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.

Snug-tight conditions must be clearly identified on the design drawing and are permitted only if the bolts are not subjected to tension loads, and loosening or fatigue due to vibration or load fluctuations are not design considerations.

Bolts used in slip critical conditions (i.e., conditions for which the integrity of the connected parts is dependent on the frictional force developed between the interfaces of the joint) and in conditions where the bolts are subjected to direct tension are required to be fully tightened to develop a pretension force equal to about 70% of the minimum tensile stress Fu of the material from which the bolts aremade.

This can be accomplished by using the turn-of-the-nut method, the calibrated wrench method, or by the use
of alternate design fasteners or direct tension indicator.


Welds
Welding is a very effective means to connect two or more pieces of material together. The four most commonly used welding processes are Shielded Metal Arc Welding (SMAW), Submerged Arc Welding (SAW), Gas Metal Arc Welding (GMAW), and Flux Core Arc Welding (FCAW).

Welding can be done with or without filler materials although most weldings used for construction utilized filler materials. The filler materials used in modern day welding processes are electrodes. Table 3.2



Finished welds should be inspected to ensure their quality. Inspection should be performed by qualified welding inspectors. A number of inspection methods are available for weld inspections.

They include visual, the use of liquid penetrants, magnetic particles, ultrasonic equipment, and radiographic methods. Discussion of these and other welding inspection techniques can be found in the Welding Handbook.

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