Civil Engineering Reference
In-Depth Information
0.003
c
d
d
-
c
3 #9 bars
(3.00 in.
2
)
21
"
d
-
c
ε
(0.003)
t
=
c
14
"
Figure 3.4
3.6
BALANCED SECTIONS, TENSION-CONTROLLED SECTIONS,
AND COMPRESSION-CONTROLLED OR BRITTLE SECTIONS
A beam that has a
balanced steel ratio
is one for which the tensile steel will theoretically
yield at the time the extreme compression concrete fibers that attain a strain equal to
0.003. Should a flexural member be so designed that it has a balanced steel ratio or be a
member whose compression side controls (that is, if its compression strain reaches 0.003
before the steel yields), the member can suddenly fail without warning. As the load on
such a member is increased, its deflections will usually not be particularly noticeable,
even though the concrete is highly stressed in compression and failure will probably occur
without warning to users of the structure. These members are
compression controlled
and
are referred to as
brittle members
. Obviously, such members must be avoided.
The Code, in Section 10.3.4, states that members whose computed tensile strains are
equal to or greater than 0.0050 at the same time the concrete strain is 0.003 are to be re-
ferred to as
tension-controlled sections
. For such members the steel will yield before the
compression side crushes and deflections will be large, giving users warning of impending
failure. Furthermore, members with
t
0.005 are considered to be fully ductile. The ACI
t
to apply to all types of steel permitted by the Code, whether
regular or prestressed. The Code further states that members that have net steel strains or
chose the 0.005 value for
t
values between 0.002 and 0.005 are in a transition range between compression-controlled
and tension-controlled sections.
3.7
STRENGTH REDUCTION OR
FACTORS
Strength reduction factors are used to take into account the uncertainties of material strengths,
inaccuracies in the design equations, approximations in analysis, possible variations in di-
mensions of the concrete sections and placement of reinforcement, the importance of mem-
bers in the structures of which they are part, and so on. The Code (9.3) prescribes
values or
strength reduction factors for most situations. Among these values are the following:
0.90 for tension-controlled beams and slabs
0.75 for shear and torsion in beams
0.65 or 0.70 for columns
