Civil Engineering Reference
In-Depth Information
The central kern is the middle third (
η
= 0.33) for a rectangle and the middle quarter
(
varies from approximately
0.3 for a solid slab with a profi le that follows the road crossfall, Figure 5.3 (a), to
0.45 - 0.5 for a typical twin rib deck, Figure 5.3 (b), to 0.65 for a well-proportioned
box section with thin webs, Figure 5.3 (c).
With experience, it is possible to guess the approximate value of
η
= 0.25) for a solid circle. For concrete bridge decks,
η
for a proposed
bridge deck. If one then calculates the cross-section area
A
and guesses the values of
y
t
and
y
b
as suitable proportions of the depth, using
I
=
A
η
y
t
y
b
, it is possible to come to
a quick fi rst approximation of the second moment of area for a bridge deck, which is
very helpful in the initial sizing calculations.
For instance, for the box section shown in Figure 5.3 (c), one may calculate the
cross-section area as 5.88 m
2
and then guess that the neutral axis was at 0.9 m from
the top fi bre and that the effi ciency was 0.6. Then
I
= 5.88 × 0.9 × 1.3 × 0.6 =
4.13 m
4
. The correct, calculated values for
y
t
,
y
b
and
η
are 0.791 m, 1.409 m and
0.636 respectively, and
I
= 4.171 m
4
. The approximate calculation was reasonably
close.
η
Figure 5.3
Effi ciency of typical deck types
