Structural safety of steel-high strength concrete and steel-normal strength concrete columns

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During the past couple of decades, several composite steel-concrete structural systems have been used in the construction of bridges and tall buildings. These systems combine the rigidity and formability of reinforced concrete with the strength and speed of construction associated with structural steel. One such system employs composite columns that consist of steel shapes encased in concrete. The concrete used for encasing a structural steel section not only increases its strength and stiffness, but also protects it from the fire damage. As a result, the use of such columns is on the rise in building construction in addition to applications in bridge and marine structures. The load factors included in the 2005 edition of the National Building Code of Canada (NBCC) and the material and member resistance factors specified in the 2004 edition of the Canadian Code for the Design of Concrete Structures (A23.3) are based on investigations of structural steel and reinforced concrete members and may not be applicable to composite steel-concrete columns in which steel shapes are encased in concrete. The objective of this study is to develop safety criteria (material and member resistance factors) for such composite columns based on accurate, elaborate, and comprehensive strength and load-effect modeling procedures, simulation and optimization techniques, and reliability analyses. The safety criteria so developed will be compatible with the A23.3 design expressions and new NBCC load factors, will be applicable to composite columns constructed with high strength concrete as well as with normal strength concrete, and are expected to be better than those currently included in the 2004 edition of A23.3. These criteria will be offered to the structural engineering profession for design office use and to the Code Committee for possible adoption in a subsequent edition of A23.3. The proposed research is needed for the development of A23.3 because the use of composite columns is on the rise as a consequence of damage suffered by many reinforced concrete columns during recent earthquakes around the world. The structural damage caused by Katrina along the US Gulf Coast is likely to further speed up the use of composite columns.