Civil Engineering Reference
In-Depth Information
Figure 7.9
Torque-twist curve for example problem 7.1
torsional ductility of this box girder is:
θ
u
θ
y
=
0
.
02406
µ
t
=
01043
=
2
.
31
.
0
.
It can be seen that the torsional ductility of reinforced concrete box sections is quite small.
This is because the warping of the wall elements and the bending of the concrete struts produce
large surface compressive strain (
ε
ds
=−
0.003) at the maximum angle of twist
θ
u
.
7.2 Design for Torsion
7.2.1 Analogy between Torsion and Bending
A prismatic tube member of arbitrary cross-section is subjected to a torsional moment
T
,as
shown in Figure 7.1. In Section 2.1.4.1 and Figure 2.6, Bredt (1896) showed that this external
torsional moment
T
is resisted by an internal moment which is the product of the shear flow
q
in the shear flow zone and twice the lever arm area
A
o
:
T
=
q
(2
A
o
)
(7.66)
A reinforced concrete 2-D element can be modeled by a truss, as shown in Figure 2.2 after
cracking of concrete and yielding of steel. In Section 2.1.2.1, the shear flow
q
is expressed by
