Question:
Hello Support,
I was running into problems looking at the beam forces and stresses of a general composite section. Specifically, I was looking at the CS: Summation results in the file I attached. As you'll be able to see, there is a My near the midspan of 194 k-ft acting on the "Total" part. My confusion comes in looking at the "Part 1" and "Part 2" forces. I am assuming the part 1 forces act at the center of my beam, while the part 2 forces act at the center of my deck. Based on my hand calc shown below, I am not able to get to the total moment given from the 2 parts (I tried to sum the moments and the axial force x moment arm length). Is the "Total" force component not what I think it is?
Additionally I am a little confused with some of the results given for beam stresses. When looking at a Beam Detail Analysis. The 4 default points attached to the section all result in 0 stress. After zooming in, it appears as though they are not actually on the section. My question is if there is an easy way to fix this/avoid this in the future. Additionally, I was wondering if there is any way to see the stress at additional stress points throughout construction stages. Based on what I have seen so far here (
http://globalsupport.midasuser.com/helpdesk/KB/View/16361922-combined-stress-from-each-of-the-construction-stages), I am limited to the 4 points which are currently not working. I would like to be able to check the stresses at both the top of my beam and top of the deck, which is why I ask. I have attached my section property calculator file for the section in case that will be of use, I also attached the excel file with the Beam Detail Analysis output.
Thanks,
Answer:
Hi,
Thank you for your waiting. There are a few things I would like to comment.
1. I don't know why but the values of Czp and Czm of composite section for C.S. are incorrect.
I imported the 8ft comp.sec file again and just redefined Composite Section for C.S. Now, the values are correct.
2. Your approach of the calculation is not right. By default, the results of construction stages are accumulative results along the stages.
Results of stage N (accumulative results) = Results of stage N-1 (accumulative results) + Current step results of stage N
You determined the resultant moment of the composite section using accumulative results. Instead, current step results should be used.
3. In order to simplify the calculation, I selected the 'Constant' option to ignore tendon for the calculation of section property.
Also, I ignored the variation of compressive strength because the neutral axis depth can be changed depending on the modular ratio of slab and prestressed beam.
4. The eccentricity from the centroid of Total section can be obtained as follows:
- Eccentricity of part 1 = Total Czm - Part 1 Czm = 3.003 - 1.736 = 1.268
- Eccentricity of part 2 = Total Czp - Part 2 Czp = 2.040 - 0.477 = 1.563
5. The current step results of each part:
The resultant moment for the current step results is
12.4231 - 1.6795 + 15.7574 x 1.268 + 1.6886 x 1.563 = 33.36
This matches the current step moment of total section as follows:
The revised model file is attached here for your reference.
[Stresses of Beam Detail Analysis]
Currently, Beam Detail Analysis does not support the stress calculation for the composite section for Construction Stage. Instead, you can check the stresses of each part from the Result Tables>Construction Stage for C.S.>Beam Stress.
Regards,
DK Lee
REQUESTION:
Could you go into more detail about what you said in point 2? It seems to us that a given cut of a beam should always satisfy equilibrium, which is what our calc was trying to check. It seems like this should be true, even after various construction stages are accumulated.
Additionally we wanted to make sure we were understanding the Part 1/Part 2/Total correctly. Our current understanding is that, for the example of this model, part 1 should have the forces and stresses in the beam. Part 2 should have the forces and stresses in the deck+haunch. Part 2 should have the combined forces, and no stresses shown. Is this correct?
I also have an additional question regarding the stress points in part 2. For part 1, the stress points can be seen in the section manager (and in other locations) as red points, as shown below. Where can I see the stress points for part 2? Additionally, are my additional stress points below (shown in blue) accurate if they are on part 2 (see point 1 below), or should these only be used to calculate stresses in part 1, I only ask because the default stress points are only on part 1, so it could be that the stress calculation is based simply on the curvature of the beam, which would be inaccurate for part 2.
Thanks,
ANSWER:
1. Point 2
You are right as long as the section remains the same along the construction stages. However, in case where the section changes before composite and after composite, it is not true any more.
2. Yes. It is correct.
3. The stress points in the section manager are only used in the Beam Detail Analysis. But, as I mention, Beam Detail Analysis does not support the stress calculation for the composite section for Construction Stage. The only way to change the stress points for the composite section is to change coordinates of default stress points of Part 1 and Part 2 from the Section as shown below. And then, check the stress of each part from the Result Tables>Construction Stage for C.S.>Beam Stress.
Regards,
DK Lee
REQUESTION:
Thanks again for your response. I was wondering if you could elaborate why the assumption you mentioned in (1.) is no longer true with a before/after composite section?
Also after looking at the "Current Step Forces", I could use some clarification on this as well. Below is shown the current step forces for stage 1h in the revised models.
Because this is the first step in the analysis, I would expect the current step and the accumulative results to be the same, but they are not (see results without "current step results" box checked below).
Why is the DC and some of the tendon/creep not included when the "Current step results" is checked? I'd assume [Results of stage N (accumulative results) = Current step results of stage N] for the 1st step.
Thanks,
ANSWER:
A stage can have additional steps (first step, user step) and last step. There is an option named 'Save Result' with which you can choose steps to be saved to view the results. In your model, DC (Self Weight) is applied at the first step of the first stage (Stage 1h PC Beams) and thus the current step results of DC are provided at the first step, not the last step. In order to view this results, you need to check on the 'Additional Steps'.
After analysis, you can see the current step results of DC by selecting the first step of Stage 1h PC Beams.
As for the creep, this effects are varying over time even in a stage. For good convergence and accurate calculation, the program generates sub-divided internal steps in a stage automatically based on the option below. Therefore, actually there are more internal steps which are not shown in the program results. So, the current step results of creep is calculated and provided considering all the steps including the internal steps. Prestress loss is also varying over time because of creep. So, the current step results of tendon is the same as creep.
Regards,
DK