World Trade Center 7 Design and Construction
WTC 7 was a 47 story office building whose height from the ground floor to its roofline was approximately 186 m (610 ft) (NCSTAR_1-9_Vol1_p142)
The south side was 75.2 m (247 ft), the east side was 45.0 m (149 ft), the north side was 100.3 m (329 ft), and the west side was 46.7 m (153 ft). On the north face, an approximately 278 ft wide, two story section of the substation extended outward an additional 42 ft along the base of the west wall. (NCSTAR_1-9_Vol1_p142)
The north face of WTC 7 was actually oriented roughly 29 º clockwise from true north (NIST NCSTAR 1-2). (NCSTAR_1-9_Vol1_p136)
WTC 7 was unlike the WTC towers in many respects. It was a more typical tall building in the design of its structural system. (NCSTAR_1A_p95)
4.5.2 Building Design and Structural Safety
- NIST found no evidence to suggest that WTC 7 was not designed in a manner generally consistent with applicable building codes and standards.
- WTC 7 was adequately designed for vertical loads due to gravity and lateral loads due to wind.
- The vertical (gravity) load resisting system comprised the core and exterior columns, which received gravity loads from the floor framing. Of particular note were the three core columns on the east side of the building (Columns 79, 80, and 81), which supported large span floor areas with approximately 15 m (50 ft) spans on at least one side.
- Above Floor 7, the lateral load resisting system comprised the exterior moment frame with a perimeter belt truss at Floors 22 through 24. There were also a perimeter belt truss between Floors 5 and 7, diaphragms at Floors 5 and 7 that transferred lateral wind loads to the core columns, and bracing in the core below Floor 7 that transferred lateral loads to the foundation. However, this system was not able to provide a secondary load path for gravity loads.
- Above Floor 7, there was no bracing, or other load redistribution mechanism, to transfer gravity loads between interior columns. The floor framing, which was the only load path between columns, could not redistribute loads between columns because the shear connections from the interior floor beams to columns were only designed for transferring vertical shear loads.
- Transfer girders, trusses, and cantilever overlays were used to transfer column loads above Floor 7 to a different column layout below Floor 5.
- The structural design did not explicitly evaluate fire effects, which was typical for engineering practice at that time and continues to remain so today. Many of the shear connections in WTC 7 were not capable of resisting lateral loads resulting from thermal expansion effects in the steel floor framing when the floor beams were heated. (NCSTAR_1A_p91)
February 19, 1989:
Before it moves into a new office tower in downtown Manhattan (WTC7), Salomon Brothers, the brokerage firm, intends to spend nearly two years and more than 00 million cutting out floors, adding elevators, reinforcing steel girders, upgrading power supplies and making other improvements in its million square feet of space.... "We were driven very much by technology. We had to find a building that could accommodate our needs, including major-sized trading floors." explained Gedale B. Horowitz, a senior executive director of Salomon. Much of the new electrical, air-conditioning and mechanical equipment will serve three double-height trading floors. To create the extra height, workers are removing most of three existing floors, using jackhammers to demolish concrete slabs and torches to remove steel decking and girders beneath the concrete. In some office buildings, that alteration would be impossible, but Silverstein Properties tried to second-guess the needs of potential tenants when it designed Seven World Trade Center as a speculative project. "@@We built in enough redundancy to allow entire portions of floors to be removed without affecting the building's structural integrity, on the assumption that someone might need double-height floors@@," said Larry Silverstein, president of the company. "Sure enough, Salomon had that need." More than 375 tons of steel - requiring 12 miles of welding - will be installed to reinforce floors for Salomon's extra equipment. nytimes.com
http://www.msel.nist.gov/Safety and Reliability.pdf