The art and science of mixed-use buildings
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.