Simplified Design for Columns - Simplified Design of Reinforced Concrete Buildings - page 163

5.5.3 Biaxial Bending of Columns

Biaxial bending of a column occurs when the loading causes bending simultaneously about both principal axes.
This problem is often encountered in the design of corner columns.

A general biaxial interaction surface is depicted in Fig. 5-10. To avoid the numerous mathematical
complexities associated with the exact surface, several approximate techniques have been developed that relate
the response of a column in biaxial bending to its uniaxial resistance about each principal axis (Reference 5.5
summarizes a number of these approximate methods). A conservative estimate of the nominal axial load
strength can be obtained from the following (see ACI R10.3.6 and Fig. 5-11):

P

P o

M b , P b

M nx

Figure 5-10 Biaxial Interaction Surface

y

e x

P u

e y

x

M ux = P u e y
M uy = P u e x

y

Reinforcing bars not shown

Figure 5-11 Notation for Biaxial Loading