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Structural Bolts Applications

 

 

< continued from A325 & A490 introduction

Structural bolts are designed to be used with nuts for the connection of structural members. The head of a heavy hex structural bolt is specified to be the same size as a heavy hex nut of the same nominal diameter, thus allowing a single size wrench or socket to be used on the bolt head and the nut. Structural bolts also have a shorter thread length so that the threads can be eliminated from the shear planes of the connection.

There are two primary ways in which structural bolts are used; Slip-Critical Connections and Snug Tightened. The AISC (American Institute of Steel Construction) publishes the "Specification of Structural Joints using ASTM A325 or A490 Bolts", which describes tightening methods for joints using structural bolts. In many cases (e.g., bearing connections, and bolts not in slip-critical connections or subject to tension loads), bolts can be used in the snug-tight condition. Snug tight is defined as the tightness that exists when all plies of a joint are in firm contact. It can normally be attained with a few impacts of an impact wrench or the full effort of a man using an ordinary spud wrench.

If full pretensioning is required, such as in slip-critical connections, then a load equal to 70% of the minimum tensile strength is referenced. Slip-critical connections, formerly know as friction-type connections, rely on the friction between the steel plies being clamped together and the high clamp load of the structural bolt/nut to prevent any movement (or slip) of the joint. To accomplish this high level of friction, the structural bots must be fully tensioned to the following minimum clamp loads;

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NOMINAL BOLT SIZE (INCHES)

MINIMUM TENSION

IN 1000'S OF POUNDS (kips)

A325 BOLTS

A490 BOLTS

1/2

5/8

3/4

7/8

1

1-1/8

1-1/4

12

19

28

39

51

56

71

15

24

35

49

64

80

102

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Atlantic Fasteners, Inc
PO BOX 19066
Greensboro, NC 27419-9066
(336) 852-0700 or (800) 233-6932 Fax: (336) 852-9730