The art and science of mixed-use buildings

Engineering mixed-use buildings is a fine art—specifiers must combine multiple engineered systems for several business and residence types into one structure. This overview offers a look at the challenges of these buildings.

By Consulting-Specifying Engineer September 16, 2013


Robbie Chung, PE, LEED AP, Senior associate, Environmental Systems Design Inc., Chicago

Raymond Holdener, PE, Senior associate, Dewberry, Fairfax, Va.

Andrew Lasse, PE, LEED AP, Associate principal/senior mechanical engineer, Interface Engineering, Portland, Ore.

Gary Pomerantz, PE, LEED AP, Executive vice president, building systems, WSP, New York City

John Sauer, PE, LEED AP, Senior director, BSA LifeStructures, Indianapolis

LeJay Slocum, Assistant director, Atlanta regional office, Aon Fire Protection Engineering Corp., Suwanee, Ga.

CSE: What challenges do mixed-use building projects pose that are different from other projects?

Robbie Chung: Mixed-used building projects face a number of challenges that one would typically not come across in a single-use project. Regulatory code requirements will vary across the usage types, requiring additional design and planning considerations. From a design standpoint, the mechanical, electrical, plumbing (MEP), and fire protection infrastructure may need to be delineated across usages to ensure that operational costs are appropriately separated. This can lead to added initial cost and reduction in leasable sq footage within the building. Furthermore, mixed-use buildings characteristically garner a diversity of project stakeholders with different goals and desires. However, with the right project professionals on board, the challenges can be effectively assessed and overcome.

Raymond Holdener: There are several:

  • Vertical alignment of outside air and exhaust ductwork risers and shafts. It is challenging architecturally to align the risers and shafts vertically when the space layouts vary from floor to floor. If the shafts do not align vertically, then the ducts must offset, which requires the horizontal offsetting of the ductwork and rated shafts. These requirements pose a challenge to ceiling heights and construction budgets.
  • Space constraints due to tight floor-to-floor conditions while maximizing ceiling heights and minimizing locations of ceilings/soffits for ductwork, sprinkler, wiring, and plumbing piping serving the space.
  • Designing outside air systems to residential units while ensuring that the code required outside air is delivered to the space and is measurable. This particularly becomes a concern if natural ventilation is used through the use of operable windows. If the operable windows are closed, then code required outside air may not be delivered to the space. When natural ventilation is used, this results in unconditioned outside air being delivered to the space, which can result in uncomfortable space conditions. Typically, we recommend that a forced air outside air system be used that provides pre-conditioned and measurable outside air to the space. This type of system may add cost to a project, versus natural ventilation, which is also a challenge.
  • Designing to maintain a reasonable air balance and relative pressurization (based on area, floor, and building overall) due to multiple types of exhaust systems (many with dynamic or variable flow), stack effect, wind conditions, and operable windows and doors.
  • Routing multiple residential utilities and services through retail spaces while routing multiple commercial retail exhaust systems through residential.
  • Fire alarm notification between different structures and occupancy uses within the same building; for example, a local smoke detector false alarm due to burnt toast in a single apartment should not cause evacuation of an entire office tower in the same complex.
  • Meeting construction budgets for all projects is a challenge, but it is often more of a challenge for mixed-use type projects.
  • Separation of utilities and metering systems to track operational cost between multiple ownership and management entities within a single project.
  • Separation of grease waste sanitary drainage lines serving restaurants from other building drainage systems. Best practice is to provide grease waste lines that extend from the restaurant to an exterior manhole without connecting to any other drainage lines. The additional piping is usually minimal in cost compared to the cost of one blockage of the building’s main sanitary waste line. 

Andrew Lasse: Mixed-use projects bring together multiple tenants and uses within the same building that in turn require special attention to the MEP systems being employed. The challenge is to cost-effectively select the MEP system that works well within these varied environments and programmatic elements. An underfloor air distribution system may be great for open office space, but how can that system dovetail with residential units in the same building? Engineers must look at the big picture and select central systems that can be flexible enough to serve varying subsystem approaches in each space, in order to best fit the clients’ needs.

Gary Pomerantz: Mixed-use projects are often larger, and due to their size, designs become more complex. Each constituent has individual systems that at times overlap with similar systems in other constituents. The base of the building is very complex because each of the constituents requires entranceways, egress points, utilities, and other infrastructure items that all converge in one area of the building. Ownership issues between the different constituents and how to divide capital and operating costs between them is often an important issue. Residential constituents are most concerned about accountability and requiring individual systems and utility metering. In some locations outside of the United States, there are laws (Strata Title Laws) that require the separation of systems and utility metering.

John Sauer: The biggest challenge presented by mixed-use facilities, like Purdue University’s Health and Human Sciences Building, is designing engineering systems for rooms that all have different requirements and needs. This is especially true at the Health and Human Sciences Building, which combines spaces for healing, learning, and discovery. These spaces all have their own needs based on differing technology, environmental requirements, lighting levels, and hours of operation. This building is epically challenging because the building is home to wet and dry labs, anatomy labs, audiology labs, a teaching kitchen, clinics, and office space. The lab spaces need optimal indoor air quality, while the audiology labs must have precisely tuned acoustics to cater to their research needs. With so many unique needs for these spaces, it makes it difficult to design an efficient operating system.

LeJay Slocum: From a fire protection standpoint the biggest challenge of working on mixed-use projects is ensuring the design incorporates sufficient initial flexibility to address the changing uses that may occur over the life of the building. With a single-use building it is easy to determine what hazards should be anticipated and design the building accordingly. With mixed-use projects, the opportunity for changes during design and construction as well as after occupancy increases substantially. Designing and specifying economical fire and life safety systems that can accommodates these changes with minimal modification is the most significant challenge of working on 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.

CSE: For super-high-rise mixed-use buildings, what challenges must the engineer overcome?

Pomerantz: Super-high-rise buildings have an added challenge of high hydraulic pressures, long distribution distances, increased life safety requirements, increased schedule requirements, and small floor plate size to overall project area ratios. Several of the 1300-ft tall buildings that we are working on require three or four pressure zones to maintain operating pressures within allowable operating conditions for equipment and that permit for the safe operation of the building. While pressure break heat exchangers can be installed, they require substantial space and reduce the overall energy efficiency of the building. The long distribution distances have larger pressure and voltage drops than conventional high-rise buildings. The piping and pumping distribution systems require more rigorous engineering and the electrical distribution is often done at high voltage. The increased voltage reduces voltage drop and reduces the construction cost and space requirements.

Andrew Lasse: High-rise mixed-use buildings make us think differently about conventions that were forged on the many mid-rise mixed-use projects that have been built. One of the big challenges is making the right provisions for retail shell spaces that are normally located on the ground level. In a 23-story building, how do we effectively accommodate a large restaurant and commercial kitchen that requires a grease duct and makeup air system? Good design dictates these systems should be routed to the roof, but installing ductwork in rated shafts for a tenant that doesn’t yet exist can be a difficult financial decision. In situations like these we need to outline the benefits and drawbacks for the building owner in order to make an informed decision.

Chung: Super-high-rise mixed-used buildings require many unique design considerations. From an early conceptual viewpoint, systems such as the mechanical, plumbing, and fire protection must be carefully coordinated with the architect to allocate the correct pressure zones in conjunction not only with the building usage break-outs, but also the elevator stacking. Electrical will also have similar concerns in regards to utility vault locations and quantities to reduce voltage drop, conduit sizing, and overall cost.

Holdener: These types of projects require multiple pressure zones for various plumbing, fire protection, and HVAC water piping systems. Pumping systems and piping schedules/pressure classes must be properly designed and specified. Stack effect must also be considered with respect to building pressurization and fan system design. Depending on building height and ambient versus interior temperatures, stack effect can have a significant contribution to system static pressure for those systems that service large portions of the building vertically. For example, a mixed-use project with a 30-story hotel and a 70-story office building connected at the base with a common retail mall has the potential to create massive amounts of air movement between the vertical structures, usually from the shorter building into the taller building via the mall area. This design challenge needs to be addressed by the architectural systems as well as the mechanical systems to achieve a successful outcome. Vertical supply and exhaust air risers should not be continuous from the top to the bottom of the building. The vertical air systems should be segmented into smaller vertical elements that minimize the stack effect. 

CSE: Describe clash detection software or tools you’ve used in a mixed-use building, and how it solved mechanical, electrical, plumbing, fire protection, and structural clashes.

Pomerantz: Documents are produced in Autodesk BIM Revit, and clash detection is done with Autodesk Navisworks. The most important issue to note about clash detection is to set realistic goals. It must be understood that consultant drawings are not shop drawings, and ultimately the contractors have the responsibility for truly clash-free documents based on actual equipment purchased and installed in locations that may be slightly different than are shown on the construction documents.

Chung: The buy-in by many architects and engineers of clash detection software and 3-D modeling has given designers a better understanding of where physical problems exist and an ease of finding a solution to them. However, there is an argument that the design process becomes lengthened due to the 3-D modeling program’s infancy. But as its prevalence increases, hopefully the programs become quicker and more advanced, allowing for more expediency in the design process and reduction in coordination issues during the construction process.

Holdener: We have used Revit MEP along with Navisworks for clash detection on projects. This allowed us to identify multiple locations where clashes existed, not only within the MEP disciplines, but also with the architectural and structural disciplines. These clash detections allowed the design team to remedy the clashes before the contractor obtained the project so as to be able to deliver a further coordinated set of documents and avoid future potential requests for information (RFI). The clash detection process is critical to the success of the coordination process. For example, if the process does not include the proper filters and clash detection parameters, the “clashes” can number in the thousands. This situation can grind the process to a halt, making the team spend more time dealing with the number of clash report items than addressing actual coordination issues. We’ve found that using a combination of Revit and Navisworks software with experienced “clash detection” teams provides the optimum approach to coordination and clash detection.

Lasse: Contractors have long used programs such as AutoCAD MEP and Navisworks to generate shop drawings and coordinate between disciplines during construction. Revit is now the go-to program for the architect, MEP engineer, and structural engineer during design. The process of generating these 3-D models and ensuring proper coordination with the contractor is unique to every project. We’ve learned the best approach is to have the contractor and subcontractor on board and engaged in the project during design to ensure the most streamlined coordination efforts and most accurate models for the building. Finding ways to weave together efforts during the modeling phase is efficient and makes good sense. Why create a Revit model in design and have the contractor generate a separate 3-D model during construction? We all need to work together.

CSE: When dealing with vertical connectivity in tall buildings, what challenges do you face, and how do you solve them?

Holdener: Stacking of MEP system piping and ductwork is critical; otherwise, the design and construction will include offsetting of pipes and ductwork, some of which can be relatively large. Fire-rated enclosures of ductwork risers must be maintained, which adds to the overall scope and physical size of these offsets. Every change in direction, or elbow, increases the system static pressure or head that the fan or pump systems must overcome, respectively. For mixed-use projects with residential above retail, vertical systems for both uses need to be considered. For example, every time a commercial kitchen grease hood exhaust duct changes direction, an accessible cleanout is required by code. For gravity drainage systems such as storm and sanitary waste, changes in direction tend to slow down the flow in the pipe, which can increase the potential for clogging the pipes and can increase sound—an important issue in residential projects. It is important to address stacking, vertical connectivity, and related impacts with the owner, architect, contractor, and other team members from the early stages of the project planning so design directions are made with due consideration to the potential impact on the project. The impact of vertical offsets in multiple MEP trades is not limited to MEP design and coordination issues. Each offset adds horizontal length to the MEP systems, architectural enclosures, and non-typical structural framing. All of this adds to the first cost of the project while reducing quality and energy efficiency.

Lasse: The golden rule in high-rise design is to stack, stack, and stack some more. Residential bathrooms and kitchens need to be aligned from floor to floor as much as possible to save significant MEP costs (most notably for plumbing) and also reduce congestion in the ceilings and maximum ceiling heights. Vertical rated shafts for exhaust and makeup air are also crucial components to stack in order to avoid costly extensions of shafts horizontally. Massive coordination and constructability challenges lie in buildings that lack vertical connectivity, and MEP engineers need to educate and remind architects of this early and often.