The art and science of mixed-use buildings
CSE: Please describe a recent project—share challenges you encountered, how you solved them, and aspects you’re especially proud of.
Sauer: The biggest challenge at Purdue University’s Health and Human Sciences Building was consolidating the systems for different uses, while insuring high operational efficiency. The facility needed one or two central systems that could support rooms with differing needs. At the Health and Human Sciences Building, BSA LifeStructures’ design team used a dedicated outdoor air system (DOAS) and heat recovery wheel to achieve energy savings. A variety of terminal unit types were then selected for use in the spaces dependent on the environmental criteria required for that space. I am especially proud of the fact that these systems achieved a 29% energy reduction compared to ASHRAE 2007-90.1.
Holdener: CityCenterDC spans a 10-acre site in downtown Washington, D.C., with Class-A office space, apartments, condominiums, a hotel, and retail. Part of the U.S. Green Building Council LEED Neighborhood Development pilot program, CityCenterDC features numerous sustainable and high-performance measures in the building systems design. The project includes 458 rental apartments in two high-rise towers totaling approximately 560,000 gross sq ft; 515,000 sq ft commercial office space in two high-rise towers, and 216 residential condominium dwelling units in two high-rise towers totaling approximately 370,000 sq ft. In addition to the immense size and complexity of the development, CityCenterDC has presented unique MEP challenges, including stepped massing and setbacks in the rental towers that require the mechanical systems to extend downward to the lower level and then offset and rise up in the “full-height” areas of the building so they can terminate at allowable high-roof locations. The flexibility required for retail tenants has required multiple commercial kitchen exhaust duct risers and an increased ability for ventilation and makeup air. The overall project has four main electrical services, 15 separate utility transformer vaults, separate utility connections, and metering systems for each tower. In terms of flexibility, sustainability, technology, comfort, and aesthetics, this high-performance project clearly sets new standards for development in the region. The advanced engineering requirements support the owner’s claim that the project is, “A model for responsible, environmentally sensitive multi-use developments.”
Pomerantz: In a mixed-use project that contained a hotel and a residential condominium, capital cost became the controlling factor for the project to proceed. The project started with a true separation of all MEP systems’ easily allocated capital costs and operating costs of both constituents. Separate heating plants, cooling plants, utility services, and essentially all MEP systems were separate. In order to meet the budget requirements of the project, the systems were combined and revenue grade meters were used to allocate operating costs between the two users. Capital costs were reduced because only one plant was built, not two. The N+1 spare equipment was provided only once and not twice. For example, only one spare condenser water pump was provided, not two. Less space was required overall and additional savings were realized due to decreasing the overall building size. The common systems were located close to the intersection of the two users, and costs associated with the system distribution were minimized.
Chung: On a recent project in Chicago that incorporated hotel usage and residential apartments, one of the challenges we faced was incorporating the needs of the hotel operator with the code-mandated regulations of the entire facility. A number of variances existed between the two, such as a 4-hour fuel storage requirement for emergency generator power backup versus a 48-hour hotel design standard requirement, fire damper provisions through rated partitions, smoke control and mechanical pressurization requirements, and so forth. By working closely with the client, we were able to direct the owner to informed decisions with operator approval on project necessities.
CSE: When re- or retro-commissioning structures, what challenges do you encounter, and how do you overcome these challenges?
Slocum: From a fire protection standpoint, changes in the available water supply and updated backflow prevention requirements often present significant challenges when trying to work in existing structures. Development growth in an area can be taxing to the municipal water supply and result in reduction to the water supply available to an existing building. Additionally, requirements to install new or updated backflow prevention on existing fire protection systems can often result in a situation where the available water supply is no longer able to meet the demand of the existing sprinkler systems. Overcoming these concerns can sometimes be as simple as changing the sprinklers for a system to ones with a greater K-factor, which require less pressure to deliver the same flow. In more complicated situations it may be necessary to modify sprinkler system piping by looping cross mains or gridding branch lines, in order to reduce the demand of the systems to less than the available water supply. In the worst cases it may be necessary to install a fire pump, which can result in significant issues, such as finding adequate space for the pump and determining the adequacy to the building power system to support a new electric motor driven pump or addressing the concerns of exhaust discharge or fuel storage which are associated with a diesel engine driven pump.
Holdener: One challenge is determining and implementing control strategies and operational sequences that have been modified over the years by the operating engineers. Some of the operational changes and setpoint adjustments benefit the project in terms of energy and occupant satisfaction, while others changes are detrimental to the project. The retro-commissioning agent must work closely with the building operating staff as well as the design engineer to develop a comprehensive commissioning plan that addresses design and operational issues.