An overview: How to engineer systems in mixed-use buildings

When working on mixed-use buildings, engineers must address many needs in one building.

By Consulting-Specifying Engineer September 30, 2012


Mark Crawford, Principal engineer, Southland Industries, Las Vegas

Christopher M. Kearney, PE, LEED AP, Project manager, exp, Maitland, Fla. 

Brian McLaughlin, Associate, Los Angeles, Arup

CSE: What sorts of challenges do mixed-use buildings pose that you don’t encounter on other projects? 

Mark Crawford: Mixed-use buildings create complexity at the interface between the occupancies. For example, high-rise hotel towers penetrating an entertainment podium with a basement-level garage require transitions from one building type to another. Structural systems usually change from steel to post-tension concrete. Plumbing may change from single-pressure zone domestic water distribution and traditional waste/vent to high-pressure domestic water with multiple pressure zones and single stack cast iron Sovent drainage system. HVAC will change, such as 100% outside air VAV with heat recovery to vertical fan coils and dedicated outdoor air system (DOAS). Utilities for one building are usually supplied from another instead of coming from the site. Code applications change from large assembly use to high-rise residential codes and must be combined into code. In short, one design concept is rarely maintained throughout a mixed-use facility.

Chris Kearney: For building engineers, some of the biggest challenges that go into a mixed-use building include fire separations between different occupancies and smoke management. Other challenges include sound attenuation requirements or acceptable sound levels for different space types and how they may affect each other. In addition, there are many opportunities to help developers build cost-effective and operations-efficient properties that some might call a challenge. 

Brian McLaughlin: The biggest challenge encountered on mixed-use buildings is often related to designing for nonseparated mixed-use provisions. In achieving this design approach, the most restrictive life safety provisions of each of the occupancies must be met throughout the building. It is important to understand how the requirements for each occupancy will impact the design. Depending upon the variety of occupancies within a building, a separated mixed-use approach may be the best option to minimize excessively restrictive provisions being applied throughout.

CSE: How have the needs and characteristics of mixed-use buildings changed in recent years? 

Crawford: Sustainable and energy-efficient designs are now standard practice because of changing codes, but more importantly, these characteristics help make the buildings appealing to the public. Interestingly, smoke control codes have become simpler with fewer requirements for complex smoke control strategies demanding complex HVAC systems. The systems can still be very involved, however; code officials have joined engineers and contractors in understanding the value of keeping systems as simple as possible. New trends are increased renovations and planning for healthcare mixed-use developments.

CSE: What do you need to take into account when engineering systems into super-high-rise mixed-use buildings? 

McLaughlin: Super-high-rise buildings are those that (per the International Building Code, or IBC) exceed 420 ft in height. The IBC has a variety of specific provisions that apply to these building. In the context of mixed-use development, it is important to holistically consider the design elements of the building. For example, if there is a multistory (let’s say fewer than six stories) podium component to the building, it may be possible to design the podium to be considered a separate building that would not need to comply with the more restrictive super-high-rise provisions. Another example of a specific system that would need to be considered is occupant evacuation elevators (OEE). IBC requirements permit OEE to be used in lieu of an additional exit stairway; however, all passenger elevators in the building would need to meet the stringent requirements for OEE. The pros and cons of applying this type of system must be carefully considered by all stakeholders.

Kearney: Super-high-rise buildings require unique design considerations. For electrical systems these might include the quantity, locations, and voltage of electrical substations and how the potential different distribution approaches relate to voltage drop, feeder sizes, penetrations, and even the supports required for the different approaches available to a designer. The decision between bus ducts and/or large electrical feeders is always a debate. HVAC, plumbing, and fire protection pressure zones will affect pump quantity/locations and system material selections. In fact, system selection and zoning can help eliminate costs associated with high-rise approaches. Other issues/decisions that need to be considered/made in design include stack effect, individual versus central systems approaches, and systems that don’t stack because of the differing uses. The decision as to how to provide utility metering (e.g., hot water, cold water, chilled water, gas, and power) also needs to be considered early in the design process. 

CSE: Please describe a recent mixed-use project you’ve worked on, and describe your success. 

Crawford: At the M Resort, a Las Vegas casino/hotel, we solved a common design issue in many mixed-use facilities. While this solution is not typical marketing fodder, the fire/life safety (FLS) systems are oftentimes incompletely coordinated during construction documents phase due to the mix of disciplines, lack of fire protection and fire alarm designer involvement in early design stages, number of interpretations of code, and lack of building official involvement. This leads to finishing the FLS system design during construction, resulting in significant issues during project commissioning and closeout. As a combination HVAC, plumbing, and fire protection design-build contractor, we led an integrated design effort that resulted in significant fire protection and code official input into the HVAC design including system airside zoning. This approach resulted in simpler execution of FLS system construction and streamlined system commissioning, and improved the system functionality and maintainability.

Kearney: The Ritz Carlton at Bank of America Corporate Center was a recent project that was part of a larger mixed-use development. Our team coordinated with the engineers designing the adjacent office tower, which shared emergency power and the chilled water system. For the chilled water system, heat exchangers were included in the hotel component to divide the hotel tower’s HVAC hydronic system into two pressure zones. Challenges included the testing and operation of the smoke control system as it was integrated into the building HVAC system. Significant smoke testing, control modifications, and system balancing occurred to bring the smoke control system into compliance while not compromising the guest experience. In addition to solving complicated smoke control scenarios, this project received LEED Gold Certification and is the first of its kind in the Ritz Carlton family.