EVALUATION OF APPROXIMATE DESIGN PROCEDURES FOR BIAXIALLY LOADED RECTANGULAR REINFORCED CONCRETE COLUMNS

Abstract

Many building codes including the Ethiopian
Building Code Standard, EBCS- 2 [l], recommend
different approximate procedures for the design of
biaxially loaded reinforced concrete columns. The
Ethiopian· code and other codes such as thef British
Standard, BS 8110 [2], recommend the design of
biaxially loaded column for uniaxial bending using
an "equivalent" uniaxlal eccentricity of load along
the axis parallel to the larger relative eccentricity.
Another commonly used approximate design
procedure adopted by many codes [2,3] is based on
the use of simplified expressions for the normal load
contour of the failure surface. Such interaction curves
have at least two rigorously determined points
corresponding to the design values of the ultimate
uniaxial moment capacities of the cross-sections
under different levels of normal forces. The
approximation according to the ACI [4,5], can also
be categorized in this group. However it involves the
determination of an additional point on the actual
interaction diagram where the magnitudes of the
moment components related to the respective
uniaxial capacities ar~ equal.
Although or:e or the other approximate procedure is
recommended by the different building code
standards, the extent to which such procedures may
lie on the safe or the unsafe side or relative \Tierits of
the different approaches is lacking in the literature.
The aim of the paper is to evaluate the different
approximate procedures by comparing the results
with the more rigorous solution for biaxially loaded
columns [7, 11 ].
The comparative result of the investigation shows
that the ACI's approach 14• SJ represents the most
accurate approximation for biaxial bending. The
approximation according to the Ethiopian Building
Code Standard, EBCS - .2 gave mostly conservative