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High-rise ofice building - Some design considerations.

  INTRODUCTION
Office facade in design process

    
High-rise office buildings, which are developed as a response to population growth, rapid urbanization and economic cycles, are indispensable for a metropolitan city development. 
    A study in the economic height of modern office buildings: “Given the high land values in central business sections of our leading cities, the skyscraper is not only the most efficient, but the only economic utilization of certain strategic plots. An exhaustive investigation… has conclusively demonstrated that the factors making for diminishing returns
in the intensive development of such plots are more than offset by the
factors making for increasing returns…” (Klaber, 1930).
   

  This statement holds true for today; however, the relationship between cost and benefit is more complex in today’s global marketplace. The current trend for constructing office buildings is to build higher and higher, and developers tend to compete with one another on heights. Tenants also appreciate a landmark address and politicians are conscious of the symbolic role of high-rise buildings. 
    Nonetheless high-rise office buildings are more expensive to construct per square meter, they produce less usable space and their operation costs are more expensive than conventional office buildings. The space efficiency, as well as the shape and geometry of the high-rise building need to satisfy the value and cost of the development equation. Space efficiency, which is determined by the size of the floor slab, dimension of the structural elements and rationalized core, goes along with the financial benefit.
    By the end of 1990s, at more than 30 stories, net to gross floor area ratios
of 70-75% were common in office buildings. However, Yeang (1995) stated in his book “The Skyscraper: Bioclimatically Considered” that net-to-gross floor area should not be less than 75%, while 80% to 85% is considered appropriate. Wherever the tall building is being constructed, achieving suitable space efficiency is not easy, since it is adversely affected by height as core and structural elements expand to satisfy the requirements of vertical circulation and resistance to lateral loads. Space efficiency can be increased by the lease span, which is
defined as the distance between the core and exterior wall.
   Factors affecting the design of high-rise buildings vary from country to country, such as local climate, zoning regulations, cultural conditions, technological opportunities, and etc. For instance, in Germany, where building codes dictate shallow floor slabs of 8.0 m, efficiencies of 60-70% are common, whereas London’s Canary Wharf Tower, can achieve a netto- gross ratio in excess of 80% with floor slabs of 2500 m2, and 11.0 m lease span.


    DESIGN CONSIDERATIONS FOR HIGH-RISE OFFICE BUILDINGS

    All of the sample buildings are landmarks of their cities, and also are designed by internationally expertise design consultants, reflecting highquality practices in respect of efficient planning. The relevant building data are provided from the clients, architects, engineers, quantity surveyors, as well as journals, books, magazines and Internet sources. The research is based on the architectural and structural design criteria affecting the space efficiency, such as floor slab size and layout, core integrity, gross and net floor areas, leasing depth, floor-to-floor and floor-to-ceiling height, and structural system. 

    The sample buildings from the world are located in seven major cities, which are Taipei, Kuala Lumpur, Shanghai, Chicago, Hong Kong, Guangzhou and Shenzhen. The height ranges of these buildings are between 367 m and 509 m, and the numbers of stories change from 69
to 114. The Empire State Building in New York, which is currently the ninth tallest office building of the world, is omitted, since it is constructed 78 years ago. The paper tends to take contemporary examples into consideration due to the rapid changes in tall building design and
construction technologies.
   Architectural and structural requirements are the basic decision making
parameters in the design of high-rise office buildings, and dictate the floor
slab size and shape, leasing depth, structural frame, floor-to-floor height,
vertical transportation and core layout. The related findings of the selected
buildings from the world and Turkey are presented and discussed below.
 

   Floor Slab Size and Shape
 
    An office building’s floor slab size and shape, on which decisions are made according to the functional requirements, client’s specific needs and various constraints, have great impact on the space efficiency and the building’s external character. Although there are no universal formulas for responding to the client’s needs or to local influences and constraints such as climate, codes or constructional conditions, the fundamental design considerations are identical almost in office buildings. The first aim is to achieve the maximum space efficiency and in order to accomplish this task, initially the floor slab shape and total floor area of the building need to be designed.
    The space efficiency of a high-rise office building can be achieved by maximizing the Gross Floor Area (GFA) and Net (usable) Floor Area (NFA) as permitted on the local site by the codes and regulations, and in order to enable the developer and owner to get maximum returns from the high cost of land, the floors must have sufficient functional space. In the initial stages of the design, the designer ascertains the extent of GFA and NFA in the proposed concept design, and these figures are used as the bases for core configuration and structural system. By the final decision, the NFA is sealed with the exact core area and the vertical structural elements. Net-to-gross floor area of a typical floor slab is of crucial economic interest to the developer, since it designates the space efficiency of the floors, at the same time as the more efficient the typical floor slab is, the more usable area the developer gets and the more income is derived from the building.
    According to Yeang (1995; 2000), floor slab efficiency of a typical high-rise office building should generally not be less than 75%, unless the site is too small or too irregular to permit a higher level of space efficiency. Floor slab designs using clever devices, such as scissor stairs, pressurized lift shafts, dispersal of toilets etc. can increase efficiency up to 80% - 85 % per typical floor. However, as Watts et al. (2007) state in their recent article, floor slab efficiency is adversely affected by the height of a high-rise office building, as the core and structural elements expand relatively to the overall floor slab to satisfy the requirements of vertical circulation as well as lateral-load resistance. Tall buildings with high slenderness ratio are inherently more expensive to build and suffer from adverse floor slab efficiency.
    Although space efficiency is simply defined as the ratio of NFA to GFA, the matter is more complicated in terms of its effects. The floor slab shape also has a vital importance as well, since it influences the interior space planning, layout of office equipments, exterior building envelope, structural system and component sizes, utilizing from natural light and air, access to escape routes, etc. Generally the more simple and regular the floor slab shape is, the easier it is to respond to user requirements in terms of space planning and furnishing. Square, circular, hexagonal, octagonal and similar plan forms are more space efficient than the rectangular plans with high aspect ratios and irregular shapes. Buildings with symmetrical plan shapes are also less susceptible to wind and seismic loads.
    The mean average value of space efficiency of the ten tallest buildings of the world is 68.5 %, whereas the mean average in Turkey is 69.5 %. Although there is a significant distinction between the number of floors and heights of the examples at abroad and in Turkey, it is observed from the analysis that the space efficiencies are very similar in terms of net-togross
floor areas.
    As shown in Figure 1a and Figure 1b, square or similar plan geometries are the most preferred shapes in examples at abroad. Seven of the ten tallest buildings at abroad have plan geometries derived from square. Since this geometry offers the same stiffness in each direction against lateral loads, square or similar configurations are the most common in the
selected examples. Petronas Towers are deemed acceptable, since they have symmetrical and regular plan forms, enabling similar planning and structural efficiency in each direction. The Central Plaza, with its triangular plan form can be regarded as regular, since it enables equal leasing depth in each perimeter, however, it is not susceptible to lateral loads in each direction, and the only column in the usable area prevents the flexibility of space. Shun Hing Square with its hybrid plan shape is the only example of irregular configurations, thus disabling equal leasing depths in each perimeter of the tower; and the workplace is separated into four regions. The selected office buildings from Turkey have different characteristics of plan geometry when compared with the ten tallest office buildings of the world. Only one case, İşbank Tower has a plan shape derived from square, however, the core geometry do not match with the plan layout, thus disabling equal space efficiencies in each perimeter. Mertim and Süzer Plaza have rectangular plan forms with matching core geometries, and though they are not symmetrical in each direction, the plan configuration enables equal and efficient work spaces in each perimeter. Sabancı Towers, Metrocity 1, Beybi Giz Plaza and Garanti Bank Headquarters are the examples of hybrid and unsymmetrical plans, whereas the Tat Towers and Tekstilkent Plaza 1 and 2 are composed of hexagonal form and similar core
configuration.

Figure 1a.Geometry of typical floor plans of
ten tallest office buildings of the world.
   There is a conspicuous intend that the contemporary office buildings must be designed with minimum or no interior columns to enable maximum flexibility, consequently a column-free floor slab from the exterior to the
core is the optimum solution for the office development. However, as shown in Figure 1a, the analyzed buildings at abroad, except for the Sears Tower and the Central Plaza, are column free in the leasing depth. Three of the sample buildings from Turkey, Tekstilkent Plazas, Beybi Giz Plaza and Garanti Bank Headquarters, have interior columns, as Sabancı Center 1 and 2, Süzer Plaza and Metrocity 1 have peripheral columns recessed
from the exterior wall. The least sufficient workplace can be observed in the typical floor plans of Garanti Bank Headquarters with multiple columns dispersed throughout the floor slab (Figure 1b). Although this building has a workplace organized into one space, the interior columns prevent the flexibility and efficiency of this usable space, presenting the disadvantage of a non-column-free floor slab as stated above.


    Leasing Depth


    Leasing depth or lease span is the distance of the usable area between the exterior wall and the fixed interior element, such as the core or the multi-tenant corridor. Although it depends on the functional requirements and is closely related with the structural frame and the material, there are considerable varieties in different markets. For example, in Germany maximum leasing depth is determined by building codes and cannot be more than 8.0 m, whereas in Japan it is typically 18.0 m (Kohn and Katz, 2002). In the United States, floor slab areas began to expand after the World


Figure 1b. Geometry of typical floor plans of
ten tallest office buildings in Turkey.
 War II with the help of technological innovations, such as air-conditioning
and artificial lighting. Today there are high-rise office buildings with 17.0 m lease span in United States and Asia. Smaller core-to-exterior window dimensions allow the users to maintain a relationship with the outside, thus benefiting from the natural light. According to Ali and Armstrong (1995) the depth of lease span must be between 10.0 and 14.0 m for office functions, except where very large single tenant groups are to be accommodated. Maximum leasing depth
has remained relatively static over the last 30 years as it is recognized that the maximum income for office development is achieved when a high percentage of the workers are located within an 8.0 m zone of the perimeter wall. Corner offices and the articulation of the façade significantly improve the ability to provide more space efficiency and quality than spaces with greater leasing depth. As floors become deeper, the marketability of the space significantly decreases (Crone, 1990).

    Floor-to-floor / Floor-to-ceiling Height


    The floor-to-floor height of an office building is typically the same for all occupied floors except for the lobby and floors for special functions. In high-rise office buildings, additional floor-to-floor height significantly entails greater cost on structural elements, cladding, mechanical risers, and vertical transportation. The floor-to-floor height of a building is a function of the required ceiling height, the depth of the raised floor (if used), the depth of the structural
floor system and material (which is dependent on the exterior-to-core distance), and the depth of the space required for mechanical distribution. Baum (1994), in his book “Quality and Property Performance”, defines quality in office buildings and suggests that the plan layout and the ceiling height are more significant than the following three determinants of building
quality: (i) Services and finishes; (ii) external appearance and (iii) durability
of materials.
   Another research project by Ho (1999) reveals that functionality of the floor slab is the most important category indicated by all the respondents of the investigation, except for users, who emphasized services as the relative importance of functionality. Designers in the same investigation rated functionality as their most important determinant of quality, because they
usually start the design process by working around constraints such as plan shape, usable floor area, and floor-to-floor heights. Commercial functions require a variety of floor-to-ceiling heights ranging between 2.7 and 3.7 m (Ali and Armstrong, 1995), and the depth of the structural floor system varies depending on the floor loads, size of structural bay, and type of floor framing system. In the case of steel floor framing, an allowance for fire-proofing must be made. However, in steel systems, increasing the structural depth will result in decreased weights
of rolled sections. Trusses, which permit the passage of ducts, provide structural depth without increase in floor-to-floor height. According to the analyzed buildings of the world, the floor-to-floor heights change between 3.73 m and 4.20 m with an average of 3.98 m (Table 4).
The floor-to-ceiling heights have a range changing between 2.65 m and 2.8 m with an average of 2.7 m. Raised floor is provided in Taipei 101 Tower, Sears Tower, Petronas Tower 1 and 2, and Two International Finance Centre. The tallest office buildings in Turkey have floor-to-floor heights changing between 3.40 m and 4.8 m with an average of 3.6 m, which is under the average of the examples at abroad. The floor-to-ceiling heights also have a range between 2.60 m and 2.80 m, with an average of 2.7, thus being close to the average value of the ten tallest office buildings of the world.


   Core Integrity

 
    The core of the building comprises all of the vertical circulation elements, such as elevators, fire-stairs, mechanical shafts, toilets, and elevator lobbies. In early office buildings, these elements tended to be dispersed on the floor rather than concentrated, while today’s contemporary buildings include all these elements in a specific zone, which is mainly the core. Many of the key structural elements, such as the shear walls that provide lateral stability,
are integrated into the core in order to simplify the architectural design.Layout of the core is critical to the development efficiency and operational effectiveness of a high-rise office building, while also playing a significant role in the way the structure copes with lateral loads (Watts et al, 2007). Building cores can be arranged in several ways. Central cores integrating
with the outer structure resist lateral loads more effectively and open up the perimeter for light and view, enabling efficient workplaces. Buildings with side cores have the advantage of homogeneous workplaces, which is usually organized into one space. This building type is very attractive to users without cellular offices and has until recently been the standard in Japan and Korea (Kohn and Katz, 2002). Multiple cores are common in lowrise buildings, which have very large or narrow floor slabs. The design of the core significantly affects the overall space efficiency of the buildings, vertical circulation, and distribution of mechanical and electrical shafts. The lifting strategy drives the core size and has a major impact in terms of design on all high-rise office buildings. One of the drivers is the acceptable period of time for users to get from ground floor to their destination. The ideal solution balances a number of factors such
as the number and the speed of lifts, group sizes, building zones and the core arrangement, considering the space usage as well as cost (Watts, et al, 2007). In order to achieve the maximum space efficiency of a high-rise office building, the core must be reduced to an acceptable ratio of the gross floor area, while coping with the fire regulations and achieving an effective vertical transportation with the elevators. In many high-rise office buildings structural elements within and around the core interact with the perimeter frame. These structural elements can be constructed with either steel or reinforced concrete, or both. In the
case of a reinforced concrete core, its structural weight can be very heavy, thus inducing an additional cost for the foundation. In United States, steel is commonly used as the structural material and lightweight fire-rated drywall is used to form the walls in order to reduce its thickness and save the foundation cost and construction time (Ho, 2007). However, in
Asian countries, the use of the structural steel with drywall forming is less common because their costs are higher than the conventional reinforced concrete construction. High-strength concrete is generally used to reduce the thickness of reinforced concrete core wall enabling more efficient spaces.   


DISCUSSION AND CONCLUSIONS
Table 4. Leasing depth, floor-to-floor and
floor-to-ceiling heights of sample buildings
from the world and Turkey (Sev, 2000).
 
    As properly-planned high-rise office buildings are discussed, space efficiency, which is one of the efficiencies like structural, constructional, energy and operational, emerges as a major concern to be focused on.
However, when the aim is to increase the rental income, space efficiency
becomes significant in comparison with other efficiencies. In this context,
this research presents important parameters for the design of high-rise
office buildings and their relationship with space efficiency. Efforts have
been made to present visual analysis, which explains the significance of
space efficiency and the relationships of parameters that impact this issue.

   The following are the major conclusions of the research:


    • Structural system and core configuration are the most important factors affecting the space efficiency of high-rise office buildings, as they are closely related with the shape of the floor slab, leasing depth, floor height and vertical transportation. Cores in high-rise office buildings are much more complex than in conventional buildings, and their design is fundamental to the development and the operational effectiveness of a tower. Key elements of the core are the structural elements and elevators while the lifting design is the major determinant of the core size and the space efficiency, and it determines the occupant travel and maximum waiting times. By the input of a specialist, dividing a building into a number of zones, each served by an appropriate sized group of lifts to decrease the core size, will increase the space efficiency. The use of sophisticated controls for elevators is also an effective way of minimizing
the number of elevators and waiting periods.As analyzed in the selected examples, space efficiency of the towers abroad are acceptable; however, most of the Turkish examples are less space efficient. The average space efficiency of two sample groups are similar,
even though the number of storeys and floor slabs of Turkish examples are almost the half the examples abroad, which originates from larger core areas and larger dimensions of vertical structural elements.
• Depending on requirements of the clients or the tenants, areas of the core elements can vary significantly, affecting the space efficiency. However, even though floor slab areas and heights of Turkish examples are almost half the examples abroad, the average ratio of core-to-gross floor area for Turkish examples is higher, thus decreasing the area of workplace and space efficiency. The vertical transportation elements, such as elevators and fire stairs require more analysis for more economic and efficient solutions of the floor plans in conjunction with the construction of high-rise office buildings in Turkey.
    • Central core approach is commonly used in the world and in Turkey for high-rise office buildings. The cores are interconnected with the main structural frame, thus resisting a substantial amount of the lateral loads in all examples, without exception. This interconnection between the core and the structural frame is provided by the structural floor system and
steel outrigger trusses in sample buildings at abroad, whereas examples from Turkey do not utilize any steel outrigger trusses. Utilization of steel outrigger trusses must be supported by designers and contractors of highrise office developments to improve the efficiency of structural system, thus affecting the size of the structural members.
    • The two common structural systems for the tallest office buildings of the world are composite mega-columns and central core with outriggers, and reinforced concrete tube-in-tube without outriggers system. Either steel or concrete structures are used; however, high-strength concrete is more common due to its lower cost, compared with steel. In Turkey, the most common structural system for the ten tallest office buildings is reinforced concrete perimeter frame with central core and tube-in-tube. High-strength concrete is not widely used in Turkey due to its higher cost and production conditions, consequently increasing the size of the vertical structural
members. Use of high-strength concrete for columns and shear walls must be supported by designers, practitioners, contractors and also must be subsidized by the government in Turkey.
    • Space efficiency could be higher, if buildings in Turkey utilize state-ofthe- art structural systems and materials, as well as elements of the vertical transportation, to have smaller vertical structural elements and smaller core areas. High-rise office buildings pose different questions for those that design, build, own and operate them. For each of these stakeholders, there is an inherent motivation for profit, generally led by responsibility for shareholders.    

    Developing high-rise office buildings to obtain this profit demands acceptance of higher risks from the outset. To minimize these risks, increasing space efficiency is of vital importance. Space efficiency is only a number of resulting from an inter-related decision making
parameters during the early planning and development of the high-rise office buildings. Efficiency of net-to-gross floor area is the key to balance construction costs and total rental values. When material choice and issues of efficiency of structure and services are integrated to assess the various options, more space-efficient solutions can be reached.



*** source: inspired by Ayşİn SEV and Aydan ÖZGEN***

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