Challenges of modeling in Ductwork

Mechanical, Electrical and Plumbing design can be intricate and require a significant amount of coordination activity in order to create the necessary detail and design components to install systems effectively. The challenges that BIM modelers face not only involve issues involved with software deficiencies, the challenge of coordinating with ductwork against ductwork but also coordinating locations of components with reference to other disciplines, and in some cases creating design information for third parties who may wish to integrate with other software.

As we know, building services projects can be complex and they require a high level of detail to make sure everything is built exactly as designed. The globally accepted Level of Development (LOD) standard has helped somewhat to define the extent of detail required for ductwork as well as other services but the challenges within the software itself or the experience of the BIM modeller are still variables that have to be overcome. The extent of detail can of course affect the time taken to complete the design but more detail means that elements such as cost estimating in which Revit can generate automatic quantity take offs, can be added to the scope. So for example a well detailed project will allow for Revit quantity take offs for quantities for duct, insulation and other materials are created.

Turning to software initially, it is certainly true that Revit MEP is now becoming the standard for ductwork modelling. However, this has not always been the case as early versions of the software omitted various components such as ductwork shoes, flanges or even circular ducts. Of course, these elements could have been created using the family set up, but that is both time consuming and costly. The latter versions of Revit have improved considerably and the tool is now becoming a defect to standard in the design industry.

With the software constantly improving the challenge then moves on to the specification and project definition, knowledge of the ductwork systems and the software skills of the users. BIM modelers face a constant challenge as they need to provide a much higher level of detail in the BIM era. Higher levels of detailing and modeling require time and effort and also a good understanding and experience of using Revit. Specific mechanical units and the associated ductwork may be unique in design that must be modeled as generic boxes, and ducts. Mechanical BIM modelers can model generic duct elements as place holders for coordination but these generic ducts may not illustrate the proper weight of a duct and that will affect the cost of material and other take-off data. Examples such as duct socks cannot be created from the fabric and therefore place holders can be modeled to coordinate locations with other disciplines. Whilst unique ductwork can be created, it adds challenges as it relies on the creation of customised families. Complex routing can also be a challenge because of possible multiple elevation changes with elbows. This routing may be created using a window of various views to see elevations and plans at the same time. These views can show how duct changes as you edit them in the model. Routing that requires different sizes along the route can be troublesome because individual runs with similar sizes need to be changed independently, however fitting may be hard to select with the various elevation changes.

As models are developed and a high level of detail is generated, coordination knowhow and experience is necessary to avoid clashes while maintaining an installable solution. Mechanical engineers coordinate with plumbing and electrical engineers to provide water and electricity to HVAC units and exhausts. Projects that involve engine rooms require coordination due to the large number of pipes and ducts that may create clashes due to limited space. Clash detection in Revit or using a tool such as Navisworks can be executed however it requires time to develop the reports and also require time to learn the process. Of course, without detail some clashes cannot be seen due to non-existent components such as small ducts or duct components that are not modelled. Smaller ducts may not be modeled because the detail is described by other means, which can result in potential problems with model. The BIM Modeller also has to give due consideration to lagging and insulation which can add 50mm (2 inches) to the width of a duct and could therefore result in clashes. Reducing the lagging or insulation is not really an option as that can affect cooling or heating capacity. One final obstacle to overcome is the duct hanging system. In some cases the brackets and hangars are modelled but in many cases, the BIM modeler has to allow for hanging and allow that in his layout/model.

As the model is detailed, essential information is developed for engineers. Modelers may be required to supply a high level of detail with duct in order to provide values for calculations. Providing essential information is challenging because it add responsibilities to model to a large level of development and requires modeling components such as ductwork to be precise based off manufacturing information. If the information is not correct then the ductwork can be under or oversized. Oversized ducts can create a lack of space in possible tight quarters or undersized ducts can lead to poor performance and circulation.

Ultimately, modeling ductwork has several challenges. The level of detail, coordinating with other disciplines, the challenges of the software itself and specifying information can create challenges but the benefit of BIM modeling can and does add more value to a project when the project is well planned and the level of design detail specified.

Kuldeep Bwail, Director at XS CAD, providing 3D Architectural Modelling,3D BIM Modeling to Homebuilder, Architect