Working to ultimate load factor - concrete prestress

[MrJB]

All, when designing a concrete prestress bed what factor should you apply to the forces geneterated by the jacking heads to convert them from working to ultimate forces? Considering between 1.25 and 1.5 yet some recommendations would greatly appreciated.

 

[Lomarandil]

AASHTO calls for a load factor of 1.2 for post-tensioning anchorage zones and 1.3 for bridge jacking (plus applicable phi factors). Neither directly applicable, but might help you calibrate your design.

 

[MrJB]

Thanks for that

 

[HTURKAK]

My approach would be ;

- If the concrete prestressing bed having full length and integrated to the both abutments and the center portion of the slab is designed to carry design axial prestressing force and vertical forces created by deflected strands and the thickened ends
are designed to transfer loads from the abutments into the slab, and to resist OT moments due to the prestress force, i would choose a FS around 1.3

- If the prestressing force resisted by both abutments having deep found , or designed for passive thrust so the concrete prestressing bed is SOG and not integrated to the both abutments, i would choose a FS around 2.0


My opinion...









If you put garbage in a computer nothing comes out but garbage. But this garbage, having passed through a very expensive machine, is somehow ennobled and none dare criticize it. ( ANONYMOUS )

 

[MrJB]

HTURKAK - The bed has 2 steel H beams that are designed to take the compression from the stressing. There will be a small eccentricity from the centroid of the H beams to the centroid of the stressing tendons. About 50mm max although will design for 100mm. Live and dead ends are inline with the H beams. Footings aren't working that hard really. The chances of the stressing forces exceeding design are very low so the 1.2 - 1.3 range sounds appropriate. Cheers