Non-linear Analysis of Circular Composite Columns

Composite columns provide many advantages for the application in multi-story and high-rise
buildings. However, EN 1994-1-1 does not provide rules for the design of circular concrete sections with fully encased steel profiles according to the simplified methods. Normally, the simplified
method for the design of composite columns is based on a normal force-moment interaction diagram which assumes plastic cross-section resistance. However, the moment resistance is to be reduced by a reduction factor M, considering among other the difference between strain-limited
and plastic moment resistance. This reduction factor is developed and calibrated for composite
compression members with concrete encased sections, partially encased sections and concrete
filled rectangular and circular tubes. Comparison of plastic moment resistance to results based on
strain limitation show that the application for circular section with encased steel section but without a steel tube may lead to unsafe results.
The paper discusses different approaches for the determination of the N-M Interaction curve and
the design on cross-section level. The results obtained from these methods are then compared to
each other for different dimensions of cross-sections of circular columns; and for different ratios
of reinforcements, various steel grades and for normal and high strength concretes. Furthermore,
the investigation focuses on the validation of the reduction factor alfa_M and shows for which section
types the values according to Eurocode 4 cannot be applied.