Welding - INSTRUCTOR'S REVIEW COPY YOU make the difference....

Info iconThis preview shows pages 1–12. Sign up to view the full content.

View Full Document Right Arrow Icon
Background image of page 1

Info iconThis preview has intentionally blurred sections. Sign up to view the full version.

View Full DocumentRight Arrow Icon
Background image of page 2
Background image of page 3

Info iconThis preview has intentionally blurred sections. Sign up to view the full version.

View Full DocumentRight Arrow Icon
Background image of page 4
Background image of page 5

Info iconThis preview has intentionally blurred sections. Sign up to view the full version.

View Full DocumentRight Arrow Icon
Background image of page 6
Background image of page 7

Info iconThis preview has intentionally blurred sections. Sign up to view the full version.

View Full DocumentRight Arrow Icon
Background image of page 8
Background image of page 9

Info iconThis preview has intentionally blurred sections. Sign up to view the full version.

View Full DocumentRight Arrow Icon
Background image of page 10
Background image of page 11

Info iconThis preview has intentionally blurred sections. Sign up to view the full version.

View Full DocumentRight Arrow Icon
Background image of page 12
This is the end of the preview. Sign up to access the rest of the document.

Unformatted text preview: INSTRUCTOR'S REVIEW COPY YOU make the difference. if you no longer require this review copy, please return it to Pearson Education and we will donate $1 to Jumpstart for children who need our help. ‘ 4 --------- —-.- ———————————————————— -=--- See back cover for details. JACK C. lVlcCORMAC CHAPTER 14 Welded Connections GENERAL Welding is a process by which metallic parts are connected by heating their surface a plastic or fluid state and allowing the parts to flow together and join (with or without- the addition of other molten metal). It is impossible to determine when welding __Ol_'_lg hated, but it was at least several thousand years ago. Metal-working, including weld‘ was quite an art in ancient Greece three thousand years ago, but welding had undoub edly been performed for many centuries before that. Ancient welding probably forging process in which the metals were heated to a certain temperature (not meltng stage) and hammered together. Although modern welding has been available for many years, it has comejm own only in the last few decades for the building and bridge phases of structur; neering. The adoption of structural welding was quite slow for several decad cause many engineers thought that welding had two major disadvantages: (1) had reduced fatigue strength, compared with riveted and bolted connections; it was impossible to ensure a high quality of welding without unreasonably e and costly inspection. I These attitudes persisted for many years, although tests began to ind neither reason was valid. Regardless of their validity, these views were widely! _ undoubtedly slowed down the use of welding— particularly for highway brid ' an even greater extent, railroad bridges. Today, most engineers agree that weld ' have considerable fatigue strength. They will also admit that the rules gove. qualification of welders, the better techniques applied, and the excellent w _ requirments of the AWS (American Welding Society) specifications make. tion of welding a much less difficult problem. Furthermore, the chemistru‘ manufactured today is especially formulated to imprbve their weldability. Cons welding is now permitted for almost all structural work. On the subject of weldi11g,it is interesting to consider welded ships. Sh'ip' jected to severe impactive loadings that are difficult to predict, yet naval a 14.2 Advantages ofWelding 447 Worker adjusts steel girder. (Courtesy of Bethlehem Steel.) all-welded ships with great success, A similar discussion can be made for airplanes and of structural welding was for railroad ted to heavier live loads, larger vibra- ridges; but are their stress situations as rips and planes? bridges. These bridges are undoubtedly subjec tions, and more stress reversals than highway b serious and as difficult to predict as those for Si 2 ADVANTAGES OF WELDING Today, it is possible to make use of the many adv fatigue and inspection fears have been larg many advantages that Welding offers: antages that welding offers, since the ely eliminated. Following are several of the 1. To most designers, the first advanta permits large savings in pounds of s ge is economic, because the use of welding teel used. Welded structures allow the elimi- nation of a large percentage of the gusset and splice plates necessary for bolted structures, as well as the elimination of bolt heads. In some bridge trusses, it may be possible to save up to 15 percent or more of the steel weight by welding. _ 2. Welding has a much wider range of application than bolting. Consider a steel pipe column and the difficulties of connecting it to other steel members by bolt- ing. A bolted connection may be virtually impossible, but a welded connection presents few difficulties. Many similar situations can be imagined in which weld~ ing has a decided advantage. 3. Welded structures are more rigid, because the members often to each other. Frequently, the connections for bolted structure are welded directly 8 are made through 448 Chapteri4 Welded Connections intermediate connection angles or plates that deform due to load transfer, making the entire structure more flexible. On the other hand, greater rigidity can be a dis— advantage where simple end connections with little moment resistance are de- sired. In such cases, designers must be careful as to the type of joints they specify. . The process of fusing pieces together creates the most truly continuous struc- tures. Fusing results in one-piece construction, and because welded joints are as strong as or stronger than the base metal, no restrictions have to be placed on the joints. This continuity advantage has permitted the erection of countless slender and graceful statically indeterminate steel frames throughout the world. Some of the more outspoken proponents of welding have referred to bolted structures, with their heavy plates and abundance of bolts, as looking like tanks or armored cars compared with the clean, smooth lines of welded structures. For a graphic it} lustration of this advantage, compare the moment—resistin g connections of Fig. 15.5, . It is easier to make changes in design and to correct errors during erection (and 3 less expensive) if welding is used. A ciosely related advantage has certainly bee illustrated in military engagements during the past few wars by the quick weldin repairs made to military equipment under battle conditions. ' Another item that is often important is the relative silence of welding. Imagin- the importance of this fact when working near hospitals or schools or when mak ing additions to existing buildings. Anyone with close-to-normai hearing who ha attempted to work in an office within several hundred feet of a bolted job canf'at test to this advantage. J . Fewer pieces are used, and as a result, time is saved in detailing, fabrication" field erection. ' AMERICAN WELDENG SOCIETY The American Welding Society’s Structural Welding Code1 is the generally recogn standard for welding in the United States. The AISC Specification clearly stat the provisions of th ‘ minor exceptions, and these are listed in AISC Specification J2. Both the AWS a' AASHTO Specifications cover dynamically loaded structures: Generally, the specification is used for designing the welds for buildings subject to dynamic l TYPES OF WELDING Although both gas and arc welding are available, almost all structural \veIdi welding. Sir Humphry Davy discovered in 1801 how to create an electric arc by ing close together two terminals of an electric circuit of relatively high vol though he is generally given credit for the development of modern welding many years elapsed after his discovery before welding was actually performed'wii : electric are. (His work was of the greatest importance to the modern structur' 1American Welding Society, Structural Welding Code-SreeI,AWS D.1.1-00 (Miami: AWS, 2006). ' tuous struc- . laced on the 14.4 Types ofWeldlng 449 sfer, making :an be a dis- nee are de‘ = hey specify. but it is interesting to note that many people say his greatest discovery was not the electric are, but rather a laboratory assistant whose name Was Michael Faraday.) Sev- eral Europeans formed welds of one type or another in the 18805 with the electric arc, while in the United States the first patent for arc welding was given to Charles Coffin oints are as less slender ; rid. Some of " 1 structures or armore In gas welding, a mixture of oxygen and some suitable type of gas is burned at the tip a graphic H of a torch or blowpipe held in the welder’s hand or by machine. The gas used in structural "' . of Fig. 15.5 Waldng usually is acetylene, and the process is called oxya fiction (an duced can he used for flame cutting of metals as well as fo ttainly bee easy to learn, and the equipment used is rather inexpensive; It is a slow process, however, lick W61 din compared With other means of Welding, and nermally 1t rs used for repair and mamte- nance 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 ma~ chine. The are is a c ontinuous 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 tem- perature of 6000 to 10,000°F is produced in the are. As the end of the electrode melts, small droplets, or globules, of the molten metal are formed and actually are forced by the arc across to the pieces being connected, which penetrate the molten metal to be- come 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 actually are propelled , arc welding can be successfully used 'for overhead work. a fairly large amount of gases in solution and, if r, will chemically combine with oxygen and nitro- om‘ewhat porous due to the little pockets formed brittle and have much less resistance to corrosion. ng an electrode coated with, certain mineral com- oating to melt and creates an inert gas or vapor apor 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 oluteiy necessary he- not protected from the surrounding ai 1 only a: gen. After cooling, the welds will be s ms an by the gases. Such welds are relatively A welded joint can be shielded by usi pounds. The electric arc causes the 0 around the area being welded. The v 2Lincoln Electric Company, Procedure Handbook ofArc Welding Design and Practice, llth ed. Part I (Cleve- land, OH, 1957), Chapter 14 Welded Connections Electrode Extruded coating Gaseous / shield K» Arc stream Base metal FIGURE 14.1 Elements of the shielded metal arc welding process (SMAW). The type of Welding electrode used is very important because it decidedly the weld properties such as strength, ductility, and corrosion resistance. Quite a num ber of different kinds of electrodes are manufactured, the type to be used for a certai job being dependent upon the metal to be welded, the amount of material that need 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 beca the melting of their coatings produces very satisfactory vapor shields around the wor Shielded metal arc welding (SMAW) and electrode just before starting an arc to fillet weidi clip angle to the beam web. (Courtesy of the American Institute of Steel Construction, I udeci 11% 30118 d - Arc stream Base metal .ecidedly affects e. Quite a num: 36d fora certain .erial that nee eneral classes q 9;. 9:. :3 on C“ (‘D S [:1 round the work“ 14.4 Types of Welding 451 as well as slag in the \veld.'Ihe resulting welds are stronger, more resistant to corrosion, and more ductiie 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 are is covered with a mound of granular fusible material and is thus hidden from view. A bare metal electrode is fed from a reel, 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 a certain rate as the weld is formed. SAW welds are quickly and efficiently made and are of high quality, exhibiting high impact strength and corro- sion 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 disadvantage 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 \veld sizes, as well as other requirements pertaining to welding procedures. Submerged arc welding (SAW). (Courtesy of the American Institute of Steel Construction, Inc.) it are at . side of aced on 3, show- ams, in— thod. A _ plied to 14.7 Classification of Welds 455 14.7 CLASSIFICATION OF WELDS joints used. 14.7.1 Type of Weld FIG URE 14.2 Four types of structural welds. (a) Fillet welds (b) Complete-penetration groove welds I“. (c) Partial-penetration groove welds Slot weld (d) Plug and slot Welds I l E 455 Chapter 14 Welded Connections plane. To use them, the members have to fit almost perfectly, and unfortunately, the av- erage steel structure does not fit together that way. Have you ever seen steel workers pulling and ramming steel members to get them into position? When members are al- lowed to lap over each other, larger tolerances are allowable in erection, and fillet welds are used. Nevertheless, groove welds are quite common for many connections, such as column splices, butting of beam flanges to columns, etc., and they make up about 15 percent of structural welding. Groove welds can be either complete-penetration welds, which extend for the full thickness of the part being connected, or partial pene- tration welds, which extend for only part of the member thickness. Groove welds are generally more expensive than fillet welds because of the costs of = preparation. In fact, groove welds can cost up to 50 to 100 percent more than fillet welds. A plug weld is a circular weld that passes through one member into another, thus 3: joining the two together. A slot weld is a weld formed in a slot, or elongated hole, tha joins one member to the other member through the slot.The slot may be partly or fully filled with weld material. These two expensive types of welds may occasionally be used' when members lap over each other and the desired length of fillet welds cannot be ob" tained.They may also be used to stitch together parts of a member, such as the taste ing of cover plates to a built—up member. A plug or slot weld is not generally considered suitable for transferring te forces perpendicular to the faying surface, because there is not usually much pen tion of the weld into the member behind the plug or slot and the fact is that resistan to tension is provided primarily by penetration. Structural designers accept plug the different parts of a member together, but many designers are not happy using th'" weids for the transmission of shear forces. The penetration of the welds from the '51 or plugs into the other members is questionable; in addition, there can be critica in the welds that cannot be detected with the usual inspection procedures. Pd‘sition Welds are referred to as flat, horizontal, vertical, or overhead—listed in order '9 economy, with the flat welds being the most economical and the overhead weld the most expensive. A moderately skilled welder can do a very satisfactory ' flat weld, but it takes the very best to do a good job with an overhead weld. the flat welds often are done with an automatic machine, most structural wel‘d' done by hand. We indicated previously that the assistance of gravity is not a for the forming of good welds, but it does speed up the process. The globu___ molten electrodes can be forced into the overhead welds against gravity, a welds will result; however, they are slow and expensive to make, so it is dcs avoid them whenever possible. These types of welds are shown in Fig. 14.3. _ Type of Joint Welds can be further classified according to the type of joint used: butt, lap comer, etc. See Fig. 14.4. .the av- Norkers = s are al- 1d fillet : ections, lake up - .ermtion ' alpene- costs 0 i-velds. rer, thus ole, tha ‘ or fully 4.8 14.8 Welding Symbols 457 Horizontal weld Overhead welds FIGURE 14.3 Weld positions Flat weld . Butt Lap FIGURE 14.4 Tee Edge i f 'Iypes of weld joints Corner WELDING SYMBOLS Figure 14.5 presents the various welding symbols developed by the American Weld- ing Society. With this excellent shorthand system, a great deal of information can be presented in a small space 011 engineering plans and drawings. These symbols re- quire oniy a few lines and numbers and remove the necessity of drawing in the welds and making long descriptive notes. It is certainly desirable for steel designers and draftsmen to use this standardized system. If most of the welds on a drawing are the same size, a note to that effect can be given and the symbols omitted, except for the off-size Welds. The purpose of this section is to give a general idea of the appearance of weld- ing symbols and the information they can convey. (For more detailed information, refer to the AISC Handbook and to other materials published by the AWS.) The in- formation presented in Fig. 14.5 may be quite confusing; for this reason, a few very common symbols for fillet welds are presented in Fig. 14.6, together with an explana- tion of each Prequalified Welded Joints Basic Weld Symbols Groove or Bun Back Fiilet Slot Square V U Supplementary Weld symbols Weld All supplementary x 1 , weld symbols, see Spacer Around Field Weld Com 6,; AWS A24 E H Standard Location of Elements of 3 Welding Symbol Backing Finish symbol Groove angle or included angle or countersink Contour symbol for plug welds Root opening, depth . of filling for plug and slot welds _ . Effective throat Pitch (c‘ to 9‘ Spacing) of welds in inches Depth of preparation or size m males Field weld symbol fl Reference line Length of weld in inches Weld-alt-around symbol Specification, Process, or other reference \ Tail (omitted whengf reference is not used) Arrow Connects reference line to arrow side Elements in [his ofjoint. Use break as at A or B to signify Basic weld symbol area remain as that arrow is pointing to the grooved or detail reference Shawn when mi] member in bevel or H-grooved joints. and arrow are reversed. Note: Size, weid symbol, length of weld, and spacing must read in that order, from left to right, along the reference line. Neither orientation of reference nor location of the arrow alters this rule. The perpendicular leg of B, V,V,lf , weld symbols must be at left. ‘ Arrow and other side welds are of the same size unless otherwise shown. Dimensions of fillet welds must be shown on both the arrow side and the other side symbol. The point of the field weld symbol must point toward the tail. Symbols apply between abrupt changes in direction of welding unless governed by the "all around” symboi or 2 otherwise dimensioned. These symbols do not explicitly provide for the case that frequently occurs in structural work, where duplicate material (such as stiffeners) occurs on the far side of a web or gusset plate. The fabricating industry has adopted this canvention: that when the billing of the detail material discloses the existence of a member on tl far side as well as on the near side, the welding shown for the near side shall be duplicated on the far side. FIGURE 14.5 Source: American Institute of Steel Construction, Manual of Steel Construction Load & Resistant: Factor Design, 2d ed. (ChicagozAISC, 2005) Table 8-2. Reprinted with the. permission of the A180 4.9 14.9 GrooveWelds 459 1 6 Fillet weld on near side (arrow 1 side). Size of weld (1 in) is given to left of weld symbol and length (6 in) to right of symbol. édn fillet weld on far side, 2~in-long intermittent welds 6 in on centers. 1 : i 2 @ 6 1 TI 6 1 . . . . Tm filiet field weld on both Sides and 6 in long. As welds same on both sides not necessary; but permissible 3 E 2 @ 6 to give sizes on both sides of line. The flag indicates it to be a field weld. A staggered intermittent g-in fillet weld 2 in long, 6 in on centers, both sides. Weld all around joint symbol. A“2 The tail used to indicate reference to a certain specification or process. FIGURE 14.6 Sample weld symbols. GROOVE WELDS When complete penetration pression, the weld stress is a weld. Three types of groove groove Welds are subjected to axial tension or axial com- ssumed to equal the load divided by the net area of the welds are shown in Fig. 14.7. The square groo l i in maximum 4 ve joint, i Reinforcement I (a) Square groove joint ([3) Single vee joint FIGURE 14_7 - i. Groove welds (0) Double vee joint ...
View Full Document

This note was uploaded on 12/07/2011 for the course CES 4605 taught by Professor Prevatt during the Fall '11 term at University of Florida.

Page1 / 12

Welding - INSTRUCTOR'S REVIEW COPY YOU make the difference....

This preview shows document pages 1 - 12. Sign up to view the full document.

View Full Document Right Arrow Icon
Ask a homework question - tutors are online