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.
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.