D.C. Mun. Regs. tit. 24, r. 24-3208
In solid metal construction, anchors, bolts, and other approved fittings shall be securely fastened to the window frame or mullion in accordance with this section.
Anchor fittings shall be attached by three-eighth inch (3/8 in.) bolts, made of the same material as the anchor.
The bolts shall pass through the frame and shall be securely held in place by nuts and washers on the inside.
The ends of the bolts shall be upset to prevent loosening or removal of the nuts.
Where the nature of the solid metal frame construction is such that the method described in §§ 3208.2 through 3208.4 cannot be followed, it shall be permissible to drill and tap the metal frame to a depth of at least three-eighth inch (3/8 in.), and to install the anchor with at least two (2) three-eighth inch (3/8 in.) screws, made of the same material as the anchor.
All anchors and anchor fastenings shall be provided with means to prevent them from turning, backing off, or becoming loose.
Anchor fittings having a single threaded section and merely screwed into reinforcing plates shall not be acceptable.
All screws used in the installation of anchors on metal frame construction shall have the threads terminate far enough from the screw head to prevent weakening due to undercutting.
D.C. Mun. Regs. tit. 24, r. 24-3208