Historic Bronze Window And Frame BlastRetrofit: Concept Development, Analysis, DesignAnd Proof Test

As part of an ongoing renovation of an historic building in the US, a number of windows will be upgraded to improve their blast resistance and to reduce the number of casualties in the event of a terrorist bomb attack. The building owner’s desire was to retain both the architectural integrity of the exterior facade and the operability of the windows. Applied Research Associates (ARA) undertook this project in which the capabilities of the current frame and anchorage system were analyzed, identified shortcomings were addressed through retrofits to the frame and anchorage, and the whole system was optimized in terms of glass thickness, frame retrofit and improved anchorage. A proof test of the retrofitted system was performed on September of 2002. Essentially no debris hazard was created on the protected side of the window. The load (7.7 Psi, 68.4 psi-ms) was approximately 5x the original capacity of the window and frame (based on applied impulse). 1 Existing window configuration The current window system is composed of a frame with two operable sashes and two fixed transom windows, as shown in Figure 1. The sashes consist of bronze tubular members that are brazed/welded at the joints. The alloy and strength of the bronze is unknown. Each sash connects to the frame with three hinges. The hinge material is probably bronze and its mechanical properties are unknown. The existing glass is annealed, 0.2 12” thick and held in place with one removable and one fixed stop. The frame is composed of bronze extrusions with open cross-sections. A different cross-section is used for the side frame member in the transom region. The same cross-section is used for the top frame member. The connections at the joints are formed by either pegged tusk tenons or brazing/welding. The sides of the window frame are attached to the wall at four locations. A bent steel plate (or clip) is connected with screws to the frame and with a bolt to masonry anchors in the double wythe brick wall behind the marble. Also, the three lower clips are directly connected to the hinges that join the sash and frame. One leg of the bronze extrusion that forms the bottom edge of the frame is inserted and is caulked into a slot that extends the full length of the marble sill. Structures Under Shock and Impact VIII, N. Jones & C. A. Brebbia (Editors) © 2004 WIT Press, www.witpress.com, ISBN 1-85312-706-X At the top of the frame, a single clip at midspan connects the top frame member to a grouted anchor in the terra cotta tile beam that spans the window opening. Figure 1. Supreme Court Window Figure 1. Historic building Window Figure 1: Existing historic window configuration 2 Performance goals for the historic windows The goal was to provide as large a level of blast protection as possible, without resorting to a completely new window system. The constraints dictated that: 1) the sashes must remain operable; 2) the existing hardware should be utilized as much as possible; and 3) any changes/additions should be visually inconspicuous.