BIM and Algorithmic Form Finding (1-2-3 Revit Tutorial)

1 Feb, 2007 By: AIA ,Rick Rundell

Parametric change engine provides advanced modeling techniques for exploring innovative real-world building designs.

The skylines of the 21st century are changing. The boxy towers that dominate our cities are getting some interesting new neighbors: buildings that owe their architectural form to an algorithmic design approach. How does BIM support the exploration of design possibilities for these new structures?

BIM (building information modeling) is the creation and use of coordinated, consistent, computable information about a building project in design -- information used for design decision-making, production of high-quality construction documents, predicting performance, cost-estimating and construction planning and, eventually, for managing and operating the facility.

A purpose-built BIM solution based on parametric modeling technology, such as Revit Building, can also be used very early in design process for conceptual form-finding and algorithmic design studies, based on mathematical parameters controlling building shapes.

This month's article focuses on a few typical techniques for using Revit Building to create complex, intricate architectural forms -- design ideas that can be the starting point for expressive building structures of the 21st century.

Designing the Rules
Revit Parametric Components (also referred to as families) offer an open, graphical system for design thinking and form making. Revit families are extremely powerful because they are driven by Revit's parametric change engine, which ensures that a change made to a family is propagated throughout the entire project. Once created, a family's parameters remain exposed and you can edit them directly within a Revit project. Ultimately, it's the parameters that drive a family's geometry. Mathematical formulas can also drive parameters in families; one example is using a formula in a window family that always makes the width of a window twice its height.

Formula-based parameters can also be used to create very complex, imaginative forms for exploring massing studies. These are not the traditional conceptual designs that dominate the architectural schematic design phase; rather, they're ideas that are quickly generated and visualized, shared with a client and perhaps even exported to Google Earth or the like to view in the context of the existing infrastructure.

Below is a step-by-step procedure for creating a parametric massing study, plus two sections with procedures on how to create a matching structural form and a pleated curtain wall.

Parametric Massing Studies
The simple Blend (shown at the top of the figure below) is the basis for this conceptual model. An integer coefficient drives algebraic formulas that determine the amount of rotation between the top and bottom of each mass as well as its orientation relative to the overall height of each mass. (See the formulas listed in the Family Types dialog box below and in the close-up in the next figure.) Later, the ability to control rotation and orientation lends a spiral effect to the overall mass.

A simple blend, driven by instance parameters associated to a single Level parameter, is used to create this formulaic massing study.

Formula-based parameters can be used to create complex forms.

In the conceptual model, multiple instances are created and stacked on top of one another. Next, the height of each instance is set to a corresponding floor-to-floor height. To create the tower form shown below, each instance's integer coefficient is set to a corresponding level number to determine its proper orientation and rotation.

Formulas control the orientation and rotation of the stacked instances of a single blend.

Structural Forms to Match
Continuing with this same general design idea, you may want to investigate the structural expression of the form as well. To do this, use the same initial mass element as a void instead of as a solid so it cannot be seen and yet is still the controlling shape. Swept forms associated to the vertices of the void create the structural system.

Use the conceptual massing tools to create and parameterize a volume that, although not visible, provides control over the structural elements once they are snapped to it. As in the example above, similar formulas and parameters let you control properties -- such as angle of rotation and floor-to-floor height -- that later help define the structure for each level.

The volume shown here, although not visible, provides control over the structural elements.

Like the previous example, multiple instances of the family are arrayed vertically and each instance is assigned a level. Based on the Level parameter, the height and angle of rotation adjusts to the proper location. Once set, each instance aligns with its two adjacent instances to form a continuous structure.

Swept forms associated to the vertices of the void create the structural system.

Curtain Walls
The last example is a pleated curtain wall. The technique is based on using a mass element as a void, to drive the form of each individual panel. Cylindrical sweeps are created by snapping to the vertices of the void. By varying the parameters of the void, you can change the height, width and amount of pleat in the curtain system.

Unlike the previous examples, this design element typically is used in an ongoing BIM. It's created as a Revit Curtain Panel Family and can be used with an overall curtain wall system.

Like other Revit Curtain Systems, the Pleated Curtain System is an array of unique Curtain Panels.

By varying the parameters of the panel element (highlighted here in red), you can change the height, width and amount of pleat in the whole curtain system.

A void blend is modeled to define the contour of the curtain Panel. Because of a void's visibility properties, the blend is not visible in the curtain panel, although it does provide a base for all model geometry in the Curtain Panel. It's parameterized to manage changes in variables such as height, width and depth.

Parameters of the blend control the shape of the base curtain panel element.

Next, a thin layer of glass is modeled on top of the void form.

A thin layer of glass is added to the void form.

Finally, the vertexes of the void create mullions.

The vertexes of the void create mullions.

Summary
BIM solutions are often associated with the most time-consuming portion of the building design process -- the detailed design and construction documentation stages. That said, a BIM solution based on parametric modeling technology, such as Revit Building. can also be used for early algorithmic form finding and supports broad explorations of design possibilities and architectural form.

Images in figures 2-10, courtesy of Phil Read, Autodesk Consulting.

About the Author: AIA

About the Author: Rick Rundell

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