Cable-Based Retrofit of Steel Building Floors to Prevent Progressive Collapse

[usman]

Cable-Based Retrofit of Steel Building Floors to Prevent Progressive Collapse

by Samuel Tan, Albolhassan Astaneh-Asl

Abstract

The project summarized here was the third phase of a three-phase research and development project. Phases 1 and 2 focused on new buildings while the third phase, discussed in this report, was primarily for the retrofit of existing buildings. The two main objectives of this study were:

1. To establish inherent strength of typical floor framing systems in steel structures to resist progressive collapse in the event of removal of a column by a bomb blast and;
2. To develop and test cable-based mechanisms that can be used to prevent progressive collapse in the event of removal of a façade column.

The test specimen was a single-story steel frame with composite floor slab. The specimen measured approximately 60 ft (20 m) by 20 ft (6 m) by 6.5 ft (2 m) tall. The steel floor beams were supported by ten wide-flange columns. The beam-to-column connections in the test area were typical shear tabs (single plates). Two columns in the middle span of the specimen were designed and constructed as drop columns. These drop columns terminated 36 inches (1 m) above the laboratory floor. During testing, the support of drop columns could be removed and a vertical downward displacement could be imposed upon these drop columns. Subsequently, the floor displaced downward under gravity load and developed tensile forces due to catenary action. This project investigated the capacity of an existing steel frame structure, retrofitted with high strength steel cables, to resist progressive collapse by developing catenary action in its floor framing members.

Three tests were conducted on the specimen. In the first test, the specimen represented a typical steel structure without any mechanism to prevent progressive collapse. The second and third tests represented the structure after two steel cables were placed on the web of the façade beams, one at each corner of web-flange intersection as a retrofit measure. The cables were anchored to the last column on each end of the floor. The columns transferred the tension force of the cable to the floor framing and the floor slab. The cables acting in a catenary action mode added to the strength of the system and prevented progressive collapse of the floor after removal of the middle column. The mechanisms performed well in the tests and proved to be very efficient and economical in preventing progressive collapse of the tested specimen.

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