LPA News Use of Concrete Filled Tube (CFT) Vertical Braces in a High Seismic Area
J.W. Oleksik, Structural Department Supervisor, Louis Perry & Associates, Wadsworth, OH 44281
T.M. Gilbert, Structural Department Manager, Louis Perry & Associates, Wadsworth, OH 44281
S.R. Malushte, Bechtel Fellow and Sr. Principal Engineer, Bechtel Power Corporation, Frederick, MD 21703
Abstract:
Concrete filled tube (CFT) vertical braces were used for an industrial SCBF structure to better balance the requirements of braces design controlled by compression loads and connection design controlled by expected brace tensile strength.
The structure is at a Coal Fired Power Plant and is designed as a special concentrically braced frame, non-building structure in Seismic Design Category D. AISC Seismic Provisions required that the vertical brace connections be designed for full tension yield of the brace. As a result, it was desired to minimize the quantity of field bolting. The CFT’s enabled the brace design to maintain a ratio of tension to compression capacity of close to 1:1 (similar designs with steel braces had a ratio as high as 4:1), thus keeping the tension area, and steel weight, to a minimum. The analysis considered that the concrete would add to the brace’s compressive strength but would not contribute toward its tensile strength.
The CFT braces were shop fabricated from plate material with headed studs attached to the inside surface prior fabrication of the tube section. Prior to erection, the CFT’s were filled with self consolidating concrete on site.
The advantage of using this type of vertical brace is to reduce the steel tonnage, the brace tension area, the connection size and amount of field bolting.
