Midas Civil BIM link – preparation of the calculation model
This article defines the principles of working with the BIM link between midas Civil and IDEA StatiCa, particularly the guidelines for properly preparing the calculation model in midas Civil to ensure a smooth import of internal forces with minimal adjustments and checks in IDEA StatiCa BIM app. The article is part of a series of three articles that comprehensively address the issue of importing internal forces into IDEA StatiCa BIM. We recommend this text as the first one to study. The next two articles are:
- Midas Civil BIM link for design of three-span post-tensioned bridge - A practical example of importing results from the calculation model of a prestressed, three-span, T-girder bridge into IDEA StatiCa BIM.
- Midas Civil BIM link – logic of load cases and combinations postprocessing in IDEA StatiCa BIM – This article defines the principles of working with a BIM link and, particularly, the guidelines for preparing a calculation model in midas Civil.
The IDEA StatiCa BIM app was designed to link the IDEA StatiCa checks (particularly the RCS app) with Finite Element Analysis (FEA) programs used for global analysis: Scia Engineer, Axis, midas Civil, and midas Gen. However, this article focuses solely on the integration with midas Civil. The IDEA StatiCA BIM app serves as a translator for complex data from midas Civil for assessment in the RCS app. A key feature is that it automatically imports the results of the construction stage analysis performed in midas Civil, including rheological and prestressing effects, etc.
IDEA StatiCa BIM imports from midas Civil:
- The model geometry, cross-sections, and material characteristics
- Prestressing parameters – tendon properties and layout
- Results of the Construction Stage Analysis
- Results of the Post Construction Stage analysis (temperature, envelopes of moving loads, uneven settlement, etc.)
- Combination rules and factors
The IDEA StatiCa BIM app cannot simply import envelope force effects from midas Civil because, for the correct assessment of a prestressed cross-section in the RCS app, it is necessary to separate the effects of prestressing, permanent, and variable loads. Therefore, IDEA StatiCa BIM works with individual load cases and re-performs their combinations according to the combination rules taken from midas Civil. To ensure correct data transfer, several principles must be followed when preparing the calculation model in midas civil. These principles will be described step by step below.
The logic of working with load cases in IDEA StatiCa BIM and evaluating envelope combinations differs from that in midas Civil. It is similar to the logic of working with combinations, for example, in Scia Engineer. However, by following a few simple principles, the re-performed combinations in the BIM app lead to identical results of internal forces as in midas Civil. This logic is described in more detail in a separate article titled Midas Civil BIM link – import of internal forces and combinations to IDEA StatiCa. For routine use of the BIM app, a detailed study is unnecessary and sufficient to follow the principles outlined below. However, when dealing with unusual tasks or discrepancies in internal forces between midas Civil and the IDEA StatiCa BIM, understanding the principles of the BIM app is important. Additionally, the underlying concepts behind the simple principles in this article will become clearer to the reader.
1. Do not use diacritical marks in midas Civil
Data export from midas Civil uses *.mid and *.mct files. Periods and commas are used as separators for individual data in these files. Periods and commas in the names of individual model entities (load cases, combinations, cross-sections, etc.) can break this structure. Accents, commas, and generally diacritical marks can cause issues when importing data into the BIM app. Therefore, we recommend not using diacritical marks when naming individual model entities in midas Civil.
2. Indicators for determining the type of ULS or SLS combination
In midas Civil, combinations are entered in the Load Combination -> General tab. In midas Civil, the user typically distinguishes the type of combination by its name, but the information about whether, for example, it is a ULS or SLS-char combination is missing in the importing data from the midas Civil. However, for the correct assessment of the limit states of the cross-section, the BIM app and, subsequently, the RCS app need to distinguish between combinations. This classification can be done manually in the BIM app after the import in the Combination Manager. Still, according to the following table, adding indicators to distinguish the combinations directly in midas CIvil in the Description field is more suitable and efficient. Combinations without indicators are automatically considered as ULS combinations.
3. Consistent distinction between permanent and variable load cases in midas Civil
For the correct operation of the BIM app with imported load cases, it is necessary to properly distinguish between permanent and variable load cases in midas Civil using the Type parameter for each Load Case. Load cases that are set in midas Civil with a type corresponding to permanent loads (Permanent Loads Case) are imported into the BIM app as permanent load cases (more precisely, the group of load cases they belong to is of the permanent type). Load cases that are set in midas Civil with a type corresponding to variable loads (Transient Loads Case) are imported into the BIM app as variable load cases. A list of individual permanent and variable types is shown in the following image.
4. Do not use permanent load cases in Envelope combinations in midas Civil
Midas Civil and the BIM app use slightly different logic when working with permanent load cases. In midas Civil, an Envelope-type combination does not distinguish between permanent or variable load cases; it simply mathematically evaluates the maximum and minimum envelope from the defined set of load cases.
On the other hand, in the BIM app, when evaluating an envelope combination from a defined set of load cases that includes both permanent and variable load cases, the effects of the permanent load cases are always added to the envelope formed by the other variable load cases. This leads to differences in internal forces between midas Civil and the IDEA StatiCa BIM. However, these differences in envelope evaluation can be easily avoided in midas Civil by not using permanent load cases in envelope type combinations and together with variable loads. This rule applies only to permanent load cases.
5. Use only permanent load cases in Construction Stage Analysis
When using staged construction in midas Civil (Construction Stage Analysis), it is important to consider how imported permanent load cases are handled. All load cases resulting from the Construction Stage Analysis (Dead load, Erection load, Tendon Primary, Tendon Secondary, Creep secondary, Shrinkage secondary) are automatically imported into the BIM app as permanent load cases, and the application considers them as part of the staged construction results. The problem arises if a load case that is not part of the Construction Stage Analysis in midas Civil is incorrectly assigned a type corresponding to permanent load cases (such as Dead load, USER, etc.). BIM app imports this load case as a permanent type and tries to "place" it among the staged construction results. However, it doesn't know where to position the load case because it wasn't part of the defined Construction Stage Analysis in midas Civil. The BIM app will then display the following message before performing the cross-section check: "The combination is not applicable for cross-section design because the permanent load cases contained in this combination do not correspond with permanent load cases defined in the construction stages."
Once again, these problems can be easily avoided by consistently distinguishing between permanent and variable load cases in midas Civil using the Type (as described in paragraph no. 3) and by using permanent load cases only within the Construction Stage Analysis. Therefore, it is essential to avoid using types in midas Civil for variable load cases (PostCS analysis) that are imported into the BIM app as permanent load cases. A common mistake, for example, is using the USER type, which is always imported as a permanent load case and thus causes the issues described in the previous paragraph.
Additionally, the BIM app has a tool for checking the consistency of permanent load cases in the evaluated combinations with the construction stages in midas Civil. This can be accessed through the Assessment 1D tab by clicking the Check of combinations button.
6. Setting up load case groups and checking after import
All load cases imported into the BIM app are automatically divided into so-called load case groups. These groups evaluate envelope combinations and ensure the BIM app's proper function; the rules defined below must be followed.
The results of the Construction Stage Analysis (Dead load, Erection load, Tendon Primary, Tendon Secondary, Creep secondary, Shrinkage secondary) are imported as partial load cases (for the start and end of each stage). They are automatically assigned to identically named load case groups, which are always set to permanent type. This process happens automatically.
The results of the PostCS analysis (variable load cases) are also automatically assigned to the respective load case groups, corresponding to the type set for the load cases in midas Civil – see paragraph 3 of this article. For correct processing of load cases in envelope combinations within the BIM app, it is important that all load cases in a single Envelope combination are in the same load case group and that the group is set to the Exclusive type. After import, it is advisable to check whether all groups of variable load cases have the Exclusive type set.
An incorrect assignment of the load case type in midas Civil will be demonstrated with the example of the Envelope combination Temperature, which evaluates the envelope of internal forces from two load cases:
- TG+ – Uneven temperature change +
- TG- – Uneven temperature change -
Let’s analyze the situation where the TG+ load case is incorrectly assigned the Temperature (T) type in midas Civil, and the TG- load case is assigned the Temperature gradient (TPG) type. This inaccuracy will not affect the results of the combination in midas Civil, and the envelope will be correct. However, after importing into the BIM app, the load cases will be placed into two (!) different load case groups, both of the Exclusive type. The Temperature envelope imported from midas Civil will be evaluated in the BIM app as the sum of the TG+ and TG- load cases. The reason for this behavior is explained in detail in this article.
7. Enable Additional steps in the construction stage settings
Just as the BIM app reconstructs the envelopes of variable loads, it also reconstructs the results of the staged construction analysis from the partial outputs of midas Civil. Therefore, it is necessary to export results from midas Civil at the end of each stage (the default setting) and at the beginning of each stage. To ensure that the BIM app has complete information to "reconstruct the staged construction," the "Additional steps" option must be checked in the staged construction settings dialog (Load -> Construction stage -> Define C.S.) for all stages. If this option is not enabled, the additional steps for the stage will be created in IDEA StatiCa BIM, but the internal force values will not filled in.
8. Adding the zero load case to the Envelope combination in midas Civil
When evaluating the Envelope combination, midas Civil strictly follows mathematical logic and evaluates the minimum value as the mathematically lowest value and the maximum value as the mathematically highest value from the defined set of load cases.
In contrast, the BIM app automatically takes into account situations where no variable load is applied to the structure when evaluating the Envelope combination of variable loads. When evaluating the envelope from the load cases with effects (in the specific section of the structure!) of the same sign, for example, positive, the BIM app will evaluate the maximum positive value as the maximum and zero as the minimum – meaning no variable load is applied to the structure.
If the individual load cases in the Envelope combination in midas Civil result in forces with the same sign, it is necessary to add a zero (empty) load case to the Envelope combination. This also considers that no variable load case may be applied to the structure from the group of variable load cases in the Envelope combination in midas Civil. It ensures consistency of results between midas Civil and the BIM app.
If the Envelope combination gives maximum and minimum values of opposite signs in individual sections (the vast majority of cases), adding a zero load case to the Envelope combination is unnecessary.
9. Consideration of the factors rsup and rinf for the SLS assessment
Serviceability limit states (SLS) checks in IDEA StatiCa RCS automatically account for the influence of the rinf and rsup factors which are defined in EN 1992-1-1; 5.10.9 (1) for the effects of prestressing. Therefore, it is not necessary to consider these factors when creating the SLS combinations in midas Civil. If these factors were applied in midas Civil, they would be applied a second time in the RCS app.
However, it is clear that when designing a bridge, an engineer needs to calculate and display, for example, the envelopes of normal stresses in the extreme fibers of the section already in midas Civil. Therefore, before exporting to the BIM app, it is necessary to deactivate these SLS combinations, including the rinf and rsup factors.
An alternative is to import the SLS combinations, including the factors, but in the RCS app, set the values of the ring and group factors to 1.0. More information on this issue can be found in the article How the fators rinf and rsup are considered for SLS combinations.
10. Envelopes check between midas Civil and IDEA StatiCa BIM
Verifying the correctness of the imported loads into IDEA StatiCa BIM is always highly recommended. This can easily be done by comparing the envelopes of internal forces between midas Civil and IDEA StatiCa BIM (BIM app). For example, the standard procedure in midas Civil is that a single "final" Envelope combination of Ultimate Limit States (ULS) combines all partial ULS combinations, and it is therefore always advisable, after import, to compare this final envelope with the "reconstructed" results in the BIM app.
However, when comparing envelopes for a calculation model with a staged (composite) concrete-concrete cross-section, a difference between midas Civil and IDEA StatiCa BIM arises regarding the considered centroid of the cross-section for evaluating bending moments. The Midas Civil program relates all internal forces (including prestressing effects) to the centroid of the currently activated cross-section. In IDEA StatiCa BIM, internal forces are recalculated to the centroid of the final cross-section throughout the entire staged construction. This difference in the reference point, to which bending moments and their envelopes are evaluated, naturally influences the results. Therefore, for staged cross-sections, it is necessary to compare, for example, the envelopes of shear forces between midas Civil and the BIM app. More information on this issue can be found in the article Composite Cross-sections - Different Internal Forces in midas Civil and IDEA StatiCa.