TECHNICAL LIBRARY

 

VIBROSCREEN Unit Helps Investigation at World Trade Center

To deter more bombings such as the February 26, 1993, blast at the 110-story tall World Trade Center, New York, NY, many bomb technicians and special investigators sifted through hundreds of tons of debris around the clock. The specialists with the FBI, Bureau of Alcohol, Tobacco and Firearms, and the New York City Police Bomb Squad carefully and painstakingly looked for evidence that led to the fatal blast which resulted in six deaths and injuries to more than 1000, as well as millions of dollars of property and business damage.

For every dump truck load of debris hauled from the blast site, about a bucketful of twisted metal fragments, wires, vehicle components and other suspicious items were tagged and set aside for further study. To speed the search for evidence at one of the three debris sifting sites located in the underground parking lot area of the World Trade Center, the investigators placed shovelfuls of the rubble onto the top screen of a Kason Circular Screen Separator.

Fitted with three screens and a gyratory motor, this 24 in. (609.6 mm) diameter unit vibrates at 1200 revolutions per minute, causing the debris materials to classify and separate by size. One of the investigators looked at the oversize materials that remained on the top screen-mainly pieces of broken concrete and refuse from the hotel. This material was larger than the 1 in. (25.4 mm) clear openings of the screen. The coarsest material from the pile of debris and from the top screen of the VIBROSCREEN was either tagged or disposed of quickly.

The more finely sized material dropped through this screen to a second 2 mesh screen with 0.437 in. ( II.I mm) openings. Again, the horizontal and vertical motion imparted by the 1/3 hp (.248 kW) 115 Volt gyrator causes the oversize material to exit through the discharge spout and the finer material dropped through the screen onto a third 6 mesh screen with 0.1318 in. (3.3 mm) openings. Material passing through this screen dropped onto a ramp which allows the fine material to reach a fourth discharge spout. Flexible connectors from the discharge spouts carried the classified debris to pails. The finest debris measuring less than 1/8 inch (3.175 mm) in size is disposed of without inspection.

The specialists took the pails of debris ranging in size from 1/8 inch (3.175 mm) to 1/4 inch (6.35 mm) and dumped them onto square screens of various mesh sizes mounted on legs for examination under powerful lights. Only after methodically viewing all the debris on the screens by leveling it to single particle bed depth and visually looking at each piece, was it disposed of properly. The specialists also used magnets to draw up metal fragments from the debris on the square screens.

The use of reliable Vibroscreen separator allowed the investigators to proceed more quickly, according to FBI Special Agent Gerry Fomino. Teams of bomb technicians and investigators worked around the clock, seven days a week. Investigators at the sifting site said that the Vibroscreen unit worked well for them. Kason sold the Vibroscreen to the FBI in 1988 precisely for this type of work and since its base and motor weigh less than 200 Ibs (91 kg), it can be easily transported in a van or pick-up truck to a blast site.

Kason also supplied a larger screen for work at the World Trade Center because the company was willing to provide separators on short notice -- an important service in aiding the investigation as well as returning the World Trade Center to operation. These circular screen separators have proven helpful for bomb technicians and investigators who must sift through debris for evidence in other countries as well as in the U.S.

Screen Tips Volume 8, Number 2