Civil Engineering Reference
In-Depth Information
Figure 2.15
Nondimensional interaction surface for axial load, shear, and bending
The ACI code uses some notations that are quite different from the symbols used in this
topic. First, the angle that defines the direction of cracking is quite different. In the ACI Code,
the angle
is defined as the angle between the direction of concrete struts (i.e.
d
axis) and the
direction of longitudinal steel bars (i.e.
θ
α
r
is defined
as the angle between the direction of cracking (i.e.
r
axis) and the direction of longitudinal
steel bars (i.e.
axis). In this topic, however, the angle
90
◦
−
α
r
and cot
α
r
.
Second, the steel areas are defined differently to resist shear and torsion:
A
v
is the total area
of transverse steel bar for shear; and
A
t
is one leg of a hoop steel bar for torsion. However,
the steel yield stresses
f
yt
are for transverse steel bar for shear, as well as hoop steel bars
for torsion. The spacing
s
is for transverse steel only, and no symbol of spacing is given for
longitudinal steel.
axis). Consequently,
θ
=
θ
=
tan
2.3.1 Torsional Steel Design
The ACI methodology for the design of torsional steel is based on the three equilibrium equa-
tions derived in Section 2.1.4.2, specifically, Equations (2.47)
-
(2-49). These three equilibrium
equations for torsion have also been summarized in Table 2.1. Equation (2.47) is used to size
the longitudinal torsional steel. Equation (2.48) is used to design the transverse torsional steel.
Equation (2.49) is used to check the stresses in the concrete struts in order to avoid the concrete
crushing before the steel yielding.
