Civil Engineering Reference
In-Depth Information
z
t
= top modulus
= 1.274 m
3
= 0.717 m
3
z
b
= bottom modulus
η
= effi ciency of section
= 0.589
where:
z
t
=
I
/
y
t
m
3
z
b
=
I
/
y
b
m
3
a
t
=
η
×
y
t
m
a
b
=
η
×
y
b
m
η
=
I
/
Ay
t
y
b
= proportion of depth of section occupied by the central kern.
5.4 Central kern and section effi ciency
The inner zone of any cross section is called the central kern, which is a very important
concept in prestress design, Figure 5.2 (a). A compressive force applied within the kern
will cause only compressive stresses on the cross section. When the force is applied
at the limit of the kern, the stress on the remote fi bre will be zero, Figure 5.2 (b).
A compressive force applied outside the kern will cause tensile stresses as well as
compressive stresses, Figure 5.2 (c). The upper and lower limits of the central kern are
called
a
t
and
a
b
and are equal to
η
×
y
t
and
η
×
y
b
respectively.
Figure 5.2
The central kern
