To someone not versed in mechanical plant construction, a project that requires only floor support for the HVAC piping system, is rare. Supporting pipe from above is the standard as it leaves the floor space free of obstruction, providing mobility for the workforce executing the project, as well as providing the end user working clearances to each piece of equipment.

Creating modular fabricated skid assemblies is a new concept to most mechanical companies, owners and design teams. Fabricated assemblies are usually a purchase item, as opposed to being fabricated locally.

Consider a project where the design requirement for a 15,000-square-foot mechanical plant requires that all system piping, larger than 4 inches in diameter, be supported from the floor. It is not unique to use floor stands for some pipe support; however, it is out-of-the-ordinary to not permit using the overhead structure to support the majority of the system piping.

The project and the challenge

On this particular museum project, mechanical contractor, Shapiro & Duncan, was faced with that challenge. A forest of floor supports, placed vertically under the horizontal piping above, would be a major installation obstacle to productivity. Not only did it create a congested area for walking, but it also restricted the ability to set equipment, as well as the use of a scissor lift, and even ladders.

In addition to installation productivity, the contractor needed to plan for required space for future equipment maintenance clearances, as well as the need for equipment replacement, which will require a clear path for the removal and move-in of new equipment.

As the mechanical contractor, we took on this challenge with the mindset that it would be a suitable project on which to maximize our many-faceted mechanical design and construction capabilities; optimize productivity through prefabrication and modular assemblies; and give the owner and design team the best mechanical solution to their unique design limitations.

The team, which was comprised of project managers, virtual design construction (VDC) specialists, fabricators, logistics coordinators, trucking operators and onsite field personnel, envisioned and created pre-fabricated modular skid assemblies to house and support all mechanical system components. The equipment included heating hot water pumps, chilled water pumps, chiller piping connections, condensing boilers, steam boilers, expansion tanks, buffer tanks, heat exchanger pumps and a heat exchanger.

However, creating modular fabricated skid assemblies was a new concept to both the owner and design team. Therefore, it was essential that we were able to get the ownership team behind its proposed solution during the initial planning phase.

We prepared a presentation and invited the owner’s team to our 51,000-square-foot fabrication shop in Landover, Maryland, for a half-day meeting to tour the shop, view our 3D BIM concept model and review the drawings that detailed how we planned to do the work. This included references to several previous projects in which we had used floor-mounted skid assemblies:

• U.S. Navy Flag Officers Quarters, Washington, D.C.
• Cato Institute, Washington, D.C.
• Rowley Hall, Marymount University, Arlington, Virginia.
• Towers Crescent Office Building, Tysons Corner, Virginia.

Once we had the ownership team on board, we were able to move forward. 

Immediately, we faced serious implementation challenges. First was the location of the mechanical room. Typically, mechanical equipment rooms are located on a lower building level supported by a concrete slab-on-grade, but since this mechanical room was on the second floor of the structure, additional weight from the steel used to build the skid assemblies, as well as sound and vibration transmission, were major engineering concerns. Because of potential vibration and acoustics concerns, the owner required a vibration and sound consultation with an acoustics engineer.

Secondly, on the design side, we had to ensure the skid assemblies would be constructed to fully support both the installed equipment and piping. A structural analysis of the skid frames was conducted to verify the structural integrity of the proposed system. This meant not only in the final installed place, but also during the 20-mile shipment to the jobsite. Accordingly, preliminary drawings were created to obtain estimated assembly weights so that they could be verified against structural and transport capacities. Our skid assemblies would have to travel from our fabrication facility to the jobsite on lowboy tractor trailers, hoisted into the building and rigged to final placement locations.

The solution
Our mechanical solution began, as our solutions usually do, with full VDC coordination. The first step was to create virtual 3D models that accurately describe the spaces in which the assemblies were to be installed. The second step was to break the room layout of mechanical equipment into modular skids that were as large as possible (in terms physical size and weight), but also liftable, riggable, and transportable. In addition, it was important for us to carefully consider the constructability constraints in our fabrication shop and any potential connection obstacles in the field.

Because all mechanical assemblies had to be hoisted off the shop floor and mounted on lowboy trailers for transport, our design team contracted a third-party structural engineer to size, confirm connection types and the optimal materials to use for the framework, support and bracing for all of the skids. DOT trailer load requirements were taken into consideration when it came time to stack skid assemblies on the trailers. Overpass clearances also had to be checked and verified on the delivery route. On the jobsite, we coordinated with the general contractor and subcontractors to ensure there were appropriate openings in the building to allow hoisting and installation of the skids. We had to create an offload and sequencing plan, so that our installation team would know which skids went into the building first and then be able to work their way out.

Supplemental cross bracing was installed to prevent racking of the skids during transportation. All braces had to be drawn and detailed in BIM and coordinated with other trades to eliminate clashes and conflicts. To further alleviate the concerns about sound and vibration transmission, the modular skid design incorporated not only concrete housekeeping pads, but the bottom of the steel framing was in-filled with concrete once the framing was set onsite.

All modular skid assemblies were pre-fabricated in our Landover facility, following lean construction principles that minimize waste, maximize time savings and ensure delivery of completed assemblies to the jobsite to help us stay on schedule. A detailed fabrication plan was created to track the fabrication of each part of each skid on a weekly timetable, to verify that we would remain on schedule. Our fabrication shop provides a controlled, safe environment that enhances productivity. Not surprisingly, this leaner, safer and more productive approach helped to build confidence of the ownership and design team in our mechanical solution. As a further quality control safeguard, all prefabricated piping assemblies were pre-tested in the shop prior to shipment.

The results

The central plant was fully up and running on time, in large measure because all of that up-front work in the fabrication shop led to faster installation in the field. This is because producing a weld in a controlled shop, at a fixed, uniform table height with a machine designed to produce an automatic weld, is faster and more efficient than welding onsite.

Once installation began, the installation plan was reviewed and updated with input from the entire team, including our field foreman and our rigging subcontractor. A great deal of communication had to occur to make sure each piece of equipment arrived on time and in the proper sequence. The rigging plan included two crane setups for two sections of the mechanical room.

All of our solution elements came together as planned, enabling our team to execute the work and bring the central plant to full operation as scheduled. The facilities director was an early adopter of our modular design concept and a valuable team member in bringing the rest of the ownership and design team on board. The final product is a show piece of which he is extremely proud; he couldn’t be happier with the results.

In retrospect, there was a considerable number of moving parts in this project and the need for a great deal of coordination. The success of this project was a testament to the perseverance shown by our entire project team. Our VDC team worked well with equipment vendors in overcoming a host of hurdles to bring this concept to fruition. Once they came up with the idea of building the fabricated modular skid assemblies, this team never gave up on making it happen.

This was a design and construction requirement that our team had not previously encountered. Our people rose to the challenge and were able to think outside the box to deliver an innovative solution that delighted the client. All the time spent on research and engineering eliminated the trial and error that would have otherwise occurred if this had been a typical stick-built effort in the field. In terms of the design, it turned out to be a very “clean” mechanical room. To the untrained eye, it looks orderly and efficient. But experienced mechanical solutions professionals can look beneath the surface and understand the planning, coordination and labor hours that went into this solution.

At Shapiro & Duncan, our people drive innovation. Thanks to the experience gained on this project, we are better able to take on unique challenges presented by owners.