Two basic types of fasteners are typically used in construction, bolts and welds.
Both are used in the fabricating shop and on the job site in connections joining individual members. Welds are also used to fasten together components of built-up members. Bolts, however, are more commonly used for field connections, and welds, for shop work. Rivets, which were once widely used for main connections, both shop and field, are essentially obsolete.
Many variables affect selection of fasteners. Included among these are economy of fabrication and erection, availability of equipment, inspection criteria, labor supply, and such design considerations as fatigue, size and type of connections, continuity of framing, reuse, and maintenance. It is not uncommon for steel framing to be connected with such combinations as shop welds and field bolts or to be allwelded.
It is usual to use field welds for column splices with bolted connections elsewhere. The variables affecting decisions on use of fasteners should be explored with engineers representing the fabricator and the erector.
Development of high-strength bolts is vested in the Research Council on Riveted and Bolted Structural Joints of the Engineering Foundation. Its Specification for Structural Steel Joints Using A325 or A490 Bolts (Table 7.1) was adopted by the American Institute for Steel Construction. Bolts conforming to ASTM A449 are acceptable, but their usage is restricted to bearing-type connections (Fig. 7.1) re quiring bolt diameters greater than 11â„2 in. Furthermore, when they are required to be tightened to more than 50% of their specified minimum tensile strength, hardened steel washers should be installed under the heads.
When high-strength bolts are used in a connection, they are highly tensioned by tightening of the nuts and thus tightly clamp together the parts of the connection.
For convenient computation of load capacity, the clamping force and resulting friction are resolved as shear. Bearing between the bolt body and connected material is not a factor until loads become large enough to cause slippage between the parts of the connection. The bolts are assumed to function in shear following joint slippage into full bearing.
The clamping and bearing actions lead to the dual concept: slip-critical connections and bearing-type connections.
For the latter, the allowable shear depends on the cross-sectional bolt area at the shear plane. Hence, two shear values are assigned, one for the full body area and one for the reduced area at the threads.
Identification. There is no difference in appearance of high-strength bolts intended for either slip-critical or bearingtype connections. To aid installers and inspectors in identifying the several available grades of steel, bolts and nuts are manufactured with permanent markings (Fig. 7.2).
High-Strength Bolt Installation
Washer requirements for high-strength bolted assemblies depend on the method of installation and type of bolt holes in the connected elements. These requirements are summarized in Table 7.5.
Bolt Tightening. Specifications require that all high-strength bolts be tightened to 70% of their specified minimum tensile strength, which is nearly equal to the proof load (specified lower bound to the proportional limit) for A325 bolts, and within 10% of the proof load for A490 bolts. Tightening above these minimum tensile values does not damage the bolts, but it is prudent to avoid excessive uncontrolled tightening. The required minimum tension, kips, for A325 and A490 bolts is given in Table 7.6.
There are three methods for tightening bolts to assure the prescribed tensioning:
Turn-of-Nut. By means of a manual or powered wrench, the head or nut is turned from an initial snug-tight position. The amount of rotation, varying from one-third to a full turn, depends on the ratio of bolt length (underside of heat to end of point) to bolt diameter and on the disposition of the outer surfaces of bolted parts (normal or sloped not more than 1:20 with respect to the bolt axis). Required rotations are tabulated in the Specification for Structural Steel Joints Using A325 of A490 Bolts.
Calibrated Wrench. By means of a powered wrench with automatic cutoff and calibration on the job. Control and test are accomplished with a hydraulic device equipped with a gage that registers the tensile stress developed.
Direct Tension Indicator. Special indicators are permitted on satisfactory demonstration of performance. One example is a hardened steel washer with protrusions on one face. The flattening that occurs on bolt tightening is measured and correlated with the induced tension.
Known in construction circles by several names ordinary, common, machine, or rough unfinished bolts are characterized chiefly by the rough appearance of the shank. They are covered by ASTM A307. They fit into holes 1â„16 in larger in diameter than the nominal bolt diameter.
Unfinished bolts have relatively low load-carrying capacity. This results from the possibility that threads might lie in shear planes. Thus, it is unnecessary to extend the bolt body by use of washers.
One advantage of unfinished bolts is the ease of making a connection; only a wrench is required. On large jobs, however, erectors find they can tighten bolts more economically with a pneumatic-powered impact wrench. Power tightening generally yields greater uniformity of tension in the bolts and makes for a betterbalanced connection.
While some old building codes restrict unfinished bolts to minor applications, such as small, secondary (or intermediate) beams in floor panels and in certain parts of one-story, shed-type buildings, the AISC specifications for structural steel buildings, with a basis of many years of experience, permit A307 bolts for main connections on structures of substantial size. For example, these bolts may be used for beam and girder connections to columns in buildings up to 125 ft in height.
There is an economic relation between the strength of a fastener and that of the base material. So while A307 may be economical for connecting steel with a 36- ksi yield point, this type of bolt may not be economical with 50-ksi yield-point steel. The number of fasteners to develop the latter becomes excessive and perhaps impractical due to size of detail material.
A307 bolts should always be considered for use, even in an otherwise all-welded
building, for minimum-type connections, such as for purlins, girts, and struts.
Locking Devices for Bolts. Unfinished bolts (ASTM A307) and interferencebody- type bolts (Art 7.3.4) usually come with American Standard threads and nuts.
Properly tightened, connections with these bolts give satisfactory service under static loads. But when the connections are subjected to vibration or heavy dynamic loads, a locking device is desirable to prevent the nut from loosening.
Locking devices may be classified according to the method employed: special threads, special nuts, special washers, and what may be described as field methods.
Instead of conventional threads, bolt may be supplied with a patented self-locking thread called Dardelet. Sometimes, locking features are built into the nuts. Patented devices, the Automatic-Nut, Union-Nut, and Pal-Nut, are among the common ones.
Washers may be split rings or specially touched. Field methods generally used include checking, or distorting, the threads by jamming them with a chisel or locking by tack welding the nuts.
Other Bolt-Type Fasteners
Interference body of bearing-type bolts are characterized by a ribbed or interrupted- ribbed shank and a button-shaped head; otherwise, including strength, they are similar to the regular A325 high-strength bolts. The extreme diameter of the shank is slightly larger than the diameter of the bolt hole. Consequently, the tips of the ribs or knurlings will groove the side of the hole, assuring a tight fit. One useful application has been in high television towers, where minimum-slippage joints are desired with no more installation effort than manual tightening with a spud wrench. Nuts may be secured with lock washers, self-locking nuts, or Dardelet self-locking threads. The main disadvantage of interference body bolts is the need for accurate matching of truly concentric holes in the members being joined; reaming sometimes is necessary.
Huckbolts are grooved (not threaded) and have an extension on the end of the shank. When the bolt is in the hole, a hydraulic machine, similar to a bolting or riveting gun, engages the extension. The machine pulls on the bolt to develop a high clamping force, then swages a collar into the grooved shank and snaps off the extension, all in one quick operation.
Welding is used to fasten together components of a built-up member, such as a plate girder, and to make connections between members. This technique, which uses fusion is a controlled atmosphere, requires more highly skilled labor than does bolting. However, because of cost advantages, welding is widely used in steel construction, especially in fabricating shops where conditions are more favorable to closely controlled procedures. When field welding is specified, the availability of skilled welders and inspection technicians and the use of more stringent qualitycontrol criteria should be considered.
Any of several welding processes may be used: manual shielded metal arc, submerged arc, flux cored arc, gas metal arc, electrogas, and electroslag. They are not all interchangeable, however; each has its advantageous applications.
Many building codes accept the recommendations of the American Welding Society Structural Welding Code (AWS D1.1) (Table 7.1). The AISC specification incorporates many of this codes salient requirements.
Weld Types. Practically all welds used for connecting structural steel are of either of two types: fillet or groove.
Figure 7.3a and b illustrates a typical fillet weld. As stated in Art. 7.27, all stresses on fillet welds are resolved as shear on the effective throat. The normal throat dimension, as indicated in Fig. 7.3a and b, is the effective throat for all welding processes, except the submerged-arc method. The deep penetration characteristic of the latter process is recognized by increasing the effective throat dimension, as shown in Fig. 7.3c.
Groove welds (Fig. 7.3d, e, and Æ’) are classified in accordance with depth of solid weld metal as either complete or partial penetration. Most groove welds, such as those in Fig. 7.3d and e, are made complete-penetration welds by the workmanship requirements: use backup strips or remove slag inclusions and imperfections (step called back-gouging) on the unshielded side of the root weld. The partialpenetration groove weld shown in Fig. 7.3Æ’ is typical of the type of weld used for box-type members and column splices. Effective throat depends on the welding process, welding position, and the chamfer angle . The indicated effective throat (Fig. 7.3Æ’) is proper for the shielded-metal-arc processes and for all welding positions.
(See also Art. 7.27.)
Welding Electrodes. Specifications for all welding electrodes, promulgated by the American Welding Society (AWS), are identified as A5.1, A5.5, A5.17, etc., depending on the welding process. Electrodes for manual arc welding, often called stick electrodes, are designated by the letter E followed by four of five digits. The first two or three digits designate the strength level; thus, E70XX means electrodes having a minimum tensile strength of 70.0 ksi. Allowable shear stress on the depositied weld metal is taken as 0.30 times the electrode strength classification; thus, 0.30 times 70 to an E70 results in an allowable stress of 21.0 ksi. The remaining digits provide information on the intended usage, such as the particular welding positions and types of electrode coating.
Welding Procedures. The variables that affect the quality of a weld are controlled by welding procedures that must be approved by the structural engineer. Specification AWS D1.1 contains several prequalified welding procedures, the use of which permits fabricators and erectors to avoid the need for obtaining approvals for specific routine work. Where unusual conditions exist, the specification requires that formal documentation be submitted for review and approval.
Base-Metal Temperatures. An important requirement in production of quality welds is the temperature of base metal. Minimum preheat and interpass temperature as specified by the AWS and AISC standards must be obtained within 3 inches of the welded joint before welding starts and then maintained until completion. Table 7.7 gives the temperature requirements based on thickness (thickest part of joint) and welding process for several structural steels. When base metal temperature is below 32F, it must be preheated to at least 70 and maintained at that temperature during welding. No welding is permitted when ambient temperature is below 0F.
Inspection of Welds
The quality of welded work is highly dependent upon the close adherence to applicable welding process and procedural requirements. This, plus attention to dimensional requirements, will generally result in serviceable welds. As a result, most welding work incorporated in building construction, other than for major structures, is inspected using visual inspection techniques. The fabricators quality personnel are responsible for adherence to approved procedures. The owners inspector observed the erectors operations and may perform any necessary visual inspection of the finished work.
Four nondestructive testing methods are commonly used to evaluate welded work. These are (1) magnetic-particle inspection, (2) liquid penetrant inspection, (3) radiographic inspection, and (4) ultrasonic inspection. The latter two methods are the most common today. Each of these nondestructive testing methods add to the cost of construction and should be used where some special service requirement justifies this added feature. Any such testing must be identified on the drawings or in the specifications.
Fasteners are indicated on design, shop, and field erection drawings by notes and symbols. A simple note may suffice for bolts; for example: 7â„8-in A325 bolts, except as noted. Welds require more explicit information, since their location is not so obvious as that of holes for bolts.
Symbols are standard throughout the industry. Figure 7.4 shows the symbols for bolts, Fig. 7.5 the symbols for welds. The welding symbols (Fig. 7.5a) together with the information key (Fig. 7.5b) are from the American Welding Society Symbols for Welding and Nondestructive Testing, AWS A2.4.
Erection Clearance for Fasteners
All types of fasteners require clearances for proper installation in both shop and field. Shop connections seldom are a problem, since each member can be easily manipulated for access. Field connections, however, require careful planning, because connections can be made only after all members to be connected are aligned in final position. This is the responsibility of the fabricators engineering staff and is discharged during the making of shop drawings. However, the basic design configuration must permit the necessary clearances to be developed.
Clearances are required for two reasons: to permit entry, as in the case of bolts entering holes, and to provide access to the connected elements either to allow the tightening of bolts with field tools or to permit the movement of manual electrodes or semiautomatic welding tools in depositing weld metal.