Combinations of Loadings

The Combinations of loadings mode enables to obtain the indicators of the stress-strain state of the system which is under the action of several loadings simultaneously. Any loading can be included in the combination with a certain factor. This enables, for example, to answer the following question: what are the values of displacements from the action of the “self-weight” loading taken with a factor of 0,9 and the "snow" loading taken with a factor of 0,7. If these loadings were given by their design values, it is not difficult to see that we will obtain displacements from characteristic loads.

If a combination includes a dynamic loading, the value of the total dynamic load obtained with the help of a nonlinear procedure for displacements or forces of all the considered modes of natural oscillations is used in the calculation of forces and displacements of this combination. For example, the following nonlinear procedure is used for the seismic loading: the squares of the results are summed for each mode, and then the square root is taken of this sum. Dynamic loadings are alternating, therefore it is necessary to select the sign of the result. SCAD selects the sign in accordance with the sign of the mode with the maximum obtained absolute value of force/reaction/... . The respective static wind loading is considered for the wind loading (the sign of the result is taken in accordance with the sign of the static wind loading). Individual oscillation modes are not included in the combination, since the result of the dynamic analysis is obtained with the help of a nonlinear procedure.

Unlike the Design combinations of forces mode where it was not known in advance which combination of loadings would be unfavorable (design) for the stress in a certain section, and the search for such a combination was performed automatically according to the rules of SNiP, you have to specify the considered combination explicitly in the Combinations of loadings mode.

Combinations of loadings can be calculated together with the whole problem, or separately, after the main calculation. In the first case the initial data are prepared after completing the generation of the design model and specifying loadings. In the second case the combinations can be specified after completing the analysis of loadings.

The mode of preparing the initial data for the calculation of the combinations of loadings is invoked from the Project tree in the Special Initial Data section. This mode becomes available only after specifying the loadings. Data are prepared in the Combinations of Loadings dialog box.

The data are specified in the following order:

When creating the combination the software enables to take into account the corresponding safety factors for load and/or fraction of duration for each loading included in it. The coefficients with which the loadings are included in a combination are divided by the safety factor for load and/or multiplied by the coefficient of the fraction of duration when the respective checkboxes in the Combinations of Loadings dialog box are checked. It enables to automate the process of creating the combinations which correspond to the characteristic, long-term, or characteristic long-term combinations. The checkboxes described above should not be used if the combination includes other combinations (the latter will be ignored). If EN 1990 is selected for calculating load combinations, these options are not available.

It should be noted that combinations created earlier can be included in the combination together with the loadings. The current combination (its number is specified in the Number of the combination field) can be deleted by clicking the Delete combination button, and all the specified combinations will be deleted after clicking the Delete data button.

When the number of loadings changes, the data for the calculation of combinations of loadings should be clarified, since some combinations can be deleted.

There are restrictions on the specification of combinations in the Erection mode: "independent" loadings of one erection stage can be included in a combination. The main loading will be added automatically to the whole combination, including additions to the main loading (given as groups of loads) with the specified coefficients.

The calculated values of displacements and forces from a combination which includes dynamic loadings are nonlinear and correspond to different moments in time. The sign may not be considered at the summation. The loss of equilibrium in the nodes can occur.

Moreover, the combinations can be created on the basis of the design combinations of forces generated in the DCF postprocessor with the help of the Create combinations of loadings for DCF operation (button — ). To do this, click the Load from file button and the standard Open dialog box of the Windows environment will appear. The loaded file with the information on the combinations has the .rsu2comb extension.

In the Variation of models and Erection modes the combinations of loadings can be specified for a single model (stage), or using loadings belonging to different design models (erection stages). It should be noted that combining loadings from different problems is usually unacceptable because it violates the superposition principle. Nevertheless, the authors left this possibility (of course, the responsibility rests with the user) in order to enable to create a combination of "the difference between similar loadings" type and to assess the differences of the stress-strain states of various models (stages).

When specifying DCF the software enables to use not only loadings but also their combinations. If such a possibility is not necessary, it is recommended to check the Do not take into account combinations in DCF checkbox when specifying the combinations of loadings.