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American Wood Council

Are self-tapping lag screws covered by provisions in the NDS?


Design provisions and design values in the National Design Specification® for Wood Construction (NDS®) are applicable to lag screws conforming to ANSI/ASME Standard B18.2.1-1981. Tabulated design values are based on lag screws conforming to ANSI/ASME Standard B18.2.1-1981 and having assumed bending yield strengths provided in the table footnotes. Note that self-tapping lag screws are not addressed in ANSI/ASME B18.2.1 and are not specifically covered by provisions of the NDS. Specifically, the NDS does not address fabrication and assembly requirements, withdrawal design values, or lateral design values for self-tapping lag screws.

For self-tapping lags screws with dimensions similar to those provided in ANSI/ASME B18.2.1, the general form of the yield equations should apply for determining lateral design values. Accordingly, tabulated design values would also apply provided that the self-tapping lag screw dimensions meet or exceed the dimensions ASME B18.2.1 and the bending yield strength equals or exceeds the assumed bending yield strength in the table footnotes. In order to use lateral design provisions of the NDS, it must be assumed that fabrication and assembly of connections using self-tapping screws permits the development of the full bearing strength of the wood beneath the lag screw or permits yielding of the lag screw (i.e. installation does not damage the wood member or connection).

Finally, it should be noted that NDS Section 7.1.1.4 indicates that connections, other than those covered in the provisions, are not precluded from use where it is demonstrated by analysis, tests, or extensive experience that the connections will perform satisfactorily in their intended end use.

 
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