INTRODUCTION INTRODUCTION TO HIGH-RISE BUILDING DESIGN Priyan Mendis and Anil Hira
1. INTRODUCTION
Tall building developments have been rapidly increasing worldwide. Traditionally the function of tall buildings has been as commercial office buildings. buildings. Other usages, such as residential, mixed-use, mixed-use, and hotel tower developments have since rapidly increased. Therefore the role of the structural engineer in design of the high-rise buildings has changed significantly primarily due to the increasingly competitive nature of the building industry. The increased competition has obvious financial benefits for the building owners, and at the same time creates an immense challenge to the design team to achieve optimum designs in terms of aesthetics, creativity, build ability and most important of all, economy. This lecture provides an introduction to tall building design, with some emphasis on its uniqueness compared with other structures, in the structural engineering context. 2. DEFINITION OF A TALL BUILDING
In terms of structural considerations, a building can be defined as tall, when its strength and behaviour, in terms of serviceability (deflections), is governed by lateral loads. The lateral loads are caused either by wind and/or earthquake. Although there is no specific value that defines a tall building, a commonly acceptable dividing line is where the structural design moves from the field of statics into the field of dynamics. The challenge for the structural engineer will increase for the new generation buildings, which will be taller, lighter and more slender. Studies have revealed that slimmer buildings have greater commercial value, confirming this recent trend in building design. With advance technologies and innovative materials, the building heights have dramatically increased in tall buildings. Therefore a new term ‘super tall buildings’, has been used for buildings which are over 300m. It is not just at the super tall end that tall buildings have been proliferating. As the graph on all tall buildings greater than 200m in existence globally shows, it has more than doubled in the last ten years. Most interesting global trend currently occurring is not the number of buildings taller than 300m that are completing, but the unprecedented number of 600m and taller buildings in planning. Therefore as a result, the term “super tall” is no longer adequate to describe these buildings. Therefore, a new term called ‘mega tall” is now officially used by CTBUH, to describe buildings taller than 600m or double the height of super tall (Ref. 8). World current tallest buildings and tall buildings completed each year over 200m , 300m and 600m since 1960 are shown in Figures 1 and 2 below. 1|Page
Figure 1: Current tall buildings in the world
Figure 2: Tall buildings completed each year over 200m, 300m and 600m since 1960 (Ref. 8)
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3. BACKGROUND TO TALL BUILDINGS
In the late nineteenth century, early tall building developments were based on economic equations – increasing rentable area by stacking office spaces vertically and maximizing the rents of these offices by introducing as much natural light as possible. Due to this global drive towards taller buildings, the past three decades began a new endeavour for countries to build the ‘world’s tallest building’. The Petronas Towers in Kuala Lumpur, Malaysia; the Taipei 101 in Taiwan; and the Burj Al Arab Tower in Dubai, United Arab Emirates; represent a few examples of high-rise buildings that have become the world’s tallest buildings during recent times. By the end of 2012, 58 of the 100 tallest buildings in the world have been completed in the past seven years, since the end of 2005 (Ref. 8). According to the Council on Tall Buildings and Urban Habitat (CTBUH) reports, the tall buildings built until 2010 are classified according to the region and the usage as shown in Figures 3(a) and 3(b). However CTBUH estimates that by the end of the decade, the number of super tall buildings in the world would more than double as shown in Figures 3(a) and 3(b).
Figure 3(a) Total number of super tall buildings in the world by region by 2010 3(b) Total number of super tall buildings in the world by usage by 2010 The world of tall buildings has changed fundamentally over past decade or two with a number of trends now evident. One of the most important trend is that the predominant location of the tallest buildings in the world has been changing rapidly. By the end of 2012, main locations have shifted to Asia (42% with 34% in China alone) and the Middle East (32%, with 21% in Dubai alone) (Ref. 8). As per CTBUH 9th World congress proceedings, 74% of the world’s 100 tallest buildings will be located in Asia, up from only 20% in 1990. Other most interesting factor about tall building statistics is how the function and structural materials of the tallest buildings have been changing. According to recent information, residential and mixed use functions influence the list, up to 53% from 12% at the turn of century, while solely steel buildings have dropped from 39% to 14% in favour of concrete and composite structures over the last decade. Figure 4 shows the trend of recent tall buildings by building location, use and structural material as shown for the 20 tallest buildings in the world.
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Figure 4: Statistical data on the current tallest 20 buildings in the world At the beginning of 21 st century Petronas twin towers held the world tallest title at 452m. Taipei 101 took the title in 2004 with 508m height. Then at the end of the decade, Burj Kalifa set new standards at 828m. Now with work started on site for Jeddah’s 1000 meter plus Kingdom tower, we can expect that in a mere two decades (2000-2020) the height of the world tallest building will have more than doubled. The most interesting aspect of future tallest 20 list is that the previous world tallest now barely make the list at all. As per CTBUH predictions, future tall buildings by 2020 with respect to building location and use, is shown in Figure 5.
Figure 5: Statistical data on the projected tallest 20 buildings in the world in 2020 4|Page
4. ARCHITECTURE AND STRUCTURE
The ultimate success of a tall building structure and design can only be judged by the community at large who have to live with the structure for the duration of the design life. Consequently the success of a tall building is primarily based on the aesthetics of the completed structure, followed by functionality, economics and cultural considerations. Traditionally the conceptual design of tall buildings is greatly influenced by the architect. After all, they are responsible for the building appearance and functionality. Aesthetics will become increasingly important to keep abreast of the increasing community demands on environmental and town planning issues. Historically, tall buildings consisted of heavy masonry cladding, providing the required stability with the principal structural system rarely exposed. Over the last two decades the buildings have become more slender and lighter, to meet the aesthetics and economical considerations. This trend will continue, ensuring the importance of the structural engineers role at the early concept stage. The Empire State Building and the Bank of China Building are excellent examples of the old and the new. The Empire State Building, where the structure is well concealed by the solid facade, is judged purely on architectural features. The structural engineers would have had minimal input at the conception stage. From a pure structural point of view an optimum building in terms of material minimisation for a square building is one that comprises of four columns located at the corners, interconnected by diagonal bracing. (Ref. 5). The Bank of China Building, closely follows this optimal solution, illustrating the benefits of close collaboration between the architect and structural engineer to conceive a structure which is elegant and economical. This commonality between structural form and aesthetics will become increasingly important in tall building design.
Figure 6: The old and the new 5|Page
5. ARCHITECTURAL ASPECTS IN DESIGN OF TALL BUILDINGS 5.1 FUNCTIONAL REQUIREMENTS
Building configurations are tremendously varied, and their derivation seems to be random. At times even whimsical, although the configuration tries to simultaneously satisfy: · The requirements of site - includes constraints in the form of site geometry, site location, geomechanical considerations, topographical considerations, climatic and seismic considerations. · The requirements of building program - includes factors imposed planning and occupancy requirements, financial constraints and construction issues. · the requirements of appearance - represents the designers desires for physical images that express the aspirations of the building owner, the users, the designers, and more recently the community's. The overall success of the building depends on numerous factors, some of the more important issues include: 1. Create a friendly and inviting image that has positive values to the owner, users and observers. 2. Fit the site, providing proper approaches to the plaza with a layout congenial for people to live, work or recreate. 3. Be energy efficient, providing space with controllable climate for its users as well as fulfilling global conservation obligations. 4. Allow for flexibility in layout and usage. 5. Maximise the views. 6. Compliments the city and its people in terms of culture. 7. Most important of all to be cost-effective. Tall buildings greatly impact the scale and context of the urban environment due to their proportional mass and height. Whether standing alone or blending into the urban context, the larger the building mass, the greater the impact. The intervention of tall buildings in urban environments affects day lighting, sunlight, shadows, and even air movements created by downdrafts and unexpected gusts. The form of the building's mass is the most crucial part in determining the quality of the disturbance and the magnitudes. The design process of a tall building should attempt to keep environmental disturbances to a minimum by the orchestration of its form. Tall buildings should respond to two primary criteria; first to a small circle of its affected users and secondly to the larger urban environment. In regard to the first criterion, the building itself must be gauged relative to its purpose, how it lives up to its expectations. The second criterion must be evaluated in its function as an element in the immediate urban setting. The degree to which tall building add to or detract from the quality of their urban surroundings is dramatic, affecting not only the immediate users but, because of their size and scale, the context of the entire city now and in the future.
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5.2 BASIC ELEMENTS
In terms of an overall tall building it comprises of three basic elements. They are:
Vertical stack of functional areas - (ie floor system) which provides the platform for its occupants at discrete levels. Vertical service riser(s) - series of vertical shafts that act as the umbilical chord to all the functional areas. This includes water, wastewater, electrical services, mechanical services, vertical transportation (lifts), stairs etc. Outside enclosure - (façade) - this skin protects the occupants from the external climatic conditions and maintains the internal conditions provided by the various services in a controlled manner.
5.3 BASIC PLANNING CONSIDERATIONS. The principal planning considerations that need early resolution includes: a. Lease span This is the distance from a fixed interior element, such as the building core, to the exterior window wall. The lease span differs in dimension depending upon the function of the space (commercial office, hotel and residential) and is a very important consideration for good interior planning. Usually the depth of the lease span should be between 10 and 14m for office function and 6 to 9m for hotels and residential developments. b. Floor-to-floor The overall economics of a building is heavily influenced by the floor-to floor height. A small difference in this height when multiplied by the number of floors and the area of the perimeter enclosure of the building has a significant influence on the overall cost.
The floor-to-floor height is a function of the required ceiling height, the depth of the structural floor system and the depth of the space required for mechanical distribution, electrical distribution (lighting) and ceiling system as illustrated in Figure 7.
Figure 7: Floor-to-floor height
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The floor-to-floor height determines the overall height of the building which impacts on the structural system, round trip times of elevators and quantities of all vertical elements in the building such as exterior walls, vertical structural elements, façade, HVAC, and all services. In some buildings "access floors" or elevated floors are provided for services such as computer/electrical cables etc. c. Ceiling height Commercial functions require a variety of ceiling heights ranging between 2.7m and 3.7m. Ceiling heights in office buildings typically is minimum 2.7m., whereas for Hotels and residential buildings the height can be as low as 2.4m. d. Depth of structural floor system. This depth depends on floor load requirements, column spacing and type of floor framing. Allowance for deflections should be allowed for. For steel structures allowance for fire rating should be incorporated. e. Elevators Vertical transportation in any tall building is totally dependent upon its elevator system. The selection of a proper system is a very critical issue in tall building design. For preliminary planning, a rule of thumb for estimating the number elevators needed is one elevator per 4600 square metres of lettable area. f. Core planning The major elements within the core are elevator shaft, mechanical shafts, stairs and elevator lobbies. Shafts for other services must also be allowed for. Core elements that pass through or serve every floor should be located so that they can rise continuously and thus avoid costly and space -consuming transfers. Stair entrances should be located as remotely from each other as possible. Mechanical fan rooms should be located where they can be easily changed in area or shape and where stairs does not surround them, shafts or electrical closets since they prevent or limit duct distribution from shafts or rooms. g. Parking In many large projects, it is essential that proper parking facilities are included within the building. 6.
STRUCTURAL ASPECTS IN DESIGN OF TALL BUILDINGS 6.1 STRUCTURAL DESIGN CRITERIA
A unique characteristic of tall building design is the significance of all three criteria in deriving a satisfactory structural solution: Strength, Serviceability and Stability. The principal contributing factor is the presence of lateral loads, which increasingly dominates the structural form with increasing height. For low-rise buildings, strength of individual components is the governing criteria however for buildings with increasing height the following features become increasingly important • The effects of lateral loads due to wind and earthquake • To assess the magnitude of the design lateral loads • Lateral displacement of the building. (Top deflection and interstorey displacement) • Accelerations • Effect of movement on non-structural elements. • Second order effects such as P-∆ effects, creep movements, differential movements. • Overall stability of the building against overturning and sliding • Importance of members governed by net tension. • Importance of correct assessment of soil/ structural interaction Later lectures will address the significance of all of the above features in tall building design.
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6.2 LOADING
Accurate assessment of the design loads is essential for the structural integrity and overall economy in tall building design. Unique characteristics of loadings applicable to tall building design include: • Importance of correct design lateral loads. Conservatism is not affordable in tall building design. • Correct assessment of live loads and dead loads • Loads induced due to differential movements due to prestressing, foundation settlements, creep and shrinkage can induce massive forces, requiring careful assessment. 6.3 DESIGNING FOR FULL DESIGN LIFE
In view of the substantial capital investment associated with a typical tall building, it is required to be functional for the whole of the design life, which typically ranges from 50 to 100 years. However we as a community have already witnessed dramatic changes in lifestyle over the last 50 years, and are expected to continue to change. With these changes, building occupants demand changes relative to their environment. Refurbishment of buildings is now a thriving building activity to meet these changes; however on economic grounds, the structure is rarely removed. An important consideration in structural design of the high-rise building is incorporating flexibility in the design for future possible changes. This requires vision, and close communication with the relevant parties, including owners, financiers, and architects. Some factors that may be considered include: • Structure - Choose material types that can cater for configuration changes. In this respect a composite steel floor for example could have definite advantages over prestressed floors. • Loads - Design floor loadings may terminate a building’s useful life prematurely. The increasing future demand for storage or heavy loading zones must be catered for. • Access floors - Provisions for access floors needs careful thought. As a minimum requirement, selfweight of the system may need to be allowed for. • Facade - The design life of the facade is typically 20 years and in some instances as low as 10 years. Due to the visual impact of the facade on the overall aesthetics of the building, refurbishment of the facade will be a growth activity. In engineering terms, allowance for future changes at the perimeter may require consideration. (e.g. future precast facade to replace existing curtain wall system.) • Vertical elements - Contingencies in loading for vertical elements and more importantly for foundations is imperative, not only to allow for future modification but for inevitable modifications made during the construction phase. Strengthening of existing foundations is not only a difficult operation but is a costly one. Provisions for future additional load is becoming increasingly important in times where every building owner wishes to add a few more floor levels in search for extra income or to install a communication tower for additional income and prestige. • Durability - This remains to be an important consideration in structural design of high-rise buildings. There still appears to be some reluctance to address this in particular for residential construction.
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6.4 Core system
The size and location of building core is largely governed by architectural requirements. The optimisation of the core element is complex and requires the structural designer to have a full understanding of the structural function of the core, and more importantly, the build ability aspects of the core. Factors needing consideration are listed below. Minimisation of material cost - The designer is required to optimise between concrete quantity, reinforcement and concrete strength. Clearly, the designer requires accurate knowledge on the cost rates for each of the components. Studies have shown that increasing the concrete strength is by far the most economical means of increasing the load capacity of a typical core wall. This is illustrated in Figure 8. Therefore given core geometry, the designer should ideally aim to minimise thickness of walls and quantity of reinforcement by using higher strength concrete.
Figure 8: Cost trends to increase RC wall strength Optimising core geometry - The core layout, whilst fulfilling its architectural functions, should be geometrically efficient to resist the derived design actions due to lateral loads and gravity loads. Figure 9 shows a typical core layout consisting of cells connected by coupling beams. Typically the core elements are subject to axial forces due to coupling action and a set of orthogonal moments in each cell. By simple statics it can be shown that the external walls are subjected to greater forces relative to the internal walls with the corner zones subjected to the greatest forces. Penetrations in these highly stressed areas should be avoided. Minimising the core area - In Australian buildings the core area, as defined in Figure 9, being outside the net lettable area, does not accrue any income. The primary aim of the designer is to minimise the core area and maximise the lettable space within a fixed building envelope. To appreciate the importance of minimising the core area the designer needs to quantify the cost benefit of decreasing the core area per square metre. In Australia, based on a rental income of $500 per square metre per annum and yield rate of 5%, the capitalised value is $10,000 per square metre, which is a significant value.
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Figure 9 - Typical core layout. To quantify the significance, consider the core shown in Figure 9. If the thickness of the core walls is reduced by 50mm throughout the height of the 55 storey building, the core area would reduce by some 11 sq. m. per floor. This equates to 605 sq. m. for the building reflecting a capitalised value of over $6 M, a sizeable value compared to the cost of the core. Minimisation of construction time - The core construction is normally a critical activity, hence its construction time must be minimised. Any delay in its construction will add to the completion time for the whole project. The cost of the delay can be quantified in terms of interest on the total project cost including land costs. For example, for the 55 storey building considered in this lecture, assuming a project cost of $300 M and an interest rate of 8.5%, a delay in construction is equivalent to approximately $0.5 million per week. The importance of construction speed will be discussed in detail in a later lecture. Core economy can only be maximised if the structural designer has a full appreciation of all the above factors. 6.5 Floor system The type of floor system adopted for a project is also a critical activity impacting on the overall completion time. For tall buildings, the designer’s fundamental aim is to minimise the floor cycle time. A single day reduction in the floor cycle time for the sample building corresponds to a 10 weeks saving in construction time. As discussed in the previous section this represents a $5 M cost benefit equal to 30% of total structural cost of the floor system. This suggests the possibility of choosing more expensive floor systems, with a quicker construction time to achieve overall economy. Minimising floor-floor height - Apart from minimising the material cost of the floor system another primary objective is to minimise the inter-storey height. For a particular occupancy type, the floorto-ceiling height is often fixed by regulatory requirement. Designers are required to focus their efforts on minimising the structural floor depth and ceiling-to-floor zone, with the latter governed by mechanical services. The importance of minimising the inter-storey height is illustrated in Figure 7, which shows that a reduction of 100mm in inter-storey height converts to a saving of $1.6 M in construction 11 | P a g e
cost. In fact, further savings associated with the reduction of vertical services including lifts would provide additional cost benefits.
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INTEGRATED APPROACH TO DESIGN OF TALL BUILDINGS
Unlike most standard structures the design of tall buildings requires a complex inter- relationship between many parties ranging from the owner, financier, real estate, architect, quantity surveyors, builder and engineering disciplines. Traditionally the collective effort expended by individual members of the design team was rarely optimised. Over the last decade, there has been an increased trend towards greater collaboration between different disciplines with the eventual goal of achieving overall economy, that is, the bottom line figure. To illustrate the benefits of close collaboration between the disciplines, specific areas of tall building design are selected. One example is to assess the benefits of collaboration between the structural and services engineers for the design of the core and floor system. The key issues requiring close collaboration with structural engineers are the floor-to- floor height, and the size and layout of vertical elements. The latter generally comprises of the core element and columns, and is greatly influenced by the structural requirements to resist lateral loads due to wind and/or earthquakes. During the conceptual stage the role of the services engineer is limited to providing advice on space allocation for the plant rooms and vertical shaft area requirements within the core and to nominating a design horizontal zone in the ceiling space. Integrated approach for floor system design between structure and services - Interaction between structural designers and mechanical service designers is required for an integrated approach to floor system design resulting in major cost savings. It is evident that design decisions affecting floorto-floor height and core area offer the biggest financial benefits.
Figure 10: Cost benefit of reducing inter-storey height by 100mm (note: indicative values only, may vary project to project). 12 | P a g e
Expressions of a tall building are uniquely affected by the structural systems within them and conversely, the form and composition of structural systems are dependent on exterior aesthetics and requirement of space. How well the two systems interact and integrate with each other depends entirely on how well the architectural designer and structural engineer interact and collaborate. Successful collaboration starts at the inception of the project at the conceptual level, where different shapes and structural systems can be brought to bear on the architectural expressions and space concepts. This collaboration should be extended to other discipline, such as mechanical and electrical engineering services. To evaluate the effectiveness of optimisation of the structural and mechanical service systems, the cost breakdown of a typical 55 storey officer tower located in Melbourne, Australia is given in Figure 11.
Figure 11: Cost breakdown of tower (Ref. 1) Figure 11 clearly illustrates the significance of optimising the floor system and the core element with the two components contributing to 80% of the total structural cost. ($A34M) A similar breakdown of the cost of mechanical services is indicated in Figure 12. The biggest impact on the capital cost of mechanical services is associated with on-floor ductwork. 8
MECHANICAL AND ELECTRICAL SYSTEMS
The mechanical systems for buildings consist of heating, ventilating and air-conditioning systems (HVAC), domestic water systems, waste water systems and fire protection systems. HVAC systems are 13 | P a g e
designed to ventilate spaces and to maintain within the spaces a certain range of temperature and humidity conditions. The system consists of several basic components: refrigeration, heating, humidification, and air distribution. The domestic water system consists of an incoming service, water meters and a vertical and horizontal distribution system. Waste and storm water systems collect water from the building plumbing fixtures roof and plaza drains, respectively. They carry it to either combined or separate waste or sanitary sewer system. The purpose of the fire protection system is to provide building users either an automatic or a manual alarm and fire fighting capability. Manual systems include pull box alarms, handheld extinguishers and water risers with both fire hose connections and fire hose cabinets for use by building personnel and the fire department. Standpipes and fire hose cabinets are always located within a fire stair enclosure. Automatic systems consist of smoke and fire detectors connected to an alarm system and automatic sprinkler systems. Electrical systems for buildings consist of high-voltage and low voltage systems. The high-voltage systems provide power for equipment , lighting and appliances for functional spaces in the building. The low voltage systems include the telephone system, the public address system and most of the control systems.
Figure 12: Mechanical services cost breakdown 9
LIFE CYCLE COSTING
A further development to integrated design approach for structural engineer in tall building application, is to consider life cycle costing for assessing options on a proposed project. Life cycle costing involves costing the chosen options over the whole of its life. A convenient measure is to bring the total cost to a present day value. Many designers base costs of various schemes on initial costs. The disadvantages of this approach are: • Ignores the operating and/or maintenance costs or assumes that these costs are independent of the initial scheme. • Assumes that all components of the building have the same design life. • Ignores the cost of money A good example of applying life cycle costing is for the building facade system. Typically a 14 | P a g e
lightweight curtain-wall system is cheaper than a precast cladding system in terms of initial cost; however the design life of the curtain wall system is considerably less than the building design life and would probably need to be replaced during the life of the building. In addition to the replacement cost, the maintenance cost associated with curtain wall would also be greater than the maintenance cost associated with precast cladding. Other factors such as heating and cooling costs for a building with curtain wall cladding would differ from the corresponding costs associated with precast cladding. When the above factors are considered it is obvious that costing based on initial cost is misleading and dangerous. For tall buildings, which comprises of a large range of components, occupying a very expensive piece of real estate, life cycle costing is imperative for a clear and accurate assessment. 10 IMPORTANCE OF BUILDABILITY AND CONSTRUCTION SPEED. The cost of construction often relates more to cost of time than to the cost of materials. This is particularly applicable to design of tall buildings where large capital funds are tied up for a relatively long construction period.
The fact that there is no universal rule governing speed of construction creates a further challenge for the structural engineers. There is now a definite role for the engineer to produce designs that not only addresses the structure in its permanent state but considers the issue on how it can be built with efficiency and speed. Both safety and sustainability aspects are important in planning and designing tall buildings (Ref. 9) 11 1.
2.
3. 4. 5. 6. 7. 8. 9.
REFERENCES Paks M., and Hira A.H., "Multi-disciplinary design approach to the design of tall buildings-the trend beyond 2001", 3rd Kerensky Engineering, Singapore, July 1994. Paks M. and Hira A.H., "Design and construction of cores for tall buildings- achieving YQM through multi- disciplinary approach", 40th Anniversary Conference, Gliwice, Poland, October 1995. Taranath B. S., "Structural analysis and design of tall buildings", McGraw-Hill Book Company. Council on Tall Buildings and Urban Habitat, 2nd volume of assorted papers, 4th World Congress, Hong Kong, Nov. 1990. Ali Mir M, "Integration of structural form and aesthetics in tall building design: the future challenge."4 th World Congress Tall Buildings: 2000 and Beyond, Hong Kong. CTBUH, "Architecture of Tall Building" 1995 CTBUH, " Tall Building Structures - A world View". 1996 CTBUH (2012) Asia Ascending- Age of the sustainable sky scraper city, proceedings of the CTBUH 9th World Congress, Shanghai Mendis, P., “Safe and sustainable Tall Buildings: Current practice and state of the art”, Key-note Address, CONCET2012, KL, Malaysia, 2012.
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