D.C. Mun. Regs. tit. 24, r. 24-3207
In hollow metal construction, anchors, bolts, and other approved fittings shall be securely fastened to the window frame or mullion in accordance with this section.
Two (2) through bolts having a diameter of not less than three-eighth inches (3/8 in.) that is made of the same material as the anchor shall pass through the entire window frame or mullion to secure each anchor. Each through bolt shall be securely fastened by a washer and nut.
The ends of the through bolts required by § 3207.2 shall be upset to prevent loosening or removal of the nuts.
As an alternate, where through bolts are impracticable, anchors shall be attached by two (2) bolts of the same material as the anchor and having a diameter that is not less than three-eighths inches (3/8 in.) that passes through the frame, and a steel reinforcing plate that is three-eighth inches (3/8 in.) thick and not less than six inches (6 in.) long placed on the inside of the frame and secured nuts and lock washers.
Where it is impracticable to provide nuts and lock washers, the reinforcing plate may be tapped to receive screws having a three-eighth of an inch (3/8 in.) diameter. The screws shall extend through the plate.
All anchors and anchor fastenings shall be provided with means to prevent them from turning, backing off, or becoming loose.
On new frames, the reinforcing plate shall be in the form of a "Z" or some other equivalent, one (1) portion of which shall reach around behind the exposed face of the frame and be bolted, riveted, or welded to a part of the frame protected by the masonry.
D.C. Mun. Regs. tit. 24, r. 24-3207