Design
CIP RC/PT Girder calculates the minimum required tendon jacking force, P-jack, and minimum required concrete strength (f'c and f'ci) to satisfy allowable stresses. These calculations are based on compression condition in Initial case, and tension and compression allowable stresses in Final case. Note that they do not include the tension condition in Initial case. The tension for the Initial case is usually controlled by maximum post-tensioning, and thus if the minimum P-jack causes a tensile overstress in Initial case, the remedy is to revise the cross section.
The required P-jack, f'c and f'ci are reported as a coordinated group, such that they should satisfy the stress requirements together. However, they are approximate values and an analysis with these values (or actual design values) is required to confirm their validity. Also, note that the actual stress calculations and capacity calculations are not based on these minimum requirements, but based on the input values.
Design Parameters
CIP RC/PT Girder uses the code-specified allowable stress and strength (phi) factors. However, the users may change these factors as desired. The design of minimum P-jack and concrete strength are based on the allowable stresses and user-specified concrete strength.
Allowable Stresses
CIP RC/PT Girder uses allowable stresses in performing design calculations. The tensile allowable stresses are used in design of minimum P-jack. These allowable stresses are based on the specified concrete strength and allowable stress factor. Tensile allowable stresses are calculated for SLD-1 (LRFD Service-3) and SLD1 with only prestress and dead loads, called PS+Perm (LRFD Service-1, PS+Perm).
Minimum P-Jack
CIP RC/PT Girder calculates the minimum P-Jack by adjusting an initial trial value to satisfy the tensile conditions for Final cases. Therefore, in order for this design to be performed, a service load combination in Final or Final w/o Sway case is necessary. The required P-jack is calculated for each POI. If the allowable stress at a POI is fa, the stress from all loads within a service load combination is fdl, and the stress from all tendons that pass through that POI is fp, then the required PT stress is calculated as fp such that fp + fdl = fa. Then, all tendons trial jacking forces (input values) are adjusted by the ratio of fp/fp to obtain the minimum required P-jack at that POI. Note that this calculation is performed for all top and bottom stresses, provided that the dead load stress is larger than the allowable tensile stress. The controlling P-jack for each tendon is the largest of all these values. Note that the allowable concrete tensile strength is calculated based on the input value of f′c. This allowable is not adjusted based on the required minimum f′c, and thus the value of P-jack obtained at this time is an approximation and a final analysis is needed to check the validity of this minimum required P-jack value.
Final Concrete Strength, f'c
The required concrete strength, f'c, is calculated to satisfy the compression conditions for the Final and Final w/o Sway cases for all service load combinations. In this case, the effect of the tendon stresses are adjusted by the same ratio of fp/fp and the combined stress is revised accordingly. Therefore, based on the allowable stress factor (say 0.6, for allowable compression of 0.6f'c), the f'c is calculated such that the revised combined stress is equal to the allowable. Note that this calculation is performed at all top and bottom flange locations provided that the revised combined stress is compressive.