In the first instance, calculate the tensile-stress area of a bolt from ASME B1.1-1989 Appendix B using The calculated value At is not based on the minimum root diameter. This value is slightly ...

Because the widths across flats of the bolt head are typically w = 1.5d, the mean diameter of stress area in bolt head is d m = (d+w)/2. Equivalent stiffness of the head in terms of the shank is ...