Three critical engineering aspects of the Carl R. Darnall Army Medical Center in Fort Hood
Since May 2013, Consulting-Specifying Engineer has been tracking Southland Industries’ progress in performing design-build mechanical and plumbing services for the Carl R. Darnall Army Medical Center located in Fort Hood, Texas. Five years after being awarded the project that would replace the existing 45-year-old hospital, the Southland team is turning over the space with a new 947,000-sq-ft hospital that will provide state-of-the-art health care service to many of our nation’s heroes and their families.
This final installment of Southland’s Project in Progress series provides a deeper look into three key engineering aspects of the project and a few challenges the team successfully overcame throughout construction.
The entrance of the Carl R. Darnall Army Medical Center Replacement (CRDAMCR) is unique in nature with respect to the rest of the facility. The area serves as the main entry point to the hospital, a pathway for people traveling between the clinics and hospital, and as the main dining area for patrons. The main north and south walls are glazed with 30-ft ceilings to provide world-class comfort and to help meet the project’s energy goals. To help attain these goals, Southland designed a radiant heating and cooling system for this entire area. Radiant heating and cooling is a great fit for high-ceiling areas, as it allows the space conditioning to be focused down low at the occupied level. During the summer months, the cooled floor absorbs the direct radiant solar heat while in the winter months the heated slab provides warmth.
The radiant system is zoned into interior and exterior zones. This allows the system to match the space usage and heat/cool where the loads are needed.
The radiant system is also a dual-purpose system. It is supplemented by the 100% outdoor-air system that serves the hospital and clinics. This system provides ventilation to the entire atrium while also providing trim heating/cooling in the high-load areas such as the south-facing curtain wall. This ventilated air is delivered to the space via variable air volume terminal boxes located in the surrounding interstitial building systems (IBS) that serve wall air-distribution grilles.
The installation of this radiant system took in-depth coordination and a great deal of planning. There are four zones and nine manifolds within the atrium. Each manifold houses a copper supply and return line, fed from the radiant mechanical rooms in the basement, which then transitions into three or more PEX-tubing loops for the specified zone. Southland used Uponor radiant rollout mats for ease of installation. Once installed, the PEX-tubing was covered with 4-in. of concrete with terrazzo topping. The two main obstacles faced with this installation were the size and complexity of the layout, paired with the fragile nature of the PEX-tubing as no sharp aggregate was allowed to be used in the concrete mix. Due to the large size of the zones, the systems took a lot of time to get the air out of the pipe for testing.
CRDAMCR was designed to be a 100% dedicated outside-air system, and at the heart of this system are the enthalpy-wheel housings. These housings are where the energy exchange occurs between the incoming outside air and the outgoing exhaust air. The enthalpy wheels are located in each of the respective building penthouses, and in the hospital’s level-three mechanical room. Each housing is made of large enthalpy wheels mounted in the center of each housing, made of double-wall insulated panels. The housings are split by a horizontal platform that separates the exhaust deck from the outside-air deck.
The lower level of the housing is the outside-air deck. Outside air is pulled in through the fixed exterior louvers and passed through MERV 9 filters before passing through the rotating enthalpy wheels. Once through the wheels, the air then flows through the opposite side of the housing unit directly into the air handling units for conditioning before being distributed to the ductwork.
The upper level of the housing is the exhaust-air deck. The exhaust air is pulled into the housing from the exhaust duct system in the opposite direction of the outside supply-air below. The air passes through the enthalpy wheels and, once on the opposite side, is drawn into the neighboring exhaust-fan housing for expulsion from the building.
The installation of these enthalpy wheels was an efficient process. Thermotech Enterprises delivered the frames and crated media. Southland hoisted the frames and crated media to the roof, setting the frames and bolting them together while Thermotech installed the motors and media. The biggest issue faced upon installation was having the wheel alignment off and slightly out-of-round. Once the media was bolted down, the wheel was within the specifications and accepted.
IBS (interstitial building systems)
The most beneficial system that was integrated into CRDAMCR was the IBS, which is composed of several zones: occupied, connection, and distribution. Each of these zones is located on a single floor, with floor-to-floor separations required to be rated to a minimum of 2-hour fire resistance. The connection zone is the interstitial space between the ceiling of the occupied zone and the 2-hour-rated floor, or walk-on platform, of the distribution zone above.
To use the IBS to capacity, these areas were coordinated with all trades involved. The walk-aisle design provides access to all of the CAV terminal boxes located in the IBS. Other equipment located in the IBS requiring accessibility was evaluated based on the frequency of routine maintenance. Access aisles were determined early in the design phase and the clear pathways were maintained during construction. The developed access aisles serve as primary and secondary access aisles. The primary aisles are intended to be 6 ft tall and a minimum of 4 ft wide, with the secondary aisles intended to be 5 ft tall and a minimum of 3 ft wide. These access aisles allow for easier maintenance and future modifications.
In addition to the walkways, the IBS was coordinated into trade zones. The requirements of each system depended on its location related to height within the IBS. Gravity-fed systems took priority and were coordinated first to accommodate required slope and any secondary utility in conflict. For the most part, each trade or utility assigned to a general zone or height was coordinated and installed accordingly.
The largest obstacle with the IBS was the lightweight concrete used for the walk-on platform. This lightweight concrete assembly was not UL-listed, and each trade using fire-stopping for penetrations had to submit a variance from the manufacturer noting that the use of the UL-listed products was acceptable to maintain the UL requirement. This concrete was also not a durable product. The consistency was not a solid product that could crack and crumble easily, so temporary covering was required during construction to prevent damage. Trade clashes in the field were rare and were ultimately solved by using the 3-D coordination model.
At the core of these critical engineering components was the strong relationship forged between Southland and the United States Army Corps of Engineers (USACE). With a level of trust formed from the design process through the commissioning phase, both parties were able to address potential issues and opportunities on a regular basis, which led to project success. A complete case study will be published in the coming months.
Kristi Koenig is a project engineer for the Southwest division of Southland Industries, a national MEP building systems firm. She can be reached at email@example.com.
Shawn Manley is a Senior Mechanical Engineer for the Mid-Atlantic division of Southland Industries, a national MEP building systems firm. He can be reached at firstname.lastname@example.org.