The Engineering of Blast Protection
Robert L. Hershey
About the Lecture
The last eight years have seen increasing interest in blast protection. The result has often been an overreaction, such as the unnecessary closing of Pennsylvania Avenue in front of the White House. However, rational engineering design can produce a better situation, providing adequate protection while preserving American freedom of movement. Often designs involve laminated tempered glass (bulletproof glass). There is a substantial record of laminated tempered glass testing in blast experiments. This lecture will address design considerations based on test data.
About the Speaker
Robert L. Hershey is a consulting engineer. He has developed techniques used to analyze structural effects of sonic boom, wind, and blast, the subject of his doctoral dissertation. He has held engineering and management positions at Booz Allen & Hamilton, Science Management Corporation, BBN, and Bell Labs. He received his B.S. summa cum laude from Tufts University in 1963, his M.S. from MIT in 1964, and his Ph.D. from Catholic University of America in 1973. He is the author of two books, “How to Think with Numbers” and “All the Math You Need to Get Rich: Thinking with Numbers for Financial Success.” He is currently the president of the District of Columbia Society of Professional Engineers.
President Collins called the 2150th meeting of the Philosophical Society of Washington to order at 8:25 p.m. on October 11, 2002. The Recording Secretary read the minutes of the 2149th meeting and they were approved.
The speaker for the 2150th meeting was Robert L. Hershey, Professional Engineer, consultant and author. Mr. Hershey is currently the president of the District of Columbia Society of Professional Engineers. The title of his presentation was, “The Engineering of Blast Protection.”
The last eight years have seen increasing interest in blast protection. The result has often been an overreaction, such as the unnecessary closing of Pennsylvania Avenue in front of the White House. However, rational engineering design can produce a better situation, providing adequate protection while preserving freedom of movement in our Nation's capital.
For almost a decade, Pennsylvania Avenue has been closed to vehicular traffic. America's main street was closed right after the 1995 Oklahoma City bombing ostensibly to protect the White House from a car borne bomb blast. The Oklahoma City bombing was the reference horror story and was presented as the technical basis for closing it. However, the speaker pointed out that straightforward engineering analyses do not bear this out.
Mr. Hershey described differences between possible White House blast configurations and the Oklahoma City blast configuration that show the White House is in little danger from a Pennsylvania Avenue car borne blast. First, he pointed out that the well-known exterior of the White House is only a façade when compared to the 660 tons of concrete and steel that were put into the White House during its 1950 reconstruction.
He also pointed out that blast pressure will vary inversely as the square of the distance from the blast. Here, the White House is at least 350 feet from Pennsylvania Avenue whereas the Oklahoma City Blast was only 10 feet away from the targeted Murrah Federal Building. This alone reduces the force of any possible blast on the White House by more than 3 orders of magnitude from the Oklahoma City blast.
Mr. Hershey then pointed out that construction differences between the White House and the referenced Murrah Building show the White House damage from a car borne bomb would be minimal at worst. Here, the building stress produced by an external blast is inversely proportional to the square of the thickness of the walls and windows.
S = P a2 b2 / 2h2(a2 + b2)
S = wall (or window) stress
P = blast pressure
a = wall (or window) length
b = wall (or window) height
h = wall (or window) thickness
The Murrah Building was essentially a large profile, continuous curtain of glass windows ¼ inch thick. Whereas, the White House walls are much shorter, approximately one foot thick, steel reinforced concrete with relatively small, widely spaced windows. Assuming these windows are laminated tempered glass, they could easily withstand a car bomb blast without breaking.
The speaker then advised that the government's own reports showed closing Pennsylvania Avenue required rerouting 26,000 cars per day. This could cost almost $50 million per year due to lost time alone–all with essentially no enhanced safety or protection for the nation's White House. One would think our city fathers might consider restoring Pennsylvania Avenue and relieving some of the downtown traffic congestion and confusion. However, the National Capitol Planning Commission is currently asking for $6.1 million to break up part of Pennsylvania Avenue and put in gravel to make this ill-advised street closing more permanent.
Mr. Hershey then closed his presentation and kindly answered questions from the floor. President Collins thereupon thanked Mr. Hershey for the Society, announced the next meeting and made the usual parking announcement. He then adjourned the 2150th meeting to the Social Hour at 9:28 p.m.