Design Tips

Strands
   Design the number of strands to meet Service III stress limits for final tension in the bottom of the beam at midspan.  Then debond strands in pairs to meet Service I stress limits for initial compression in the bottom of the beam at the ends.  Debond strand lengths according to where the actual stress exceeds the limit while remembering transfer lengths are accounted for.  For example, if the stress limit is exceeded at 10.5 feet from the end, 10 feet of debonding will probably satisfy the code requirements (transfer lengths are approximately 2 feet for a 1/2 inch strand).

Mild Steel
Prestressed concrete beam design requires consideration for five different types of mild steel: anchorage (bursting), confinement, shear, interface, and longitudinal ties.     Usually four steps of reinforcement is adequate, the first being of anchorage and the next three for shear.  For anchorage steel the following design steps are suggested:
   - begin with five #4 bars spaced at 4 inches, if No Good
   - add one bar at a time until a maximum of eight is reached, if No Good
   - use five #5 bars spaced at 4 inches, if No Good
   - add one bar at a time until a maximum of eight is reached, if No Good
   - use five #5 bars spaced at 3 inches, if No Good
   - add one bar at a time until a maximum of eight is reached.
The number of confinement bars is calculated by the program.  Reinforcement for confinement is assumed to be #3 bar with spacing the same as used for anchorage steel. 
Shear steel is designed starting at the critical shear location specified at some distance from the support.  The program assumes that same reinforcing size and spacing designed at the critical shear location is also used between the critical shear location and the end of the beam (the additional confinement and anchorage steel is not added for shear).  For shear steel the following design steps are suggested:
   - begin with #4 bars and use the graph as a design guide as to the spacing
   - if spacing falls below 6 inches consider using #5 bars
   - do not mix #4 and #5 for shear
If maximum stirrup spacing controls, a maximum of 1 foot spacing needs to be used for some distance from the critical shear location. 
Interface steel may be required in some designs.  If required, simply extending the shear reinforcement into the deck may not meet the code requirements and additional interface bars may need to be designed and detailed on the plans.  For wide flanged girders, a minimum of four legs per row is recommended.  For designs where no interface steel is required, general practice is to extend the shear steel into the deck as a precaution in the event the interface surface is not roughened as expected.
No longitudinal steel is assumed by the program.  The requirement for longitudinal steel is met through the addition of shear steel and prestressing steel.  

