Tall order for super high-rises
Super high-rise buildings pose unique challenges to engineers. The number of these buildings is increasing, which is creating more demand for specialized engineers with high-rise expertise.
Meet the roundtable participants
- Paul Bearn, PE, Associate, Electrical Services Engineer, Kling Stubbins, Philadelphia
- Michael J. Ferreira, PE, Senior Engineer, Hughes Associates Inc., Baltimore
- Mehdi Jalayerian, PE, LEED AP, Executive Vice President/Managing Principal, International Environmental Systems Design Inc., Chicago
- Lalit N. Mehta, PE, Vice President, Mechanical Engineering, Syska Hennessy Group, New York City
CSE: What engineering challenges do super high-rise buildings pose that are different from other structures?
Paul Bearn: The vertical distribution and transportation requirements are quite different. The riser space required to provide power, communications, conditioned air, and elevator access is considerable, and at odds with the desire to maximize usable building area. Super high-rise buildings must comply with building code requirements for increased structural integrity and additional sprinkler risers. The challenge is compounded by the change in space usage from floor to floor, which is typically encountered from parking levels, to lobby areas, to occupied spaces, to mechanical floors.
Michael J. Ferreira: From a life-safety standpoint, the primary engineering challenge is enabling safe egress over the extended period of time it takes to evacuate a super high-rise building. Unlike a typical low-rise building,where full evacuation usually is accomplished in as little as 20 to 30 min, evacuation from a super high-rise can take more than two hours.
Mehdi Jalayerian: The MEP systems for the super high-rise buildings must be developed in close coordination with the structural design and adapt to vertical configuration to optimize system working pressures. The stacking configuration is generally overlapped with stacking of the MEP system. It is common for the building’s structural cross-bracing to coincide within the same floors as the main MEP systems servicing vertical zones of the building. Successful super high-rise designs are created through an integrated process between architecture, structure, and MEP systems. A fully integrated approach also provides for economical solutions that are buildable and require less coordination during the construction phase.
Lalit N. Mehta: The largest, and often the most overlooked, challenge in designing super high-rise buildings is understanding local practices, culture, codes, and standards governing each region. Another is integrating sustainability, reliability of systems, stack effect, wind effect, infiltration, life safety, and security.
CSE: How have the needs and characteristics of super high-rise buildings changed in recent years?
Ferreira: Prior to Sept. 11, 2001, super high-rise buildings were designed with a defend-in-place mentality. Occupants were only evacuated from the fire floor and immediately adjacent floors, limiting the loading on the exit stairs. The psychology of evacuation has changed. Even if directed to stay in place, occupants may choose to evacuate the building, overloading egress components that were designed for staged evacuation. And where super high-rise buildings used to predominately contain office space, there has been a trend toward residential super high-rises; the potential for delays due to alerting sleeping occupants could prolong evacuation time.
Mehta: High-rise buildings have evolved into multi-use, market-driven facilities. Having various facilities under one roof generates additional revenue for the owner and allows efficient delivery of required services.
CSE: With the challenging economy, are project owners more or less likely to have super high-rise buildings for you to tackle?
Mehta: We have seen a setback after Sept. 11 in the U.S. However, it is human nature to build higher and higher. We are seeing this trend of building higher in many more countries.
Bearn: Economic cycles are constantly in flux. High-rise construction in the US has slowed, but in other areas construction is booming. Five years ago Dubai was hot; high-rise construction has shifted more recently to Asia.
Ferreira: Super high-rise buildings are often signature buildings with more robust funding sources and political pressures to complete. I have not seen a substantial reduction in signature projects.
CSE: Please describe a recent project you’ve worked on—share problems you’ve encountered, how you’ve solved them, and aspects of the project you’re especially proud of.
Ferreira: I’m particularly proud of our work on the Statue of Liberty, a rather unconventional high-rise structure. We performed a fire modeling and evacuation analysis to demonstrate the impact of a number of proposed fire protection and exiting enhancements aimed at significantly increasing the safe egress time for a range of fire events. The proposed changes, scheduled to be completed in 2012, improve life safety for visitors accessing the statue and allowed visitors to resume accessing the crown observation platform for the first time since 2001.
Mehta: We are working on several super high-rise buildings in South Korea. The challenges are to meet or exceed the local practices, culture, codes, and standards. We are especially proud of providing sustainable and innovative design to conserve energy.
CSE: When working on high-rise buildings in other countries, what differences or innovations have you witnessed? What can engineers working on American buildings learn from this?
Jalayerian: High-rise buildings in other countries put a larger emphasis on life safety than previously considered in older North American buildings. It is generally required that high-rise buildings provide a dedicated area of refuge for occupants located at a designated floor throughout the building to facilitate extended exiting time. Innovative approaches to sustainable design are also required to achieve energy efficiency without significant installed cost premium.
Mehta: The culture and lifestyle dictates the types of systems used. For example, in Korea, radiant floor heating is common in residential areas, and there are strict requirements for the smoke exhaust system. There need to be stair and vestibule pressurization during fires, equipment and distribution system redundancy, and you need to provide renewable systems for the building to minimize utility and operating costs.
Bearn: The electrical systems can be strikingly different, even when working in countries which reference U.S. building codes. Cable tray is used for power distribution far more frequently than the conduit and wire typically encountered in the U.S. Systems may be designed to the National Electric Code (NEC) but use International Electrotechnical Commission (rather than Underwriters Laboratories) panelboards and other equipment, with wire in metric sizes. Multistandard convenience outlets, accepting European, U.K., U.S., Asian, and other plug configurations may be specified, or multiple outlet types may be installed.
CSE: What are some common missteps that engineers might make on such a project?
Mehta: Engineers need to familiarize themselves with the design, equipment available, and construction practices in the region. Local practices typically determine required deliverables, which usually are very different from what we may be accustomed to in the U.S. Understanding local practices can significantly improve the engineer’s rate of success. Since the final design on most of these projects is completed by the local engineers, it is important to engage local engineers early in the design process.
CSE: In seismically active zones, how do you work with the structural engineer (or others on your team) to ensure the mechanical, electrical, plumbing, and fire protection systems are sound and meet code?
Mehta: Codes for super high-rise buildings are still evolving. It is important to collaborate with the structural engineer to secure life-safety systems as well as other essential systems, and to coordinate the support system and expansion/contraction of the distribution system with the structural engineer to meet good engineering practices and local code requirements.
CSE: What factors do you need to take into account when designing building automation and controls for a super high-rise building?
Mehta: You must provide a high-speed network system that enables and ties into different entities. This will provide effective communication and control for all systems. In addition, the selection and location of central command stations is key for overall management and control of building-related functions.
CSE: What are some common problems you encounter when working on such systems?
Mehta: Typically, the common problem is selecting the required hardware and software communication protocol to allow proper integration with each of the various building systems. Identifying the proper communication interfaces with each building’s subsystem will reduce commissioning and maintenance related issues.
CSE: How have changing HVAC and/or electrical codes and standards affected your work on super high-rise buildings?
Jalayerian: Our design practice for super high-rise buildings has anticipated the intent of subsequent local code changes by considering collections of worldwide applicable codes. The designs have included best international practices with respect to codes and standards for initial provisions, and have considered and implemented the anticipated enhancements. Every new generation of super high-rise establishes an improved application of codes and advances the standards of practice.
Mehta: New energy code requirements and changes to codes result in challenges on the project. To overcome these issues effectively, the owners, architects, and engineers should collaborate from the beginning.
CSE: How do such codes/standards vary from region to region? Describe any international projects you’ve worked on.
Mehta: Even though international projects use the International Building Code (IBC), we find they still have their local requirements. Designing smoke exhaust for office areas in New York City, we could use ceiling plenum. In South Korea, the smoke exhaust will have to be ducted to each area of the floor. Hotel guest rooms and residential space over 50 sq m of space requires both supply and smoke exhaust systems.
Jalayerian: Being the first to design a super high-rise in a developing jurisdiction presents the opportunity to help the local community of building authorities, utility companies, fire “brigades,” contractors, and owner-operators to become familiar first-hand with solutions which are typically outside the scope of local codes and experience. An example is location of transformers above-grade versus at-grade: in the U.S., common practice includes transformers located high above-grade on mid-level mechanical rooms and near the “centroid” of the load for the section of the tower it is serving. This method of distribution economically provides for high reliability and flexibility. European standards somewhat limit this; Chinese standards recognize and require this; Middle Eastern jurisdictions did not allow this approach but are evolving toward such concepts.
Bearn: In countries such as Saudi Arabia, both U.S. and European codes coexist, and it can be unclear exactly which code is applicable–and there’s often a desire or requirement to adhere to both sets of codes, despite the conflicts which exist between them. A quite different problem is faced in China, where the Guo Biao (“National Standard”) codes are written in a style less like the strict requirements of U.S. codes and more like a design guideline, which can lead to difficulty in interpreting and understanding the codes. Similarly, the U.S. cities where most U.S. high-rises are built often adopt their own codes and standards, which can deviate greatly from national U.S. codes.
CSE: Which codes and standards prove to be most challenging in super high-rise work?
Bearn: One of the biggest challenges we’ve encountered has been applying European lightning protection codes to high-rises. IEC 62305-3 paragraph 5.2.3 requires that, for buildings over 60 m high, the top 20% of the building façade be protected against side flash, with air termination system requirements identical to those for the roof protection. While the façade protection system may be invisible from ground level, it is quite visible to the building occupants–even more so to the building architects.
Jalayerian: Most building codes are written for typical buildings. The most challenging issues are lack of coordination of one requirement of code with respect to performance of another requirement of the applicable code or standard. Integrating a locality’s preference for operable windows while mitigating the stack effect performance of super high-rise buildings will be a significant challenge and imposes considerable installation and operational cost on the super high-rise building.
CSE: Discuss fire and life-safety codes in high-rise buildings. What new standards are changing the design of these buildings?
Ferreira: The National Institute of Standards’ investigation of the World Trade Center disaster resulted in a number of recommendations that were implemented in both the 2009 edition of the IBC and the soon-to-be-released 2012 edition. Among the changes impacting super high-rises are the addition of a third exit stairway, 50% increase in stair width, provision of a emergency voice communication system, hardening of stairways and exit enclosures, provision of redundant sprinkler water supply risers, and providing the framework for using elevators for occupant evacuation.
Jalayerian: Fire and life-safety codes generally do not cover all provisions for construction of a super high-rise building. Every new building advances the fire and life-safety application to a more enhanced level. Oftentimes these advancements become baseline for local codes. Today’s super high-rise buildings include enhanced elevators specifically configured for evacuation and designated as “lifeboats” providing for significantly improved occupant exiting during an emergency event. This approach also places additional loads on the emergency/standby system. Typically elevators operate sequentially when supplied from the emergency/standby power plant, so the circuit from the emergency source to an elevator bank’s automatic transfer switch would potentially be much smaller than the circuit from the “normal” source. A bank of lifeboat elevators can require as much emergency power as in the normal condition. Additional provisions such as flood lighting and closed-circuit TV cameras on top and bottom of the lifeboat elevator cabs for unmanned survey of hoistway conditions are also required.
Mehta: Generally, NFPA and IBC codes (with local codes) are used for the fire and life-safety systems. Fire life-safety plays a major role in high-rise buildings. Fire and smoke alarm, sprinkler system, method of evacuation, refuge area and lifeboat concept, egress stairwell pressurization, fireman elevator lobby pressurization, and elevator lobby pressurization are key elements. The recent change to the IBC code which allows all elevators to be used for the evacuation system has posed a challenge to architects and engineers. The elevator hoist way has to be waterproof. This requires a method to keep the water away from the elevator door opening in case of the activation of sprinklers in the area.
CSE: What’s the one factor most commonly overlooked in electrical systems in super high-rise buildings?
Mehta: Redundancy and reliability of the electrical distribution system.
Jalayerian: We have peer reviewed some super high-rise designs that have overlooked the coordination of a heavy lift elevator cab for moving substation transformers and overlooked provision of adequate equipment access. A critical aspect of the design process for super high-rises requires close coordination of all MEP spaces with architecture and structure.
Bearn: Backup power systems, including both life-safety systems (emergency lighting, fire pump, smoke evacuation) and standby power systems for business continuity, need to be reviewed in detail early on in the project’s conceptual phase. Such systems can have a substantial physical requirement for building footprint, fuel storage, and air intake, and can have a high cost associated with them. In addition, the code and product listing requirements vary greatly from country to country. Local expectations vary as well: in the Mideast, it is common knowledge that domestic water pumps are “always” provided with generator power, even if there is no written requirement to do so; this may surprise uninitiated Western designers.
CSE: What types of products do you most commonly specify in a super high-rise, and why? Describe the UPS system, generators, etc.
Jalayerian: We commonly design medium-voltage distribution to avoid extremely long “utilization voltage” circuits; highest quality products are also considered to maximize power reliability/availability. Electrical system configuration generally uses double-ended substations with automatic transfer switches local to each load. Most super high-rise installations also include a spare substation transformer on each upper substation level for quick replacement in a failure event.
Mehta: An uninterruptible power supply is provided for the critical facilities such as main telecommunication rooms. The system is designed to maintain power supply for approximately 15 min. The system is always backed up by the emergency generator, which is designed to start within 10 sec of power failure. The generators provides backup power for life-safety systems, controls, fire pumps, telecommunication systems, domestic water tank fill pumps, boilers, hot water pumps, etc.
CSE: How have sustainability requirements affected how you approach electrical systems?
Bearn: The biggest shift in electrical systems design has clearly been in the areas of lighting and lighting controls. The palette of lighting sources available on the market and selected by lighting designers is radically different from that of only 10 years ago (when incandescent was still prominent), and the rate of change continues to accelerate. Lighting power densities have dropped, due to the impact of ASHRAE 90.1, U.S. Green Building Council LEED, and International Energy Conservation Code (IECC) guidelines and requirements, and the widespread adoption of energy codes means that automatic lighting controls are now commonplace or mandatory. It doesn’t hurt that lighting and lighting controls have some of the quickest paybacks of any energy conservation measures. Beyond lighting, there is clearly an interest in photovoltaics and energy usage monitoring like never before.
Jalayerian: Aside from energy efficiency, sustainability in a super high-rise is also achieved through extending the medium-voltage primary distribution to as close as possible to the load center as well as implementing standardization (modularization) of equipment into design solutions. This concept reduces the material required to distribute power and provides for consistent installation process.
CSE: What common mistakes do you see in electrical systems in super high-rise buildings designed by other engineers?
Mehta: There is not enough coordination with other trades, and redundant distribution systems are overlooked.
Jalayerian: Most super high-rise buildings include mixed-use occupancy. The MEP systems for such buildings utilize distributed system configurations tailored to meet the operational requirements of each occupancy or building zone. Considering all life-safety loads to be running simultaneously on the emergency/standby generator plant, instead of considering just the equipment required for the single worst-case scenario, can be a common mistake. Through careful review of operational requirements, the electrical system can include diversities that provide for most economical installation tailored specifically to the needs of the building operation. Similar arguments can be made with regard to not considering loads on the generator plant during a non-life-safety-event utility blackout, overlooking thermal stability effects on MV cable sizing, not coordinating transfers/offsets for risers between floorplates of differing occupancy/core layout, not coordinating sufficient clearance for equipment removal, replacement, etc.
Bearn: Shortcomings often become apparent in tenant improvement projects subsequent to the initial building construction. Insufficient consideration has been given to future needs, such as space for additional power distribution equipment or supplemental cooling. Or worse, there are often limitations or miscoordination in the architectural design (the building passageways, floor loading, elevators, and access points), which make the replacement or addition of major electrical equipment a difficult and costly proposition.
CSE: What trends and technologies have effected changes in fire detection/suppression systems in super high-rise buildings?
Jalayerian: Today’s super high-rises include duplicate risers for closed fire alarm system network “backbone” loops. They also generally feature duplicate fire command centers to facilitate efficient and effective operation of such facilities considering mixed-use occupancies.
Mehta: The super high-rise building will require intermediate tanks and pumps to fill the tank at the highest level to provide gravity feed for the suppression system. Water booster pumps at the intermediate level are provided for fire department use. For safety, dual risers are provided with cross connection at the floors. For multi-use buildings that include office, hotel, residence, retail, etc., a fire command center is provided for each entity in addition to the central command center.
Ferreira: The 2012 IBC code change requiring provision of an emergency voice communication system in addition to an audible fire alarm system in super high-rises will improve occupant notification as to the nature of an emergency event. This increased awareness should reduce the tendency of occupants to seek to evacuate for noncritical events, lessening the loading of exit stairs. In addition, mass-notification systems are seeing increased use, whereby building occupants are reached via multiple modes of communication, including: e-mail, text messaging, cell phones, and visual message boards.
CSE: Discuss the new egress strategies in high-rise buildings. How have they modified your design, and the architect and team’s design?
Ferreira: The trend toward full-building evacuation in lieu of a zoned evacuation/defend-in-place mentality factored into the recent IBC code changes requiring a third exit stair, minimum distances between stairs, and a 50% increase in stair width has the potential to significantly impact space usage and planning for high-rise buildings.
Mehta: Super-tall buildings are vertical cities. Occupant safety and evacuation are major factors. The trend is to direct people to refuge areas and use elevators to move people from these areas. This approach provides infrastructure reliability to support the elevators and the refuge area. Control of smoke migration through the building is another challenge engineers are facing in these structures. The revised code allows all elevators to be used for evacuation; this requires proper controls to ensure the doors do not open at the fire floor.
CSE: What changes in suppression systems have you seen recently in super high-rise buildings? What do you see changing in the near future?
Mehta: Fire suppression systems are still heavily dependent upon water. The super-tall structures have significant quantities of water stored at high levels to insure gravity supply when needed. Also, engineers need to provide reliable means to replenish the water during a fire. Engineers need to insure that areas which cannot be protected by the gravity water flow are equipped with fire pumps. Some clients have asked for gaseous protection for selected parts of the buildings instead of water suppression systems. These requests must be carefully evaluated on a case-by-case basis, and many factors need to be considered.
Ferreira: Advances in high-pressure/high-efficiency water pumps are allowing sprinkler systems in much taller buildings to be served using ground-level water supplies. In recent years there has also been an increase in interest in the use of high-pressure water mist sprinkler systems to protect high-rise buildings; these can result in a substantial reduction in the amount of water needed to suppress a fire and can greatly reduce the quantity of water that needs to be supplied to the uppermost portions of super high-rises.
CSE: What are some important factors to consider when designing a fire and life-safety system in a super high-rise facility? What things often get overlooked?
Ferreira: In today’s post-Sept. 11 environment, engineers may be asked by clients to consider design scenarios over and beyond the traditional fire scenarios, such as intentional acts that might potentially impact super-tall iconic buildings. As such, there is a greater emphasis being placed on the analysis of failure modes for fire and life-safety systems and providing redundancies should portions of these systems be damaged during an event. Features such as redundant sprinkler water supplies, and the additional stairwell (incorporated in the soon-to-be-released 2012 IBC) are aimed at addressing these issues.
Jalayerian: Survivability, physical protection of components, avoiding “eggs in one basket,” provide redundant source, configure redundant pathways for system distribution.
CSE: What sustainability issues concern your super high-rise projects?
Jalayerian: Aesthetic design is generally a primary requirement for a successful super high-rise building. Integrating sustainable design solutions and features into an aesthetically desirable development can be challenging and requires early integration of all project team members in the design process.
Mehta: Super-tall structures are iconic elements at any location. These buildings put high demands to incorporate environmental stewardship. Many ocal authorities have started to implement very specific requirements for building energy performance, installation of renewable energy sources, etc. Many clients are still looking at economic feasibility of various sustainable approaches such as cogeneration, solar power, wind power, geothermal energy, rainwater harvesting, gray water systems, daylighting, external shading, etc. Implementation is driven by economic performance and desired LEED rating.
CSE: With changing awareness of sustainability issues and an increased number of products, has working on green structures become easier, or more challenging?
Mehta: The awareness of sustainable design has increased and become an important factor in many regions. Some countries are more advanced than others, and availability of products and materials has certainly increased in many regions.
Jalayerian: Sustainability for the super high-rise (also true for low-rise) must be measured holistically. Application of green products alone cannot complete sustainability requirements of a super high-rise building. Super high-rises, due to large build area, impose a significant energy impact on existing infrastructure serving the development site. They also include a large surface (building envelope) to volume ratio which interacts with the outside environment (solar and temperatures).
Bearn: Sustainability is a key owner consideration as never before, which certainly makes it easier to incorporate green elements into a design. The challenge lies in evaluating the various green options available and selecting those that provide the biggest impact and value without breaking the budget.
CSE: Systems associated with super high-rise buildings, such as advanced ventilation, can challenge the energy efficiency of such structures. How do you maintain a balance?
Mehta: Indoor air quality in the space is extremely important. Buildings seeking LEED certification could achieve credit for the higher rate of ventilation. High ventilation will use more energy. To balance the overall energy of the structure, a CO2 monitoring system can be installed. The system will allow us to reduce the outside air quantity when the indoor air quality is at the acceptable level.
CSE: What unique requirements do HVAC systems in super high-rise buildings have that you wouldn’t encounter in other structures?
Jalayerian: Stack effect mitigation in super high-rise buildings is an important consideration in the configuration and design of HVAC systems. Stack effect forces are significant in a super high-rise building, especially for buildings that are located in colder climatic conditions and can impose significant energy consumption to overcome uncontrolled air movement into and out of the building. The architectural space planning, building riser shafts, and HVAC system design, must take into account this unique phenomenon associated with super high-rise buildings.
CSE: How can automated features and remote HVAC system control benefit super high-rise clients?
Jalayerian: Most super high-rise buildings include mixed-use occupancies, with a majority of high-rise zones of the building dedicated to residential occupancies. These residential occupancies are not generally occupied year round, and therefore automated control technologies that operate remotely can significantly reduce the operational cost of the building and must be considered an essential feature of the design.
CSE: What types of water pumps, valves, etc. do you have to specify to ensure water is adequately moved around the structure? What issues are involved?
Mehta: Due to the building height, all water systems are split into pressure zones. Pressure in these zones is determined by occupancy type and equipment to be used in this zone. The actual pump, valve, etc. selection is made based on the increased need for reliability, required pressure ratings, and availability of materials in the region.