There are several specifications that provide equations to consider the effect of creep and shrinkage on the estimation of prestress loss. As some of the references state, creep and shrinkage equations for precast segmental concrete bridges differ from that for the other types of concrete bridges. For example, one of the references states that the creep factor for precast construction is normally around 1.6, while for in situ construction this factor would normally be between 2.0 and 2.5. Since most of the references are related to the bridges other than segmentally constructed ones, I would really appreciate it if you kindly provide me with some guidance and references related to the following points:
1. Equations for creep and shrinkage calculation in precast concrete.
2. Equations for prestress loss estimation in precast concrete bridges.
Thank you in advance.
Creep and shrinkage depends on the age of concrete. Usually in precast segmental structures prestressing force is applied to the "older" concrete, therefore the final creep wil be lower. Determining the creep and shrinkage put into formulas appropriate age of concrete at the moment of prestressing and service load application.
Dear Rafal,
Thank you for your answer.
If I want to put my interpretation on your comment, I would say, the equations of creep and shrinkage presented in specifications can be used for both cast-in-situ and precast concrete and the only difference between these two cases could be the age of concrete at loading. But there is still a question.
AASHTO LRFD Bridge design specification states that “In order to determine the effects of shrinkage and creep on the loss of prestressing force in segmentally constructed bridges, a more precise estimate shall be made, including the effect of: specific materials, structural dimensions, site conditions, construction methods and concrete age at various stages of erection.” and I found no further information related to segmentally constructed bridges.
The question is, is there any equation which is presented specifically for prestress loss estimation in precast segmental bridges?
Agree that using the correct age of concrete and the normal equations is valid for hand calculation scheme design, but if the client can afford a segmental bridge then you should be using bridge specific software such as CSI Bridge, Sofistik or Midas Civil. This will do all the hard work of the staged construction analysis and comply with the AASHTO requirement for a more precise estimate.
Dear Richard,
Thank you for your answer.
Since I am working on a research project related to the mentioned subject, I need to know the theories of the staged construction analysis implemented in the softwares you mentioned. Could you please tell me if there is any reference for the theories?
Details of the theories used are included in the software user manuals, some of which may be available online. Most use a stepwise implementation of the equations in the design code selected such as EN 1993, ACI 318-14 etc
Amin Iranpour ... Good question… Please check references below.. hope it will be helpful for you…
Regards…
http://www.dot.ca.gov/des/techpubs/manuals/bridge-design-specifications/page/section9.pdf
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