BIM and Energy Modelling – Interoperability Incomplete

Why do a job once when you can do it three times? If you are only being paid to do the job once then the answer should be obvious. Even still, this triplication of work is common practice in many mechanical-electrical-plumbing (MEP) engineering offices. Three different tools for three different tasks: HAP or TRACE for load calculations, Revit or AutoCAD for design/production, eQUEST or IES<VE> for energy modelling. Three different models developed by three different departments for three different tasks on a single building.

Although the software tools have improved dramatically, there is still a disconnect between expectations and reality. There is no single tool that can do everything well. And the many tools that do exist often have trouble getting along.

High performance buildings are complex systems. Within an MEP engineering office many specialized professionals are involved in putting forward an efficient, coordinated and cost-effective design. Engineers, designers and energy modellers work together behind the scenes to provide all the sophisticated systems that we expect to find in the built environment. From HVAC, plumbing and fire protection to power, lighting and controls, it is the role of the MEP engineer to make these systems work within the overall context of structure, architecture and site.

Furthermore, the expectations for new construction in this day and age are very high. All this work must be done within a breakneck schedule, at the lowest possible life-cycle cost, with the lowest possible environmental footprint.

BIM takes CAD to a whole new level

Among the biggest changes that have been happening in the construction industry over the last five years or more is the increased use of Building Information Modelling (BIM). BIM has taken the traditional 2D CAD tools to a whole new level. While in 2D CAD the lines drawn in model space are just lines, with 3D BIM the shapes modelled have information associated with them, as suggested by the ‘I’ in BIM. Instead of an air terminal being just a square drawn on a page, the BIM model will know that it is an air terminal and there will be information associated with it, such as the flow rate it is handling and the static pressure drop. If the air terminal is connected by ductwork to a VAV box, the BIM model will know that the VAV box is serving that air terminal and it will be able to size the VAV box accordingly.

A big reason why 3D BIM has been so widely adopted is that it has the potential to reduce coordination time between disciplines. For example the model will know if the user is trying to draw a length of ductwork through a structural column, unlike a 2D CAD drawing which will not make this distinction.

As technology changes, so does the idea of BIM. Many architects, engineers, contractors, owners and operators are starting to look beyond 3D BIM toward 4D (time), 5D (cost) and even 6D (life-cycle management). BIM models created by architects and engineers during design are being used by contractors during construction and then passed along to the owner and operator for use during the life of the building.

Energy modelling has become mainstream

Another major change in the industry recently is the focus on energy modelling. With the popularity of the LEED rating system and the stringent energy efficiency requirements of some building codes, developers, owners and designers are paying close attention to the energy performance of their new buildings. No longer is energy modelling only done on so-called high performance buildings. It has become a completely mainstream part of the design process.

And like BIM models, energy models are starting to bridge the gap between design tool and owner/operator tool. Many building owners are opting to design sub-metering systems with the intent of using the data to calibrate the design energy model after a year of operation. When this is done, the energy model becomes a valuable tool the owner can use to perform “what-if” analysis on proposed changes over the life of the building.

There are many different energy modelling software options, all of which have strengths and weaknesses. Some are better at modelling hydronic systems; some are better at modelling air-side systems; some are more visual; some have better reporting features; some have codes and standards built in and can automatically generate certain reference models. With so many options, it takes knowledge and experience to determine which is the most appropriate for a given project.

Combining BIM and energy models

A lot has been made of the benefit of combining BIM and energy models. The BIM model created by the architect can be exported to the energy modelling software, thereby eliminating the time-consuming area takeoffs and geometric modelling aspects of the energy modelling process. Not only does this streamlined process have the potential to save time, it also eliminates the opportunity for mistakes in the translation from the BIM model to the energy model. Programs like Google Sketchup and AutoDesk Revit have plug-ins with direct links to IES<VE>. And beyond these direct links, many BIM and energy modelling programs can use the gbXML schema to import/export model data between programs.

It has been suggested that with the advanced inter-operability of today’s BIM and energy modelling software tools, there is essentially one model required for design, engineering and energy modelling and this model can be passed around between team members, each of whom can perform the tasks required to do their job.

You need a toolbox, not a Swiss Army knife

The reality is that there are still multiple models. On BIM projects it is still very common to see multiple models developed within the same office for different aspects of the engineering/design process. For example, although Revit can do load calculations, there will still quite often be a HAP or TRACE model. And although HAP and TRACE can do annual energy calculations, there will still quite often be an eQUEST or EE4 model. People like to use specific tools and certain tools are better than others for certain tasks.

As the Swiss Army Knife modelling approach is replaced with the toolbox approach, the focus has to shift from finding software that can do every task, to finding software that is inter-operable. Can the output of one model seamlessly become the input for the next model and back again?

Interoperability is something that is heavily touted by software developers as a selling point for their products. It is said that this interoperability can allow the team to react quickly to design changes and “easily fit analysis into the design workflow” (www.iesve.com/software/interoperability/revit).

An uneasy union

Unfortunately, it seems as though achieving interoperability between building energy models and BIM is nowhere near as easy as the “marketing people” make it seem.

For those fluent in BIM, you hear the phrase “out of the box”; so what does it mean? Does it mean it’s simple? Typical? That most of the work is already completed by the program for you? In reality, when preparing a BIM model to be exported to an energy modelling program, it is anything but “out of the box.”

Take for example a BIM model created with Revit which is to be exported to IES<VE>. If no rooms are defined in the Revit model, the model will not export. If rooms are defined in the Revit model but not properly bounded, the surfaces will export to IES<VE> as shading elements, not spaces. If the rooms are properly bounded but not connected, for example only bounded from floor to ceiling, the rooms will export to IES<VE> as spaces, but the interior ceilings and floors will be treated as exterior roofs and exposed floors. If the rooms are not connected from centre-of-wall to centre-of-wall the rooms wi
ll export to IES<VE> as spaces, but the interior surfaces will be treated as exterior walls.

Even if everything is set up properly in Revit for a successful export to IES<VE>, the rooms that were defined in Revit may not be appropriate for energy modelling. The zoning schemes for load calculations and energy analysis are not based on defining a separate zone for every unique room. Rather they are based on three general rules: unique heating/cooling load profile; unique schedule; and unique space function. All that is to say that even if the Revit model can be exported to the energy model, the engineer will still have to spend time revising the zoning scheme to suit his or her requirements.

There are many settings that do not transfer unless the modeller, whether it be the architect or engineer, puts in the extra leg work. Depending on the level of development of the project, the architect will often go to great lengths to establish the construction assemblies for the exterior walls, interior partitions, etc. Unfortunately all that hard work does not export through the gbXML schema and these elements must be created from scratch in the energy modelling software.

One particular element that does tend to be transferred over is the structural column – as a room. When rooms are automatically added in the Revit model by the architect, Revit takes ALL enclosed spaces, including structural columns. These columns then show up in IES<VE> as Spaces, which causes problems.

Things are improving

Things are better now than a few years ago. For one thing the tools are more robust. Also, the technical support is better. Interoperability issues seemed to be a grey area that no one among the BIM and energy model software developers wanted to take responsibility for. That has changed. There are very good sources of information available on how to properly set up a Revit model so that it will export properly to IES<VE>, including a couple of white papers that can be downloaded from the IES<VE> website (see above). If the steps in those documents are followed closely it is possible for this workflow to be successful.

Certainly there is a lot of potential for time savings, cost savings and a more streamlined process with BIM-energy modelling interoperability. At MCW, we have had success taking early stage architectural Sketchup models and exporting to programs such as AutoDesk Vasari for early stage energy analysis. This has allowed us to provide feedback to our architectural clients at a very elementary stage on building orientation, building massing, external shading and other issues that cannot wait for a detailed energy model to be developed.

However, we have only been able to successfully export a Revit model to IES<VE> after considerable manipulation of the Revit model. It is our experience that most energy modellers will not bother with this, but will create the geometric model from scratch in the energy modelling software in order to have full control over the finished product.

When it comes to exporting complicated architectural Revit models to IES<VE>, the industry is still a long way from the kind of real time, out-of-the-box interoperability that you read about in the brochures. But the upside advantage to making it work is such that we are going to keep trying. Maybe one day these tools will learn to get along.cce

Brian Tysoe, P.Eng. is an Associate and National Manager of Energy Modelling Services, and Laura-Lee Moran is the BIM Manager, both at MCW Consultants in Toronto. www.mcw.com