Using Revit MEP for Ductwork Modelling and Ductwork Fabrication

Revit is a useful tool in the AEC industry since it was introduced. It has effectively enabled organisations to minimise the possible on-site concerns and helped to lower the overall project cost. Revit MEP covers of all of the main services including ductwork modelling which is the subject of this particular article and specifically how it is used for modelling and how it is now being used for fabrication.

Focusing firstly on ductwork as a discipline, ductwork is a core feature of MEP design projects as it facilitates heating, ventilation and air conditioning (HVAC) for regulating the air flow and to maintain acceptable indoor air quality as well as thermal comfort.

Duct work essentially comes in three types of shape – circular, elliptical and rectangular and they are designed to be fitted at varied elevation levels. Revit has provided an array of options in its latest update for design ductwork models, yet there are many challenges when modelling ductwork in Revit. Ductwork is unique in design for each of the structures and all element expose different challenges while designing. BIM services modellers must create ductwork models to accommodate customised frames, windows, fittings and complex routings. Specific outlets are also required to be designed for ductwork to fit in the outlets as well as to leave adequate space for electrical and plumbing requirements.

As well as different shapes, ductwork has to modelled in different sizes as well, to suit the design requirements along with the factors such as flow of air and mode of discharge or exhaust. An oversized duct may pose design challenges such as failing to accommodate the electrical and plumbing requirements and under sized duct may result in a serious design flaw and may not be able to hold the desired amount of air flow. Ductwork modellers typically model to a high level of detail (LOD), typically at LOD 300 and increasingly at LOD 350.

Although it is only a component of MEP design ductwork modelling does require expert inputs and precise layout plans to create a clash free duct network for building and engineering projects. This can seamlessly accommodate plumbing and electrical lines without causing costly design changes at construction level. Pre-Revit, the industry used AutoCAD MEP, a 3D tool, as well as other specialist tools and add-ons such as CAD Duct to create ductwork models. When Revit was introduced it had a number of shortfalls and incomplete areas for mechanical services and indeed for ductwork. However later versions have addressed earlier shortfalls and the tool is at a stage where it can be used for detailed design to address the challenges and requirements detailed above but also for interfacing with fabrication level detail, which will be discussed further below.

As well as modelling capability, Revit also provides other advantages such as providing quantity take-offs at an early stage, which helps to avoid costly design changes in the later part of the design process and provides accurate quantities for ductwork, insulation and other materials. Even though seamless ductwork models are prepared using Revit, the functionality of the design files across other platforms had remained limited. For instance, when the ductwork model files designed in Revit had to be used for fabrication purposes on related software, due to its incompatibility on these softwares, the duct layouts had to be re-modelled leading to time delays and a more expensive fabrication process, resulting in fabrication errors which can have negative implications while assembling the ductwork.

To overcome the limitations for fabrication, Autodesk updated Revit features for ductwork modelling in its latest version - Revit 2017. Revit 2017 now has tools to design duct fabrication which are included within the package. This enables designers and modellers to create the ductwork models, layouts and designs for the entire project lifecycle, including fabrication teams. The files which have wider compatibility for fabrication tools such as FABMEP help designers and modellers to design ductwork seamlessly over various platforms and save the project in a single file without affecting the actual design. Historically, this level of interoperability has not been experienced from design, detailing and finally through to fabrication and in the future - facilities management.

In summary, Revit MEP is an established tool for ductwork modeling and it does address the core elements of a ductwork system and allow a reasonably sound set of design drawings to be issued. The challenge for fabrication from Revit was always a concern and as briefly discussed, Autodesk have now started to address this and we are seeing fabrication interoperability at last. Added to this is the fact that Revit is working more closely with fabrication tools in its own right and therefore the fabrication (manufacturing) industry is now starting adoption of Revit models in a way that has not previously been experienced. The end game will surely mean accurate designs, delivered faster and therefore more efficiently - helping to reduce costs and improve timescales in the engineering and building industry.

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