Load cases distinguished from Dead load

Question:

Dear Support Team,

I would like to ask you to increase the number of load cases that could be distinguished from the dead load for CS. now the limit is 10, and it is much better than the previous limit (3), however, it is still slightly not enough. When we are designing a balanced cantilever bridge we have more stages/load cases that need to be separated from the dead load. It would be really great if that limit could be increased with the new Midas Civil version to let say to 20 load cases.

Kind Regards


Answer:

Hello,

Thanks for getting in touch.
Each separated load case in the "Distinguish from the dead load for CS" could hold up to 15 different static load cases.  So, we can in fact distinguish up to 150 different static load cases in those said 10 erection loads.
Could you please let us know with an example for a better understanding of this?  Maybe we can propose an alternative.  If not, we'll definitely consider it for a future update.

Regards,

Question:

Hi,

You said that each of those 10 erection loads could hold 15 different load cases and this way we can distinguish up to 150 different load cases - unfortunately, I can not agree with that opinion, because in the end we still have only 10 erection load cases. Each of those 10 erection cases could be added to the combination (or not), for each of them we can applied different load factors, etc and it is true. However, we are not able to apply different load factors for each of these 15 static load cases held by each erection case. Only one load factor could be applied to each of that erection loads, and it does not matter how many simple load cases this erection load can hold. So we are really able to distinguish only 10 erection cases.

Now I will try to explain why 10 erection cases are not enough especially when we look at the balanced cantilever bridge. 
1. So 2 (sometimes even 3 depends on the code etc) of 10  erection load cases we are using to distinguish static load cases like: 
     a) Track slab, concrete box
     b) Rails, Noise Barrier, Utilities, OLE, etc
We need at least two cases because based on the code we need to applied different load factors for these erection cases, but as I mentioned sometimes we need to use even 3 of that 10 erection cases.

2. Next 2 of the load cases we need to distinguish forces from Lifting frames, separate for both side of the cantilever (LF-L, LF-R) to be able to apply different loads factor (for example 1.2 and 0.95) separately for both of the liftings frames

3. Next 2 of the load cases we need to distinguish forces from the segment's weight during lifting, and the same as for the lifting frame we need to distinguish these forces separately for both of the segment ends to be able to define different load factors but also to be able to add segment (forces) at one of the cantilever ends.

So far we have used 6/7 of 10 erection cases unfortunately this is not the end yet.

4. We need at least 6 erection load cases for wind load, let's say we are using 2 cases for each wind load direction: Z+, Z-, Y+, Y-, X+, X- (and this is a very positive assumption since based on Eurocode for vertical wind load should be used 4 cases). Of course you can say the wind load does not have been applied at construction stage analysis user can at that as superposition load after bridge erection and this is true but only partial because we can do that only for wind load at service situation. But when we are looking at wind load during erection then the best way is to add this load at construction stage analysis. Because you need to remember that during erection we are applying wind load also to the Lifting frames, and the position of the lifting frames during erection is changing. So only when we applied wind load at construction stage analysis we are able to easily correlate wind load on the lifting frame with the current position of the lifting frame, and this way have concurrent forces. 

So as you can see in the best scenario we need at least 12 erection load cases, but we are engineers so we should look for the worst scenario:) and in the worse scenario, we need 15 erection load cases. My experience is also limited so I understand that it is possible that someone needs a few more cases it depends mostly on the code that we are using for design, so this is why I wrote that it would be really helpful to have 20 erection loads. I hope I managed to explain why we need more erection cases.

Kind Regards,

Answer:

Hello,

So, I had been thinking about this and I've generated a sample model here.  The lifting frame, segment weight during lifting, and wind load have been defined in 3 erection cases.  The wind is considered in x, y & z directions.  For simplicity, the wind is considered in +x & + y directions only, while for wind in z, 4 different cases for positive and negative direction are considered.  Again for the same reasons, I've considered 1.25 & 0.95 segment weight to be acting simultaneously at both ends, which will not be the case practically, but that too could be simulated in a similar way.  Mainly, the load groups have been created to consider the right loads at the right time.  With this, all possible loads could be very well simulated within 10 erection load cases.  However, if I've misunderstood the requirement in some way, kindly elaborate on it.
Kindly go through the model and do let me know if further assistance is required in understanding the process.

Regards,

Question:

Hi,

First of all the solution that you proposed theoretically could even work (at least for wind load) however it is highly impractical, just for wind load we will have 16 cs (2x*2y*4z) and this is for one lifting frame position, but during erection, we will have more that one lifting frame position, let assume 5 (and 5 it is not much at all). Please also noted that you assumed symmetrical load for both ends of the cantilever when you try to create CS with unsymmetrical load the number of  CS will increase and we look only at wind load so far so if I would like to try your solution just to wind load I will get 16x3x5=240 cs (just from wind load!). Basically, all load applied to the cantilever need to be distinguished between the left and right side of the cantilever because I need to have the option to apply different factor for the left and right side of the load, (this is why in previous mail I say that we need to distinguish two load case for lifting frame LF-L -left side and LF-R -right side, so we are able to applied factor 1.05 for LF-L and 0.95LF-R, but it is also valid for other loads like segments, etc.

Due to what I wrote above I figured out a different approach to the problem however it requires more erection load cases. Please see the attached file with the model with my approach to the problem. I used all 10 available load cases, 2 for Lifting frame: LF-L & LF-R, 2 for segments: SE-L & SE-R, and 6 for wind load (but only X and Y direction because it was not enough erection load cases for Z-direction). Please also noted that in this model I was not able to distinguish any superimposed dead load due to the fact that I have already used all available erection load cases.

The only disadvantage of that approach is that when I want to look at the results/combination results at any specific stage then I need to select the stage that I am interested in - as below and rerun the analysis.
However, if I distinguished all load cases that I defined in CS then I am able to create almost any combination that I need. I am also able to look at SLS, ULS, and any defined combination so it is really good.
Of course, it would be much better to be able to create a load combination for the selected construction stage without rerunning the model (some software is able to do that). I think that You should think about it for further Midas development, however, I think that it is a rather bigger task than adding an additional 10 erection load cases.
 

Regards,

Answer:

Hello,

Thanks for the explanation again.  Yes, I agree that if all these loads are to be considered in construction stages, then the number of construction stages would be too many.  However, I don't see any other way around this, since even with the additional erection load cases, the problem activating and deactivating (so more stages), as well as the problem of re-analyzing, will remain.  Generally, that problem of reanalyzing is a deal-breaker for most, but if you are ok with that, then yes, this can also be done.  Though, if the loads are activated and deactivated with factors in the construction stage itself, then the problem of reanalyzing is resolved and the complete thing can be done in 10 erection stages as well.  As for the total number of construction stages, that's generally not an issue, since the number of stages that can be considered is not limited.  So this is how our clients generally do it.
As you've mentioned, it'd be really helpful if the load combinations could be generated for loads applied in construction stages.  That would considerably reduce the number of construction stages to be considered.  We've received such a request from other clients as well and will definitely consider it in future updates.
However, we haven't received any request to consider more than 10 erection load cases.  We will still add this to our development wish-list and this too would be taken up in a future update, if more clients request such a feature.
Kindly let us know if further assistance would be required.

Regards,
Creation date: 5/26/2021 9:33 AM (nandeep)      Updated: 5/26/2021 9:34 AM (nandeep)
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