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Structural steel detail
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1. History of High-strength Bolts

The joints obtained using high-strength bolts are superior to riveted joints in performance and economy and they are the leading field method of fastening structural steel members. C. Batho and E. H. Bateman first claimed in 1934 that high-strength bolts could satisfactorily be used for the assembly of steel structured, but it was not until 1947 that the Research Council on Riveted and Bolted Structural Joints of the Engineering Foundation was established. This group issued their first specifications in 1951, and high-strength bolts were adopted by both building and bridge engineers for both static and dynamic loadings with amazing speed. They not only quickly became the leading method of making field connections, they also were found to have many applications for shop connections. The construction of the Mackinac Bridge in Michigan involved the use of more than one million high-strength bolts.

Connections that were formerly made with ordinary bolts and nuts were not too satisfactory when they were subjected to vibratory loads because the nuts frequently became loose. For many years this problem was dealt with by using some type of locknut, but the modern high-strength bolts furnish a far superior solution.

2. Advantages of High-strength Bolts

Among the many advantages of high-strength bolts , partly explaining their great success, are the following.

(1) Smaller crews are involved as compared with riveting. Two two-person bolting crews can easily turn out over twice as many bolts in a day as the number of rivets driven by the standard four-person riveting crew. The result is quicker steel erection.

(2) In comparison with rivets, fewer bolts are needed to provide the same strength.

(3) Good bolted joints can be made by people with a great deal less training and experience than is necessary to produce welded and riveted connections of equal quality. The proper installation of high-strength bolts can be learned in a matter of hours.

(4) No erection bolts are required that may have to be later removed (depending on specifications) as in welded joints.

(5) Though quite noisy, bolting is not nearly as bad as riveting.

(6) Cheaper equipment is used to make bolted connections.

(7) No fire hazard is present, nor danger from the tossing of hot rivets.

(8) Tests on riveted joints and fully tensioned bolted joints under identical conditions definitely show that bolted joints have a higher fatigue strength . Their fatigue strength is also equal to or greater than that obtained with equivalent welded joints.

(9) Where structures are to be later altered or disassembled, changes in connections are quite simple because of the ease of bolt removal.

3. American Welding Society

The American Welding Society's Structural Welding Code is the generally recognized standard for welding in the United States. The LRFD Specification clearly states that the provisions of the AWS Code apply under the LRFD Specification with a very few minor exceptions, and these are listed in LRFD Specification J2. Both the AWS and the AASHTO Specifications cover dynamically loaded structures. Normally, however, the AWS Specification is used for designing buildings subject to dynamic loads unless the contract documents state otherwise.

4. Types of Welding

Although both gas and arc welding are available, almost all structural welding is arc welding. Sir Humphry Davy discovered in 1801 how to create an electric arc by bringing dose together two terminals of an electric circuit of relatively high voltage. Although he is generally given credit for the development of modern welding, a good many years elapsed after his discovery before welding was actually performed with the electric arc. (His work was of the greatest importance to the modern structural world, but it is interesting to note that many people say his greatest discovery was not the electric arc, but rather a laboratory assistant whose name was Michael Faraday.) Several Europeans formed welds of one type or another in the 1880s with the electric arc, while in the United States the first patent for arc welding was given to Charles Coffin of Detroit in 1889.

The figures to follow in this chapter show the necessity of supplying additional metal to the joints being welded to give satisfactory connections. In electric-arc welding the metallic rod which is used as the electrode, melts off into the joint as it is being made. When gas welding is used it is necessary to introduce a metal rod known as a filler or welding rod.

In gas welding a mixture of oxygen and some suitable type of gas is burned at the tip of a torch or blowpipe held in the: welder's hand or by an automatic machine. The gas used In structural welding is probably acetylene, and the process is called oxyacetylene welding. The flame produced can be used for flame cutting of metals as well as for welding. Gas Welding is rather easy to learn and the equipment used is rather inexpensive. It is, however, a somewhat slow process as compared with other means of welding, and normally is used for repair and maintenance work and not for the fabrication and erection of large steel structures.

In arc welding, an electric arc is formed between the pieces being welded and an electrode held in the operator's hand with some type of holder, or by an automatic machine. The arc is a continuous spark that upon contact brings the electrode and the pieces being welded to the melting point. The resistance of the air or gas between the electrode and the pieces being welded changes the electrical energy into heat. A temperature of somewhere between 6,000 and 10.000T is produced in the arc. As the end of the electrode melts'; small droplets or globules of the molten metal are formed and are actually forced by the arc across to the pieces being connected, penetrating. the molten metal to become a part of the weld. The amount of penetration can be controlled by the amount of current consumed. Since the molten droplets of the electrodes are actually propelled to the weld, arc welding can be successfully used for overhead work.

A pool of molten steel can hold a fairly large amount of gases in solution, and if not protected from the surrounding air will -chemically combine with oxygen and nitrogen. After cooling the welds will be relatively porous due to the little pockets formed by the gases. Such welds are relatively brittle and have much less resistance to corrosion. A weld can be shielded by using an electrode coated with certain mineral compounds. The electric arc causes the coating to melt and creates an inert gas or vapor around the area being welded. The vapor acts as a shield around the molten metal and keeps it from coming freely in contact with the surrounding air. It also deposits a slag in the molten metal, which has less density than the base metal and comes to the surface to protect the weld from the air while the weld cools. After cooling, the slag can easily be removed by peening and wire brushing (such removal being absolutely necessary before painting or application of another weld layer)*] ^Shielded metal I arc welding is frequently abbreviated here with the letters SMAW.

The type of welding electrode used is very important as it decidedly affects the weld properties such as strength, ductility, and corrosion resistance. Quite a number of different types of electrodes are manufactured, the type to be used for a certain job being dependent upon the type of metal being welded, the amount of material that needs to be added, the position of the work, etc. The electrodes fall into two general classes—the lightly coated electrodes and the heavily coated electrodes.

The heavily coated electrodes are normally used in structural welding because the melting of their coatings produces very satisfactory vapor shields around the work as well as slag in the weld. The resulting welds are stronger, more resistant to corrosion, and more ductile than are those produced with lightly coated electrodes. When the lightly coated electrodes are used, no attempt is made to prevent oxidation and no slag is formed. The electrodes are lightly coated with some arc-stabilizing chemical such as lime.       '

Submerged (or hidden) arc welding (SAW) is an automatic process in which the arc is covered with a mound of granular fusible material and thus hidden from view. A bare metal electrode is fed from a reel and melted and deposited as filler material. The electrode, power source, and a hopper of flux are attached to a frame that is placed on rollers and that moves at 8certain rate as the weld is formed. SAW welds are quickly and efficiently made and are of high quality, exhibiting high impact  strength and corrosion resistance and good ductility. Furthermore, they provide deeper penetration with the result that the area effective in resisting loads is larger. A large percentage of the welding done for bridge structures is SAW. If a single electrode is used, the size of the weld obtained with a single pass is limited. Multiple electrodes may be used, however, permitting much larger welds.

Welds made by the SAW process (automatic or semiautomatic) are consistently of high quality and are very suitable for long welds. One disadvantages is that the work must be positioned for near flat or horizontal welding.

Another type of welding is flux-cored arc welding (FCAW). In this process, a flux-filled steel tube electrode is continuously fed from a reel. Gas shielding and slag are formed from the flux. The AWS Specification (4.14) provides limiting sizes for welding electrode diameters and weld sizes, as well as other requirements pertaining to welding procedures.

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