Highrise Elevator Cores
Fire resistance of the building core elements can be determined from the applicable building regulations. All penetrations for services through the walls of the service core need to be designed to maintain fire integrity for the prescribed period of time.
The core configuration is normally finalized at an early stage of design development because of its implications for the functional layout of the building. Traditionally, the configuration is greatly influenced by the architect. The design optimization process is subsequently carried out within the allocated zones during the preliminary design phase by the design team's experts in the individual disciplines.
The cost of a core for a typical highrise office building is estimated to be around 38 percent of the total structural cost, or 4 to 5 percent of the total development cost. Clearly, if the optimization of the service core by the structural engineer is limited only to structural optimization, the potential savings in terms of the overall cost is relatively small.
In contrast, an optimization of the building's structure which affects the costs of other systems and which takes into account the speed of construction may result in more significant financial benefits.
Elevator Shaft Configuration
In determining the internal configuration of the service core, one of the first elements to identify is the extent to which vertical transport will be provided within the building. A highrise building requires a set of elevators and therefore a specialist elevator consultant in the design team.
In conjunction with the structural and building services engineers, the architect will look at the elevator grouping and arrangement — including people lifts, goods lifts, and fire lifts — that meet design criteria such as average waiting times, handling capacities, and so on.
These criteria differ depending on the building type — hotel, apartment block, or offices. A large bank of elevators is the main element in a service core design and all other elements are designed around it. Vertical transport solutions are complex, requiring computer simulations of people movements, predictions about users within buildings, and historical data from existing buildings.
The vertical transportation of people within a highrise building will also depend on local fire regulations. The fire department may require fire compartmentation between the elevator lobby and elevator shafts. A separate fire-fighting elevator — capable of moving firefighters around a burning building when all other lifts have returned to their neutral position — is often required.
The relationship between these elements may affect the leasable area of the building, hence the need for the design team to find the optimum solution while not restricting themselves to traditional and conventional elevator shaft configurations.
There are, of course, conventional rectilinear layouts, but they should not preclude other more experimental options. It is also important to remember that fire-protection considerations must be taken into account in the organization of elevators and escape stairs. Requirements vary depending on local building regulations.
Elevator Shafts within the Service Core
Once the location of the service core on the floor plate has been determined, the exact size of the core (internal shaft dimensions, wall thickness, etc.) needs to be established to calculate the area efficiencies.
It is next necessary to define design criteria for the services shaft and the elevator system. Early liaison with the fire officer is important in establishing life-safety requirements. Elevator shaft dimensions can easily be obtained for all the components of the elevator system. Common fire compartmentation of all vertical shafts can minimize wall thickness provided the structural designer is satisfied with the core stability in the overall design of the building.
Elevator shafts are sized according to car shapes and sizes and door sizes, with due consideration given to space requirements for guide rails and brackets, counterweight systems, running clearances, and ancillary equipment. Sufficient air space should always be provided around cars and elevator counterweights to minimize buffeting and airborne noise during operation.
In organizing the configuration of elevator banks in the service core, it is necessary to ensure that a bank of two, three, or four elevators in line shares a common fire-protected shaft without a dividing structure, so avoiding a single enclosed elevator shaft. If single enclosed elevator wells are necessary for structural reasons, the designer must ensure that air relief slots (ideally, full-height vertical slots) to allow adequate air relief.
Elevator Lobby Configurations
"Outward facing" elevators (elevator-bank openings that face directly into the net usable area) are the most efficient. This is because the lobby is accountable as part of the net usable area on most typical floors. However, in certain countries, local building codes permit this layout only if the service core has fire-rated elevator doors and pressurized elevator shafts. Such an arrangement also allows for good access, with wide elevator lobbies at ground-floor level to handle traffic peaks more efficiently.
The lobby of "inward facing" elevators (or two banks of elevators facing each other) can be included as part of the core but the arrangement is less efficient in terms of net usable area versus gross floor area. For inward facing elevators, the designer must ensure that both ends of the lobby are kept open.
As a general guide, the width of the elevator lobby should be twice the depth of the elevator cars it is servicing. For a single line of elevators, a minimum lobby width of 8.2 feet (2.5 meters) is generally recommended. When designing the service core in relation to the floor plate, the designer must ensure that the lobby will not be used as a common or public thoroughfare at ground-floor level.
In multiuse buildings, care should be taken to provide separately identified lobbies for each group of elevators, particularly on the ground floor where clear signage is essential. This is also applicable to highrise buildings where the lower floors are served from a separate bank of elevators.
Ken Yeang is an architect and principal (with his partner Tengku Dato Robert Hamzah) of the firm T. R. Hamzah & Yeang Sdn. Bhd. He is a professor at the University of Sheffield and holds a doctoral degree in architecture from Cambridge University, where he specialized in ecological issues applied to planning and building design.
This article is excerpted from Service Cores: Detail in Building, copyright © 2000, available from John Wiley & Sons and from Amazon.com.
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