Self-tapping screws that are hardened so that their threads form or cut mating threads in one or both of the sheet steel parts being connected are frequently used for making field joints. Such screws provide a rapid and efficient means of making light-duty connections. The screws are especially useful for such purposes as fastening sheet-metal siding, roofing, and decking to structural steel; making attachments at joints, side laps, and closures in siding, roofing, and decking; fastening collateral materials to steel framing; and fastening steel studs to sill plates or channel tracks. The screws may also be used for fastening bridging to steel joists and studs, fastening corrugated decking to steel joists, and similar connections to secondary members.
Since 1996, the AISI specification included design rules for determining nominal loads for shear and tension. The safety factor to be used for computing the allowable load is 3.0.
Several types of tapping screws are shown in Fig. 8.12. Other types are available. There are many different head styles—slotted, recessed, hexagonal, flat, round, etc. Some types, called Sems, are supplied with preassembled washers under the heads.
Other types are supplied with neoprene washers for making watertight joints in roofing.
All the types of screws shown in Fig. 8.12 require prepunched or predrilled holes. Self-drilling screws, which have a twist drill point that drills the proper size of hole just ahead of threading, are especially suited for field work, because they eliminate separate punching or drilling operations. Another type of self-drilling screw, capable of being used in relatively thin sheets of material in situations where the parts being joined can be firmly clamped together, has a very sharp point that pierces the material until the threads engage.
Torsional-strength requirements for self-tapping screws have been standardized under American National Standards Institute B18.6.4, ‘‘Slotted and Recessed Head Tapping Screws and Metallic Drive Screws.’’ Safe loads in shear and tension on such screws can vary considerably, depending on type of screw and head, tightening torque, and details of the assembly. When screws are used for structural loadcarrying purposes, the user should rely on experience with the particular application, manufacturer’s recommendations, or actual tests of the type of assembly involved.
Essential body dimensions of some types of self-tapping screws are given in Table 8.12. Complete details on these and other types, and recommended hole sizes, may be found in ANSI B18.6.4 and in manufacturers’ publications.