AGG recently spoke with Dennis A. Young, associate, structural engineer with Leo A. Daly in Omaha, Neb., who has worked extensively on security sensitive projects. 

AGG: What affect has blast/security requirements of the government had on your company’s design for federal and government buildings? 

Young: The government’s blast requirements have required us to spend considerable time and effort on support systems for the windows. When dealing with window frame reactions approaching 30,000 pounds, the support structure needs to be considerably stronger than what we historically have used. Furthermore, column spacings on the exterior walls can be no more than 30 feet. This is mainly due to progressive collapse requirements. However, when the window support system needs to span between building columns, a 30-foot long window support beam becomes rather deep. 

AGG: What has been the impact of blast/security requirements on glazing in new and renovated federal/government buildings? 

Young: It is fairly easy to provide the necessary structure needed to support blast windows in new construction. Usually, a connection is needed at each of the horizontal window mullions. When there is not a column or wall near a horizontal window mullion, a secondary steel support system needs to be provided and hidden in the walls on either side of the mullions.  For renovated buildings, it becomes much more difficult to provide the necessary window support framing. Existing window supports are more than likely inadequate to support the new windows. A new support framing system needs to be inserted into the openings and connections provided at the main building structure for the secondary window supports. Sometimes, this requires the main building structure to be modified and enhanced as well. 

AGG: Is there a difference in the amount and type of glass used? 

Young: The amount of glass used should not change from what was used before, because the window designers can engineer many different sizes and shapes to fit the architectural requirements. Sometimes, however, the window designers have to be cognizant of the amount of structure needed to support the frames and to reduce the window size accordingly. The type of glass used has changed. Before the blast requirements, an insulated window would consist of two panes of 1/8-inch thick glass separated by an air gap. Blast-resistant windows now require two thicker panes with the outside pane being solid and the inner pane being laminated. The panes are still separated by the air gap, which is usually ½ inch. The outer pane is usually ¼-inch thick and so is the inner pane. For most blast resistant applications, this is adequate. For higher blast loads or where the performance condition requires the panes not to fail, thicker windows may be necessary.

Architects' Guide to Glass & Metal
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