What is the difference between full diameter and reduced diameter body lag screws (also known as lag bolts)?

Full diameter screws have a larger unthreaded portion than the root diameter. Reduced diameter body screws' shank portion is the same as the root diameter of the screw. See the figure in Table L2 of the 2005 NDS for more clarification.

The reason the root diameter was used in the 2005 NDS was to better address the use of "reduced body diameter" lag screws (vs. "full body diameter") - and to better address the condition where the full length of the fastener is threaded.

Because "reduced body diameter" lag screws have a shank diameter approximately equal to the root diameter of "full body diameter" lag screws, design values for these fasteners are smaller than those provided in the 1997 NDS edition for "full body diameter" lag screws.

Root diameter (Dr), rather than the shank diameter, is used to calculate the tabulated lag screw design values, such as the ones shown in Table 11J. 

Please refer to Section 11.3.6 Dowel Diameter in the 2005 NDS where it states:

" When used in Tables 11.3.1A and 11.3.1.B, the fastener diameter shall be taken as D for unthreaded full-body fasteners and Dr for reduced body diameter fasteners or threaded fasteners except as provided in"

where states:

" For threaded full body fasteners (see Appendix L), D shall be permitted to be used in lieu of Dr when the bearing length of the threads does not exceed 1/4 of the full bearing length in the member holding the threads...Alternatively, a more detailed analysis accounting for the moment and bearing resistance of the threaded portion of the fastener shall be permitted (see Appendix I)."

Design Aid 1 - Application of Technical Report 12 for Lag Screw Connections provides one alternate method of accounting for the moment and bearing resistance of the threaded portion of the fastener and moment acting along the length of the fastener as provided in Technical Report 12 (TR12) - General Dowel Equations for Calculating Lateral Connection Values.