessitating extra expense and loss of material.