In general, a connection with a few large-diameter fasteners costs less than one of the same capacity with many small-diameter fasteners. The fewer the fasteners, the fewer the number of holes to be formed and the less installation work. Larger-diameter fasteners are particularly favorable in connections where shear governs, because the load capacity of a fastener in shear varies with the square of the fastener diameter. For practical reasons, how- ever, 3⁄4-and 7⁄8-in-diameter fasteners are preferred.
Maximum Fastener Diameters in Angles. In bridges, the diameter of fasteners in angles carrying calculated stress may not exceed one-fourth the width of the leg in which they are placed. In angles where the size is not determined by calculated stress, 1-in fasteners may be used in 31⁄2-in legs, 7⁄8-in fasteners in 3-in legs, and 3⁄4-in fasteners in 21⁄2-in legs. In addition, in highway bridges, 5⁄8-in fasteners may be used in 2-in legs.
FASTENER HOLES
Standard specifications require that holes for bolts be 1⁄16 in larger than the nominal fastener diameter. In computing net area of a tension member, the diameter of the hole should be taken 1⁄16 in larger than the hole diameter.
Standard specifications also require that the holes be punched or drilled. Punching usually is the most economical method. To prevent excessive damage to material around the hole, however, the specifications limit the maximum thickness of material in which holes may be punched full size. These limits are summarized in Table 5.5.
In buildings, holes for thicker material may be either drilled from the solid or subpunched and reamed. The die for all subpunched holes and the drill for all subdrilled holes should be at least 1⁄16 in smaller than the nominal fastener diameter.
In highway bridges, holes for material not within the limits given in Table 5.5 should be subdrilled or drilled full size. Holes in all field connections and field splices of main members of trusses, arches, continuous beams, bents, towers, plate girders, and rigid frames should be subpunched, or subdrilled when required by thickness limitations, and subse- quently reamed while assembled or drilled full size through a steel template. Holes for floorbeam and stringer field end connections should be similarly formed. The die for sub- punched holes and the drill for subdrilled holes should be 3⁄16 in smaller than the nominal fastener diameter.
A contractor has the option of forming, with parts for a connection assembled, subpunched holes and reaming or drilling full-size holes. The contractor also has the option of drilling or punching holes full size in unassembled pieces or connections with suitable numerically controlled drilling or punching equipment. In this case, the contractor may be required to demonstrate, by means of check assemblies, the accuracy of this drilling or punching procedure.
TABLE 5.5 Maximum Material Thickness (in) for Punching Fastener Holes*
AISC
|
AASHTO
|
AREMA
|
|
A36 steel
High-strength steels
Quenched and tempered steels
|
d 1⁄8†
d 1⁄8†
1⁄2‡
|
3⁄4§
5⁄8§
1⁄2§
|
7⁄8
3⁄4
|
* Unless subpunching or subdrilling and reaming are used.
† d fastener diameter, in.
‡ A514 steel.
§ But not more than five thicknesses
of metal.
Holes drilled or punched by numerically controlled equipment are formed to size through individual pieces, but they may instead be formed by drilling through any combi- nation of pieces held tightly together.
In railway bridges, holes for shop and field bolts may be punched full size, within the limits of Table 5.5, in members that will not be stressed by vertical live loads. This provision applies to, but is not limited to, the following: stitch bolts, bracing (lateral, longitudinal, or sway bracing) and connecting material, lacing stay plates, diaphragms that do not transfer shear or other forces, inactive fillers, and stiffeners not at bearing points.
Shop-bolt holes to be reamed may be subpunched. Methods permitted for shop-bolt holes in rolled beams and plate girders, including stiffeners and active fillers at bearing points, depend on material thickness and, in some cases, on strength. In materials not thicker than the nominal bolt diameter less 1⁄8 in, the holes should be subpunched 1⁄8 in less in diameter than the finished holes and then reamed to size with parts assembled. In A36 material thicker than 7⁄8 in (3⁄4 in for high-strength steels), the holes should be subdrilled 1⁄4 in less in diameter than the finished holes and then reamed to size with parts assembled.
A special provision applies to the case where matching shop-bolt holes in two or more plies are required to be reamed with parts assembled. If the assembly consists of more than five plies with more than three plies of main material, the matching holes in the other plies also should be reamed with parts assembled. Holes in those plies should be subpunched 1⁄8 in less in diameter than the finished hole.
Other shop-bolt holes should be subpunched 1⁄4 in less in diameter than the finished hole and then reamed to size with parts assembled.
Field splices in plate girders and in truss chords should be reamed or drilled full size with members assembled. Truss-chord assemblies should consist of at least three abutting sections. Milled ends of the compression chords should have full bearing.
Field-bolt holes may be subpunched or subdrilled 1⁄4 in less in diameter than finished holes in individual pieces. The subsized holes should then be reamed to size through steel templates with hardened steel bushings. In A36 steel thicker than 7⁄8 in (3⁄4 in for high- strength steels), field-bolt holes may be subdrilled 1⁄4 in less in diameter than the finished holes and then reamed to size with parts assembled or drilled full size with parts assembled. Field-bolt holes for sway bracing should conform to the requirements for shop-bolt holes.
If numerically controlled equipment is used to punch or drill holes, requirements are similar to those for highway bridges.