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.
- Understand the interrelationship of mechanical, electrical, plumbing (MEP), and fire protection systems in interstitial space.
- Know when building information modeling (BIM) can assist in design and clash detection.
- 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
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 18x18-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 48x48-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.