Modeling Post-tensioned Concrete I-Girder Bridges using Wizard Application and Construction Stage

Question:

Hello,


I am preforming the load rating of a bridge superstructure composed of post-tensioned concrete I-girders and concrete slab. I have followed the video published by MIDAS to use Wizard application for modeling the bridge:

Prestressed Concrete Girder Composite Bridge: Wizard Application (midasoft.com)


I have a few questions in this regard and would appreciate your help. I have attached both Wizard file and the final file of my model just in case they are needed.


1) My first set of questions is about the autogenerated Construction Stages by the Wizard application. I have included below a screen shot of the Construction Stages (please see Figure 1 below). Here are my questions:


A) What is the philosophy behind the assumption of 10-days duration for Stages 1, 2-1 and 3? Does it make any difference if I change them?


B) I have highlighted Stage 4 in Figure 1 below. In this stage, long-term effects are to be considered (note that I have defined Time Dependent Materials considering Creep and Shrinkage as mentioned in the tutorial video). The issue is that I believe that Stage 4 should be defined before Stage 3. This is my reasoning for this statement: in Stage 3, after composite-actions loads are defined. With the sequence shown in figure below, the bridge will be analyzed based on the bridge stiffness computed in Stage 2 where there is not long-term consideration. I believe that with the sequence shown in figure below, only post construction stage loads such as live loads will be analyzed considering the long-term effects which is not correct. Please note that, according to the AASHOT LRFD 2017 provisions, dead loads after composite actions must be analyzed considering long-term effects and live loads should be analyzed without considering these effects. However, the sequence that the Wizard application generates is in contradiction with this requirement.



Figure 1)


2) Figure 2 below shows the definition of the composite section based on the Wizard application and the tutorial video. I have the following questions in this regard:


A) In the video, it is mentioned that we need to input “28” and “7” days for the ages of Part I (i.e., the girder) and Part 2 (i.e., the concrete slab), respectively. What is the philosophy behind these numbers?


B) Why the parameter “v/s” is always zero even after I “Update Long Term” as shown in Figure 3 below?


Figure 2)



Figure 3)


3) My last question is regarding the “Load Rating Cases”. Figure 4 below shows the definition of load rating cases for PSC Bridge in MIDAS. This is my question: this post-tensioned bridge is a composite bridge. We considered this feature when defining construction stages which were shown in Figure 1 above. However, when defining Load Rating Cases as shown in Figure 4 below, there is no “DC After” and “DC Before” like what we define for Steel Composite bridges in MIDAS. As you can see, for PSC bridges, we only have “DC”.  Why do we have this inconsistency in MIDAS between Steel Composite and Pre-stressed Composite? For PSC bridges, what type of DC should we define in Load Rating Cases as “DC”? Should we define all DC loads i.e., both DC before and after composite action?





Figure 4)



I look forward to hearing from you.

Best Regards,


Answer:

Hi,

1) My first set of questions is about the autogenerated Construction Stages by the Wizard application. I have included below a screen shot of the Construction Stages (please see Figure 1 below). Here are my questions:

A) What is the philosophy behind the assumption of 10-days duration for Stages 1, 2-1 and 3? Does it make any difference if I change them?

The input data of the wizard generates construction stages with the 10-days duration in the model as shown below. 


If you look at Stage 1, prestressed beams and cross-beams are activated. 10 days of duration in Stage 1 means that 10 days have passed before Stage 2-1 starts. The duration of the stage will make concrete beams gain ages. Thus, the change of the values will affect the time-dependent behavior of concrete, i.e. creep, shrinkage, compressive strength of concrete. The value of 10  is just the default value. It should be updated depending on your actual project.


B) I have highlighted Stage 4 in Figure 1 below. In this stage, long-term effects are to be considered (note that I have defined Time Dependent Materials considering Creep and Shrinkage as mentioned in the tutorial video). The issue is that I believe that Stage 4 should be defined before Stage 3. This is my reasoning for this statement: in Stage 3, after composite-actions loads are defined. With the sequence shown in figure below, the bridge will be analyzed based on the bridge stiffness computed in Stage 2 where there is not long-term consideration. I believe that with the sequence shown in figure below, only post construction stage loads such as live loads will be analyzed considering the long-term effects which is not correct. Please note that, according to the AASHOT LRFD 2017 provisions, dead loads after composite actions must be analyzed considering long-term effects and live loads should be analyzed without considering these effects. However, the sequence that the Wizard application generates is in contradiction with this requirement.

You have defined time-dependent material properties.  Thus, the long-term effects of permanent loads will be taken into account by calculating creep effects directly. For example, the stresses due to creep can be viewed as shown below. This approach does not use short-term or long-term section properties based on the modulus ratio. In Stage 4, the long-term section properties are not calculated. Instead, creep effects are calculated based on the time-dependent properties. So, Stage 4 should not be defined before Stage 3.


2) Figure 2 below shows the definition of the composite section based on the Wizard application and the tutorial video. I have the following questions in this regard:

A) In the video, it is mentioned that we need to input “28” and “7” days for the ages of Part I (i.e., the girder) and Part 2 (i.e., the concrete slab), respectively. What is the philosophy behind these numbers?

This concrete age represents the period of concrete curing before the concrete member starts to resist the loads. During the period of the concrete curing, no stresses are assumed to be introduced in the member by the loads. But the concrete age is necessary to calculate the creep, shrinkage, and compressive strength of concrete. 28 and 7 days are just an example. The user needs to update the values depending on the actual construction.

One of our tutorials is recommended for you to understand the duration and concrete ages. I have attached the tutorial here.


B) Why the parameter “v/s” is always zero even after I “Update Long Term” as shown in Figure 3 below?

This needs to be defined by user. the 'Update Long Term' option is just applied to calculate 'h' value of the section.


3) My last question is regarding the “Load Rating Cases”. Figure 4 below shows the definition of load rating cases for PSC Bridge in MIDAS. This is my question: this post-tensioned bridge is a composite bridge. We considered this feature when defining construction stages which were shown in Figure 1 above. However, when defining Load Rating Cases as shown in Figure 4 below, there is no “DC After” and “DC Before” like what we define for Steel Composite bridges in MIDAS. As you can see, for PSC bridges, we only have “DC”.  Why do we have this inconsistency in MIDAS between Steel Composite and Pre-stressed Composite? For PSC bridges, what type of DC should we define in Load Rating Cases as “DC”? Should we define all DC loads i.e., both DC before and after composite action?
In the prestresssed composite rating, long-term effect is not considered by long-term properties, but it is considered by time-dependent material properties. On the other hand, the steel composite girder uses long-term properties to consider long-term effects. So, DC After and Dc Before are necessary for the steel composite rating but not in the prestressed composite rating. You need to include both DC before and after composite action under the DC load case.

Regards,
DK Lee
Technical Manager

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Creation date: 5/4/2021 8:30 AM      Updated: 5/4/2021 8:30 AM
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