The Singer Tower addition of 1906–1908 had a steel skeleton and weighed . The tower's columns were spaced apart on their centers. Because the three center bays on each side contained windows, only the corners used diagonal bracing and, as such, were treated as square prisms. Inside, there was another structure for the central elevator shafts, which were connected to the corners of the tower via longitudinal beams. A girder supported the columns at the tower’s corners at the fourth floor, while 36 columns rose from the basement into the tower. Four pillars were placed at each corner of the tower and six more pillars were placed in the elevator shafts. Each truss extended upward for two stories, causing the columns and braces to act as wind-resistant cantilevers. The braces on the north and south contained eleven panels each while those on the east and west contained ten panels. The four columns at the center of the tower supported its dome.