Retrofit Options for Historic Facades

Building design criteria requires that government buildings be designed for a variety of extreme loads including blast, hurricane, and impact conditions. The main goal of blast-resistant window and wall design is to minimize hazardous debris to prevent human injury and minimize the response of load-bearing members to prevent progressive collapse and human fatalities. New buildings can utilize hardened facade systems designed for extreme loads. However, retrofit design of existing buildings can be more challenging—especially when historic buildings are involved. Retrofit of historic buildings requires that original facades remain as much as possible. Therefore, interior retrofits must be utilized that provide the required thermal and energy efficiency, in addition to resistance to extreme loads.

Retrofit options for windows include anti-shatter films, storm windows with laminated glass, (fabric) blast curtains, and glazing cable or bar catcher systems. Another option is use of a heavy-duty curtain as a facade catcher system. A Cascade Coil Drapery product was designed for use as a catcher retrofit for a curtainwall system. Shock tube tests had been conducted and were used to validate modeling techniques employed to represent material-level and structural-level response modes of the woven coil drapery. After validation, finite element models (using LS-DYNA) were utilized to create resistance functions for different Cascade Coil products in this application. Variables included: coil orientation, coil gauge, coil pitch, connection type, and span. The resistance functions (or load vs deflection curves) were used in the Single-degree-of-freedom Blast Effects Spreadsheet (SBEDS) distributed by the US Army Corps of Engineers Protective Design Center (USACE PDC). The fast running models allowed for quick evaluation of all retrofits subjected to all applicable blast loads for a “mock” entry curtainwall in a representative 1930’s to early 1940’s government building. Connection design to existing structural systems was also considered during the design process. An explanation of the model validation and selected Cascade Coil drapery system for this application is provided.