Using the available sources, in particular chapter 2 of the NIST document NIST NCSTAR 1-3: Mechanical and Metallurgical Analysis of Structural Steel, a set of basic structural data on WTC towers 1 and 2 has been gathered and is used in the model of a "typical" story of those buildings. A typical story is defined as one that is above the 7th story and is not a mechanical floor or in the hat truss area. Note that this model is meant to begin the task of establishing the geometry of the problem, it is not necessarily intended that it is the model used for simulation. Further work will be required to determine how each of the elements is to be simulated. For these floors, the basic structural elements and the associated dimensions are given in the following table.
| Parameter | Value (lengths in m) |
|---|---|
| building width | 63.14 |
| building depth | 63.14 |
| core width | 41.8 |
| core depth | 26.52 |
| perimeter column count | 236 (59 per side) |
| core column count | 47 |
| story height | 3.6576 |
| perimeter column spacing | 1.02 |
| number of long span trusses | 60 (30 per side) |
| number of short span trusses | 28 (14 per side) |
| truss spacing | 2.04 |
| number of transverse trusses | 8 |
| transverse truss spacing | 4.08 |
The long span trusses connect the perimeter columns to the core along the sides of the buildings that have the greatest distance from the perimeter to the core. The short span trusses are along the other two sides. The transverse trusses are also called the bridging trusses and are orthogonal to the long and short span trusses. In the table above, the transverse trusses refer only to those that are orthogonal to the long span trusses. If the transverse trusses orthogonal to the short span trusses are determined to be structurally significant, they will be added to the model.
Perimeter Column Data
Chapter 2 of the NIST document NIST NCSTAR 1-3: Mechanical and Metallurgical Analysis of Structural Steel indicates that the perimeter columns were 14 in (35.56 cm) in width and 13.5 in (34.29 cm) in depth for all of the columns from the 9th to the 107th floor. This data is shown in the following figure from the NIST document.
Regarding the thickness of the steel used in the perimeter columns, the NIST document (NCSTAR 1-3) makes the following statements:
PC&F of Seattle, Washington, fabricated the perimeter wall panels. The perimeter panels were comprised of three important subassemblies: the columns, the spandrels, and the > truss seats. The structural plans called for the columns to be fabricated from 14 grades of steel with Fy = 36, 42, 45, 46, 50, 55, 60, 65, 70, 75, 80, 85, 90, and 100 ksi,
although PC&F received approval to upgrade all 85 and 90 ksi steels to 100 ksi. Above the 75th floor, more than half of the columns had yield strengths between 55 ksi and 70
ksi, inclusive. The spandrels were fabricated from twelve grades of steel with Fy = 36, 42, 45, 46, 50, 55, 60, 65, 70, 75, 80, and 100 ksi (again with all 85 ksi steel upgraded
to 100 ksi). The truss seats were specified to be fabricated from steel with Fy = 36 ksi minimum.
Additionally we are told:
Perimeter columns in the upper stories were typically fabricated of lighter gauge steel, most commonly 0.25 in (6.35 mm) …
In contrast to the upper stories, in the lower stories, the perimeter column flanges were as thick as 3 in. (76 mm) and typically made of lower strength steels.
Spandrel Data
Chapter 2 of the NIST document NIST NCSTAR 1-3: Mechanical and Metallurgical Analysis of Structural Steel gives the following information regarding the spandrels which connected perimeter columns:
Adjacent columns were interconnected at each floor level by deep spandrel plates, typically 52 in. (1.32 m) deep. The spandrels formed an integral part of of the columns: there was no inner web plate at spandrel locations. Spandrels were generally specified with a yield strength lower than that of the column webs and flanges, as well as a heavier guage that the adjacent inner webs. Spandrel thicknesses ranged from 0.375 in. to 1.375 in. … Twelve grades of steel were specified for the spandrels, with the same strength levels as the columns but without the two highest strength steels.
Referring to the information given on the perimeter columns, the twelve grades of steel used in the spandrels have minimum yield strengths of 36, 42, 45, 46, 50, 55, 60, 65, 70, 75, 80 and 85 ksi.
Core Column Data
Note: the following was written prior to the collection of the core column data from the NIST SAP2000 model. That data can be viewed on the core column data pages.
There were two types of core columns used in WTC 1 and 2, box columns and rolled wide flange shapes which will be referred to here as I-beams. The yield strengths of the core box columns were 36 ksi or 42 ksi. The I-beams were one of four grades, but the yield strengths were primarily 36 ksi or 42 ksi. About 1% of the core columns were 45 ksi or 50 ksi steel. The columns in lower floors were almost exclusively large box columns up to 12 in. by 52 in. (0.30 m by 1.32 m). NIST indicates that the plates of the box columns were up to 7 in. (178 mm) thick. There seems to be a problem with this thickness since columns 12 in. in width could not accommodate sides of 7 in. in thickness. The following figure shows NIST's understanding of the layout of the core columns and the floors of transition from box columns to I-beams.
The architectural drawings differ from the NIST data in the floors at which the column transitions are made. The following figure gives the transition floors from this source.
There are a few floors for which the data have not yet been determined. As was noted in the discussion on data sources, discrepancies in data will be resolved by accepting the architectural drawings as authoritative unless it can be shown otherwise.
