Interfaces and Relevant Features for BIM-Oriented Planning

Technical Article

This article discusses the most common BIM interfaces. During the transition to the structural branch-specific model, adjustments are often necessary. The occurring tasks and tools for their successful and fast solution are presented.

BIM Interfaces

In order to be able to use created BIM models also for structural design, defined interfaces are required with which structural components can be transferred to the respective structural engineering software for analysis and design. The standard interfaces usually allow the transfer of structural objects (members, plates, walls, solids). Moreover, interfaces specifically tailored to structural analysis include structural objects such as support conditions, releases, loads, load cases and load combinations. These structural objects are not really visible and tangible data resulting from the pure geometry of the structure, but depend on its use and applied standards.

The structural engineer also decides whether, for example, a support or release is acting rigid, semi-rigid or completely hinged. If, in addition to the physical BIM model, which clearly describes the geometry (visible), the mechanical structural model (idealised model for the calculation) is also available, these are ideal conditions for starting directly a structural analysis. Instead of the physical BIM model, the mechanical structural model is then used directly. Interfaces which also contain structural objects are:

  • IFC Structural Analysis View
  • SDNF Format
  • Product interface steel construction
  • CIS/2

The development of these interfaces took place at the national level in the first instance in the late 1990s. However, it quickly became clear that international standardisation was required, and therefore it was agreed that further developments will only be based on Industry Foundation Classes (IFC). Therefore, new developments can only be expected in the IFC Structural Analysis View. Nevertheless, the interfaces mentioned above are still widely used and make an important contribution to BIM scenarios in many companies.

In addition to these interfaces based on exchange files in text format, BIM and structural engineering software for analysis and design are also coupled via direct interfaces, where data is transferred via APIs (Application Programming Interfaces). Such interfaces do not depend on interface specifications (such as IFC or SDNF) and therefore it is up to the participating companies which possibilities and limits of transfer may exist. Dlubal Software developed interfaces to the following programs based on APIs:

  • Tekla Structures
  • Autodesk Revit und AutoCAD
  • Bentley ISM
  • AVEVA Bocad

Figure 01 Design Revit - Calculation Dlubal RFEM - Construction Tekla Structures

In addition to the previously mentioned interfaces, the DXF format also plays an important role, but the object orientation is missing. If no mechanical structural model is available, only physical BIM models can be transferred. For this, the IFC-Coordination View 2.0, based on IFC 2x3, is the most important interface. Since mid-2017, buildingSMART offers the possibility to certify the next generation of this standard - IFC 4 Reference View - and further implementations of the software companies are to be expected. If Coordination View or Reference View models are transferred, it is necessary to create the mechanical structural model in the structural engineering software for analysis and design. Various tools are necessary in structural analysis software for this work, and some of them are presented in detail below.

BIM-Relevant Features in Structural Analysis Software

IFC Compatibility
IFC Coordination View models serve as a model for a mechanical structural model. The IFC objects must be transferred to the native objects of the respective structural analysis software (beam, surface or solid elements) so that a calculation is possible. A pure referencing (visualisation) is not sufficient.

Figure 02 - RFEM Conversion IFC Object to Structural Object

Material and Cross-Section Mapping
In structural analysis, characteristic values for materials and cross-sections (modulus of elasticity, safety factors, moments of inertia...) are required, depending on design standards. In a BIM software which focuses on architecture and quantity surveying, these parameters are not needed and/or too little attention is paid to them. The calculation software has very sophisticated own databases adapted to the requirements of the design. The materials and cross-sections must be translated from the import data into the data of the structural analysis software by means of mapping tables. This translation should also be flexible enough to define free dimensions for defined cross-sectional basic shapes (any rectangular, I-section, or channel cross-section, and so on) via parameter mapping.

Alignment and Connectivity
Furthermore, it is necessary to ensure correct connectivity of the elements and to define releases and support conditions. It is maybe necessary to displace and align the imported objects.

Figure 03 - Extension of Free Member Ends and Merging to a Common Node

For this purpose, it is necessary that many nodes and thus the connected objects can be moved in any reference levels with just a few steps. No duplicate or quasi-identical nodes may occur, which usually cause difficulties in meshing.

Figure 04 - Vertical Orientation of the Wall at the Ceiling Level

Plausibility Checks
Problems must be found quickly and eliminated with the help of the program. For this purpose, plausibility checks for double or close nodes, overlapping lines, short lines or possibly free unused nodes are important.

Clean-Up and Merging of Nodes
Essential functions such as merging of multiple nodes with automatic adjustment of affected dependent objects must not be missing. It is especially helpful if you can work graphically and use drag-and-drop functionality.

Figure 05 - Identification/Clean-Up of Almost Identical Nodes

Physical Components and Their Relevance
By necessary intersecting of crossing lines or members, physical components such as beams or columns are divided into multiple FE elements. In doing this, it may happen that important reference lengths such as the actual column length which plays an important role for the design or for the update of the original BIM file get lost. The structural analysis software should offer an option to maintain the original length.

Figure 06 - Sets of Members of RFEM Combine Individual Members and Correspond to the Physical Components in the BIM Model

Identification of Changes
When importing/exporting data several times to respond to changes, the software should offer the possibility to display the individual states of revision or at least to filter the changes made in the current step.

Figure 07 - Marking and Visualisation of Changes in the Structural Analysis Software RFEM by Means of Visibilities

Offset Modelling, Customization Tools
For simple referenced models (possibly partial models), picking up and modelling models has proven effective. It is then very helpful if the centre lines of the walls can be designed by using the global offset of half the wall thickness. Tools for extending or shortening lines relative to other lines (trimming/extending) must not be missing.

Figure 08 - DXF Background Layer: Wall Centre Line is Created by Picking up the Outer Edge With Offset

Unique Identifiers
For roundtrip and thus necessary update scenarios unique IDs are required. The software must be able to save these IDs. It may be necessary that IDs of other programs are imported or their databases are updated with regard to the IDs.

Copy & Paste
A lot of working time is required for BIM and derived structural models in the advanced stages. It must be ensured that the respective applications do not overwrite existing data of other branch-specific models. Therefore, it may be helpful if changes such as new members or surfaces are only imported by simple copy & paste. This allows the user to select specifically which changes are taken into consideration.

Structural Results in the BIM Model
The improved data-based communication is a major advantage of BIM. This results in the fact that the structural analysis software provides, amongst other things, internal forces, cross-sectional changes or reinforcements. This information needs to be digitally available so that other special applications can handle further tasks based on it. In the simplest case, DXF-based result plots can be, for example, referenced, with which the BIM designer models reinforcement drawings or connections.

Figure 09 - Export of the Amount of Reinforcement From RFEM and Visualisation in Revit

If the BIM software offers open databases, this communication is raised to a much higher level and subsequent applications can access this data automatically via special APIs.

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RFEM Main Program
RFEM 5.xx

Main Program

Structural engineering software for finite element analysis (FEA) of planar and spatial structural systems consisting of plates, walls, shells, members (beams), solids and contact elements

Price of First License
3,540.00 USD
RSTAB Main Program
RSTAB 8.xx

Main Program

The structural engineering software for design of frame, beam and truss structures, performing linear and nonlinear calculations of internal forces, deformations, and support reactions

Price of First License
2,550.00 USD