Electrical Design For Tall Buildings

Electrical Design for Tall Buildings: Designing electrical systems for high-rise buildings is an exercise in engineering coordination and cooperation. By Mark Bendix, P. ENG., Senior Director, Operations, Giffels Assocs. Ltd., Toronto -- Consulting-Specifying Engineer, 8/1/2007 12:00:00 AM MT There has been a sharp increase in the development of extra tall buildings, usually multi-purpose and often consisting of a retail and/or entertainment podium and towers of commercial offices, hotels and residential facilities. A good example is the iconic Emirates Towers complex in Dubai, comprised of a below grade parking area, a retail podium and one tower of commercial office space. The other tower houses The Jumeirah Emirates Towers Hotel. The complex electrical systems installed in these tall buildings present to the engineer a number of design challenges, including space constraints, limitations of physical structure and the integration of multiple systems. To successfully overcome these challenges, careful planning, collaboration with other professionals and coordination of systems are essential. First of all, every tall building is supplied with multiple sources of electricity, including feeds for normal power, usually supplied by the local electrical utility company (LEUC), and an emergency or standby source of power, usually supplied from on-site engine-generator sets. The LEUC supplies the building with medium-voltage power from one or more utility substations. Ideally, the supplies are fed from multiple sub-stations to increase the reliability of the main electrical system. The utility supply will enter the building from below grade and usually terminates in a main switch room. Often, the location of this electrical room depends on the demands of the LEUC. A significant concern is that each LEUC has its own idiosyncrasies. In some jurisdictions, the LEUC has no requirements for the main switch room at all, and it can be located anywhere within a basement area. In other jurisdictions, the LEUC may require that the main isolation equipment (switchgear or ring-main units) be located as close as possible to the outside wall where the service enters the building. Other LEUCs, such as the Dubai Electricity and Water Authority (DEWA), in the case of Dubai, may demand that this isolation equipment be located in a room at street level, directly accessible from the outside or in a completely separate building at the site property line. Therefore, it is essential that the electrical engineer contacts the LEUC as early as possible to determine if there are specific requirements for the service entrance equipment and its location. At this early stage, the engineer also should determine the codes and standards that the LEUC requires for electrical system design. The engineer will likely find that the LEUC has a set of additional design requirements specific to local conditions and local practices. Often these are not obvious, and if the engineer does not uncover these early, it can be costly to the engineer and to the owner. At a late stage in the design of the Burj Lofts and Burj View buildings in Dubai, DEWA insisted on the addition of a low-voltage main isolation room at the ground level. This change required close cooperation between the engineer and architect to accommodate the room with the least impact on the design and loss of leasable space.

A similar situation occurred during the construction of the Emaar Residential Towers in Dubai. In this case, DEWA changed its high voltage regulations during construction, so that a ground floor ring main unit (RMU) room had to be added, with a significant impact on the construction process and, once again, loss of leasable space. The medium-voltage supply must be transformed down to the utilization voltage (480 volts in the United States, 600 volts in Canada, and 400 volts or 380 volts in much of Europe and Asia). In standard buildings, the transformers are located at or below ground level. In extra tall buildings, transformers at low levels are insufficient. At some height, the voltage drop caused by the impedance of the supply conductors will become significant and the supply voltage will fall below acceptable values. The architect's design must therefore accommodate service levels in the upper parts of the building, in which additional transformers are located. Selecting the location of the service level requires cooperation between the engineer and the architect. The engineer will require the service levels to be located where they can adequately service selected floors. The architect will consider issues such as the impact on the esthetics of the façade, the space requirements and space constraints, the impact of the service space on adjacent spaces and the transportation of equipment to and from the service room. In the Emirates Towers, the transformer rooms at the upper level are located close to the elevator shafts, so that transformers can be transported through the elevator shaft in the event that one must be replaced. Medium-voltage cables must be fed to the transformers on the upper levels. Frequently, the owner or LEUC will demand that the medium-voltage cables be kept completely separate from any low voltage equipment and routed up the building in separate accessible spaces. The access is necessary so that the cables can be secured and supported at regular intervals so as to relieve stress on the cables, and to limit their movement under short circuit conditions. But what about emergency power? To ensure safety in the event of a normal power outage, emergency power generation is required. The emergency generators also may be used to provide a reduced level of service to non-critical items. When selecting the generation system, the electrical engineer must decide between low-voltage or medium-voltage generators. Selecting medium-voltage generators allows for the use of fewer, larger generating units, all of which can be located at a low level. However, a medium-voltage emergency system will require a sophisticated transfer scheme—more costly than low-voltage equipment. In addition, some LEUCs will not permit the use of such an arrangement. Conversely, selecting low-voltage generators will require more generator units. And due to voltage drop, they will have to be distributed throughout the building on service floors. Moreover, electrical and mechanical engineers must coordinate their work to ensure that sufficient combustion air and ventilation is provided to the generator rooms, and to ensure that the exhaust, fuel and cooling systems are correctly designed.

Service rooms, spaces and risers In any building, service rooms and spaces present a design coordination challenge. The architect will strive to maximize the use of space for which the building is being provided and will attempt to minimize the space loss caused by service spaces. The electrical and mechanical engineers must work closely with the architect to ensure that an adequate number of service rooms and spaces are provided to support the building requirements. They also must ensure that these spaces are large enough and practically located, that is, close to the point of utilization. The location must allow for easy movement of equipment in and out of the room and to the outside.

Finally, the spaces must be configured to accept the equipment they are to house and provide sufficient space for equipment maintenance. The architectural design will include service cores such as elevator shafts, electrical and telecommunications rooms, mechanical rooms and risers, garbage and linen chutes and other such utility spaces. The cores may extend the complete height of the building or they may rise to a specific level and then transfer and continue in a different location. Where such an offset occurs, the engineers must find a horizontal space in which the services can be transferred to the new location. In many cases, the electrical engineer will find that the electrical spaces provided in the initial design are irregular, undersized and impractical, and may request larger and more practically located spaces that will provide maintenance and service staff with a convenient and comfortable working environment. The architect will accommodate these requirements as long as the loss of usable space is minimized and the overall building costs are not increased. For example, when Giffels Assocs. was designing Atlantis, The Palm, the engineer and architect made several changes to service rooms as the design matured. The size and shape of some rooms in the original architectural concept proved to be too small, oddly shaped or not practically located. To resolve the problem, the engineer provided the architect with scaled layout drawings showing the equipment to be accommodated, as well as minimum space requirements and technical limitations. As design progressed, the solution was further refined based on the specific needs of the facility. The space requirements for electrical and telecommunication riser rooms and spaces in tall buildings are significant. These rooms will house equipment for many different systems, including power distribution panels, feeder and plug-in busways, lighting control panels, emergency lighting supply panels, fire alarm transponder panels and their associated battery cabinets, security system equipment, voice and data distribution racks and cabinets, building management system panels and cable and conduit risers. To minimize the space demands, it may be possible to spread the equipment among several floors and to serve multiple floors with one piece of equipment, but this solution is not practical for all types of equipment. Services will be supplied radially from each service room to the point of utilization. The routing of cables and conduits that exit these service rooms presents another challenge. The electrical and telecommunications rooms in the service cores may be located adjacent to an elevator shaft and riser shafts for air distribution or linen or garbage chutes. Service raceways may not penetrate such risers, so the electrical engineer may be faced with a limited exit window. Usually, these raceways will exit into a public area and must be routed above a ceiling. These constraints may limit the size of the raceway window to the extent that it cannot accommodate all required raceways, resulting in the need for additional service risers on each floor. Cabling limitations also may result in additional service risers. The length of power distribution conductors will be limited by overall voltage drop. Horizontal telecommunication cables (category 5 and category 6) are limited to a maximum length of 295 ft. to comply with acceptable standards. It is clear that the architect and electrical engineer must work together closely so that the electrical equipment can be accommodated and safely serviced without excessive loss of usable space. This requires a significant amount of cooperation and compromise from both parties.

Structural constraints

It is common to embed electrical conduits into a concrete structure, which creates a special challenge for the structural engineer, especially in high-rise construction. The electrical engineer must consult with the structural engineer to determine the maximum size and concentration of conduits that may be embedded in the slab. The type of slab construction and the thickness of slab will determine the extent to which the conduits can be accommodated. The complex structure of tall buildings usually includes levels with deep transfer beams. The electrical engineer must be familiar with the structural elements, as it is likely that they will cause interferences in routing of services. The structural engineer will usually accommodate limited, minor penetrations through structural elements; however, when these become numerous and large, careful coordination and planning will be required. The structural and electrical engineers must cooperate closely with respect to major vertical and horizontal structural penetrations. It is important that large penetrations and openings are identified early so that they can be designed into the structure. When these elements are identified late in the design process, they usually result in unsatisfactory results and project delays. The height of a structure is determined by the number of floors that the owner or developer decides are necessary to make the building financially successful, together with the floor-to-floor height of each floor. The greater the floor-to-floor height, the greater the cost of construction. The architect's challenge is to limit the floor-to-floor height while maintaining a pleasant environment and esthetically attractive appearance. At the same time, the architect must work with engineers to ensure that adequate space is provided for routing of services and that the integrity of the structure is maintained. Usually distributed through common public areas above ceilings, the electrical and mechanical services must be carefully coordinated between the electrical and mechanical designers, because the services compete for the same limited space. Factors such as branching services and crossover of these services must be carefully considered. The electrical engineer must know whether the mechanical engineer is using the ceiling space as an air plenum, which limits the types of cables that can be installed in this space, based on their flammability and toxicity rating. High-rise building design requires a mix of talented professionals who are able to work as a homogenous team that is cooperative, communicative and willing to compromise to ensure the best solution is delivered to the owner. Rather than working in isolation, the electrical engineer must play an integral role in the design team, coordinating decisions with all the other disciplines.

Lighting tall buildings In high-end tall buildings, the design of the lighting treatment is usually undertaken by a lighting design specialist who will develop a design that enhances the architectural features of the building. The electrical engineer must work closely with this designer when developing the power and control systems for the lighting. Frequently, the electrical engineer is responsible for coordinating with the other design professionals with respect to the lighting design. For example, the structural engineer may be required to ensure that the structure can adequately support large chandeliers and the mechanical engineer must ensure that the cooling system accounts for the heat generated by the lighting.

Aircraft warning lights Aircraft warning lights are an essential requirement on any tall building. In the United States, these systems must comply with Federal Aviation Administration requirements. In much of the rest of the world, the ICAO standards apply. The red (sometimes white) warning beacons are located at the top of the building and at intermediate levels as dictated by the standards. By their nature, these beacons are conspicuous, which often conflicts with the architect's vision for the building. The architect and engineer must work together to locate these lights so that they fulfill their intended purpose and limit the impact on the esthetics of the building.

Lightning protection Inevitably, extra-tall buildings are occasionally struck by lightning. To protect life and property, it is important to provide a well-designed lightning protection system that complies with the local standards. Once again, the electrical engineer must cooperate closely with the architect and structural engineer. The lightning protection system will form a Faraday cage around the building. The architect and engineer must work together to ensure that the copper grid placed at roof level is adequately secured. The electrical engineer must work with the structural engineer to locate the down conductors that direct the lightning energy to the earth. Copper conductors within the structure can be used; however this added cost is unnecessary since the rebar that exists within the building columns already is also a good conductor and is commonly used for this purpose.

Extra-low-voltage systems The electrical engineer is responsible for the design of extra-low-voltage (ELV) systems. The ELV systems may include the fire detection and alarm system, voice evacuation system, voice and data communication systems, public address systems, access controls systems, intrusion detection systems, CCTV systems, audio-visual systems, cell phone and wireless distribution systems and other such auxiliary systems. Increasingly, because ELV systems are converging and systems communication takes place at a high level, the electrical engineer must ensure that the various ELV systems are coordinated and are able communicate flawlessly with each other, as required. The electrical engineer must have a clear understanding of how each system operates and how they are to interact and support each other. Today's ELV systems are continuously evolving and being upgraded rapidly, so that the system that the electrical engineer selects during the design of the building may be outdated before installation. The electrical engineer's design must be flexible enough to limit changes to the physical infrastructure when the ELV systems are updated.