By Rita Wong, P. Eng. Halcrow Yolles
BIM -Promises And RealitiesBuildings Building Structure Software & Applications
Previous articles in this series have shown that BIM (Building Information Modeling) software is not simply a 3D modeling and design documentation tool for generating innovative design solutions. Behi...
Previous articles in this series have shown that BIM (Building Information Modeling) software is not simply a 3D modeling and design documentation tool for generating innovative design solutions. Behind BIM’s geometric representation of objects is a database of extensive information that can be retrieved for reference or manipulation. As such, BIM is an integrated model that can be used for the design, documentation, coordination and management
of a project. One of the benefits structural engineering firms expect is the improved workflow efficiency that they might achieve through the use of an integrated BIM model. To date, however, Halcrow Yolles has found that exchanges between BIM and other engineering software applications have not been as efficient as anticipated due to limitations in the interoperability of different software.
Challenges with transferring data
Possible categories of BIM integration include:
(1) links to third party structural analysis and design applications
(2) customized structural analysis and design functionality in BIM software
(3) links to other construction-related software.
The bi-directional interaction of BIM software with structural analysis and design packages aims to eliminate the need for multiple models among office team members. It also aims to facilitate the coordination of engineers’ design work with design documentation.
Exchanges between BIM software (Revit Structure, Tekla Structures) and various engineering analysis/design software (ETABS, SAP2000, RAM Structural System, SFrame, Fastrak Floor Designer) have been applied on numerous Halcrow Yolles projects with varying degrees of success. Types of data that can be transferred (some are limited to uni-directional transfers only) include non-complex structural geometry, material properties, member section sizes and end releases, loads and supports.
Although the transfer process appears straightforward, the effort required is more involved.
The most common issue with the data exchange is the differences in modelling scope and nomenclature between BIM and analysis software. For example, dimensional accuracy required in the BIM model for construction purposes could be too complex for analytical purposes, and manual adjustments to create a simplified analytical model need to be thoroughly tracked. On the other hand, finite element meshing in the analytical model might lead to extra structural members in the BIM model, giving an augmented piececount.
In terms of nomenclature, the definition of various parameters usually differs among software packages. Material and section profile names require proper mapping to be transferred correctly. As well, contrasting methods of defining floor levels can lead to inaccurate results. For instance, a Pratt truss with sloping chords may generate redundant levels at each node elevation when imported into BIM software; a mezzanine level might be lost in translation; or a sloping member might be converted to a horizontal beam depending on what constitutes a floor level in each software.
In addition, certain parameters may not be passed back and forth between programs due to restrictions in the software’s Application Programming Interface (API). Translation using neutral file formats aimed at data interoperability — Industry Foundation Classes (IFC), CIMSteel Integration Standards (CIS/2), Steel Detailing Neutral File (SDNF), etc. — is not flawless. The reason is that BIM and engineering software may support different versions of the standard and some of these formats may not be governed by conformance programs to ensure accurate results.
Another observation from using third party links is that there is usually a few months’ lag from the release of the BIM software update to the availability of the revised link to suit. If other firms in the design team promptly upgrade their BIM software, the structural engineers’ reliance on using an integrated structural model for analysis and design is impacted.
Structural offices are left with several options to deal with the issues mentioned above: they might develop workarounds for the limitations until third party vendors produce more comprehensive interface programs; they might consider using alternative analysis and design packages with superior integration performance; or they might create their own in-house links between the BIM and engineering software used by their office.
Case in point: Canadian Museum for Human Rights
Halcrow Yolles’ experience on the Canadian Museum for Human Rights project, where deliverables are specified to be on the Revit platform, illustrates some of the limitations in current BIM software. Located in Winnipeg, the project has a complex geometry that posed a challenge to creating a precise structural model using Revit Structure as the sole modelling tool.
As a result, CATIA was used to generate the structural model from the design architect’s Form-Z surfaces. Various methods were used to import each portion of the model into Revit Structure 2009, as this version has limited capability to deal with complex geometry.
The Excel-based model generator was used for sloped or curved structural steel framing, but the orientation of members had to be manually adjusted in Revit. Segmented sloping concrete walls following radial curves in plan could be imported as masses via SAT files, but temporary walls had to be modelled to create Revit wall objects with precise geometric configurations at wall joints.
As for the analytical model, a third party link between Revit and SAP2000 was not available, and the workaround method of transferring via ETABS would risk unintentional changes to the complex geometry. In addition, not all assignments made to the analytical model in SAP2000 would be carried back to Revit, which meant re-assignment of data would be necessary in subsequent exports of the updated Revit model. This difficulty resulted in the creation and manual updates of three separate models in the office, contrary to the purpose of using BIM technology. The project schedule did not allow for the production of in-house links to overcome the interoperability hurdles, but we anticipate that the newer versions of Revit, combined with internal efforts, will lead to a smoother integrated process for future projects.
Using alternative customized options
Engineering firms may make use of alternative integration options that operate on various components of the building.
They can customize engineering functionality in the BIM application to design individual structural elements by employing model data and user-defined criteria to affect parametric objects. Examples include joist depths that can be assigned in the BIM model based on deflection criteria, applied loads and joist spacing; or footings that are sized and reinforced based on column loads and soil conditions. Another method of incorporating engineering data is to associate Excel files containing design results with schedules produced by BIM software for contract documents.
Opportunities to expand scope of services
The additional dimensions of BIM offer integration opportunities beyond structural engineering. BIM models can be linked to other construction- related software, such as applications for producing specifications, construction scheduling, quantity take-off, cost estimating, energy analysis and building envelope design. These capabilities can be used to expand an engineering firm’s scope of services, or used by construction firms to simplify their processes.
One main feature that distinguishes BIM software from other 3D programs is its underlying database. The ability to access and connect this information with various engineering packages is expected to increase efficiency for structural engineering firms and allow for earlier design decisions. Although data transfer with engineering applications is not yet seamless, with
continuing development of the interfaces both by third party software vendors and in-house efforts, a truly integrated model is achievable in the near future.
Having an integrated model is only the technical half of the equation, however. Changes to the office workflow and production process are required to complete the BIM transformation in a structural engineering firm.
Rita Wong, P. Eng., is with Halcrow Yolles, international structural consulting engineers based in Toronto.