AISC LRFD 1.pdf

246 BOLTS AND THREADED PARTS [Comm. J3.10. Bearing Strength at Bolt HolesProvisions for bearing strength of pins differ from those for bearing strength ofbolts; refer to Section J8.Bearing values are provided as a measure of the strength of the material upon whicha bolt bears, not as a protection to the fastener, which needs no such protection.Accordingly, the same bearing value applies to all joints assembled by bolts,regardless of fastener shear strength or the presence or absence of threads in thebearing area.Material bearing strength may be limited either by bearing deformation of the holeor by block shear rupture of the material upon which the bolt bears. Recent testingby Kim and Yura (1996) and Lewis and Zwerneman (1996) has confirmed the bearingstrength provisions for the former case wherein the nominal bearing strength R nis equal to CdtF u and C is 2.4, 3.0, or 2.0 depending upon hole type and/or acceptabilityof hole ovalization at ultimate load as indicated in LRFD Specification SectionJ3.10. However, this same research indicated the need for more accurate bearingstrength provisions when block-shear-rupture-type failure would control.Appropriate equations for bearing strength as a function of clear distance L c aretherefore provided and this formulation is consistent with that adopted by RCSC inthe Load and Resistance Factor Design Specification for Structural Joints UsingASTM A325 or A490 Bolts (RCSC, 1994).Frank and Yura (1981) demonstrated that hole elongation greater than 1 4 in. (6 mm)will generally begin to develop as the bearing force is increased beyond (2.4dtF u ),especially if it is combined with high tensile stress on the net section, even thoughrupture does not occur; for a long-slotted hole with the slot perpendicular to thedirection of force, the same is true for a bearing force greater than (2.0dtF u ). Anupper bound of (3.0dtF u ) anticipates hole ovalization (deformation greater than 1 4in. (6 mm)) at maximum load.Additionally, to simplify and generalize such bearing strength calculations, the currentprovisions have been based upon a clear-distance formulation. Previous provisionsutilized edge distances and bolt spacings measured to hole centerlines withadjustment factors to account for varying hole type and orientation, as well as minimumedge distance requirements.11. Long GripsProvisions requiring a decrease in calculated stress for A307 bolts having longgrips (by arbitrarily increasing the required number in proportion to the grip length)are not required for high-strength bolts. Tests (Bendigo, Hansen, and Rumpf, 1963)have demonstrated that the ultimate shearing strength of high-strength bolts havinga grip of eight or nine diameters is no less than that of similar bolts with muchshorter grips.J4. DESIGN RUPTURE STRENGTHTests (Birkemoe and Gilmor, 1978) on coped beams indicated that a tearing failuremode (rupture) can occur along the perimeter of the bolt holes as shown in FigureC-J4.1. This block shear mode combines tensile strength on one plane and shearstrength on a perpendicular plane. The failure path is defined by the center lines ofLRFD Specification for Structural Steel Buildings, December 27, 1999AMERICAN INSTITUTE OF STEEL CONSTRUCTION