Figure 1: Sequential film strip images from video production showing the planned erection sequence for a pedestrian bridge.
As building information modeling (BIM) based project delivery methods continue to take hold in our industry, an intriguing and sophisticated modeling technique is beginning to emerge. It is often referred to as 4D BIM or Simulation-Based Modeling and relies on integrating components of the 3D BIM with time- or schedule-related information. The use of the term 4D is intended to refer to the fourth dimension: time. The construction of these so-called 4D models enables one to visualize the entire duration of a series of events and is being used by a variety of project participants from designers to contractors.
These 4D models expand the value of the traditional 3D models developed during the BIM-enabled design process, together with a project’s scheduling systems, by improving understanding and collaboration for all project participants. Some of the key attributes associated with these models will improve site planning by enabling "what if" scenarios to test and improve different schemes. Simulations of installation conflicts, design clashes, and workflow management can be performed before the work begins. Construction sequences can be simulated to facilitate quick and effective decision-making by the contractor, design team, and owner. They can be performed continuously for an overview, or stepped through to show the project at particular points in time. Because these sequences are linked to actual project schedules, they provide valuable context. The same technology can be used to resource space planning scenarios and evaluate various equipment placement options, to name but two examples. Real-time navigation through the modeled environment is supported during the simulations to enhance exploration.
The process of creating a 4D model can be relatively straightforward so long as the BIM models have been developed with sufficient granularity. Individual elements (or groups of elements) in a discipline’s BIM model must be linked with discrete, date-driven construction activities that are generally contained in the contractor’s schedule. The 4D model creator uses a simple process to link model elements with line items in the construction schedule. If the 3D model has been developed in relatively close correlation with the intended fabrication or erection plan, then the process of assigning individual model components (or groups of components) to time line events can proceed unencumbered. However, if the 3D model has been created with no regard for such construction-related segregation, then some remodeling effort will likely be required to ensure an accurate linking of model components to time line activities. For this reason, it is important for developers to align the model as close to actual conditions as possible if it is known that a 4D model will eventually be needed for a project.
Modeling for expediency alone may not serve well if a 4D model is desired. For example, consider the modeling practice for the deck of a parking garage structure. Depending upon the structural configuration of the garage, it may prove expedient to model a large slab monolithically. If a construction simulation model is desired for the project, the monolithic garage decks will need to be remodeled and segregated to match the placement schedule envisioned by the contractor. Similarly, if one models a multistory column as a single element, instead of several floor-to-floor elements, rework on the model may be required in order to obtain an accurate visual depiction of the erection process. A good tag line to remember in this regard is to "model with the end in mind."
Once completed, the animated simulation of the 4D Model can be played back to visualize the actual occurrence of events in the proper chronological sequence. Current 4D creation technology can support a wide variety of BIM and scheduling platforms.
Figure 1 illustrates a series of images rendered from a 4D model of a circular pedestrian bridge. The model was constructed to illustrate the erection of a series of individual section lifts over a major transportation thoroughfare. Each section of the bridge, shown in green, can be seen coming into its final position. These images can be annotated and then streamed together to produce an animated video showing the actual erection sequence. The clarity and enhanced understanding that these models bring to the project stakeholders provides a significant benefit and makes a compelling case for the use of 4D models.
The ability to visualize the construction sequence in the context of the actual project site provides the design team, owners, contractors, municipalities, regulatory agencies, and other interested parties with a bird’s-eye view of the process, simplifying their understanding of the events to occur. In many instances, this improved understanding can facilitate the decision-making process often required when scheduling work in logistically challenging environments. It provides visualization that results in a more intuitive understanding of the process.▪