Structural Steel

Carbon-Steel or Unfinished (Machine) Bolts

‘‘Secondary connections may be made with unfinished bolts conforming to the Specifications for Low-carbon Steel ASTM A307’’ is an often-used specification. (Unfinished bolts also may be referred to as machine, common, or ordinary bolts.) When this specification is used, secondary connections should be carefully defined to preclude selection by ironworkers of the wrong type of bolt for a connection (see also Art. 5.1). A307 bolts have identification marks on their square, hexagonal, or countersunk heads (Fig. 5.2), as do high-strength bolts.
Use of high-strength bolts where A307 bolts provide the required strength merely adds to the cost of a structure. High-strength bolts cost at least 10% more than machine bolts.
A disadvantage of A307 bolts is the possibility that the nuts may loosen. This may be eliminated by use of lock washers. Alternatively, locknuts can be used or threads can be jammed, but either is more expensive than lock washers.

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