Civil Engineering Reference
In-Depth Information
Figure 8.7
Representation of an element of unit length of a cracked member by a model
composed of uncracked and fully cracked parts such that the extension or
curvature is the same as in the actual member.
8.6.1 Note on crack width calculation
The value of
ε
s2
, to be used in the crack width calculation by Equation (8.48),
is the steel strain due to
N
and
M
on a fully cracked section, ignoring the
concrete in tension. Here it is assumed that the stress on concrete is zero prior
to the application of
N
and
M
. If the section is subjected to initial stress due,
for example, to the e
ect of shrinkage occurring prior to the application of
N
and
M
, the forces
N
and
M
should be replaced by
N
2
ff
=
N
−
N
1
and
M
2
=
M
−
M
1
; where
N
1
and
M
1
are two forces just su
cient to eliminate the initial
stress. The values of
N
1
and
M
1
may be calculated by Equations (7.40) and
(7.41), which are used to calculate the decompression forces in partially
prestressed sections.
8.7 Time-dependent deformations of
cracked members
Partially prestressed members are often designed in such a way that cracking
does not occur under the e
ect of the dead load. Thus, cracking due to the
live load is of a transient nature; hence the e
ff
ff
ects of creep, shrinkage
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