Civil Engineering Reference
In-Depth Information
Figure 7.23
Torque-twist curve for series C
the web reinforcement of the
spandrel
bea
m design
ed by the ACI limit design method using
a torsional stress
33
f
c
(MPa) (4
f
c
(psi)) was just enough to provide the required
ductility, and that the most economical design had been achieved.
Specimen C1 behaved as a specimen with excessive reinforcement web reinforcement in
the spandrel beam. The process of moment redistribution and the mode of failure were similar
to those of specimen C2. However, the development of torsional plastic hinges in the spandrel
beam was accompanied by a continuous increase of torque, and some of the web reinforcement
in the spandrel beam did not yield at failure. Specimen C1 behaved satisfactorily
from the
view
point o
f the limit design concept, but the design torsional stress
τ
n
of 0
.
50
f
c
(MPa)
τ
n
of 0
.
(6
f
c
(psi)) is obviously higher than necessary.
In contrast, specimens C3 and C4, which were designed by neglecting the interaction of
torsion and shear, behaved as specimens with inadequate web reinforcement in the spandrel
beam. Instead of a flexural failure in the floor beam, collapse of these specimens was caused by
a premature shear-torsion failure of the spandrel beam. As shown in Figure 7.23, the torques
reached their maximum at the yielding of stirrups (see circled number 3) in the spandrel
beam and then decreased rapidly. Since the angle of twist of the torsional plastic hinges was
insufficient to allow a redistribution of moment from the spandrel beam to the floor beam, no
yielding was observed in the bottom longitudinal steel of the floor beams, and the design floor
beam load
w
f
of 114 kN (25.6 kips) could not be reached.
In summary, we
can conc
lude
that th
e ACI limit design method of using a design torsional
stress
33
f
c
(MPa) (4
f
c
(psi)) is applicable to high shear regions of a continuous
spandrel beam near the columns. In the regions of high torsion and high shear, the torsional
web reinforcement calculated from the specified torsional stress should be added to the web
τ
n
of 0
.
