When creating a design model for a structure, one of the steps is to prepare combinations for defined load cases. Most often we use the mechanism of automatic generation of combinations, according to the rules of the standard set in the project configuration. If necessary, we can modify the rules and relations between cases or case families using the available mechanisms in the Concomitance between load cases dialog window (available for Eurocode combinations).

However, while working on many projects there are situations when we want to define a set of our own combinations. Of course, in the case of a small number of load cases and a small number of required combinations, this can be done directly in the combination dialog. Nevertheless, in this article we will take a look at more effective methods, which will work especially well in case of a larger number of cases/combinations and when we want to use our own combination definition in other projects. These will be two solutions: using the mechanism to export and import combination definitions using an Excel spreadsheet and the mechanism to import combination definitions as a CBN text file. 

Using an Excel spreadsheet

Starting with version 2020.1 Advance Design allows for easy and quick exchange of combination definitions between the current project and an Excel spreadsheet. For this purpose, there are two dedicated buttons available in the Combinations window.

The Export button exports definitions of all existing in the project load combinations to a new Microsoft Excel spreadsheet. For this a dedicated xlsx file is created on the path selected by the user. The Import button reads definitions of load combinations from the selected spreadsheet and adds them to existing definitions on the current project.

Let’s look at the data structure in the spreadsheet. The first row contains the column names, while the following rows contain the definitions of the subsequent combinations.

• ID column

This column contains the identification numbers of the successive load combinations.

• CODE and TYPE columns

These columns contain the combination type text code (CODE) as well as the name of the combination type (TYPE). The names and type codes depend on the settings of the project location – working language and standard for the load combinations. Below is an example of codes and types for the Eurocode settings:

ECULSSTR	ULS StrGeo
ECULSEQU	ULS EQU
ECSLSC	SLS characteristic
ECSLSF	SLS frequent
ECSLSQ	SLS quasi-permanent
ECELUA	AULS

It is important that when editing or creating a worksheet, the combination types in the TYPE column are consistent with the types available for the current standard. Therefore, in the TYPE column, the cell values are selected from the list.

• CAS and COEFF columns

These columns are always defined in pairs and specify the load case ID number (CAS) and the corresponding coefficient (COEFF). The number of pairs of these columns’ headers should be equal to the maximum number of load cases in the most extended combination. Of course we can reduce or increase the number of pairs of headers if necessary. The number of pairs of values in a given row with the definition of the combination depends only on the given combination, but at least one value pair should be defined.

When you import load combination definitions from a selected spreadsheet, these combinations are added to the list of existing combinations in the current project. If combinations with the same ID number already exist in the project, then the ID number of the imported combinations is changed to the first free number. If the combination definition in the Excel file is incorrect (e.g. contains case numbers that do not exist in the model), then the combination is omitted during import. In these cases a warning appears.

Using the possibilities of a spreadsheet, including the possibility of using complex formulas, macros or cooperation with other programs, we have almost unlimited possibilities to prepare and automate the creation of our own load combinations.

Using CBN files

CBN files are text documents containing combination definitions. Generation of the predefined load combination with using the CBN file is based on loading the file from the disk using the Loads CBN button, located in the Combinations window. We can also preview the contents of the file without the generation of combination by using the View button.

To define your own file with the definition of a combination, you need to create a text file similar in structure to the predefined .cbn files. Lines starting with # or // are used to enter comments / notes and are optional. The remaining lines are treated as definitions of subsequent combinations and consist of case codes and coefficients separated by spaces. Additionally, at the end of each line there is space for the combination code and for the comment. Let’s look through the contents of the sample file:

In this example there are 3 lines of combinations, so 3 combinations will be generated. CASE1, CASE2, CASE3 are the type codes for 3 different load cases. Next to each code there are coefficients given with a character. Codes such as ECELUSTR are codes for the generated combinations. They can be arbitrary texts, in which case they will be used as an additional description, or they can be created in the naming convention for a given standard, so that they are automatically recognized as suitable combinations for design calculations. The text at the end of each line of a combination is optional and is not used during combination generation. It can be our additional description visible only during file preview. Note that in the last combination, in the second and third case coefficients equal to zero appear, which means that this combination will only contain the first load case. Exactly the same effect you get if there are no codes and coefficients for these two last load cases in this line. In the picture below you can see the effect of combination generation for the above example.

When loading the file, combinations are generated according to custom definitions, and the combination code is assigned to combinations accordingly. The combination name is generated based on the combination definition.

Note that when using an Excel file, to identify load cases their ID number is used. This allows you to precisely determine the relationship, regardless of the type of load case. However, in this solution we need to know the ID numbers of these cases before the combination is generated.

In case of CBN files, load case codes are used. Thanks to the codes it is easier to prepare a universal combination definition, which can then be used for many projects, but at the same time we have to be careful about the compatibility of the codes in the CBN file and in the project.

In every new project the load case codes are the same for all load cases. They can then either be modified manually by us or updated automatically by the program during combination generation (the program asks for this during combination generation).

Manual modification of the codes is useful when we have prepared the combination definitions in the CBN file using our own codes. In such a case, we can mark each case explicitly in a given project, making sure that they match the codes in the CBN file.

If you allow the program to automatically complete the codes during combination definition, remember that the same codes are assigned to all cases of a given type. This means that all fixed load cases will have the same code, similarly all snow load cases etc. This solution is convenient when there is no need to combine cases inside the same type.

Let’s see the effect of one exemplary row defining the combination in the CBM file:

 CASE1  +1.10 CASE2  +1.20 CASE3  +1.30 CombA

Example 1. There are 3 different load cases with names and codes respectively:  A – CASE1, B – CASE2 and C – CASE3.   The result will be one combination with all cases:

A*1.1 + B*1.2 + C*1.3

Example 2. There are 2 different load cases with names and codes respectively:  A – CASE1 and B – CASE2.  The result will be one combination:

A*1.1 + B*1.2

Example 3. There are 3 different load cases with names and codes in the project:  A – CASE1, B – CASE1 and C – CASE3. Note that the first two cases have the same code. The result will be two combinations:

A*1.1 + C*1.3

B*1.1 + C*1.3

As you can see both mechanisms, either using an Excel spreadsheet or using CNB text files, allow you to easily and quickly create your own sets of load case combinations in Advance Design. I encourage you to get closer to these solutions and use them while working with your own projects.

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