Out On a Ledge

Three-dimensional BIM model snapshot of the ERCWP full plant build-out without perimeter walls and screenwalls.

It’s supposed to be a simple box containing air conditioning equipment. But this time, the designers made their way out of the box and onto a ledge — literally.

The new East Regional Chilled Water Plant (ERCWP) at The Ohio State University departs from the traditional look and feel of utilitarian facilities by incorporating an all-glass building envelope topped with a metallic perforated screenwall around 12 rooftop packaged cooling towers. This unique structure is split into two large rectilinear forms, with the lower glass-enclosed plant housing the mechanical and electrical equipment while the cooling tower enclosure hovers over and around three-quarters of the plant. The fritted glass walls will give passersby a unique view inside the inner workings of a chiller plant. By combining Building Information Modeling and strategic, forward-looking thinking, the architectural and engineering teams were able to design a centralized chilled water plant that will meet the long term cooling capacity demands of the Academic Core North section of the campus.

Workers are installing a perforated metal screen panel system on the cooling tower enclosure frame. Two pairs of packaged towers have been installed behind the enclosure. The fritted glass wall panel system around the plant allows passersby a view into the inner workings of a chiller plant.

The use of BIM was key to successful design of the ERCWP. The project was made possible through integration between Autodesk Revit and Bentley Systems AutoPLANT software 3D BIM models. The building’s structural and architectural models were developed in Revit, while the mechanical and electrical team used AutoPLANT to create a series of models. Each plant piping system had its own 3D model, which could be reviewed independently and easily imported into the overall model for multidiscipline coordination.

Similarly, the electrical components were broken out into conduit, lighting, and equipment models. The mechanical and electrical modeling was enhanced by incorporating manufacturer-supplied equipment drawings directly into the BIM model. This reduced drafting time and increased model reliability. Plus, since AutoPLANT is spec-driven, all pipe, valve, equipment, and conduit system material information is contained for easy reference. BIM modeling also played a crucial role in overcoming the main structural challenge for the project — the cooling tower enclosure.

A view of the framing inside the tower enclosure. W12x50 tension braces extend down over a 36-inch-diameter basin equalizing pipe to support the towers, access platforms and screen wall hung 20 feet off the south face of the plant. Note the four, 5-foot-diameter exhaust air stacks positioned between the horizontal truss members in the tower basin framing level.

The 45-foot-tall cooling tower enclosure is roughly 100 feet square in plan and stretches halfway down the sides of three of the glass building walls. The structural engineering team at RMF Engineering was tasked with finding a way to support cooling towers, large-bore piping, and screenwalls that extend 10 to 20 feet out from the edges of the building. Forces from all the bracing suspended off the edges of the building were transferred into a concrete roof diaphragm and a horizontal truss system elevated 10 feet above the roof.

The BIM model was used to carefully lay out almost every beam, brace, and pipe within the enclosure. Hundreds of feet of water piping fill the space between the roof and tower bottoms. Steel framing needed to be located accurately to avoid interferences with pipe and equipment. Four, 5-foot-diameter exhaust air stacks were modeled to fit precisely between the members of the main horizontal truss system. Conflicts between steel and tower basin sumps were discovered early so a solution could be found before construction. The BIM model also allowed us to vary steel framing to accommodate different-sized cooling tower arrangements available from multiple vendors.

Ultimately, the team decided to centralize the chilled water generating equipment into one plant (as opposed to installing the equipment in smaller systems throughout campus) because centralized plants allow for better system reliability, accessibility, and maintainability. The entire facility can be laid out with specific consideration to material handling and maintenance access and a standby unit can be shared among all of the connected buildings (standby units are often not economically feasible or cannot be physically accommodated when the equipment is distributed in all of a campus’ buildings). Furthermore, the centralized chillers will be staged on as needed, so they will typically operate close to full capacity, where they are most efficient.

A view looking up at the underside of the tower enclosure outside the southeast corner of the plant. Framing extends out 10 feet to the east and 20 feet to the south to support the screenwalls, piping, and cooling towers, visible through the grating. The last pieces of fritted glass window wall have yet to be installed around the corner building column.

Pathways were designed within the BIM model, allowing equipment to be moved and positioned where needed throughout the building (this will aid the initial construction, as well as equipment replacements over the life of the facility). Bridge cranes, monorail hoists, and gantry cranes were also included in the design to allow the owner maintenance access to all critical equipment. The BIM model made it possible to verify clearances for all the hoists and cranes, including the two upper gantry cranes that serve the tower tops and access the operating floor 65 feet below via a roof hatch opening. Designing to facilitate maintenance activities is critical because it increases the likelihood that those activities will actually be performed, extending the life of the equipment and keeping it operating at optimum efficiency.

Designing and reviewing the model in 3D was essential for finding and working around conflicts in a tight multilevel space. The 20-foot-tall plant basement is filled with equipment and water piping for plant operations, along with distribution piping that exits into a new tunnel system to serve campus buildings. Also in the basement are large ventilation fans, numerous controls, and power conduits and cable tray. The dense layers of piping, conduit, and ductwork were carefully laid out in the BIM model to avoid impacts with the equipment pathways and building structure. The general contractor for the project, Shook Construction, continued BIM model development to incorporate modifications and additions during construction.

Shrink-wrapped chillers, delivered under the plant build-out phase, wait to be piped up like the chiller at the bottom of the picture. A 10-ton bridge crane hangs from runway girders connected to the underside of the roof structure to provide maintenance access to the chillers and material handling into the basement pump room.

Finding an optimum framing system and making sure it accommodated all the mechanical, electrical, and architectural demands within the plant BIM model was not the only structural challenge to this project. The ERCWP is partially located on the site of the original Ohio State University football field, now a grassy plaza. Underneath the old gridiron were foundation and subgrade hazards — expansive clay soils and expansive shale rock. To make matters worse, the shale had a high sloping profile. The shale had to be cut down in the northwest corner of the site to bear foundation walls and pier caps directly on it. Elsewhere, the rock sloped off and required close field observation to dictate either stepped or extended footing revisions. Where the rock was deep, drilled piers were used to penetrate the soils before socketing into rock. The potential for soil heaving from wet expanding shale subgrades below the basement floor was a major concern.

Also of concern were the dense wet clay subgrades. If the exposed clay subgrades dried out during the excavation phase, they could later become saturated by groundwater or a broken pipe, swell, and heave the basement floor. Therefore, it was decided to keep the clay portion of the excavation wet and the shale portion dry. This was achieved by spraying the exposed shale with an asphalt coating and laying a polyethylene membrane covered by 6 inches of stone on the wet clays. To limit the possibility of water migration into the shale, the site was sloped to a deep perimeter foundation drainage system.

When fully complete, the ERCWP will provide The Ohio State University campus with 15,000 tons of efficient cooling capacity to meet existing and future demands. Further build-out of the plant is underway as the university has already opted to install three more chillers and six more cooling towers. The project is currently tracking LEED-certified with anticipated completion set for early 2015.

James Riches, P.E., an associate at RMF Engineering, Inc., has 20 years of experience in various aspects of structural engineering design and construction. He specializes in the structural design of campus utility plants and utility distribution systems utilizing Building Information Modeling. He can be reached at jim.riches@rmf.com.

Posted in | August 12th, 2014 by

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