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, fck

The required concrete strength, fck, 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.

Initial Concrete Strength, fci

The concrete strength at initial condition, fcj_initial, is calculated in a similar fashion to the calculation of fck; however, the combinations considered for this calculation include the service load combinations in the Initial case.

Effective Width of Slab Calculation

For T-beams and slab bridges, the effective compression flange width is calculated as specified in IRC21:305.15 and IRC21:305.16, respectively.

For Slab bridges, the moments and shear reported are for the full width of the bridge. For live load distribution the effective width resisting a wheel load is first calculated. The live load moment per unit width (effective width) is then multiplied by the width of the slab bridge to compute the full-width live moment for design purposes. The same effective width is used to compute all live load effects (Max/Min Moments and Shears) for the full- width.

Five possible combinations of vehicles are considered as follows:

Class A + Class 70R (Default and the most common in practice).
Class B + Class 70R.
Class A + Class AA.
Class B + Class AA.
Custom Vehicle.

For Class 70R and Class AA vehicle types, the vehicle could be either wheeled or tracked.

Additionally, Class 70R vehicle may have three different wheel configurations as shown in the table below. The table also shows the default tread width assumed which can be changed by the user.

Two basic vehicle configurations are considered: (A) One vehicle by itself or (B) Two vehicles side-by-side with the minimum spacing between them as specified by IRC. The two side-by-side vehicles could be either of the same type or of different types.

Tread Width = Tire Width-50 mm

The parameter, b1 representing the dispersion of the wheel load through the wearing surface, in the equation for effective slab width is calculated as follows:
- The tire contact area is assumed to be dispersed at 45 degrees thru the wearing surface. If there is no wearing surface, b1 is equal to zero.
- The dispersion per wheel is calculated. It is reduced to account for overlap from either the adjacent wheel of the axle of the same vehicle or that of the adjacent vehicle when two side-by-side vehicles are being considered.
- beff is then calculated for each case of b1 calculation.
- The moment due to each vehicle is assumed to be distributed equally among all wheels on an axle.
- Moment per wheel per effective width is then calculated.
- The vehicle combination producing the maximum moment per unit width is used to calculate the governing effective width.

The above procedure is used to calculate all Live Load effects (Max/Min Moments and Shears) at each Point of Interest (POI).