Civil Engineering Reference
In-Depth Information
Figure 7.20
Torsion failure of specimen B5
Therefore, the ultimate design load cannot be reached. Such a design would be unsafe from
the viewpoint of both strength and ductility.
Comparison of the three specimens shows clearly that spe
cimen B
2 ha
s the m
ost desirable
design, assuming a torsional design stress
33
f
c
(MPa) (4
f
c
(psi)). This spec-
imen behaves in a ductile manner, can reach its strength, and can satisfy the serviceability
condition on crack width control. This limit design method is not only very economical, but
also very simple. As a result, it has been adopted in the ACI building Code (ACI 318-77) since
1977.
τ
n
of about 0
.
7.2.5.2 Interaction of Torsion and Shear
Figure 7.21 shows a reinforced concrete space frame, where three floor beams are framing into
each span of a continuous spandrel beam. The T-shaped test specimen described in Section
7.2.5.1 and Figure 7.15(a) is indicated by heavy dotted lines. The spandrel beam in this T-
shaped specimen represents the portion of the spandrel beam near the midspan where the
flexural shear stresses are sm
all. The fl
exur
al shea
r stresses in all three test specimens B1, B2
and B5, are less than 0
166
f
c
(MPa) (2
f
c
(psi)), the contribution of concrete. The critical
sections, however, are near the column faces where large shear stresses
.
v
n
as well as large
torsional stresses
τ
n
occur simultaneously. How should we design the web reinforcement to
resist the combined actions of large
τ
n
? In order to answer this question, we will
now study the portion of the space frame adjacent to a column, i.e. the U-shaped specimen
consisting of the negative moment region of the spandrel beam and the two floor beams, as
shown by the heavy solid lines in Figure 7.21. Such a test specimen with two floor beams,
however, is difficult to manufacture and to test.
v
n
and large
