Curator's field notes
This is an excerpt from field notes written by NMAH's curator of engineering, Jeffrey Stine, on 7 January 2002, describing his tour of the Pentagon reconstruction site three days earlier. These field notes convey information obtained through discussions with the building's construction manager, and have not been vetted or verified. This 'internal document' was produced as part of the museum's 9/11 fact-finding effort, and is presented here as an example of that exercise.


To assist in NMAH’s efforts to document the terrorist attacks of 11 September 2001, the museum’s engineering specialist and I arranged to be given a tour of the Pentagon reconstruction project, so that we might see for ourselves what physical damage had been done to the building, how the original building had been designed and constructed, what had been done to harden the outside of the building in “wedge 5" prior to the attack, and what changes are being made to the design, construction, and use of materials in that part of the building now being entirely replaced.

Our tour was arranged by the public relations office of the multi-year Pentagon renovation project (the project that is hardening the outside of the entire building; but that project is now also charged with replacing the large section of the building destroyed by the jet crash, explosion, and fire). We were joined by the manager of the new construction—this effort has been named “Project Phoenix”—an engineer working for the federal government; he has been involved in the overall renovation of the Pentagon, and was pulled from that assignment to head up the construction effort.

The building-wide renovation project was a response to the Oklahoma City bombing and other anti-American terrorist bombings around the world. The goal was to harden the Pentagon against an external explosion, one that was envisioned most likely to be caused by a truck bomb. Elements of the U.S. Army Corps of Engineers helped in studying the Pentagon and determining just what needed to be done, such as relocating the large Metro bus transfer station at least 280 feet from the building.

The other major component of the renovation was hardening the outside of the building, to lessen any damage that might be caused by an external explosion. One of the key components was the replacement of all of the Pentagon’s outside windows with blast-proof glass. Because the building is on the National Register of Historic Places, the replacement windows were designed to look just like the originals. Those original windows could be opened (they appeared to be double-hung windows—the type where the lower half could be raised, or the top half lowered), so the replacement windows looked as though they, too, could be opened, but of course they could not. Although most of the building is reinforced concrete, the outside of the building is faced with Indiana limestone (apparently President Roosevelt had prohibited the use of granite for this purpose), and backed up with bricks and mortar. Thus, the outside walls were the most vulnerable, in terms of withstanding an explosion. The new shatter-resistant windows, therefore, could not simply be put back in place, as an explosion would just blast them (presumably unbroken) into the building. The design solution was to use channel steel (steel rectangles—it appeared that two different sizes were used; a larger size running up and down the walls, with the cross sections being somewhat smaller) extending from floor to ceiling, to which steel members were welded to hold in the new window frames. This was meant to strengthen substantially the windows and their surrounding frameworks. The concern then was with the brick, which could potentially turn into flying shrapnel by a large explosion. The design solution here was to cover the insides of all the external brick walls with a super-strong woven fabric, which was bolted into the floor and the new steel framework. This fabric (Kevlar, or something similar) was thus intended to work as a “catcher’s mitt.”

We were told that the original Pentagon was built as five separate buildings, which were joined together (expansion joints separated the five parts, or wedges). Each of the five sections was independently provided with various utilities. And this was the pattern that was being replicated in the renovation: the job has been divided into five parts, coinciding with the five original wedges. Because the renovation team wanted to learn what it was doing while working in a part of the building holding the least senior and critical elements of the Pentagon (rather than in the wedges overlooking the Potomac River, where the Secretary of Defense, the Joint Chiefs of Staff, et al. maintain their offices), they started in wedge five in the back (facing west, toward Arlington National Cemetery). They had only recently completed this section of the renovation, and had moved about half the people back into their offices, when the jetliner crashed into this very section. (By all accounts, this was the least vulnerable part of the Pentagon to be attacked in this manner, both because it had been hardened and because it was only partially occupied—factors that substantially reduced the number of casualties.)

The construction manager explained that there were three destructive factors associated with the terrorist attack: (1) the kinetic energy of the plane itself hitting the Pentagon; (2) the subsequent explosion inside the building (at one place, he noted, the force moved up an elevator shaft, blowing out the doors on the fifth floor); and (3) the aviation-fuel-fed fire. We were told that the force of the crash was absorbed by several elements of the building: the new external hardening/reinforcement surely helped in this respect; and the various expansion joints in the original reinforced concrete building also worked to absorb this energy—perhaps most of the energy. (We asked if other parts of the building were pushed off line because of the crash, and were told no, essentially because the Pentagon worked as five separate structures. Photographs of the original crash site show how one side was jagged, the other seemingly sheared in a straight line. That “clean” shearing took place along one of the expansion joints.) The plane penetrated the three outer rings of the building. Workers and/or investigators had painted a bright-orange diagram of where the plane’s nose would have struck the next ring, had it continued forward. We were told that one element of the original building that helped keep the structure standing for some 35 minutes was the spiral rebar wrapped around the concrete columns. The rebar helped hold the concrete together. It worked so well, in fact, that in replacing this segment of the building, they have special ordered the same type of spiral rebar for the new columns (apparently, this is not standard construction design). Unfortunately for our collecting effort, workers cleaned up the site in one month and one day, trucking the debris straight to a landfill, where it was immediately buried.

Our tour guide explained the new construction. A much larger portion of the building was demolished than had been destroyed by the plane crash and fire. This was done for several reasons: at some points, to clear out to a place where it was easiest to tie into the undamaged structure (say at an expansion joint); at some points, the heat from the fire had changed the characteristics of the steel beams and rebar, so the engineers decided it was better to remove and replace these sections; and at other points, the bottom floors were extensively damaged, but the top floors were not—thus, they could have temporarily supported the top floors while they rebuilt the bottom floors, but this would have taken longer and been more expensive. Once the debris was removed, they used the original plans of the building to locate the numerous pilings, which supported the building’s weight.

They saved as much of the limestone facing as they could, but much of it was badly broken and/or scared. They returned to the same vein of limestone in Indiana to find matching stone (stone that was both the same color and the same texture). We were told that there were about five different quarries located on this limestone vein. They approached the original quarry used by the Pentagon in the early 1940s, and while the quarry could provide stone of the same color, it was unable to match the texture. Thus, they went to another quarry, which was able to provide them with what they needed. To match the slate roof, they returned to the same slate quarry in Vermont where the original slate shingles were acquired. The owners were able to provide what they needed, and even offered to donate the materials—a generous and unusual offer the government had to decline.