Clarifying the theoretical basis of the global geometric imperfection analysis (out-of-plumb columns)

Abstract

The concrete structures present several geometric deviations, some of them considered indirectly in the safety factors of the design. However, the imperfections of the element axes, especially columns, should be explicitly considered in the structural analysis because they have great influence over the building stability. The NBR 6118:2014, Brazilian standard code for the design of concrete structures, defines a global geometric inclination to the columns, also called "out-of-plumb columns", that causes additional forces not presented in the "perfect" structure, which must be considered as a permanent action. The code also sets the design equations, as well as the load combination criteria. Although the expressions for the out-of-plumb angle calculation are used in the designs, they are not well understood by the technical community or even really explained in Brazilian publications. This paper discusses the theoretical basis that supports such expressions and compares what is presented in NBR 6118:2014 to other international codes. Structural analysis results are discussed, considering buildings with different heights and loadings, to evaluate how the global out-of-plumb effect seems to be relevant. The main objective is to contribute to a better understanding of the concrete building design principles and, specifically, fill a gap in national technical literature about this global imperfection.