美国建筑行业BIM标准：进展报告

这是一篇《BIM Handbook》英文原著作者，美国乔治理工大学Chunk Eastman教授及其同事写的关于美国建筑行业制定BIM相关交换标准进展情况的报告。文章首先概述了美国制定行业BIM标准的一些基本情况，并以作者主持的“预制混凝土”行业BIM标准制定为例，详细阐述了预制混凝土涉及的各专业之间的数据交换。数据交换涉及5种预制混凝土相关软件（Vectorworks Architect, StructureWorks, Tekla Structures, Scia Engineer, and Nemetschek Precast Part Manager.），均以IFC作为数据交换的中间格式。交换图如下：

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Introduction

Significant efforts are being undertaken to “solve” the interoperability problem by developing IFC model views that carefully define the contents needed in specific exchanges. This approach is laid out in the National BIM Standard (seeNational Building Information Modeling Standard, Version 1.0 Part 1 Overview, Principles, and Methodologies, Technical Report, National Institute of Building Sciences athttp://www.buildingsmartalliance.org).  Examples of such progress include COBie (Construction Operations Building Information Exchange, developed by U.S. Army Corps of Engineers) for contractor hand-off to owner at the end of construction for operations and maintenance (seehttp://www.wbdg.org/resources/cobie.php) and the General Services Administration (GSA) development of GSA-2010 BIM standard for government design and review (see GSA BIM Program Overview  at http://www.gsa.gov/portal/content/102276).  These efforts have largely targeted single or a small set of exchanges. None of the National BIM Standard efforts have yet completed a set of exchanges that address a large AECO domain.

Potential significant advantages exist for a domain-wide specification of exchanges:  more effective use of domain experts’ time, efficient identification of subtle nuances dealing with different lifecycle stages that can be missed in a single exchange, the ability to re-use specifications and implementations, and integrated development and testing by the software companies. There are economies of scale of doing multiple exchanges together within a target domain.

The first effort to address such industry-wide exchanges is the Precast Concrete National BIM standard, sponsored by the Precast/Prestressed Concrete Institute (PCI) and the Charles Pankow Foundation (CPF).  The PCI effort was initially led by Michael LaNier of BERGER/ABAM Engineers Inc, who was succeeded by Jason Lien of EnCon United and an advisory committee of over 20 precast, engineering and other company representatives. The technical work of the committee was supported by a team from Georgia Tech and the Technion, led by Chuck Eastman and Rafael Sacks.

This industry-wide effort was defined to address all the major exchanges dealing with precast concrete elements throughout the product lifecycle, addressing a range of contract delivery methods with different up-front workflows. The exchanges are between different actors in the process, including architects, structural engineers, precast engineers/detailers, general contractors/construction managers, precast plant managers and erectors. The effort followed the procedures of the National BIM Standard referred to earlier. Twenty-six exchanges were initially identified. 

Workflows in different projects each have their own specific context, leading to variations in details. But the varied exchanges can be well documented and specified by addressing who are the actors and what stage of the building lifecycle are involved, plus the intent of the senders/receivers. While the exchanges were each specified separately in sufficient detail to guide implementation (see Introducing a New Methodology to Develop The Information Delivery Manual For AEC Projects in Proceedings of the CIB W78 2010: 27th International Conference – Cairo, Egypt, 16-18 November by Aram V, Eastman C, Sacks R, Panushev I, and Venugopal M.), many duplications were found. Some exchanges were needed for different purposes, but had the same precast content. Also, there were many repeating modules of information specification, dealing with objects, embeds, geometry, properties, finishes and so forth. Consolidating the exchanges based on the defined criteria reduced the number to only a few major exchanges (between eight and ten, depending on the degree of similarity imposed).

Demonstration

On October 25 2011, at the PCI Convention in Salt Lake City, Utah, initial results of the National Precast BIM Standard were demonstrated. Five precast concrete software systems participated: Vectorworks Architect, StructureWorks, Tekla Structures, Scia Engineer, and Nemetschek Precast Part Manager. Six exchanges were made between these applications using Industry Foundation Classes (IFC) as the neutral data schema. To visualize the exchanged data, two other applications were used: Solibri Model Viewer and Nemetschek IFC Viewer.  The exchanges are those shown in Figure 1.

Exchange One: Architectural Model to Structural Engineering

An architectural design model was created in Vectorworks Architect software, which consisted of a part of precast utility building. The brick-walled building had two storeys each to be erected of two slab bays; this makes four slabs total and six columns in addition to cast-in-place slab on grade. The architectural model of the structure was extracted as shown in Figure 2. The architectural design model defined each slab as a monolithic container for the second floor and generic double-tee slabs as placeholders for the roof slab. Apart from the generic slabs, columns and beams, the architect also designed corbels on each column, with the corbels being attached features on the column. 

Figure 2. The precast test model designed in Vectorworks Architect.

This architectural model was exported into Industry Foundation Classes (IFC) in two different representation formats, as B-rep and extruded geometry, each having different uses. B-rep is a simple face set representation useful for volume calculations and clash detection. However, for more complicated tasks such as editing and parametric model elaboration, extruded geometry is required. The IFC file exported was visualized in Solibri Model Viewer to verify the model information. Figure 3 shows the hierarchical structure of the model displayed in Solibri model viewer.   

The model hierarchy shown in Figure 3 is embedded as part of the IFC model using the relationship entities available in IFC. This shows that the model export is not just plain geometry but also includes important information assigned to objects in the form of attributes such as hierarchy, material, quantities, metadata, etc.

Exchange Two: Structural Engineer to Architect for Review

The structural engineer received the design model from the architect and designed the elements structurally. He performed structural analyses on the designed engineering structure. This was achieved in the demo by importing the IFC file into Scia Engineer. This step was performed in a semi-automated manner. The engineer imports the entire IFC model; however he has control over which entities are to be converted into analytical members in the Scia Engineer format. Once this conversion operation is complete, the engineer can perform various analysis tasks such as shear and bending and complicated analysis such as non-linear cracking, time dependent analysis, dynamics, etc. Figure 4 illustrates two of the analyses performed on the precast piece members for the demo.

Based on the analysis results, the structural piece members were modified for structural integrity; in this case, two corner columns were extended with short shear walls. The revised structurally safe model was passed back to the architect for review. For this purpose, the model is again exported into IFC and imported into Vectorworks. This completes the initial set of exchanges between the Architect and Structural Engineer.

Exchange 3 and 4: Design Model to Precast Detailing System

Traditionally, the exchange of data between architects and precast concrete fabricators occurs in the format of Contract Documents. The architect provides the contract documents to the general contractor, who then passes them to the precast fabricator. The precast fabricator typically re-creates all the information in the form of a new set of drawings showing the details of the precast pieces. These are called the Precast Assembly Drawings and they are used to derive the piece drawings for production of the precast pieces. After detailing, the precast assembly drawings are passed back to the architect for design intent validation. The rework and time spent generating the same information in two different set of applications can be considerably reduced or eliminated by providing interoperability between design applications such as Revit, ArchiCAD, Bentley and Vectorworks to detailing packages such as Tekla, StructureWorks, AllPlan Engineering etc.

In order to achieve this, the architectural design model is passed to the precast detailer. At the detailing stage, accurate information about precast piece detailing, including connections, finishes, joints, reinforcing, tensioning cable layout, pre-tensioned pieces, embeds, lifting hardware, etc., is modeled. In terms of model progression and level of detail, the detailed precast model should include all discrete components, in contrast to the monolithic elements in architectural design model. Hence, slab containers are replaced with individual precast planks, connections, topping, and with provision for camber in-place. Figures 5 and 7 show the details of individual hollow core and double tee planks in place of the monolithic slabs in Figure 2. The individual entities are aggregated into a parent slab, maintaining the assembly information of the slab.

Figure 5. Individual hollow core and double tee planks in place of the monolithic slab entities in the architectural model.

For the purposes of this demo, the IFC model was imported into two detailing systems, namely Tekla and StructureWorks. There are differences in the way each system imports the IFC data and converts it into its own native file format. Some rebuilding of the parametric structure of the precast pieces was required in order to make them editable in native form. This was dealt with explicitly in StructureWorks (a SolidWorks add-on), applying the constraints to the edges and surfaces or the assembly. Tekla uses an object converter that partially automated the conversion process. Some of the specific details added by StructureWorks are shown in Figure 6, such as the connection of corbels and the cut notches and rebar for attaching the column to the foundation.

Figure 6. Details for precast column connection: corbel connection(left image), and notches for baseplate connection (right image).

The detailed model helps the precast detailer to generate general arrangement shop models and drawings that can be guaranteed to be consistent with the production piece drawings and the bill of material. An example piece drawing is shown in Figure 7.

There are two exchanges where the detailed model is passed back to the architect and structural engineer for design intent validation and structural review (these were not included in this demo). Architects review the detailed model with corrections as required in terms of the joints and alignments of the precast panels, materials, topping, and visible surface finishes. Structural engineers review the model for structural integrity. There are many cases in the industry where different precasters each have their own standard member dimensions. Hence, there might be changes in the thickness and width of the precast elements added to the model during this stage. It is important for the structural engineer to review the load bearing capacity of these modified pieces and also the temporary erection loads.

Figure 8 shows the same detail in Tekla and in StructureWorks. The articulated slabs show the fabrication detail added by different detailers with the floor slab made up of precast hollow core planks with topping, beams, columns, and corbel system.  The two models are slightly different, based on different hollow core profiles, defined by different detailers. In the figures, certain beams or columns are made transparent to visualize the internal connection hardware and rebar.    

Figure 8. Detailing of slab, beam, column and corbels in Tekla (top image) and StructureWorks (lower image).

Exchange 5 and 6: Precast Detailed Model to Management

There are different downstream uses for the detailed precast model. Construction coordination and clash detection, plant and part management, fabrication and erection are examples. According to a 2009 McGraw-Hill survey ( see The Business Value of BIM: Getting Building Information Modeling to the Bottom Line by N. Young, S. Jones, H. Bernstein, J. Gudgel, The McGraw-Hill Companies, 2009), the BIM-based coordinated activities are generating significant benefits to the construction industry. In practice, the general contractor brings together the models from different subcontractors and checks spatial coordination between systems to avoid clashes before actual construction begins. B-rep geometry is sufficient for this task. At this time or earlier, the contractor determines the construction sequencing and schedule.
 
The detailed precast model is passed to the plant management system to coordinate the fabrication and delivery of the precast pieces with other project elements being produced at the same time. Project sequencing is applied throughout these steps, so pieces are produced in the order they will be erected. Two different part management systems were included in the demo: the StructureWorks Part Tracker and the Nemetschek Precast Part Manager. The StructureWorks system is a web-based system that imports the IFC model and extracts all the piece detail information. The Nemetschek Precast Part Manager imports the IFC files and allows for graphical planning of the project and plant resources, as shown in Figure 9.  These systems allow allocating parts to the fabrication beds, based on the plant schedule and also orchestrating a delivery schedule. Part management systems also integrate with ERP systems and can also pass information back to the general contractor.

Figure 9. Production management in Nemetschek Precast Part Manager.

Using the IFC model as the input into production planning streamlines the planning process, helps to maximize the resources and minimize material waste so the plant can run at  peak  efficiency, and  allows a predictable and efficient delivery schedule of all necessary detailed components, with needed product information.

Summary

This demonstration offered an important example of the potential of “openBIM,” showing a sequence of exchanges over a large portion of the precast concrete workflow. While the complexity of the elements was not great, the demonstration showed the benefits that precast concrete software companies can provide to support smooth and effective workflows. It also showed by example how exchange interfaces can support on-the-fly adjustment of exchanges regarding types of geometry, what level of detail to include, and which properties. An advantage of supporting a set of exchanges is that combinations of exchange components with different functionality are implemented together and can be controlled by the user. Other industry-wide exchanges are now in development, including efforts led by the American Institute of Steel Construction (AISC) and the American Concrete Institute (ACI).

Acknowledgements

We would like to thank the presenters of the demonstration: Robert Anderson of Nemetschek Vectorworks, Mark Flamer of Nemetschek Scia , Mark Potter of StructureWorks, Alistair Wells of Tekla, and Dan Monaghan of Nemetschek Precast for their strong support and help. We also thank Rafael Sacks for his consistent support and advice.

 

About the Authors

Chuck Eastman is a Professor in the Colleges of Architecture and Computing at Georgia Institute of Technology, Atlanta, and Director of the Digital Building Laboratory, a joint industry-university research center. His career has been spent making building models a practical reality, starting in the 1970s. He has held positions at UCLA and Carnegie-Mellon University, and been funded to advise industry associations on their development and deployment of BIM, including AISC, ACI, PCI, GSA, NIBS and FIATECH. He has authored 5 books and over 100 journal papers.  In 2006, he was awarded the buildingSMART Open Data Award for his work on CIS/2. He can be reached at:chuck.eastman@arch.gatech.edu. 
 

Manu Venugopal recently completed his Ph.D. defense in the School of Civil and Environmental Engineering at the Georgia Institute of Technology. His doctoral research is titled “Formal Specification of Industry Foundation Class Concepts using Engineering Ontologies.” It aims to improve the model exchange semantics and was funded by a National Institute of Standards and Technology (NIST) grant. At Georgia Tech, Manu is involved in a variety of engineering projects involving Building Information Modeling (BIM) at the Digital Building Lab headed by Professor Chuck Eastman and emerging technologies for the construction industry at the RAPIDS Laboratory headed by Dr. Jochen Teizer. He has been involved with the development of a Precast National BIM Standard sponsored by the Precast- Prestressed Concrete Institute (PCI) and the Charles Pankow Foundation.
 

Shiva Aram is a Ph.D. candidate in Design Computing program at Georgia Institute of Technology. She also has an MBA degree from Georgia Tech. Her research is about Building Information Modeling (BIM) implementation in the AEC industry, focusing on BIM-based interoperability and managing transactions in BIM servers. During the last three years, she was a member of the technical advisory team on a project implementing National BIM Standard (NBIMS) for precast concrete funded by Charles Pankow Foundation and PCI. She has also worked with AISC on a project to enhance steel model exchanges which is still in progress.  Shiva received her master’s degree in Project and Construction Management from Shahid Beheshti University.