9 - 1 1 R e s e a r c h

an attempt to uncover the truth about September 11th 2001
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Michael N. Biscotte, P.E., and Keith A. Almoney

News reports dramatically related a turning point in American cultural life: "April 19, 1995, about 9:03 a.m., at the Murrah Federal Building in downtown Oklahoma City, the unthinkable happened. ... A stunned nation watched. ... 168 people were dead. ... Two and a half tons of ammonium nitrate, common farm fertilizer mixed with fuel oil, packed into a rental truck brought the world to an end. Terrorism made simple. ... it was so easy. So cheap. So destructive."

Architect Jim Loftis was quoted at the time as saying "buildings can be made bomb-resistant but not bomb proof. We can't afford it, and it would be a miserable way to live." CNN reported that "lost in the debris, with all the lives, has been any illusion of safety. ... The last casualty of the Oklahoma City bombing may be the loss of our sense of security, now scarred forever."

Terrorism's Ongoing Threat

The scars from the Oklahoma City bombing are still fresh in the minds of government officials six years later when assessing the physical security of likely targets for terrorist attacks. The potential for attacks against the U.S. government was made even more immediate with the bombing of the USS Cole in Yemen last October and the killing of 17 of its sailors and the wounding of 39 others. With this escalation in terrorism came the desire to develop a reasonable protective shield for the nation's foremost symbol of military strength, the Pentagon. The need to reduce the building's vulnerability to a terrorist attack was high on the list of renovation priorities for the sixty-year old headquarters of the Department of Defense.

"When the Pentagon was designed and built in the early 1940s," reflected Walter Lee Evey, director of the Pentagon Renovation Program Office, "there were a number of concessions made to a country at war. The original designers exercised economies in construction to lessen the impact on strategic materials needed to equip the military." The extensive use of reinforced concrete and non-reinforced masonry was one concession. Certainly the threat of any kind of terrorist attack on the building was far from the thoughts of the original designers. As a result, the Pentagon was constructed with a thin limestone facade over a brick infill between reinforced concrete floors, structurally supported by a reinforced concrete beam and column frame. Enough to protect from the elements but not from the potential forces of significant blast events.

Architecturally, the designers of the huge office building also opted for the extensive use of windows. This feature helped connect workers with the outside world, and further reduced the demand for critical wartime construction materials. Along each 924' (281.6 m) exterior wall, there are approximately 400 windows, roughly 5' wide by 7' tall (1.8m � 2.1 m). Together, the lightly constructed facade and large number of windows offer little resistance to terrorist attack.

A reasonably forceful blast from any close point along the Pentagon's surrounding network of public roads would create broad personnel risk inside the outermost of the building's five concentric office rings and could cause severe property and structural damage as well. According to Evey, "The Renovation Office recognized this shortcoming and was determined to address it effectively by incorporating improved personnel safety features into the overall renovation program." The blast protection task was included in the new design work for the first of the Pentagon's five "wedges" and is now a "template" for the follow-on renovation of the other sections.

Designing for Safety

The U. S. Army Corps of Engineers, Omaha District, Protective Design Center evaluated possible threats to determine a "most likely" bomb blast scenario, calculating dynamic, time-varying forces for various blast sizes and locations on the building's perimeter. From this analysis, the Renovation Office established blast resistance structural design criteria for the project. The next step in the process was to develop the design, incorporating the established criteria.

The Renovation Program architect/engineer for Wedge One, Hayes, Seay, Mattern & Mattern, Inc. (HSMM), worked closely with the Protective Design Center in assessing the blast resistance of the existing walls and the proposed replacement fenestration. During the preliminary evaluation, the engineers studied the outermost (E) and innermost (A) ring walls. The resulting data helped formulate initial cost analysis data for reinforcing outer ring walls and provided a foundation for the final design.

While HSMM considered various mitigation approaches, the options were few due to the Pentagon's existing exterior design, the window size/alignment and limestone facade, being considered architecturally significant and historical. Protective improvements, therefore, had to be made on the interior side. Other guidelines restricted new construction to a narrow area behind the exterior wall to avoid consuming significant amounts of interior, occupiable floor space.

Finding A Solution

Merely replacing existing windows with blast-resistant ones anchored to the existing brick infill would not be a workable solution. The newer windows would direct blast pressures to the window support connections at the infill. The tensile and shear loads would overwhelm the existing infill walls and allow the panes and frames to separate from the walls as potential missiles harmful to personnel working in the building. A typical bay with window is illustrated below.

The idea of supporting the brick infill walls with a reinforced concrete wall "backing" was rejected as a "typical" approach because of the Pentagon's extensive fenestration (although this design was accepted for "blank" wall panels with no window openings).

Another proposal suggested dropping a continuous, structural tube through roof openings behind the walls and through the floor slabs. Grouting the floor slabs around the tubes would allow the tubes to transfer explosive loads horizontally into the slabs. This idea was rejected, primarily because of the building's structural irregularities. The Pentagon's windowless fifth floor, a late-construction addition, has a large, thick capstone running along the bottom third of the brick wall. This unusual, non-structural feature extends 17 inches (432 mm) into the interior from the wall. A design where the tube penetrates the stone would be costly as well as difficult to construct. Another irregularity is the second floor spandrel beam, which is located under the other spandrels and protrudes several inches beyond the wall into tenant space. The vertical tube would have to bypass this feature, too. To compensate for these irregular features and achieve effective structural protection, large spacers would have to be added to the continuous tube on every floor. This factor made the approach too costly.

Belying its regular-looking, geometric appearance, the Pentagon has a number of as built dimensional and structural irregularities. Many of these were never documented during construction and were only discovered during renovation. These unknowns forced HSMM to pursue a general solution that would be cost-effective and feasible for every floor and wall section.

The resulting general design solution called for erecting structural reinforcements around the windows, anchoring at the top and bottom to structural concrete floor slabs and not the non-structural brick infill walls. This general solution also accepts blast forces from the walls themselves and transfers both window and wall loads into the horizontal slab diaphragms.

This solution has a tubular frame for each windowed wall panel, consisting of two vertical tubes (HSS 6 � 6 � 0.25 on floors two through four, and HSS 8 � 8 � 0.1875 on the first floor) horizontally braced with tubes (HSS 4 � 4 � 0.375) welded at each window's head and sill. (See the above picture for a rendering of the structural frame at a typical second floor window.) The frame becomes the structural support for the blast-resistant windows, with the vertical tubes giving new blast protection to the infill walls.

To make the solution work, HSMM designed a practical floor-ceiling connection scheme for the vertical tubes. The tubes must withstand large deflections to perform their intended function of absorbing blast loads. Large deflections, however, with their inherent shear, create significant tensile force on connecting hardware in a blast situation. This condition eliminated the more direct "top/bottom" approach of connecting the vertical tubes to the concrete slab above and below with expansion anchors.

With the stringent design criteria minimizing intrusion into tenant space, the solution had to work in the narrow space between the tubes' interior face and the interior face of the brick wall, a matter of only a few inches. The answer was to weld the tubes to long, narrow plates running along the floor and ceiling. These plates connect to their counterparts on floors above and below with through-bolts, using 3/4"-diameter A36 threaded rods. To maintain good connection to the slab for constructibility and to compensate for variations in tube length, the vertical tubes are also welded to opposed double gusset plates, which in turn are welded to the floor/ceiling plates. This solution connects the window frames from floor to floor. This design directs dynamic horizontal blast forces through the flexible tubes into the floor diaphragms. This approach was uniformly applied to the window panels on the second, third and fourth floors.

The first and fifth floors, due to the existing construction mentioned earlier, posed special problems. Because the first floor is slab on grade, connecting the tubes to the floor slab by through-bolting would not work. And since anchor bolts could not take the calculated tensile and shear forces, a different approach was required. A core-drilled hole in the slab on grade accommodates the tube bottom so the tube will bear horizontally against the floor through a bearing plate. After inserting the greased end of the tubes, the holes are grouted to create a bearing surface and protect the capped tubes' ends from soil moisture corrosion.

The first floor ceiling configuration created a separate connection challenge. The spandrel beam from the second floor protrudes approximately 4" (102 mm) from the interior wall for about 17" (432 mm) down from the ceiling, forcing the vertical tubes several inches away from the infill wall and window frames. The resulting gap below the spandrel had to be "closed" to maintain structural integrity by using a dry-pack, non-shrink grout and by welding a 3" wide (76 mm) spacer tube (HSS 6 � 3 � 0.3125) along the length of the HSS 8 � 8 tube face. The tubes' top connection used the same through-bolt scheme as the other floors, aligning with the bottom plates on the floor above.

The other "non-conforming" area, the windowless (along the outer wall) fifth floor, also required a unique approach. The capstone mentioned earlier prevents alignment of the fifth floor's vertical tubes with those from the floor below. To gain structural benefit for the vertical tubes, again the design turned to spacers to fill the in-space between the verticals and the masonry walls. A wide-flange beam (W16 � 36) spacer was used above the capstone and a somewhat smaller one (W12 � 26) above. Four of these new structural tubes with spacers were placed evenly between existing concrete columns to provide improved bracing for the wall section. Since the fifth floor ceiling is also the roof slab, a through-bolt connection would have to go through existing slate or copper roof material. This approach was unacceptable because it would alter the historically protected exterior. The design decision was to use expansion anchors.Though not acceptable for other areas, a design modification allowed their use on the fifth floor. To avoid the tensile strength problem and shear potential on the ceiling anchors, the design was altered to eliminate the tensile force. The first thought was to core-drill as on the first floor. However, the construction contractor, Morse Diesel International, Inc., suggested using instead a slip-insert assembly to reduce costs,one of the firm's many practical suggestions for successfully installing these various structural supports. The sleeve design allows the tube to slip into the sleeve and bear against it to transfer shear forces to the slab. Because the tube itself is not physically anchored to the sleeve, the tube can slip upward to deflect blast loadings, thus diminishing the tensile forces and allowing the expansion anchors to maintain their integrity.

The Windows

Eventually, all of the nearly 8,000 windows in the Pentagon will be replaced with fixed double-pane glass mirroring the original architecture but offering improved thermal and ultraviolet filtering properties. However, the new exterior outermost E-Ring windows facing the perimeter roadways and the innermost A-Ring windows (at the courtyard center of the complex), being the most vulnerable, will be blast resistant. The new windows are an insulated, laminated, fully-tempered assembly that is designed to absorb and resist the blast loads without shattering into small projectiles or leaving the frame as a single unit. This design meets the client criteria for translucency and energy efficiency, as well as for safety in a blast event.

The Walls

Another HSMM design consideration was the projectile potential of the brick infill walls in the event of a terrorist bomb. The solution incorporated a system developed by Protective Design Center to mitigate this concern. The Protective Design Center system employs an extremely tough mesh geotextile material, normally used to stabilize highway embankments, to arrest wall debris loosed by a blast. For the proposed solution, the fabric ends are wrapped around steel plates, which are then bolted to the sill tube and to the support plate at the floor slab below the window. The fabric is also installed between the vertical tubes and the existing concrete columns with the wrapped plates bolted to the support plates at the ceiling and the floor. Masonry Arts, Inc., was the contractor for this portion of the work, and likewise offered a number of practical solutions when circumstances varied from the design. These renderings show the fabric as loosely woven to allow the viewer to see the wall beyond. In reality, however, the material is woven much more tightly. This taut screen deflects to absorb missile energy if brick wall masonry is loosed in a blast, allowing the masonry material to fall harmlessly to the building floor.


The complete blast window and wall reinforcing system, when fully in place along all five of the Pentagon's sides, will significantly diminish the Defense Department headquarters' vulnerability to blast damage from a terrorist attack. The design solution is constructible, does not have significant impact on tenant space and does not affect the building's historical appearance. The design is flexible in its adaptability to the varying installation conditions and in its systemic connectivity and provides effective structural response to projected scenarios. The design solution met all Renovation Office and User criteria, did so in a cost-effective and straightforward manner and offered a viable solution to a difficult situation. It is a design, however, that all involved earnestly hope is never tested.

Michael N. Biscotte is a Vice President and registered engineer, and Keith Almoney is a staff structural engineer with Hayes, Seay, Mattern & Mattern, Inc., at its Roanoke, VA, headquarters. Either author may be contacted through the company's website at www.hsmm.com.

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