Moving loads generator 1D

Applicable to load 1D members.

Icon of the command:

Setting the configurations of the moving loads on 1D members via the generator

Upon opening of the "Moving load generator" dialogue, the default window is depicted in the figure below. Parts #A - #I are further explained.

#A - The moving load generator 1D is actually a library of "CA" entities (combination assembly). The dialogue behaves as dialogue of standard load case, it is possible to add, delete, edit !CA! entities similarly to load cases. Each new "CA" is by default named CA1, CA2,... In general, user might define several CA entities (the number is not limited). However, keep in mind, that these CA usually are much more demanding on calculation than standard linear load cases. Depending on the setting, one CA entity might internally require several 1000 (or even more) linear load cases to be analysed by the FEM solver in the background. Everything depends on setting of the corresponding CA.

#B Two main approaches are available, based on the setting "Type of generated load cases", the approach might be "Envelope" or "Influence lines". The options of further settings are different for both approaches. In general the "Envelope" uses brute force to determine the envelop of the extreme result values - internally, vast number of load cases are calculated, each with slightly different position of the load, and the results of these load cases are internally processed into envelope. In case of Influence line, user needs to determine in which position (section) of the beam he is interested in extreme values of specific internal force (this requires more user control, but is less calculation demanding).

Type of generated load cases = Envelope

#B - If set "Type of generated load cases" = Envelope, this means, internally several linear load cases will be analysed, respecting varying positions of the load sets within the load systems, possibly assigned to several traffic lanes. This might lead to vast numbers of internally analysed load cases. Out of this vast number of load cases, a resulting envelope will be provided (hence the name "envelope"). For this option, it is also required to define, which internal forces, #H, and / or which support reactions, #I, user wants to investigate through the whole structure. At least one component out of the options in #H,#I needs to be selected.

Within the results for the "CA" entities, the envelope of extreme values will be provided for all the internal forces, no matter what setting is selected in #H,#I, as shown on example in the figure below:

However, in order to process these results further and combine with the results of another load cases (self weight, permanent loads etc.), for each CA entity, 2n result load cases will be provided within the results of the load cases. 2 stands for the maximum and minimum, and the nstands for number of selected items (internal forces, reactions) from available options in #H,#I (1 ≤ n ≤ 12). These 2 load cases are envelope results of the maximum and minimum effects of the selected entity (#H,#I), where for each section, the corresponding values of the remaining (non selected in #H or #I) results (either internal forces or reactions) are provided (in order to combine correctly). See example below:

Note, that for the result load case of the internal forces (#H), only results of the internal forces are available, and results of "corresponding" reactions are empty, as there would be no corresponding reactions actually. For results of the "reaction result load cases" (#I), it is analogical - only results of reactions are provided, and the results of the internal forces are empty. See example below. Keep this in mind when creating further combinations with the other load effects (self weight, permanent load,...). Depending on what is to be checked, the corresponding result load cases should be included in the combination.

#C - Assigned traffic lanes - by selecting the "..." button, it is required to assign traffic lanes to be loaded within the corresponding CA entity. No traffic lane is assigned by default. For each selected traffic lane, separate section in the dialogue is provided (#C1, #C2). At the top of this section, it is noted for which traffic lane the following setting belong.

Furthermore, for each traffic lane, it is necessary to assign at least one load system. By clicking "..." within each considered traffic lane (#C1, #C2) a selection dialogue is provided, from which available load systems might be assigned to traffic lanes. One load system might be used by several different traffic lanes. Based on the number of assigned load systems, as many separate subsections of the dialogue are provided (#C1a,#C1b, #C2a). In example below, 2 load systems (named LS1 and Model SW/0) are assigned to traffic lane TL1, and one load system (LS1) is assigned to TL2, hence there are 3 separate subsections. In general, the number of subsections is not limited, but again, keep in mind that the larger the number, the more internal combinations (or in this case rather variations or permutations, as the order also plays role here) of mutual load positions exist, what might significantly increase the computational time.

Each load system within each traffic lane might act on some eccentricity e_y or e_z. E.g. if whole bridge beam is modelled by one beam member, hence oneTraffic lane 1D component in SCIA model, but there are actually multiple traffic lanes (transport terminology) on this bridge (e.g. two way road, highway,...), each such traffic lane would be considered by a uniquely assigned load system (#C1a,#C1b) within this traffic lane (#C1). For each such assigned load system, the appropriate eccentricity e_y should be defined (#C1a,i,#C1b,i), in order to consider traffic loads correctly.

Additionally, each unique assignment of a load system to traffic lane (#C1a,#C1b, #C2a) might be considered within some synchronization group (#C3). It is possible to synchronize loads not only within one traffic lane (e.g. within #C1), but also between several traffic lanes. Hence the synchronization group acts through whole the "CA" entity. It is possible to set 6 different synchronization groups numbered 1-6, or "none". All the loads through one "CA" entity within specific number of synchronization groups are "moving together". If "none" is set, loads within this load system within specific traffic lane are not synchronized with any other load within the corresponding "CA" entity.

Examples of specific configurations are provided in this chapter.

#D - Step - in units of length - this defines the step size, which will be considered as span of the varying load positions. The finer the step, the larger number of internal load cases (each with different position of the load) need to be analysed. Set accordingly respecting the structure size (the length of traffic lane).

#E - Load group - set the load group in which the generated result load cases will be assigned.

#F - Load on traffic lane:

Complete = internally considers only such load cases of varying load positions, where all the loads are applied on the defined traffic lane. This setting should be used when modelling e.g. crane loads, as the crane wheel cannot go outside the track.

Partial = internally considers also load cases, where only part of the load is applied on the defined traffic lane. This should be used when modelling traffic loads on bridges for example, where if the train enters the bridge, just part of the train is loading the bridge (and analogically when it exit the bridge). Hence this setting results in more internally analysed load cases.

#G - Compute with influence lines - This option automatically creates vast number of calculations using influence lines in the background and merges the results into the envelope. In order to understand how this feature exactly works, first study how the explicitly defined influence lines work in this chapter and also chapter below (what happens when within the CA entity the "type of generated load cases" is set to "influence lines"). If this check box is activated, along the traffic lane in each position (respecting the defined step size, e.g. with 1m span), influence line for each selected 1D internal force result(#H) will be calculated. Hence, in each position along the traffic lane, the corresponding linear load cases using the Unit deformation of 1D member will be analysed firstly (in the background). For each such influence line, two linear load cases with extreme values (min and max) for the corresponding 1D internal force will be firstly determined (to find the position of these extremes based on the influence line) and subsequently analysed. The envelope will be provided out of the results of these internally calculated load cases. This solution offers possibility to use the reduction of the load in places when the loads acts in favourable direction (decreasing the corresponding internal force at the considered position) - hence the load multipliers for favourable and unfavourable positions defined within the Load Systems 1D are valid for the "envelope" solution only when this check box is activated (when the influence lines are used).

Type of generated load cases = Influence lines

If the "type of generated load cases" #Bis set to "Influence lines", specific influence line needs to be assigned to this CA entity - #B2. All the other parameters, as the logic of assignment of the traffic lanes and load systems for specific traffic lanes belonging to some synchronization groups, - #B3, remains the same as in previous case (see above "type of generated load cases"="envelope").

Notes:

- the property "Compute with influence lines" is not present, as when already type of generated load cases is set to "influence lines", this setting has no additional sense.

- "Step" is still present, and now represents step size, which is considered when numerically evaluating the extreme positions of the load based on the corresponding influence line. The finer the step, the more precise is the exact position of the applied load, but the calculation might take longer of course.

The detailed work flow, of how to define and use the influence lines - #B2, is in this chapter.

Example - using influence lines

#A - A simple load system is prepared, jut two line loads, 1 m apart.

#B - Within the moving load generator, the load system and traffic lane (#C) are assigned to specific "CA" entity

#C - The traffic lane consist of 3 beams, it needs to be defined prior it is used within the moving load generator (#B)

#D - In order to use influence lines (hence assign a special load case into the moving load generator - in this case named LC6 - IL_My), the corresponding unit load needs to be defined within the special "influence line" load case. In this example the load case is set for influence line of internal force, entity My (bending moment along local y axis of beams), and the position is in the middle of the second beam.

After the analysis, except the special load case named LC6 - IL_My (the u_z result of this load case is the influence line - #D) there are two generated result load cases:

#E - maximal value of the bending moment My in the defined position

#F - minimal value of the bending moment My in the defined position

It is possible to check the internal forces of these two generated load cases, e.g. bending moment My (see figure below). The result of the "CA" (#B1) entity are envelope out of these two load cases (#E1and #F1).

Analogically the results of other internal forces might be checked - e.g. Vz:

#E2is the corresponding Vz for the load case of the maximal value of My at the defined position;#F2 is the corresponding Vz for the load case of the minimal value of My at the defined position.

The envelope of these Vz might be verified for the "CA" entity (see #B2).

In order for further result processing, the generated load cases should be used rather then the "CA" results - as these "CA" results do not contain information about the corresponding values between the internal forces. The generated load cases contain this information (e.g. My + corresponding Vz in specific position, etc.)

Example influence lines - influence of number of sections on 1D member on precision of the result

The considered structure resembles a bridge of 3 spans. By using of the influence line analysis, position of certain load (defined in load system) to obtain the maximal My in the middle of the second span (see the unit load for the special influence line load case) is to be determined:

The settings within the moving load generator 1D (step is 0.25 m):

After the analysis of CA entity using the influence lines, two result load cases are generated. Focus on the "max" one, the one where the position of the load will be considered to obtain the maximal My at the given point. The maximal My for this generated result LC_CA_Influence_LIne_max is 1.23 kNm.

If the same load as considered within the load system is explicitly positioned on beam (respecting the 0.25m steps from moving load generator), it is found the same result (just mirrored, as the structure and the load are symmetric) is obtained for the manually defined LC12 - in this load case the maximal My is 1.23 kNm, it appears this load position is considered to obtain the maximal My value. However, just one 0.25m step before, the position of the load is set within the LC11, and the maximal moment My is even larger 1.24 kNm. At the first glance it appears that not the position to obtain maximum My is considered within the generated result load case, but rather one position aside.

However, the reason of this is just little numerical non precision. The difference between 1.23 and 1.24 is rather small. The non precision is caused by the fact, the influence line (first picture of this example) should be precisely 3° curve, but it is considered as linear poly-line for the purpose of numerical integration. During this integration, the number of 10 sections is considered along each beam (what is default setting - see below).

In order to increase the precision, larger number of sections might be used, hence instead of 10 for example 30. If this is used, the manually defined LC11, which yields maximum My 1.25 kNm (also changed a little due to higher precision now) is exactly same as in the generated result load case - see figure below.

Note: the default number of sections 10 is robust enough for most of the cases. The numerical non precision is expected to be rather small, as shown in the example above. Increasing the section number a little will improve the precision. However, due to small difference it is not considered as necessary. This example is shown only to see the influence of this setting, and to show that indeed the load position in generated load cases is expected in order to yield the maximal effect of the corresponding internal force (or deformation) in the given point, respecting certain numerical precisions.