Structural drafting and the design of details

This book has been prepared especially to meet the requirements of engineering students, structural draftsmen, and apprentices in structural drafting. It corresponds in scope to the duties of the structural steel draftsmen, and it therefore covers, not only the preparation of the detailed working drawings for steel structures, but also the design of the details of construction. It is a text-book in structural Drafting, and it may be used as a text-book in elementary structural Design. As a reference book for structural draftsmen, it gives practical points as well as theory. A knowledge of the use of drawing instruments is presupposed, but the fundamentals of structural drafting are fully presented. The application of these fundamentals is illustrated by the drawings of many different types of members of steel structure. Exceptionally exhaustive are the chapters on the design of beams and the component parts of plate girders. The tables at the end of the book are sufficiently complete for most student courses, so that no steel manufacturers' handbook need be used. Many of the tables are arranged more conveniently for both students and draftsmen than the tables in the usual handbooks, particularly the tables for I-beams, channels, and angles. A more complete outline of the book is given in Chapter I.

1. The structural draftsman is not concerned directly with the manufacture of steel or even with the rolling of the commercial steel "shapes." His drawings show how these shapes are cut, punched, and assembled to form members which in turn go to make steel structures. But every draftsman should understand the processes which are allied to the work of his company. The student has no time for a careful study of the different operations, but he should have a general idea of how steel is made and used. For his convenience an abstract is presented in this chapter and in the two subsequent chapters. Later he may acquire further knowledge from books or from inspection trips to rolling mills, to structural shops, and to erection sites.

2. In the Estimating or Designing Department of a structural company are made the preliminary design and the estimate of cost of a proposed structure. These may be based upon the customer's layout, or upon an original design submitted to the customer for approval. Usually several different companies furnish estimates in competition. After a contract is awarded the design sheet is forwarded to the Detailing or Drafting Department. The design sheet usually shows the main form and dimensions of the structure, the principal stresses, and the sizes of all main members, together with special instructions regarding the details. The work of the Designing Department is explained more fully on page 20: 2.

3. In the Drafting Department the detailed working drawings are prepared for use in the shop. Various diagrams and lists are also made, such as the preliminary bills of material from which the steel shapes are ordered from the rolling mills. As far as possible, the material must be ordered before the drawings are made so that the mills may roll the steel while the drawings and the templets are being prepared. As soon as the drawings are made and checked, blueprints are sent to the templet shop, to the structural shop, and to others concerned. The work of the Drafting Department is described more fully on page 20: 4.

4. The drawings are first sent to the templet shop where templets are made for most of the members. These templets are virtually patterns for cutting and punching the component pieces. They are usually made of wood. Not only can the work be laid out on wood with greater facility than on steel but the templets can often be completed before the steel arrives from the rolling mills and thus the completion of the structure is hastened. Furthermore, the work may be laid out on wood once and then the templets may be used repeatedly in marking many steel pieces which are alike or similar.

5. The manufacture of steel and the rolling of the structural shapes are described in the next chapter. The finished shapes are shipped to the structural shop where all cuts and holes are first indicated on the steel by means of the templets. The steel is then taken to shears to be cut and to punches to have the rivet holes punched. The component parts of each member are then assembled, being held together temporarily by bolts until the shop rivets are driven. Other processes may be required on some members before they are painted and shipped, as described more fully in Chapter IV, page 27.

1. Erection. The different members of a structure are shipped to the site as far as practicable in the proper sequence for erection. The methods of erecting them differ with the size and the type of the structure and with its location. Usually buildings are made self-supporting from the first, but truss bridges must be supported by "false work" or by other means until they are nearly complete. Locomotive cranes are used extensively in the erection of mill buildings, girder bridges, and viaducts. Derricks are used for office buildings and "travelers" for truss bridges. Main members are usually placed in position first and secondary members are filled in afterwards. Enough erection bolts are used to hold the members in position until the "riveting gangs" can drive permanent rivets.

2. The Engineering Department includes the Designing or Estimating Department and the Drafting or Detailing Department. Both departments are in charge of a Chief Engineer and often one or more Assistant Chief Engineers, although these officers are usually more directly concerned with the work of the Designing Department. The organization of the Designing Department differs in different companies and the procedure depends upon the organization and also upon the nature and the magnitude of the proposed structures. Some companies have a special contracting Department which acts as intermediary between the Designing Department and the customers. Some designs are made by the customer's engineers or by consulting engineers, and the structural companies simply estimate the cost and submit bids. Some structures are so simple or so similar to other structures that the designers, or the contracting engineers in charge of branch offices, can make quite accurate estimates quickly without complete designs. Often times the customer has little conception of the type of structure best suited to his needs and the structural companies prepare alternate designs from which the customer may make selection.

3. Design sheets or stress sheets are made by the designer or by a draftsman under his direction to illustrate the proposed structure. They show the general form of the structure, the principal dimensions and stresses, and the composition of each main member, as illustrated in Fig. 21. The design is made according to specifications approved by the customer. An estimator must have an intimate knowledge of drafting room methods and of shop methods and costs. He must know from experience how much to allow for the details of construction such as connection plates and angles. He must be familiar with the methods of erection and be able to determine the method best adapted to a given structure in a given location. The estimator may be assisted by draftsmen, tracers, and computers. As soon as a contract is awarded, the design sheet is adapted to the needs of the Drafting Department and any necessary alterations or additions are made. The design sheet should give all information necessary to enable the draftsmen to make the detailed drawings. It may be supplemented by an "information sheet" which gives the principal terms of the contract such as the time of delivery and whether the cost is quoted at a "lump sum" or at a price per pound.

4. The Drafting Department is in charge of a Chief Draftsman. His subordinate draftsmen are usually divided into squads, each in charge of a "squad foreman" or "squad boss." The drawings for each contract are usually all made in one squad, the drawings for other contracts often being carried on simultaneously. The size of each squad varies with the amount of work being done under the direction of a single squad foreman from 3 or 4 to 16 or 20, the normal number being from 6 to 8. In each squad are checkers, detailers, tracers, billers, and computers. The detailers make the working drawings, and design the details. The drawings show how the standard shapes are cut and combined to form the different members. The number and the spacing of the rivets are given so that the members may be properly constructed and so that they may be easily connected to other members in the structure. As far as practical the parts are combined in the shop in order to reduce the number of members to be shipped and to be erected at the site. The detailers often prefer to trace their own drawings or else to draw with ink directly upon the tracing cloth, thus making no use of the tracers. The detailers are often called upon either to make or to check shop bills and shipping bills and other lists of material. Most of the billing and the calculating of weights are done by younger men of limited experience often working in a separate squad under a chief biller. The checkers "check" or verify the drawings and indicate the mistakes. They are often called upon to make drawings also. They are men of greater experience than the detailers and they assist the squad foreman in laying out new work preparatory to ordering the material. The term "draftsman" has a double meaning. Some limit its use to detailers because they do the actual drafting, while others refer to everyone in the Drafting Department as draftsmen, particularly those who have reached or passed the rank of detailer.

1. Method of Procedure. When a new contract is received in the Drafting Department the Chief Draftsman studies the general character of the structure, notes the time limit if any, and assigns the work to the squad foreman who can best handle it. The squad foreman makes a careful study of the whole contract and adopts the method of procedure by which the work under his direction can be carried out most efficiently. In general his first aim is to have all main material ordered as soon as possible because of the usual delay at the mills. In some types of structure either he or his more experienced men can list most of the material directly from the design sheets. In certain classes of work such as truss work it may become necessary to make small layouts of connections, or even to begin the working drawings in order to determine the lengths of the material; later these preliminary drawings may be given to detailers for completion. In other types of structure such as office buildings it may prove feasible to make the plans and diagrams before listing the material. These diagrams may be made so complete that the material may be listed from them quickly and accurately, and the detailed drawings may become routine work which can be done by men of limited experience. The preliminary lists of material are usually sent to an Order Department where the material is summarized and the short pieces are grouped in multiple lengths to be cut after they arrive from the mill. The squad foreman divides the drafting among the detailers to the best advantage so that the work may be carried on efficiently and in logical order. Part of the drawings may be sent to the shop before all are completed and as far as practical an attempt should be made to complete the drawings in approximately the same order that the corresponding members will be erected. For example, the drawings of the ground floor beams in an office building should be made before the drawings of the roof beams. Erection diagrams should be made as soon as possible so that the marks of all members may be recorded as soon as determined. Drawings should be checked shortly after they are made and the shop bill for each drawing should be made as soon as the drawing is completely checked. A shop bill is a summary of all the material required to make all the members represented by a drawing. Later shipping bills and lists of rivets and blots to be used in erection are prepared.