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.
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
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. These allowable stresses are based on the specified concrete strength and allowable stress factor.
Minimum P-Jack
CIP RC/PT Girder calculates the minimum P-Jack by adjusting an initial trial value to satisfy the stress checks specified by the user in the Load Combinations For example, 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 fck. This allowable is not adjusted based on the required minimum fck, 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 stress checks specified by the user in the 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, 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.