All parts of a connection should be held tightly together during installation of fasteners.
Drifting done during assembling to align holes should not distort the metal or enlarge the holes. Holes that must be enlarged to admit fasteners should be reamed. Poor matching of holes is cause for rejection.
For connections with high-strength bolts, surfaces, when assembled, including those adjacent to bolt heads, nuts, and washers, should be free of scale, except tight mill scale. The surfaces also should be free of defects that would prevent solid seating of the parts, especially dirt, burrs, and other foreign material. Contact surfaces within slip-critical joints should be free of oil, paint, lacquer, and rust inhibitor.
Each high-strength bolt should be tightened so that when all fasteners in the connection are tight it will have the total tension (kips) given in Table 6.18, for its diameter. Tightening should be done by the turn-of-the-nut method or with properly calibrated wrenches.
High-strength bolts usually are tightened with an impact wrench. Only where clearance does not permit its use will bolts be hand-tightened.
Requirements for joint assembly and tightening of connections are given in the Specification for Structural Joints Using ASTM A325 or A490 Bolts, Research Council on Structural Connections of the Engineering Foundation. The provisions applicable to connections requiring full pretensioning include the following.
Calibrated-wrench Method. When a calibrated wrench is used, it must be set to cut off tightening when the required tension (Table 6.18) has been exceeded by 5%. The wrench should be tested periodically (at least daily on a minimum of three bolts of each diameter being used). For the purpose, a calibrating device that gives the bolt tension directly should be used. In particular, the wrench should be calibrated when bolt size or length of air hose is changed.
When bolts are tightened, bolts previously tensioned may become loose because of compression of the connected parts. The calibrated wrench should be reapplied to bolts previously tightened to ensure that all bolts are tensioned to the prescribed values.
Turn-of-the-nut Method. When the turn-of-the-nut method is used, tightening may be done by impact or hand wrench. This method involves three steps:
1. Fit-up of connection. Enough bolts are tightened a sufficient amount to bring contact surfaces together. This can be done with fit-up bolts, but it is more economical to use some of the final high-strength bolts.
2. Snug tightening of bolts. All high-strength bolts are inserted and made snug-tight (tightness obtained with a few impacts of an impact wrench or the full effort of a person using an ordinary spud wrench). While the definition of snug-tight is rather indefinite, the condition can be observed or learned with a tension-testing device.
3. Nut rotation from snug-tight position. All bolts are tightened by the amount of nut rotation specified in Table 5.9. If required by bolt-entering and wrench-operation clearances, tightening, including by the calibrated-wrench method, may be done by turning the bolt while the nut is prevented from rotating.
Direct-Tension-Indicator Tightening. Two types of direct-tension-indicator devices are available: washers and twist-off bolts. The hardened-steel load-indicator washer has dimples on the surface of one face of the washer. When the bolt is torqued, the dimples depress to the manufacturers specification requirements, and proper torque can be measured by the use of a feeler gage. Special attention should be given to proper installation of flat hardened washers when load-indicating washers are used with bolts installed in oversize or slotted holes and when the load-indicating washers are used under the turned element.
The twist-off bolt is a bolt with an extension to the actual length of the bolt. This extension will twist off when torqued to the required tension by a special torque gun. A representative sample of at least three bolts and nuts for each diameter and grade of fastener should be tested in a calibration device to demonstrate that the device can be torqued to 5% greater tension than that required in Table 6.18.
When the direct tension indicator involves an irreversible mechanism such as yielding or fracture of an element, bolts should be installed in all holes and brought to the snug-tight condition. All fasteners should then be tightened, progressing systematically from the most rigid part of the connection to the free edges in a manner that will minimize relaxation of previously tightened fasteners prior to final twist off or yielding of the control or indicator element of the individual devices. In some cases, proper tensioning of the bolts may require more than a single cycle of systematic tightening.
Welded connections often are used because of simplicity of design, fewer parts, less material, and decrease in shop handling and fabrication operations. Frequently, a combination of shop welding and field bolting is advantageous. With connection angles shop welded to a beam, field connections can be made with high-strength bolts without the clearance problems that may arise in an all-bolted connection.
Welded connections have a rigidity that can be advantageous if properly accounted for in design. Welded trusses, for example, deflect less than bolted trusses, because the end of a welded member at a joint cannot rotate relative to the other members there. If the end of a beam is welded to a column, the rotation there is practically the same for column and beam.
A disadvantage of welding, however, is that shrinkage of large welds must be considered.
It is particularly important in large structures where there will be an accumulative effect.
Properly made, a properly designed weld is stronger than the base metal. Improperly made, even a good-looking weld may be worthless. Properly made, a weld has the required penetration and is not brittle.
Prequalified joints, welding procedures, and procedures for qualifying welders are covered by AWS D1.1, Structural Welding Code Steel, and AWS D1.5, Bridge Welding Code, American Welding Society. Common types of welds with structural steels intended for welding when made in accordance with AWS specifications can be specified by note or by symbol with assurance that a good connection will be obtained.
In making a welded design, designers should specify only the amount and size of weld actually required. Generally, a 5â„16-in weld is considered the maximum size for a single pass.
The cost of fit-up for welding can range from about one-third to several times the cost of welding. In designing welded connections, therefore, designers should consider the work necessary for the fabricator and the erector in fitting members together so they can be welded.