Elastic Shortening Losses

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Weiyang
Elastic Shortening Losses
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Hi, I am currently modeling a splice girder bridge with a time-step loss method. A 1406 kips pretension force is assigned to each segment. When I check the axial force which is causing by pre-tensioning at the analysis result diagram, the magnitude of axial force is same as the pretension force. Could you please confirm that the elastic shortening losses associated with pretensioning are not accounted for? Where can I look for the losses information? Thank you.

Rick Brice
Elastic Shortening Losses

Elastic shortening loss is computed. The best way to see it graphically is by looking at the effective prestress graph.

The plot for the Prestress Release interval (typically Interval 2) has two lines. One is the effective prestress just prior to release, and the other is after release. The difference is the elastic shortening effect.

You can see the computations in the Time Step Details Report. Look at the analysis for the Prestress Release interval. The effect of elastic shortening is the change in prestress force due to the application of the prestressing as an external load to the concrete section.

Weiyang
Hi Rick. Thank you for your

Hi Rick. Thank you for your reply. In the effective prestress graph, it does change in the pre-tension stress along the time-steps. However, in the Time Step Details Report, the stress on the concrete due to pre-tension does not change between "Interval 2: Prestress Release" and "Interval 4: Place segments into storage" and the after steps. Please see the new upload screenshot. Can you verify it? please. Thank you.

Wei

Rick Brice
There is no elapsed time

There is no elapsed time between Release and the beginning of storage. Time dependent effects are 0.

Weiyang
Hi Rick, Thanks for your

Hi Rick, Thanks for your patience. I still get confused. Please see the highlights in the uploaded PDF file. I select 2 locations in the girder and compare their pre-tension stress in 3 different time-steps: Interval 2: Prestress release (For temporary stresses before losses); Interval 4: Place segments into storage (Stresses at Service Limit State after losses); and Interval 29: Load Rating (with time dependent effects). The pre-tension stress is similar/same. Since Interval 2 is before losses, I am expecting that it has higher pre-tension stress than Interval 4 & 29. Unless the pre-tension stress in Interval 2 is after losses. Thank you for your time.

Wei

Rick Brice
Elastic Shortening Losses

You are expecting the stress in the extreme fibers of the girder concrete, due to pretensioning, to change over time. You are thinking of the effective prestress method. PGSplice is using the time-step method.

For typical pretensioned girders analysis, girder stress due to prestressing is computing using the effective prestress at various times. The effective prestress is the jacking force reduced by time dependent losses. Girder stress due to creep, shrinkage, and relaxation are not explicitly computed because they are "built-in" to the effective prestress (if there is continuity, long term creep effect may still need to be accounted for, but these are secondary effects).

Time-step analysis is different. The girder stress in the concrete due to the prestressing force is constant with time (assuming one application of force). The stress caused by the prestressing doesn't change. However, the stress in all of the parts of the girder changes with time due to creep, shrinkage, and relaxation. These effects are explicitly computed at combined in the service limit state combinations.

The stress demand for time step analysis is Prestress + Post-tensioning + Service I. Service I is DC + DW + CR + SH + RE + PS(secondary effects).

The time step details report lists every change in stress, at every interval, in every component (rebar, prestressing, pt-tendon, deck, deck rebar, and girder) for every cross section analyzed. Review this report to get the full time-step analysis results.

Also see the discussion in the Technical Guide and read the papers by Tadros that show the development of the time-step method.
https://www.wsdot.wa.gov/eesc/bridge/software/Documentation/PGSplice/4.1...

Weiyang
Thanks for your excellent

Thanks for your excellent explanation. Finally, I understand the program's analysis. Thank you for all the help. I appreciate.

Wei

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