BOLTS AND WELDS : STRUCTURAL FASTENERS BASIC AND TUTORIALS

STRUCTURAL FASTENERS BASIC: BOLTS AND WELDS
What Are Structural Fasteners?


Steel sections can be fastened together by rivets, bolts, and welds. While rivets were used quite extensively in the past, their use in modern steel construction has become almost obsolete. Bolts have essentially replaced rivets as the primary means to connect nonwelded structural components.

Bolts
Four basic types of bolts are commonly in use. They are designated by ASTM as A307, A325, A490, and A449. A307 bolts are called unfinished or ordinary bolts. They are made from low carbon steel.

Two grades (A and B) are available. They are available in diameters from 1/4 in. to 4 in. in 1/8 in. increments. They are used primarily for low-stress connections and for secondary members. A325 and A490 bolts are called high-strength bolts. A325 bolts are made from a heat treated medium carbon steel.

They are available in three types: Type1—bolts made of medium carbon steel; Type 2—bolts made of low carbon martensite steel; and Type 3—bolts having atmospheric corrosion resistance and weathering characteristics comparable to A242 and A588 steel. A490 bolts are made from quenched and tempered alloy steel and thus have a higher strength than A325 bolts.

Like A325 bolts, three types (Types 1 to 3) are available. Both A325 and A490 bolts are available in diameters from 1/2 in. to 1-1/2 in. in 1/8 in. increments. They are used for general construction purposes.

A449 bolts are made from quenched and tempered steel. They are available in diameters from 1/4 in. to 3 in. A449 bolts are used when diameters over 1-1/2 in. are needed. They are also used for anchor bolts and threaded rod.

High-strength bolts can be tightened to two conditions of tightness: snug-tight and fully tight. Snug-tight conditions can be attained by a few impacts of an impact wrench, or the full effort of a worker using an ordinary spud wrench.

UNFINISHED BOLTS IN STRUCTURES BASICS AND TUTORIALS

UNFINISHED BOLTS TREATMENT IN STRUCTURES BASIC INFORMATION
What Are Unfinished Bolts?


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 1⁄16 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 better balanced 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 interference body-type bolts 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.

HIGH STRENGTH BOLT AND NUTS INSTALLATION BASICS AND TUTORIALS

GUIDE IN INSTALLATION OF HIGH STRENGTH BOLTS AND NUTS
What Is the Proper Way Of Installing Nuts and Bolts?


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 aretabulated 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.