Effects of Head Size on the Performance of Twist-Off Bolts
Schnupp, Keith Otto
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This study examines a specific application of button-head type twist-off bolts. Currently, the Research Council on Structural Connections Specification (2000) removes the requirement for ASTM F436 washers (ASTM 2000a) under the bolt head of twist-off bolts where the head diameter equals or exceeds that of an ASTM F436 washer when oversized and slotted holes are used. The need for washers is also removed for A490 strength bolts used on steels with specified yield strengths less than 40 ksi provided that the head diameter equals or exceeds an ASTM F436 washer. The ASTM F1852 Specification (ASTM 2000b) allows for head diameter dimensions that are slightly smaller than an ASTM F436 washer. Following the RCSC Specification, manufacturers that produce bolts using the ASTM F1852 dimensions are required to use ASTM F436 washers under the bolt head. The discrepancies between the specifications lead to this study, which involved the testing of button-head type twist-off bolts with two different head diameters, both of which were smaller than an ASTM F436 washer. Five bolt diameters between 5/8 in and 1-1/8 in. were tested in standard, oversized, and long-slotted holes. The performance of the twist-off bolts was determined by measuring and comparing the achieved relaxed pretension force in the bolt after tightening. It was found that twist-off bolts with head diameters less than an ASTM F436 washer had no trouble attaining their required minimum pretension force. Bolt head diameter and hole size were found to have no significant influence on the pretension force that was achieved for all bolts tested.
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