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)

Stresses at transfer 347
(9.3-6)
where, as in eqn (9.2-1), Pis related to the effective prestressing force Pe
by the loss ratio
Table 9.3-l Allowable compressive
stresses at transfer (BS 8110: Clause 4.3.5)
(9.3-7)
Nature of stress distribution
famaxt
Extreme fibre 0.5fc; 8
Uniform or near uniform 0.4fci
• /c; = cube strength at transfer.
In practice, the stress criteria in eqns (9.3-1) and (9.3-3) are almost
invariably met; but eqns (9.3-2) and (9.3-4) must be checked. BS 8110
limits famaxt to the values in Table 9 .3-1. famint for Class 1 members should
not be less than -1 N/mm 2 (i.e. tension not to exceed 1 N/mm 2 ); famint
values for Class 2 members are as in Table 9.2-2, where feu is now to be
interpreted as the cube strength lei at transfer.. As explained in note (c)
following Table 9.2-2, designers usually do not set famin for the service
conditions to the limiting stresses in that table; however, stresses at
transfer are of a temporary nature, and designers are more willing to set
!amint to the limiting values.
Example 9.3-1
Given that for the Class 2 post-tensioned beam in Example 9.2-2, the
prestress loss ratio a is 0.8 and the cube strength fei at transfer is 40 N/mm 2 ,
determine the permissible tendon zone for an effective prestressing force
Pc of 837 kN, considering both the service stresses and the stresses at
transfer. Allowable stresses are as in BS 8110.
SOLUTION
(For the service condition, the permissible tendon zone has been worked
out in Example 9.2-4 and plotted in Fig. 9.2-4.) At transfer, P =Pel a=
837/0.8 = 1046 kN. Consider midspan section: from eqns (9.3-2) and
(9.3-4),
f <., Md
1t - Jamaxt + Zt
:5 20 (Table 9 3-1) + 34·5 X 106 < 21 82 N/ 2
· 19.0 x 106 - · mm
ht 2: !amint -
Md
z2
2: -2.3 (Table 9.2-2) - ~i:~ ~ !~: 2: -3.89 N/mm 2