PART TWO: Analysis of Graphic Evidence
Bomb Damage Analysis Of Alfred P. Murrah Federal Building
Oklahoma City, Oklahoma
PART TWO
Analysis of Graphic Evidence

Tab 2 is a cross section view of the building looking from the west. The very large header or cross beam is shown at the north edge of the third floor. A large but smaller header is seen at the recessed north edge of the second floor with a brace beam extending out to the large columns in Row A. The front of the whole building is glass.

Tab 3 shows the architectural layout of the first floor of the Murrah Building and the location of the truck bomb with superimposed circles of roughly equal levels of damage potential. The explosive force drops rapidly (initially proportional to one over the distance cubed) as the shock front travels farther and farther away from the truck bomb. After the release wave, the shock front will propagate proportional to one over the distance squared.

The maximum possible yield from 4800 pounds of ammonium nitrate would be obtained if it were in a compressed sphere and detonated from the center. That would produce a 4.4 foot diameter sphere of detonation products at about 500,000 pounds per square inch. By the time the blast wave hits the closest column, the pressure would have fallen off to about 375 pounds per square inch. That would be far below the 3500 pound compressive yield strength of the concrete. Any column or beam failure from the truck bomb would therefore have been from blast wave structural loading and not from any wave of deformation in the concrete.

The basic building structure consists of three rows of columns (35 feet apart) with eleven columns in each row (20 feet apart). The four corner columns have an external clamshelllike structure for air ducts, etc. If we label the column rows A, B. and C from front to back, and number the columns 1 through 11 from left to right, then columns A2, A3, A4, A5, A6, A7, A8, and B3 collapsed, essentially vertically. Tab 2 shows a very large reinforced concrete header at the floor level of the third floor of column row A. Much larger columns extend from the header down for the odd-numbered columns, i.e., A3, A5, A7, and A9. The even- and odd-numbered columns extended from the top of the building down to the header. The foundation of the building is a heavy, reinforced concrete slab with no sub-levels.

From the potential damage contours on Tab 3, and assuming the single truck bomb, the pressure and impulse for collapsed columns B4, B5 and A7 are all in the 25 to 35 pounds per square inch region. However, the much smaller and closer columns, B4 and B5, are still standing, while the much larger column A7 is down. Column B3 is down with 42 percent less pressure and impulse than columns B4. These facts are sufficient reason to know that columns B3 and A7 had demolition charges on them. Moreover, there is not sufficient blast impulse at that range to collapse any of the three. In fact, columns B2, B4 and Bs all have the sheet rock and furring strip finish still intact on the second and third floors except where damaged by falling debris.

The large header across the front of the building at the third floor of Row A was not blown back into the building as one may expect from such a large bomb. The header came straight down but rolled backward 90 degrees because the columns above the header rested off center toward the back.

...next PART THREE: Analysis of Photographic Evidence OKC Bomb Report Index Page

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