Slip-Critical Bolted Connections

The results of sixteen experiments of bolted slip-critical connections with fillers were explored.  Fifteen of the connections used oversized holes and one connection used standard holes to establish a baseline comparison.  Such connections with oversized holes are commonly fabricated for use with structures such as long-span trusses, since the use of oversized holes allows erection in-place rather than first assessing fit-up on the ground.

Filler plates are used to connect members of different depths or widths. The Specification for Structural Steel Buildings of the American Institute of Steel Construction required connections with oversized holes to be designed as slip-critical at what is termed the “required strength level,” for which a resistance factor (LRFD) of 0.85 and a safety factor (ASD) of 1.76 were specified. These slip strengths were typically below values that had been used for years in the Specification for Structural Steel Buildings: Allowable Stress Design and Plastic Design. In addition, when fillers are used in these connections, the AISC (2005) provisions did not require changes in the strength calculations, whereas if standard holes were used, options are provided for connection design that included reduction of the bolt shear strength or development of the connection.

The sixteen experiments highlight the behavior of bolted steel connections with oversized holes in which fillers were included and were undeveloped, partially developed, or fully developed. Both single-ply and two-ply filler were investigated, as were welded fillers, and specimens fabricated using either turn-of-the-nut or tension-controlled bolts. Extensive instrumentation was used on the specimens to document the flow of forces through the connection.

The research documented the slip and shear strengths of these connections, proposed formulas for assessing these strengths for the different conditions investigated, provided revised recommendations for design of these types of connections, and included suggestions for further work.

Specimen 455h Shear Failure

Bolted slip-critical connection research. Specimen 455h bolt shear at 2,197 kips.

Specimen 455n1 Slip

Bolted slip-critical connection research. Specimen 455n1 slip at 1,388 kips.