Case study: Coordinating interstitial spaces in an existing building

An existing building—destined as a medical facility—held a few surprises during renovation. BIM and clash detection helped smooth the process.

By Chris St. Cyr and J. Patrick Banse, Smith Seckman Reid Inc., Houston October 27, 2013

What do you do with an existing 1.2 million-sq-ft empty shell building when the owner says it wants to build out about 200,000 sq ft on three different levels and place some of the highest profile areas such as a high-volume education center and an imaging center on level 1, which just happens to have the shortest floor-to-floor height (14 ft) in the building? You accept the challenge, of course.

The program defined fitting out of about 70% of level 1 with an entry lobby, café, imaging center, facilities offices, education center, and emergency department. About 50% of level 4 was fit out with an operating room (OR) suite, post-anesthesia care unit (PACU), endoscopy, central sterile, and patient beds. Level 10 included a clinic and filled about 70% of the available space.

More specifically, the challenges were:

  • The level 1 imaging suite with two MRI rooms, a nuclear medicine area, a PET/CT room, a CT room and a conventional radiology room with support spaces
  • Place a high-profile education center consisting of two large conference rooms and pre-function space with 10-ft ceilings and several smaller conference rooms adjacent to the larger rooms
  • On level 5, fitting large ductwork between existing hangers of a chilled water pipe rack
  • Field investigate existing conditions, integrate as-built information, and create a 3-D design model in Autodesk Revit MEP to aid the design process.

The coordination process

The project was designed in Autodesk Revit and Revit MEP, which allowed for real time 3-D modeling of mechanical ductwork and piping with architectural walls and ceilings. The model was updated weekly by the architect. The existing structure information was imported into the project model and used to avoid beams while using the open pans for duct and piping offsets. Importing and being able to see sloped sanitary waste and vent piping aided in the design coordination efforts.

The owner-provided “as-built” drawings from the previous installing contractors were used and useful to some degree, but they also proved that as-built documents do not always show all conditions. For example, electrical conduit and racks were not shown or indicated on the as-built drawings. Field investigation aided the process, but the project schedule and design team scope of services did not allow for extensive field verification work.

The design team used Autodesk Navisworks collision detection, which identified major element clashes that were then adjusted to fit, but did not allow for complete coordination due to unknown vendor conditions, such as pneumatic tube and low-voltage cabling not being part of the architecture/engineering design model.

Reality during construction

The contractor’s responsibility as part of its scope was to create a fully coordinated multi-discipline 3-D model (virtual design and construction) to be used for clash detection and installation. Each subcontractor (mechanical, plumbing, electrical, fire protection, low-voltage cabling, pneumatic tube) laid out its systems as indicated on the design drawings but with more detail, and included required offsets, hangers, conduit with required radius bends, and cable tray. The design team participated in these coordination sessions to make design decisions as needed while the coordination proceeded.

While looking at the contractor’s model, some areas were quickly identified as needing modification to fit in the available space due to undocumented existing conditions (large conduit racks in the middle of available above ceiling space) and unforeseen conditions. In the level 1 area, this created a domino effect: When one duct moved, so did three others, which identified the need to relocate some ductwork up to the floor above to shorten the level 1 horizontal duct runs while still making connections to the duct risers at the chases (see Figure 1).

When an existing chase wall was opened during construction, the team discovered a large vertical conduit rack installed within a duct chase against an exhaust duct riser negating the planned duct connection. 3-D modeling could not have noted that condition as the conduit was not visible nor indicated on existing drawings (see Figure 2).

The coordinated model in Figure 3 shows the ductwork, terminal units, piping, and offsets along with light fixtures to fit the limited above-ceiling space of level 1. The contractor’s coordination model provided the guidance for larger items to fit and meet the design intent and criteria while leaving the means and methods of installation to the individual trades. It also confirmed the need for this model type to be part of the project construction scope.

Level 5 (the mechanical floor) had a 20-ft floor-to-floor height, which by most standards (and imagination) seemed more than adequate to route ductwork and piping from the existing air handlers to the central core chases. However, existing conditions, such as chilled water piping routed at low elevations (10-ft above finished floor close to the AHUs) and supply and return ducts serving the multiple electrical and intermediate distribution frame (IDF) rooms adjacent to the shafts, made coordination more difficult than expected. Supply ductwork from two air units had to route above 20-in. chilled water piping but between hanger rods. Design drawings including 3-D modeling of systems do not typically show placement of hangers, but in this case working with the contractor during field coordination allowed the duct placement to be shown and the design duct dimensions accurately reflected with the existing conditions (see Figure 4).

Another area on level 5 that required a coordinated effort involved duct and pipe routing between an existing main electrical room with floor mounted AHUs outside the room and adjacent air handlers with existing disconnects and variable frequency drives (VFDs). Differing field conditions from existing drawings pre-empted accurate 3-D modeling, but the field determined solution to relocate chilled water piping serving the electrical room air handlers and removing and relocating the large air unit disconnects and VFDs allowed the ductwork and piping to be installed while maintaining proper clearances with the resultant conditions reflected on the contractor’s coordinated model.

Successful coordination

Working in a 3-D Revit model provides many opportunities to coordinate during the design process, but even with Navisworks collision detection, it does not provide contractor installation drawings, nor should it be expected to. Because not all trades are incorporated into the model, it will not show all items needed for installation by the contractor. What the design model does, however, is more clearly show the design intent to allow for early coordination with architectural design (e.g., ceiling heights) with MEP above ceiling needs prior to the construction administration phase and also allow the contractor to better implement its means and methods for a workable installation. New construction models will differ from existing condition models in that existing buildings will always have unknown conditions that cannot be accurately reflected in a model and must be field determined and resolved as the construction proceeds.

Dealing with existing building conditions

Existing air handling units (AHUs) were set in place on level 5 and had no piping or ductwork connected. Supply air and return air duct risers were in place located in a central core chase with ductwork directed down to the lower floors. Exhaust risers began at various levels and routed up to fans in the level 11 penthouse within 2-hour rated chases. Domestic water risers, medical gas risers, chilled water, and heating water risers routed vertically from level 1 up and from level 5 down, also in the central core chases. Strategically placed exit stairs, elevators, electrical rooms, and intermediate distribution frame (IDF) rooms were also placed adjacent to the vertical duct chases in the core. Existing duct taps with fire smoke dampers were installed through the chase walls at varying intervals.

Chris St. Cyr is a senior mechanical designer with Smith Seckman Reid. He has more than 24 years of mechanical design experience, with the past 15 years in the design of health care facilities. J. Patrick Banse has more than 35 years of experience in the consulting engineering field with the past 30 years in health care design and engineering. He is a member of Consulting-Specifying Engineer’s Editorial Advisory Board.