PERMISSIBLE FLEXURAL STRESSES IN TENDONS & CONCRETE ACCORDING TO ACI CODE PRESENTED BY TATHEER AKHLAQ 2006-MSC-CIVIL-STRU-08
Most specifications for prestress concrete construction impose certain limitations on stresses in the concrete and steel at particular stages
While tensioning the steel, immediately after transfer of prestress force to the concrete.
At full service load.
These stress limits are intended to avoid damage to the member during construction, and to insure servicibilty by indirectly limiting crack width and deflection.
The resulting design must be checked for strength to ensure adequate safety agianst failure.
Often deflections must also be calculated explicitily at particular load stages of importance.
PERMISSIBLE STRESS IN CONCRETE
Permissible Stresses In Concrete
Concrete stress limits imposed by the ACI are summarized in the tables given below.
F ci ’ = compressive strength of the concrete at the time of initial pretress
F c ’ = specified compressive strength of the concrete
Stage 1: Permissible Stresses After Transfer Of Prestress Force
The allowable stresses of Table 1 apply immediately after
Transfer of prestress force to the concrete
Loss due to friction
Elastic shortening of the concrete have been deducted
But before time-dependent losses due to shrinkage, creep and relaxation are taken into account.
Table 1 Permissible Stresses After Transfer Of Prestress Force
Stage 2: Service Load Stress Limit
The service load stress limits apply after all losses have occurred and when the full service load acts.
Table 2: Service Load Stress Limit cover requirements are met. with requirements stated elsewhere in the code, and where special realtionships shows that immediate and long time deflections comply on transformed cracked sections and on bilinear moment deflection of members ( except two way slab systems ) where analysis based 12√Fci' c. Extreme fibre stress in tension in precompressed tensile zone 6√Fci' b. Extreme fibre stress in tension in precompressed tensile zone 0.45 F ci ' a. Extreme fibre stress in compression shall not exceed the following 2. Stresses at service loads (after allowance for all prestress losses)
Stage 3: Escape Clause
The “ escape clause” permits higher stresses to be used when tests or analysis indicate that satisfactory performance can be expected.
The permissible stresses of table 1 and 2 may be exceeded if shown by test or analysis that performance will not be impaired.
PERMISSIBLE STRESSES IN STEEL
Permissible Stresses In Steel
The permissible stresses in steel are given in table 3.
F pu = Ultimate strength of the steel
F py = Specified yield strength of steel
Table 3: Permissible Stresses In Steel
The code commentry warns that designers should be concerned with setting a limit on final stress when structure is subjected to corrosive condition and repeated load.