Interstitial spaces: Managing the dark zones of the building

Engineers must pay close attention when coordinating mechanical, electrical, plumbing, and fire protection system design in interstitial spaces.

By Erin McConahey, PE, and Jamey Lyzun, PE, Arup, Los Angeles October 27, 2013

Learning objectives

  1. Understand the interrelationship of mechanical, electrical, plumbing (MEP), and fire protection systems in interstitial space.
  2. Know when building information modeling (BIM) can assist in design and clash detection.
  3. Understand how digital tools and new technology can facilitate the coordination process. 

In the conventional solution – combining the claims of structure and services – the ducts that carry air to and from the center are hung from the floor, then hidden behind a false ceiling. This zone of darkness is further stuffed with equipment for lighting, electricity, smoke detectors, sprinklers, computers, and other building “controls.” The section is no longer simply divided by the discrete demarcations of individual floors: it has become a sandwich, a kind of conceptual zebra: free zones for human occupancy alternate with inaccessible bands of concrete, wiring and ducts … Idealism vs. philistinism: the section becomes battlefield; white and black compete for outright domination. The dark zone is not only strictly “useless” for the future inhabitants of the building; it also become conceptually inaccessible to the architect, who has become an intruder in his own project, boxed in, his domain a mere residue of the others’ demands.”

–Rem Koolhaas, Pritzker Prize-winning Architect, in S,M,L,XL, The Monacelli Press, 1995 

Interstitial: Of, forming, or occupying interstices: "the interstitial space."

Interstice: A space that intervenes between things; especially: one between closely spaced things

–Merriam-Webster Dictionary

As mechanical, electrical, plumbing (MEP), and fire protection engineers, it’s easy to say that we are responsible for ensuring building functionality, public health, and comfort while maintaining the highest affordable energy efficiency. In reality, most of our time is spent trying to fit large things into small spaces (ducts, equipment), or multiple small things in the same space (pipes, conduits). All of this co-existing complexity happens in the dark zones of interstitial spaces. The success of the production side of our work depends on playing a cooperative, proactive, and intelligent game in that shared sandbox because dreaming up the ideas behind our designs and analyzing their effectiveness is less than half the battle and less than half the cost. 

Most of us have experienced that these dark zones of the building usually do not attract architectural attentiveness until there is a cost driven need to reduce floor-to-floor height or to increase ceiling height.

With façade costs running $100 to $150/sq ft, the appeal of shaving 6 in. off each floor can begin to have multimillion-dollar repercussions and the drive to increase ceiling heights can be directly linked to a desire for more daylight penetration and higher property values. 

In either case, the burden of such change is primarily borne by the services team because the redesign implications are much greater than just moving a plane in elevation. Moreover, due to the ever-more transparent 3-D environment in which we all work, there is a great pressure from our clients to “optimize” the space we use, without a full understanding of the constructability, support, or future maintenance concerns that should be inherent in good design.

The ‘old school’ approach

There are times when building information modeling (BIM) is touted as a panacea for all coordination problems. Having worked in 3-D virtual environments for the past 13 years, these authors believe that the key to good coordinated design is … well, good coordinated design. We see that engineering teams often start too quickly in 3-D models, routing through apparent voids and filling space “because it’s there,” leading to crazy incoherent distribution routes that stem from the luxury of technology-enabled intellectual sloppiness. As with all computer-based things, “garbage in = garbage out.” We still have to see the solution in our heads in order to draw the solutions in our software. So we advocate for early phase old-school approaches that remind us of the true inter-relationships that arise from first principles (see Figure 1 above). 

As noted in the quote above, the section is the key flashpoint of coordination, if not a battleground, then at least an area requiring multilateral agreements. Agreeing on logical interstitial space depths does not require fancy software, but it requires a sketch that includes real dimensioned sizes, inclusive of support, insulation, and slope. Because ducts are often the largest things in the ceiling void, engineers must understand that ceiling depth is directly driven by the heat load in the area served, the distance between risers, and the type of HVAC system used. 

Starting with the last item (type of HVAC system), it is clear that air as a heat transfer fluid takes up much more cross-sectional space as compared to water: an 18×18-in. supply duct on a recent job was replaced with a 1.25-in. chilled water pipe once the engineering team changed from a variable air volume (VAV) system to a local water-based cooling system (chilled beams) and a 48×48-in. duct is equivalent to a 2–in. pipe. The size of ducts is directly driven by the heat per sq ft to be absorbed multiplied by the floor area served. On a recent job, we provided this simple table (Table 1) to our clients to demonstrate that it was within their capacity to choose how deep the ceiling ducts would be based on how many risers they provided to us.

Table 1 was provided as a simplified space planning exercise for a supply only duct condition. This table can be used on its own as a quick way to convey the relationship between the number of vertical distribution points (risers) relative to the horizontal distribution and ceiling height requirements. The same strategy can also be applied in combination with other duct and piping systems to provide a more in-depth understanding of this relationship. It is up to the designer to determine the level of information needed to achieve the appropriate horizontal-vertical balance when negotiating space demands with the architect and owner’s facility planners.

On other jobs, at early phases, we use hand sketches or simple computer generated sketches to further demonstrate the complexity of the combination of all services in the ceiling (Figure 3), taking into account cable tray, sloped piping, ductwork, and ceiling depths. 

With early diagrams and concerted effort on a recent museum project, we worked with structural engineers to determine how to best optimize routing of services through structural elements, where lower voids were left clear for sprinkler and electrical routing. In Figure 4, the rendering represents a 3-D model of ductwork mains integrated with structural trusses, and the photograph was taken of this space from below during construction.

The danger of drawing sketches as in the past was that one would never have the ability to imagine every nonstandard conflict such as those places where there were inevitably crossing services that would limit access. Engineers should continually review non-standard areas to ensure that these locations will work as per the originally envisioned design. It can be useful to keep the following list close by when reviewing these sectional relationships: any architectural voids, mechanical/electrical/IT rooms, risers, elevators/stairs, slab depressions, pattern-breaking geometric forms in the building, braced frames and shear walls, and entries into raised floor cavities. Catching conflict in these types of areas is a specific benefit of using the BIM features that are available in current software tools.

Using modeling tools 

Computer processing, software packages, and cloud-based storage have developed over the past decade to allow new opportunities for architects and engineers to coordinate and manage the dark zones at the design phase of a building project (Figure 5). New digital tools allow the design team to better understand 3-D constraints, to produce a more accurate model, and to track and resolve conflicts with other disciplines as the design is developed. With the appropriate amount of time and fee, new software literally allows for a design team to leave no stone unturned. However, most project schedules must limit the amount of stone turning to what is reasonable and required. 

The latest digital tools can provide a designer with vivid renderings and instantaneous sections to give all model users a better awareness of the unique conditions of the building project. Both the engineer and the architect can use renderings to better understand how systems will fit or co-exist with the architecture and with other systems. The section becomes augmented into three dimensions with the ability to rotate a view to truly understand the constraints and conflicts for any specific location. This has changed the way designers and BIM technician draw solutions from the start. For documentation purposes, it is quite easy in most current software to draw planes to generate working sections and cut-lines along a model as it develops. 

Advancements in software have allowed for the inclusion of “smart” components that have embedded installation clearances, inlet/outlet dimensions, and handedness (Figure 6). With the inclusion of actual components and fittings, the likelihood of conflict will be reduced. The long-winded annotation that described how to connect a system together within a 2-D plan has now been replaced by a coordinated 3-D model that shows the same information more clearly to other stakeholders. An added benefit is that this model can now be viewed in the field using a tablet. Intelligent building components improve the accuracy of the design model, and allow for more accurate equipment schedules and quantity takeoffs. Smart components allow disciplines to extract data for load calculations and also to have embedded characteristics to facilitate other disciplines to complete their own calculations. The use of smart components is one progression as the design industry transitions from 3-D coordination to more comprehensive BIM. 

On a recent 500,000-sq-ft health care project, Autodesk Revit MEP was used at early stages to improve the accuracy of the mechanical load calculation model and reduce the data entry required. The software enabled the team to export floor areas, room names, and other characteristics into a format that could be read by the load calculating software. 

As the design progressed to construction documents, a strategy was developed with the architect to assure that changes to room names and dimensions would enable the team to easily update the load calculation software with limited rework. Dynamic sections were used by the team throughout the drawing production to determine bottlenecks and coordinate with the other trades. Finally, the use of smart components allowed for the development of air balance tables and schedules. For a project of this size and type, these systems were found to significantly improve the accuracy of the calculations and schedules and to reduce time required for manual efforts. 

For example, on a large museum project, scripts that interrogated the model were used to establish sheet metal quantity and pipe lengths in order to expedite the work of the cost estimator. This not only allowed for a more accurate understanding of the amount of ductwork and pipework, it also gave the engineering team an accurate understanding of insulation, jacketing, and hanger costs. On a courthouse project, partial models were submitted with requests for information (RFIs) in order to help visualize actual conditions in the field and to facilitate more accurate solutions. On multiple projects, the shop drawing review process also has been made much easier by using overlay tools to compare contractor shop drawings to the original design documents. And finally, on a recent college project, the Arup team used equipment room renderings to convey equipment room access to the school’s facility personnel.

One additional advancement is the use of remote file servers and cloud-based storage that allow teams to download other discipline models and track progress in real time. If all team members are working in the same platform, conflicts can truly be resolved as the design evolves. Third-party software with clash detection can also be used to compare multi-trade models (even those developed with different software) and identify interference. Now, more than ever, software tools allow designers to be proactive in coordinating with other trades as they work while ensuring that nothing is missed through clash detection and diagnostics.

The risks

With so much power in the hands of a modern design team, there is an argument that mistakes may never again be made. Clearly, that is not true. Just like the adoption of the typewriter, computer-aided design (CAD), and word processing, there are changes to the entire design process that require attention of all project team members. Tried-and-true methods must still be implemented. For instance, the use of BIM does not replace the need for strong planning and communication among the design team. In fact, because there is often an exchange of progress models, there can often be a need to increase the level of communication. 

It should also be acknowledged that 3-D design and BIM frequently can be more time consuming than 2-D drawing production, especially at early stages. 3-D and BIM design can take additional time to coordinate fittings and review sections in 3-D. Because of this, it is essential to match the level of development of the model with that of the other trades to minimize rework. To revisit the previous point of “garbage in = garbage out,” in the integrated model, if any one component is poorly conceived, it can impact all other services. While cloud-based storage does allow for access to the latest information, it does not necessarily guarantee that all information has been fully considered. Good designers know that the design process can be iterative. Thus, it is important to ensure that the designers try to increase the level of development of their own model in step with those of the other disciplines. 

In traditional procurement methods, changes to the mechanical or plumbing system often may not have been appropriately conveyed to the electrical engineer in the haste of a project deadline. The risk of neglecting to communicate key coordination items is always greater when time is short or project teams are large.  On more recent projects, we have experienced expedited schedules and integrated project delivery (IPD) procurement. In these cases, the contractor detailed coordination can overlap the design schedule. Thus, engineers must not only determine when to begin detailed coordination within the design trades, but they also must ensure that design iterations do not impact completed coordination by the contractor trades. Even with cloud-enabled project access, judgment and communication are essential factors that contribute to a successful process. Particularly at early stages, team members must coordinate allowances based on their judgment of where the model will end up as opposed to the exact dimensions at which it currently stands. 

One way to balance engineering allowances with design model integration in a way that other trades can follow is to develop a BIM execution plan that defines the level of development (LOD) at each stage of the model. The level of development may be defined uniquely within the execution plan or refer to specific language on LOD as defined by the American Institute of Architects within the E202 BIM Protocol document (see Figure 7). With the latter, LOD 100 through LOD 500 is defined to give other model users an understanding of the precision of the existing model in terms of scheduling, pricing, fabrication, and construction.

The LOD of the model should also inform the confidence that a team may have in optimizing floor-to-floor and equipment room sizes. As identified in Figure 8, some of the components that would be incorporated in the final installation would not be identified within the design model. For instance, duct/pipe accessories and anchorage are often described in equipment schedules, specifications, or typical details as opposed to in the plans. In seismic zones, there can be even greater challenges in incorporating diagonal bracing for seismic support. Even at later stages, contractor shop drawings may not consider all clearances required for phasing, installation, or future maintenance. Because of this, good judgment must still be used when evaluating the benefits of reduction in floor heights or in reducing spatial allowances of equipment rooms.

Working as a team

It is essential that design teams and BIM operators have a strong awareness of how much information is needed and how extensively tools are to be used at early stages of a project. In general, many of the tools require additional setup. For instance, additional effort may be needed to define a family of parts for a particular smart component or to establish a process to export information into another type of software. The team must determine which processes will be used and how information must be transferred between your firm and outside entities. 

Because there must be much more cohesion between all models, the design team must invest early, not just in establishing initial setup of the files, but also in establishing a process for importing and merging future, more detailed data. It is recommended that a BIM execution plan be used to document the process for file transfer, the level of detail that will be identified in the model, and the area of responsibility for each entity. BIM managers must also be named to take ownership of each firm’s model, to monitor file sizes, and to assure that automated processes continue to function. 

The authors have experienced substantial benefits in the use of BIM through the automation of manual processes and the accuracy of the model. Further, BIM allows the development and visualization of alternate solutions, which then can be communicated to our clients. Spatial access in ceiling voids and mechanical rooms can now be communicated quite easily by directly exporting a screen shot from the maintenance personnel’s point of view. Scripts can be used to determine actual quantities of components to facilitate more efficient and accurate costing. At later stages of a project, as the models become more stable, we see substantial benefits in reviewing contractor shop drawings and eliminating or reducing RFIs from the field. 

At this point, we have not found research to identify whether the time cost of production is in fact offset by the more efficient construction administration process that is enabled by BIM. The value of the process is heavily dependent on management of change during construction. While the software does present new challenges and require new and more defined roles, it is essential to our profession to assure that efficiency is maximized. We continue to use pilot projects to advance the automation and integration processes available. We also have experienced the benefit of using external consultants to assure the project models are correctly set up and that the most efficient functions within the BIM tools are being fully used. 

BIM is here to stay, and our profession must continue to explore software capabilities to fully use and optimize all of the benefits and to advocate with software providers to develop tools to fit our industry needs.

Erin McConahey is a principal in mechanical engineering at Arup’s Los Angeles office. During her 18 years with Arup, she has worked internationally and now leads multidisciplinary design teams on a wide variety of project types. She served on the editorial board of Consulting-Specifying Engineer for 6 years and was a 2008 40 Under 40 award winner. Jamey Lyzun has more than 12 years of experience on projects throughout North America, Asia, and the Middle East. In a range of projects types and sizes, Lyzun has applied many of the techniques outlined to assure that integrated strategies at the design phase can lead to trade coordination at construction phase. Lyzun is a 2013 40 Under 40 award winner.