The exterior of the building had applied granite spandrels and pilasters. Pilasters were omitted on the north and south wall, which makes for easy identification in the debris pile:
Building 7 had three unique features which are important in understanding what the evidence shows happened to it. First, it had a trapezoid footprint, which played a role in how it collapsed. Second, it was a “large span” design, meaning normal spacing of vertical columns was increased, creating the need for large floor span trusses from the inner core structure to the outer perimeter wall, or “curtain”. Third, it was built on top of a Con Ed (electrical) substation, which required a 7 story complex network of trusses. All three played a role in how the building fell, confirmed by video of collapse and structural remains.
Vertical support consisted of two components; the outer curtain and the core columns:
The visual collapse of WTC7 that we all have seen is primarily of the outer shell only. The core collapsed in advance of the perimeter walls, as will be shown in future chapters. The curtain wall consisted of columns and I-beam lintels:
Above photo is from the east wall. The north wall, shown in the next photo, had more spacing between columns. The interior during construction shows the floor truss span from core to wall on the north side of the building. The larger truss spanned from Core Column 79 to the north wall, near the NE corner. The north side was the longest span in the building. approximately 50 feet :
Bottom trusses to span Con Ed:
More diagrams of the truss work spanning Con Ed can be found at the FEMA site, “World Trade Center Building Performance Study”. Detailed study is not required in order to understand the basic concept of building failure. Familiarization , however, helps in identifying original location of structural remains. The FEMA study provides other valuable information regarding Building 7, and is worth at least a quick look at the diagrams.
The streets and buildings around WTC7 will be referred to throughout this blog in order to locate areas of the site. Directional notations will also be used, such as SE (south east) for building corner locations. For a full understanding of information in this blog, familiarity with the site is necessary in order to correlate what occurred at one corner of the building, and what the debris evidence reveals. Building 7 was not oriented due north/south, but the four sides are generally referred to by their primary direction. Knowledge of surrounding buildings is instrumental in knowing what you are looking at in the pictures. A map: (Click for enlargement)
A close up. Note location of Fiterman Hall, Pedestrian Walkway, and Promenade for future reference:
Surrounding Buildings looking east:
The immediate neighborhood: