Structure > ILM Preprocessor

General

For several reasons, the erection procedure for an incremental launching bridge requires specific support from the software. The amount of construction stages is higher than for other erection procedures, the launching process itself must be simulated to get the typical envelopes of results and finally the structural model must be completed by additional elements such as launching nose, casting yards and temporary supports (spring elements).

Basically, bridges erected using the incremental launching method, are modeled like all other bridges: The final structure with supports and prestressing conditions is modeled in accordance with the standard modeling rules, following the restriction that the boundaries of the concreting segments must coincide with structural element boundaries. Additional intermediate supports have to be defined as spring elements (ILM-springs) in their real position, detached from the final structural system.The points of the superstructure, which are supported in the individual launching steps, need not essentially coincide with superstructure element ends. Based on input launching sequence the function Recalc ILM will create a refined superstructure system with new elements and new nodes in a new database. The numbering of these new elements and nodes is done automatically.

Additional construction stages and loading cases have to be established for every launching phase. The construction stages related to a launching steps are defined as empty stages. (All launching steps of a particular segment are usually in the same construction stage.) The function Recalc ILM will then place all related actions in the referenced construction stage: The loading information related to the launching steps (removal of previous ILM supports and activation in the new position) will be assigned to previously created empty load sets and load cases. to and start the actual analysis. The calculation in the new directory with updated data can be immediately started, because all empty load sets, load cases and stages have been filled up by the ILM Preprocessor.

However, the application of the ILM Preprocessor module is partially restricted by the following conditions:

The road alignment in plan view must be straight or circular.
The cross section depth should be constant.
The bridge decks structural nodes (bridge and nose) should be on one level.
ILM node series (additional node series that is linked to ILM supports) have to be along one line.
ILM springs have to have a stiffness in all DOF (if there’s no stiffness input a very small value).
To define the launching direction the ILM structure in the ILM Preprocessor has to consist of a segment defined as 'nose' and at least one additional 'girder' segment.

ILM Preprocessor - Segment Definition

The moved superstructure is defined by a segment of type 'nose' and at least one segment of type 'girder'. All ILM-springs that are moved along the structure have to be defined as individual segments.

Following Segment types are used in this context:

Nose segment – Synonym for the first launched element series. All related girder segments are linked to this segment. In case of 'Straight launching' the launching direction is defined by the relative position of the nose segment related to the girder segments.
Girder Segment – Element series that is added to the structure in the casting yard and that is launched together with the related nose and girder segments.
Fix support / Temp. support – Additional spring elements for the launching procedure itself. Each ILM-spring has to be defined as an appropriate segment.

ILM Preprocessor - Schedule

The major difference to the standard procedure is, that in case of an ILM simulation separate construction stages have to be specified

for the launching process itself, and
for the actions performed in between

I.e. we will have alternating stages for every concreting phase, and for every subsequent launching phase. E.g. stage 1 for positioning the nose segment and pouring the 1^st segment (activating the self weight, stressing the tendons, creep and shrinkage till the start of launching, etc.), stage 2 for launching the 1^st segment with a given number of steps, stage 3 for pouring the 2^nd segment and so on. The removal of the launching nose is modeled in a final stage after the last launching stage.

All superstructure elements and spring elements are activated and deactivated in the usual way in the respective stages. These are the stages where the newly launched segment has arrived at the current location and the related schedule actions, such as calculating the load cases self weight, pre-stressing, creep & shrinkage or performing design code checks are done. The specifications for these stages have to be made by the user according to the actual needs.

The intermediate launching stages consist of a predefined number of launching steps and are defined as empty stages in the schedule, i.e. no specification of activation and schedule actions are required from the user. These empty stages will be used for the ILM simulation later.

Further, additional empty load sets and load cases have to be created, in order to be used in the launching stages. The loading information related to the launching steps (removal of previous ILM supports and activation in the new position) will be assigned this load set and load case. This reference load case and load set may be the same for all launching steps, or different. Usually, all steps of a launching stage are related to the same reference load case and load set, but different load cases and load sets are prepared for the different construction states.

ILM Preprocessor - Recalc ILM

If the option Export to RM Bridge is set, this function will create a new database containing the (refined) structural system and the actual loading and schedule data to be used in the analysis. The user can then switch to the new database and start the actual analysis. Please note that all further actions such as traffic load, final creep and shrinkage, etc. need to be performed in the new database on the refined system in order to guarantee compatibility of the results.

Additionally this function offers some checking facilities, which may be activated when the option Export to RM Bridge is switched off.