The limiting capacity for most prestressing lines is either jacking capacity or overturning of the anchorages (stressing abutment). The magnitude and eccentricity of the prestressing force combine to cause the overturning moment that the casting bed must resist. Four factors contribute to the increased demand on stressing beds than have been experienced in the past:

1. girder sections that have been optimized for high performance concrete (HPC) contain more strands than have been traditionally used,
2. strand size has been increased from 0.5" to 0.6" diameter (0.7" diameter strand may become more commonly used as well),
3. a greater total jacking force is required to stress these strands, and
4. long span HPC girders are notably taller than previous standard girders (6 to 8 feet in depth) resulting in larger eccentricities and harped strand exit locations that are well above the floor of the stressing bed. The combination of increased jacking force and larger eccentricities combine to tax the overturning capacity of casting beds.

Long span HPC girders tend to be laterally unstable during lifting and transportation. The use of temporary top strands to improve the stability is a common and effective practice. The temporary top strands are well above the floor of the casting bed with an eccentricity that is approximately equal to the height of the girder. The jacking force in 4 to 6 temporary top strands is small compared to that of the permanent strands; however their large eccentricity produces a significant overturning moment.

Temporary top strands are used to ensure stability of prestress girders during shipping and handling. These strands may be either pretensioned or post-tensioned. When temporary top strands are required for shipping it is most advantageous to pretension them along with the permanent strands. The use of pretensioned temporary top strands results in reduced release strength requirements, reduced long term camber, and reduced slab haunch requirements. However, the capacity of some prestressing lines is insufficient to withstand the overturning moment. Permitting the fabricator the option of post-tensioning the temporary top strands will reduce the demand on the prestressing bed.

The following procedure is used to design precast-prestressed girders for optimized fabrication:

1. Design for Final Service Conditions (estimates total number of prestressing strands and final concrete strength)
2. Design for Lifting without Temporary Top Strands (estimates release strength and lifting location)
3. Design for Release without Temporary Top Strands (estimates concrete strength at time of form stripping)
4. Estimate Temporary Top Strand Requirement (skip to step 7 if not required)
5. Design for Lifting with Pretensioned Temporary Top Strands (estimates release strength and lifting location)
6. Design for Lifting with Post-Tensioned Temporary Top Strands (estimates release strength for lifting from step 2)
7. Design for Shipping (estimates support locations and final concrete strength)
8. Check Final Service and Strength Conditions

It should be noted that, while this process is most critical for long, slender girders made of HPC, it is also beneficial to optimize girders of "normal" design. If temporary top strands are not required for shipping, steps 5 and 6 are simply skipped.

The fabrication optimization chapter and report lists the various combinations of lifting locations and release strength for lifting with and without temporary strands and for pretensioned and post-tensioned temporary strands.

More detailed information on designing for optimized fabrication is available from Brice, Richard, B. Khaleghi, and S. J. Seguirant, 2009, Design optimization for fabrication of pretensioned concrete bridge girders: An example problem. PCI Journal, V. 54, No. 4 (Fall 2009): pp. 73-111.