Civil Engineering Reference
In-Depth Information
Solving these two equations for
P
and
e
gives the answer that
P
= 22.1 MN and
e
= 0.258 m.
Of course, it would not be economical to more than double the prestress force in
this beam. However, it is a good example of how familiarity with the concepts of centre
of pressure and central kern allow one to control the effects of prestressing. There
are in fact situations where designing the prestress to control dead load defl ection
is economical. One such is for precast car park beams, where any difference in the
defl ection of adjacent beams would cause diffi culty in connecting them.
5.23 The shortening of prestressed members
Typically, prestressed members are compressed at 5 MPa, although this may be as low
as 2 MPa or as high as 10 MPa. A member compressed at 5 MPa will shorten elastically
by approximately 0.15 mm/m and will then continue shortening for, typically, a further
0.25 mm/m due to creep, giving a total shortening of 0.4 mm/m. This is in addition to
shrinkage of the concrete and temperature movements.
It must never be forgotten that prestressed members must be free to shorten for the
prestress to be active. Although this is generally the case for the longitudinal prestress
of bridges that are carried by sliding bearings or fl exible piers, it is not so obvious
Figure 5.21
The structure must be free to shorten
