Structural Concrete - Hassoun

11.17 Parme Load Contour Method 403
11.16.2 Bresler Load Contour Method
In this method, the failure surface shown in Fig. 11.26 is cut at a constant value of P n ,givingthe
related values of M nx and M ny . The general nondimension expression for the load contour method is
( ) α1
( ) α2
Mnx Mny
+ = 1.0 (11.35)
M 0x M 0y
Bresler indicated that the exponent α can have the same value in both terms of this expression
(α 1 = α 2 ). Furthermore, he indicated that the value of α varies between 1.15 and 1.55 and can be
assumed to be 1.5 for rectangular sections. For square sections, α varies between 1.5 and 2.0, and an
average value of α = 1.75 may be used for practical designs. When the reinforcement is uniformly
distributed around the four faces in square columns, α may be assumed to be 1.5:
( ) 1.5 ( ) 1.5 Mnx Mny
+ = 1.0 (11.36)
M 0x M 0y
The British Code assumed α = 1.0, 1.33, 1.67, and 2.0 when the ratio P u ∕1.1P u0
is equal to
0.2, 0.4, 0.6, and ≥ 0.8, respectively.
11.17 PARME LOAD CONTOUR METHOD
The load contour approach, proposed by the PCA, is an extension of the method developed by
Bresler. In this approach, which is also called the Parme method [11], a point B on the load contour
(of a horizontal plane at a constant P n shown in Fig. 11.28) is defined such that the biaxial moment
capacities M nx and M ny are in the same ratio as the uniaxial moment capacities M 0x and M 0y ;thatis,
M nx
M ny
= M 0x
M 0y
or
M nx
M 0x
= M ny
M 0y
= β
A
B
M ny /M 0y
M nx /M 0x
Nondimensional load contour at constant P n
(straight-line approxima-
Figure 11.28
tion).