F. K. Kong MA, MSc, PhD, CEng, FICE, FIStructE, R. H. Evans CBE, DSc, D ès Sc, DTech, PhD, CEng, FICE, FIMechE, FIStructE (auth.)-Reinforced and Prestressed Concrete-Springer US (1987)

342 Prestressed concrete simple beams
_ _ 250 X 10 6 + 34.5 X 10 6 _ 2
max. perm. h - 16.5 2 1. 7 x 106 - 3.39 N/mm
Substituting into eqns (9.2-14) and (9.2-15) (or eqns 9.2-2 and
9.2-3),
Pernax = 1243 kN
es = 122 mm
Example 9.2-3
Equation (9.2-16) shows that the minimum required effective prestressing
force is independent of the dead load. Explain how you could have arrived
at this conclusion by common-sense reasoning.
Tension
ComP.ression
0
Fig. 9.2-3 Stress distributions when Pemio is used
SOLUTION
Figure 9.2-3 shows the stress distributions when the effective prestressing
force has the minimum required value Pernio·
Line EGF represents the condition (Pernio + Mirnax + Md)
Line HGJ represents the condition (Pernin + Mirnin + Md)
The limiting stress conditions, namely that points F and H fall on the farnin
stress ordinate, are completely defined by the (Pernin + M;rnax + Md) line
and the (Pernin + M;rnin + Md) line. For specified values of M;rnax and
M;rn;n, these two lines are fixed relative to the (Pernin + Md) line.
Irrespective of the value of the dead load moment Md, the (Pernin + Md)
line can be held in a prescribed absolute position by rotating the Pernin line,
i.e. by changing the angle p in the figure. Since changing P necessitates
changing only the tendon eccentricity, it means that the prestressing force
P ernin cannot be affected by the dead load.
The permissible tendon zone
In practice the designer usually tries to keep the prestressing force constant
along the beam. If the effective prestressing force has the value Pemin as
calculated from eqn (9.2-16) for the critical section, then at this section the