Protecting Buildings from Bomb Blasts
JACOBS SCHOOL STRUCTURAL ENGINEERS originally developed composite overlays
to protect buildings efficiently from earthquakes. The material is about
as thin as a cotton shirt, and is made up of carbon threads woven to increase
the strength and flexibility of a structure. This prefabricated material
is applied like wallpaper to walls, floors or columns. The composite overlays
allow the building to absorb horizontal forces and prevent key structural
components from cracking and causing the building to fail. Some 2,000
hospitals, parking garages and commercial and residential buildings worldwide
have been retrofitted using the technique.
“We see this as a real opportunity to make a difference to help save
lives,” says Gil Hegemier, Professor of Structural Engineering, who along
with Structural Engineering Professor and Jacobs School Associate Dean
Frieder Seible, is working with industry partners and the government on
research to safeguard buildings against blast loads.
Hegemier says that after the bombing of the Oklahoma City federal building
in 1995, he began thinking about the similarities between earthquakes
and blast loads. “In Oklahoma, three of the five ground floor columns
collapsed, the load on those columns could not be redistributed, and then
the first floor fell in. Finally, the entire building collapsed on itself
in pancake fashion. We’ve seen this same kind of failure over and over
again when we’ve visited sites damaged by major earthquakes.” Karagozian
& Case (K&C) went on to create a computer simulation of the blast
on the building, and predicted that had the building been retrofitted
with composite overlays, it may have withstood the catastrophic collapse.
In 1998, these compelling predictions resulted in a research and development
contract from the Defense Threat Reduction Agency (DTRA) to investigate
measures to retrofit U.S. Embassies and other critical structures worldwide
against blast loads. One of the first tests was a full-scale blast test
on a four-story building at the White Sands Missile Range in New Mexico.
The testbed reinforced concrete building is typical of embassies and of
low-rise office facilities on the East Coast. Without the retrofit technique,
first floor columns in the building were basically destroyed by a blast
from C-4 explosives.
But when the columns were rebuilt and retrofitted with carbon overlays,
the same bomb caused little structural damage.
The UCSD team continues tests at White Sands, but is concentrating on
design equations and methodologies to optimize blast retrofit. They have
created a second testing site at the Kirkland Air Force Base near Albuquerque,
New Mexico where they can confirm their theories by conducting blast tests
on full-scale building components.
Ironically Professors Seible and Hegemier were preparing for a blast test
at Kirkland on the morning of September 11 and the base was closed off
due to the infamous terrorist attack.
“Clearly, our retrofit technology was developed to protect against blast
loads and would not have saved the World Trade Center,” says Seible, who
explains that the collapse of the towers was caused when jet fuel from
the airplane burned at extremely high temperatures and melted the steel
structure. “However, we are developing systems for new construction which
may also offer further protection to highrise buildings.”