BIM Brings Changes to Façade Design

The following post is an excerpt from Technological Changes Brought by BIM to Façade Design.

IT and Façade Design

To meet the requirements for energy-efficient, green, and sustainable buildings, and respond to the increasingly serious shortage of contractors and necessary increase of cost, industrialization of façade design engineering is one of the development trends of the architectural envelope industry. Industrialization of façade design engineering must be promoted by the industry and, in turn, its improvement can boost promotion.

CAD and VR

Since the birth of CAD in the last century, engineers have abandoned manual drawing and turned to electronic drawing. This has changed traditional design methods and brought the first revolution in the façade design engineering field.

To manage the CAD data, information technology also underwent a three-stage evolution from CAD file management, to CAD database management, to Product Data Management (PDM). PDM is an integrated working mode that can provide a façade design collaboration environment for sharing, so that designers can work on the same database, reducing unnecessary condition transfer and confirmation and fully sharing information resources.

The rapid development of three-dimensional modeling and virtual reality technology helps improve communication efficiency among the multiple parties involved in façade design.

The visual expression of a three-dimensional model can help architects and façade designers freely exchange ideas and deliberate on the designed volume, shape, façade, and exterior space throughout the design, but the aforesaid façade surface model cannot hold more design information for construction and installation.

In addition to the physical dimensions of the architectural envelope and required materials, design information also includes wind pressure resistant strength, seismic resistance, air tightness, water tightness, transformation, construction technology, heat transfer coefficient, etc. Indetermination of such information will lead to poor efficiency in subsequent work, such as building estimate and budget, fabrication, field installation, etc.

VDC and BIM

Industry demands promote the continuous integration of three-dimensional geometry-based modeling technology, Virtual Reality (VR) technology, and Building Information Modeling (BIM) technology, and development toward integrated application.

Virtual Design and Construction (VDC) is another concept becoming popular in the engineering construction industry. It requires, through the multidisciplinary parametric models provided by design, construction, operation, and maintenance teams in project construction, integration of building facility information, the construction process, and the management organization to ensure the achievement of general management objectives for the project.

If the VDC management concept can be implemented, we will be able to capture and reuse data from conceptual design to prefabrication and even in downstream processes, and apply the data to the entire process from concept design, to modular construction, to component prefabrication.

Then, we will have the chance for “building industrialization.”

BIM is one of the core technologies of implementing VDC. BIM technology is highly correlated with industrialization of façade design engineering logically.

Based on BIM, the carrier of building information, not only the visual design, multidisciplinary integration correction, panel optimization analysis, and quantity calculation for the architectural envelope, can be possible.

A breakthrough can be made, specifically, the mode of documentation in design delivery in the traditional building industry serves no one well. Instead, the panel fabrication drawing can be directly generated and the component fabrication data can be directly extracted from the design model, which makes possible the paperless design and plant fabrication of the curtain wall unit.

BIM technology has become the inevitable choice for industrialized development of the architectural envelope industry.

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BIM-based façade design features Parametric design

BIM parametric design changes all the elements of a façade fabrication to a functional variable, and then, by changing the function, or to say, by changing the algorithm, drives the façade panel shape to change, thus creating different building design schemes.

As an art of building, façade design fundamentally has an anti-logic basis. As aesthetic theory goes, there is no debate for taste. Sticking to conventional thoughts will never lead to the palace of art.

However, parametric design is not contradictory to traditional building design. It is oriented to the future, and has many unimaginable forms. It is a tool that can inspire designers.

In BIM parametric design, all real attributes of façade fabrication components are given a parametric simulation and calculation, as well as related data statistics. In BIM parametric design, a façade component is not only a virtual geometric component, but also has other geometric attributes, such as component material, thermal performance, cost, as well as purchase information, weight, installation number, etc.

The significance of BIM parametric design is that we can, according to different design parameters, quickly conduct calculations and statistical analyses on modeling, layout, energy conservation, evacuation, etc. and then give priority to the most appropriate scheme.

This is where BIM parametric design differs from ordinary parametric design that is only for the realization of geometric modeling.

SKY SOHU Project BIM models

It is noteworthy that the concept of parameterization is different from the concept of parametric design.

Parameterization refers to the modeling capability of BIM software, which is an important guarantee for realization of parametric design. BIM software applicable to façade design must, first of all, provide “accurate” BIM capability to ensure that the modeling accuracy of small BIM components, such as round hole, bent piece, etc., is within the allowable plant fabrication error range.

The American Institute of Architects (AIA) uses level of detail (LOD) to define the accuracy of building components in BIM. According to LOD, BIM evolves from an approximate model to an accurate completed model through the progress of project, and the model accuracy is from the rough to the subtle:

LOD100. Conceptual.
LOD 200. Approximate Geometry (scheme and enlarged preliminary).
LOD 300. Precise Geometry (construction drawing and detailed construction drawing).
LOD400. Fabrication.
LOD 500. As-built.

For an ordinary building design, when the model accuracy is within LOD100 to LOD300, the design delivery can be completed; but for a façade design, to ensure the design delivery model can be applied in subsequent plant fabrication, the modeling capability of the BIM software must reach LOD400.

Knowledge-based visual design

A BIM-based three-dimensional virtual design environment helps quickly transfer design information and simulated information to project partners, so as to improve their communication efficiency, make possible WYSIWYG (what you see is what you get), and reduce economic losses caused by redesign.

Visualization can be used for design clarification of detail structure joints, such as the façade panel edge, corner, hole, junction, and beam bottom flashing and trim. Besides, visual display can help quickly discover any conflicts among disciplines and improve design quality.

Curtain Wall Joints of CABR Research Building (Source: CABRTECH BIM Team)

BIM visualization is automatically generated via the information for entity components. We can have the multi-view sectional drawings and axonometric drawings of a curtain wall model automatically generated to transfer information.

There are correlative and feedback relations among the components in such a “tridimensional wall.”

When a façade design engineer modifies a component, all the views in relation to the component will be automatically updated, saving us the trouble of modifying the plan, elevation, and section respectively.

The “correlative” visual feature is conducive to improving communication efficiency and also improving design engineer work efficiency, solving the long-standing problem of discrepancies among, omissions in, and incompletion of drawings.

“Top-down” design

From concept design, to modeling design detailing, to plant fabrication, to the final installation, the curtain wall is involved in many steps and covers both building and mechanical fabrication fields. In most cases, data cannot be smoothly connected, and data chain breaking may occur.

Based on data chain inheritability underlined by detailed BIM design, and with BIM as carrier, the “top-down” design idea can help accurately get the upstream curved surface modeling data and also accurately coordinate fabrication.

For “top-down” BIM design, it is required to first construct a “top basic skeleton” of the design, and then make copies, modifications, and detailing based on this “top basic skeleton” in the subsequent design process, finally completing the detailed design.

For example, the entire façade design engineering of a project is the highest-level “top basic skeleton”. The tower, floor and other parts can be broken down into several levels of “top basic skeleton” and each can show the geometric shape and spatial position of the façade panels in the part, and reflect the geometric constraint relation with other “top basic skeletons.”

Thus, the “top basic skeleton” is the core of the detailed “top-down” façade design development, and also the bridge and link for the interrelation among façade panel components.

The parametric modeling capability of BIM software is the basis for the smooth development of detailed “top-down” design.

In BIM-based parametric modeling, design can be automatically modified by parameter driving. There is an obvious corresponding relation and global correlation between the parameters and the controlled sizes of the model, so that the transfer of model data changes from and to different levels enjoys uniqueness and instantaneity. The “top-down” BIM design has two main characteristics:

The geometric modeling of the façade fabrications can be easily transformed to building components with real attributes. When we change parameters to make geometric shape change, building components change accordingly, which relates visual shape to real façade fabrication components, so that the visual model is transformed to a real “information model”. In the detailed design of a metal curtain wall, for instance, BIM-base technology can help, according to architect’s requirements, generate a large complex curved surface, easily divide the curved surface, and cut the shape into small, simple-technology, material-saving curved panels suitable for mass production.

Then, via sheet metal unfolding, turn them into drawings of plan view size, and make perform cutting and blanking with fewer errors or error free. Furthermore, using the building components with real attributes, an enterprise can gradually enrich and complete its parametric curtain wall component library, conducive to accumulation and reuse of enterprise knowledge.

After the shapes and positions of “top basic skeleton” are satisfactory, optimizing special-shaped curved panels can be done to meet the requirements of complex surfaces for fabrication, transportation, installation, and cost.

The façade designer modifies the shapes of curved panels by parameter driving, and, within the allowable visual error range, replaces double curved surfaces with single curved surfaces, and curved surfaces with flat surfaces, to generate a standard, simple façade fabrication wherever possible. In the meantime, the façade designer must give consideration to cost, construction difficulty, physical performance, and nice appearance.

For instance, the façade designer must take into consideration the supply situation for panels and the fabrication parameters of numerical control machine tools, and calculate the maximum sizes of panels, for purposes of gradual optimization and balance between nice look and cost efficiency.

The reason why BIM software can enable panel optimization lies in its excellent parametric modeling capability and its real-time data extraction capability.

Because the façade model contains geometric information (about panels, keels, connecting pieces, supports, and embedded parts), material information, and management information, when the façade panel shape changes, the corresponding material list and cost information is generated quickly. Because the cost of a special-shaped, curved-surface façade panel is uncertain, the adjustment of the curtain wall shape will definitely change a series of factors, including component cost and fabrication requirements.

BIM can correlate all the factors to form a data model with a dynamic update function, which is used to continuously improve and optimize the previous curved-surface façade models, compare the cost indicators for different design schemes in real time according to the output tabular bills of quantities, and, through a step-by-step iterative loop, finally make possible the balance between nice look and cost efficiency.

Automatic professional correction

As façade design engineering becomes more and more complex, there is a trend of cross-disciplinary cooperation in façade design. When the main structure is nearly completed, façade engineering can start, together with electromechanical engineering.

Façade design is closely related to other disciplines in terms of spatial position. BIM can change the traditional mode-of-work coordination among architects, structural engineers, and façade design engineers, and integrate the BIM models of different disciplines for interdisciplinary collision detection to discover the geometric position conflicts among different disciplines in advance.

For instance, through interdisciplinary collision detection, we can determine:

the reserved room between curtain wall keel and concrete structure
whether the structure leaves adequate room to the façade fabrication
whether the positions of embedded parts are accurate
whether there are any conflicts with decor and electromechanical positions

In another instance, we can examine whether the positions of large trimmings and logos are matched with the façade fabrication structure to determine whether there are any conflicts between the building and the façade fabrication.

Changes brought by BIM-based design to subsequent processes

Model-based design delivery

Model-based design delivery is one of the important means of industrialization of the architectural envelope industry. As façade fabrication is mostly customized in plants, design and fabrication are closely combined. Compared with traditional manufacturing, façade panel units enjoy a higher degree of customization, which is reflected by not only different designs for different projects but also different façade panels even in a project, fast and flexible production is needed.

Apparently, the mass production based on standardization and regularization of façade panel units is not the mainstream direction of industrialization of façade design engineering.

BIM-based design delivery can help avoid information loss in the transformation of two-dimensional design to three-dimensional fabrication model, and accurately transfer the façade design data to the numerical control machine tools, directly used for façade fabrication.

Therefore, the complete, accurate transfer of design data and automatic digital fabrication can not only improve building quality, but also reduce the huge waste arising from the design to different fabrication steps, which may be the industrialization development trend of the future architectural envelop industry.

Virtual assembly

Recently, unitized façade fabrication is increasingly being applied. Because unitized façade panels are fabricated and assembled in workshops, the builders working at the construction site must have a good knowledge of the façade panels for different façades and of different floor heights and types, so as to have them correspond to the right positions.

After the detailed BIM-based façade design is completed, the components, such as unit panels, keel frames, and irregular profiles, can be given unique codes according to data planning; then the façade model of the whole building is assembled, and the data can be extracted to generate a material list. In the material list, each component has a unique number.

The Installation Number of Façade Panels Generated From the BIM Models of GALAXY SOHO Project [2]

Blanking, fabrication, and management of material placement are done according to the numbers and the units are quickly assembled according to the standard unit template drawings.

It is required to note the information about fabrication, transportation, and installation direction and sequence in the façade panels. BIM means can then be employed to preassemble the façade panel to better arrange the multi-operational construction plan and installation sequence, scientifically create site planning, reasonably arrange the construction period, improve installation quality, and reduce the idling of the labor force.

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Excerpted from Technological Changes Brought by BIM to Façade Design