SPECIAL CONSIDERATION IN DESIGN OF SLENDER COLUMNS USING MOMENT MAGNIFIER METHOD IN SWAY FRAMES
Civil Engineering Department, Cyprus International University, Lefcoşa, Cyprus
Corresponding Author e-mail : Ahmad_younis_2012@yahoo.com
Abstract: The moment magnification method for design of slender reinforced concrete columns (RC) is permitted by the current ACI code and many other codes. This paper presents the design for slender reinforced concrete column in sway frames based on Moment Magnifier Method (MMM) in which the member’s stiffness EI calculated immediately before failure used. Rarely, secondary stresses become significant when the column height is longer than typical story height and/or the column section is small…show more content… At the time, the new provisions allowed for the use of structural analysis methods made available with the advent of computer analysis software to assess P-∆ effects in slender columns. Instead of a computer analysis, the 1971 code for approximating slenderness effects introduced the moment magnification procedure. The moment magnification procedure w replaced the reduction factor method included in the previous 1963 version of ACI 318. In ACI 318-63, slenderness effects were considered by dividing the axial load and moments by a reduction factor less than one. The reduction factor method was based on the recommendations of Broms and Viest (1961). The researchers studied the inelastic buckling behavior of reinforced concrete and analytically derived the factor accounting for it. The moment magnification method was based on a similar procedure used by the American Institute of Steel Construction (AISC) for approximating slenderness effects. Overall, the basic premises introduced in the 1971 code are present in the 2011 code with slight modifications based on recent…show more content… During that time, there were more than 10 research papers being published in academic journals in North America and a formal design method, named as Moment Magnifier Method (MMM), was introduced into the ACI building code (1971). Today, this Moment Magnifier Method (MMM) is still being used in the latest ACI building code (1999).2
In the analysis and design of short columns, it was assumed that buckling, elastic shortening, and secondary moment caused by lateral deflection had small effect on the ultimate strength of the column: therefore, these factors were not considered in the design procedure. However, when the column is long, these factors must be considered.
Slenderness effects are more pronounced in columns that exist within the frames that do not have adequate bracing against lateral loads (unbraced frames or sway frames). Unbraced frames show excessive sway which jeopardizes the stability of columns while Adequate bracing in frames helps to stabilize secondary deformations at column ends and produces more stable columns. Because of the different behavior between braced and unbraced frame, columns are treated in different way depending on the bracing conditions of their