Overview — BIM Design Process

Design is typically an iterative process, where ideas are successively refined in a series of phases or stages, each focusing on specific aspects of the design. While the process followed by every project team will have its own unique features, the stages typically include:

Each of these design phases a specific focus and questions that are typically answered before moving on to later stages.

As design activities move through these stages, a top-down approach is often used. The early stages focus on high-level decisions with major impacts that will shape the options available during later stages. The later phases focus on developing and exploring the selected options and filling in the needed details.

The conceptual design stage typically starts with a program statement that enumerates the requirements that must be fulfilled by the design. Using this program statement as a guide, the design team typically develops and evaluates several rough concepts for the overall shape, massing, orientation, and positioning of the building on the project site.

In developing these design concepts, many constraints must be considered that bound the potential building envelope. These often include site setbacks, floor area or building footprint limitations, daylight plane and shading restrictions, and other requirements imposed by local authorities and approval agencies.

Balancing the client requests and requirements and the site constraints can be a tricky task. Abstract concepts of the overall building shape and placement will be developed and evaluated, often with little detail about the precise features of the building envelope or the configuration of the rooms inside. In order to keep the design problem manageable, this top-down approach focuses on making the big decisions first, saving more detailed design decisions for a later phase.

Using the power of BIM tools, many aspects of these conceptual designs can be evaluated at even this earliest stage. For example, building performance analysis can be performed to evaluate the potential energy use of various building shapes and orientations. Design teams can use this early feedback to help guide their recommendations about the best alternative to choose.

The conceptual design options will be evaluated and presented to the clients, and once a preferred option is selected and approved, the process can move on to the next stage.

Having established the overall shape and form of the building envelope, design teams often focus on area and space planning—allocating the space within the building envelope to the various program needs and considering various layouts for how these spaces might be organized.

During the preliminary design phase, designers will also typically articulate the features of the building envelope, proposing the exterior wall materials and assemblies, preliminary window placements, daylighting and shading features, and the roof form.  Preliminary room layouts are also be created to develop the scheme for the interior spaces will be used.

As information about the proposed design is refined and added to the building model, analysis of the proposed design can continue to guide the design. For example, during the preliminary design phase, cost estimates are often developed based on the quantities and materials specified in the model. These estimates provide valuable feedback that helps design teams evaluate the impact of potential design decisions and ultimately make better decisions.

During the design development phase, the ideas and design features selected during the preliminary design phase are developed and explored in more detail.

For example, once the room layouts have been approved, a wide variety of interior details and finishes can be added to flesh out the design. Design alternatives for casework options and fixture placement can be considered. Special architectural treatments for interior walls or ceilings can be evaluated. While these features could have been added during an earlier phase, it makes more sense and decreases the likelihood of wasted work to add them after the preliminary design decisions are made.

Another common design activity during the design development phase is the evaluation of design options—identifying key features where the design team would like to develop and explore several alternate approaches. The flexibility of the BIM modeling process allows teams to develop alternate design ideas and evaluate them in the context of the overall proposed design. By making it easy to consider design options, teams have greater latitude to test innovative ideas and find better solutions.

During the design development phase, analysis of the building model can continue at greater detail.  Cost estimates can be updated and refined. Analysis tools can be used on the building model to assist teams in creating preliminary designs for the structural and building systems (mechanical, electrical, and plumbing systems), and the impacts and interactions with these systems can be evaluated and used to improve the proposed design.

Construction Documents and Details

The construction document and detailing phase focuses on adding information to the building model to fully describe the proposed design through drawings and details that can be used to guide the construction process.

Fully documenting a proposed design at the level of detail required to guide the construction process can be a monumental task.  Every feature of the building assemblies, the connection details, and the components that will be installed must be documented and presented on sheets for distribution to the project delivery team.

BIM tools can assist in this phase of the design process in several ways. The building model provides an overall framework for generating the details that must be created.  Rather than drawing each detail from scratch as a series of lines, views of the model can be created that form the basis for details. Annotations and notes are added to complete and fully explain these details, but the building model serves as a valuable starting point as well as a consistency check. Since model views are live, any changes to the building model are automatically reflected in the derived details.

While the output of the construction document phase is typically a set of printed plan sheets, the building model can be shared with the project delivery team to facilitate automated quantity takeoffs as well as conflict and interference checking. This powerful application of the BIM design process requires innovative approaches to organizing project teams and sharing risk, and these topics are explored in curriculum Unit 5, Integrated Project Delivery.

The use of BIM tools creates the opportunity to radically change and improve the design process in several ways.  Whereas traditional 2D CAD-based design approaches focused on increasing the productivity of the construction document phase, a BIM-based design workflow changes the process in a more fundamental way by enabling the sharing and incremental enhancement of design information through all project phases.

Building modeling enables design teams to systematically assess and evaluate the performance of their designs at even the earliest stages of a project. This early feedback enables critical and impactful design decisions to be make earlier in the process, when there is still an opportunity to make greater impact at a lower cost.

The use of a BIM-based workflow also provides a vehicle for sharing proposed designs that allows members of the design team to easily collaborate using a live version of the building model. Rather than working in silos and passing paper-based snapshots of the design, all members of the design team can access the latest model changes and assess the impact of their design recommendations in the context of the overall design.

Beyond the direct benefits to the design team, a BIM-based design workflow also benefits the full project team and project delivery process.

Design is just one aspect of the total project lifecycle. Capturing building design information in a BIM model also benefits other key stakeholders including the procurement team, the building team, and the facility managers. Each of the project functions is responsible for producing certain information assets, which all originate from the building information captured in the design model:

Traditionally, each of these functions has operated in silos, without much collaboration or communication until there was a need for information handoff. In paper-based workflows, those handoffs occur manually via physical construction or design documents. And the regression of the project information through these paper handoffs results in “backflows” that require rework to rebuild the project by reinterpreting the information from the drawings.

Figure 2.1 BIM improves the integrity of information between functions

Figure 2.1 illustrates how at each project phase, a set of digital assets (for example, CAD drawings) is created and knowledge is built up. In traditional paper-based workflows, these assets are used to their full potential. Converting the information into paper forms, then reinterpreting the drawings to create new digital representations creates inefficiency and invites misinterpretation.

A BIM-based design process enables the use of the building model as a conduit that can capture and coordinate knowledge developed by all participants in the process in a format that can be shared throughout the project lifecycle. In this way, information flows from one function to another occur more smoothly and with greater integrity and reliability.