Common Structural Forms for High-rise Buildings in Hong Kong

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1 Topic : Common Structural Forms for High-rise Buildings in Hong Kong Prepared by Raymond Wong wind Building bends or deflects under lateral force The more rigid of a building structure, the less it deflects Deflection is a comfort consideration for building users. Building regulation has a basic criteria on deflection. The structure of a building should be so designed to have a deflection less than 1/500 of its height The problems of tall and slim building 1

2 Various deflection modes under wind tunnel test Structural forms suitable for buildings in HK 1. Column and beam (framed) structure in RC sometimes called framed buildings which are structures formed by the framed elements usually in the form of columns and beams, as well as further strengthened as necessary by the introduction of rigid floor membranes and external walls. - Simple formwork is required - lower cost of construction - easy to satisfy various spatial arrangement and shapes - may not be very able to cater for the structural needs of tall and slim buildings - more suitable to construction low-rise or horizontalspread buildings such as schools, podium or carpark structures 2

3 Example of layout design evolution from a 5 x 3 grid system Example of In-situ RC frame building 3

4 Structural forms suitable for buildings in HK 2. Load bearing wall (shear wall) structure in RC - More complicated formwork is required - cost of construction may increase especially for building with more internal walls or in complicated shape - the load bearing walls can strengthen the rigidity of the building and able to resist building against deflection - building examples: public buildings such as the Harmony Blocks Example of load-bearing wall structures 4

5 Structural forms suitable for buildings in HK 3. Load bearing wall plus core wall structure - Complicated formwork is often required especially in forming the core - cost of construction is higher, especially for buildings with complicated shape (e.g. lots of set-back or light wells - has very strong rigidity to resist deflection - this is the most common structural form for use in HK s residential buildings due to:. walls between residential flats can be best used as a load bearing wall (fire resistance is one reason). To fulfill building regulation for prescribed window areas. To satisfy the rigidity of high-rise buildings Example of load-bearing wall structures with building area below 500 sq m 5

6 Example of load-bearing wall structures of building area around sq m Example of load-bearing wall structures with several blocks linked together to form a connected structure 6

7 Structures in load-bearing wall with a RC core Structural forms suitable for buildings in HK 4. RC Centre core plus a RC frame structure - Confine to more regularly shaped or building with simpler layout, such as usual tower type buildings - fair rigidity performance but may not be sufficient for building higher than, say 50 storeys. - rigidity can be improved by the adding in of other stiffening members - plenty of window areas (it is only a frame in the external face the building) - The internal core may have difficulty in getting natural ventilation or sunlight. 7

8 Example of cored structure with an external frame for towertype office building Various layout arrangement for building using core and outer frame structure 8

9 Various layout arrangement for building using core and outer frame structure Example of cored structure with an external frame Central Plaza in Wanchai 9

10 Example of cored structure with an external frame Langham Place in Mong Kok It is sometimes quite difficult and costly to construct the RC core 10

11 Structural forms suitable for buildings in HK 5. Center core plus a outer core structure (tube in tube) - Formwork usually very complicated with the floor added in at a second stage - building shape confined to tubular forms - area of windows limited due to the provision of a structurally strong external tube (wall) - quite strong in rigidity and be able to construct to very tall structure - building examples Hopewell Center (HK) or the previous World Trade Center in New York Examples of tube-in-tube structures Jardin House in Central 11

12 Hopewell Centre in Wanchai Examples of a framing plan showing the location/layout of walls, columns and beams 12

13 Structural forms suitable for buildings in HK 6. Steel frame structure - building is lighter than usual RC structure due to the relative higher weight/strength ration of steel - construction cost is lower especially for buildings adopting high level of standardization - building is weak in rigidity due to steel is a more flexible (elastic) material when compare to RC - additional stiffening elements in the form of internal/external trusses, out-rigger or bracing members are often required to make the building structure more rigid - addition work is required to fire-protect the steel structure - building examples: The Center, China Bank Building Examples of building constructed in pure structural steel frame 13

14 Construction of Structural steel frame building Structural forms suitable for buildings in HK 7. Composite structure (RC Core plus steel frame, hybrid structure) - building can be lighter than usual RC structure but with a more acceptable rigidity than pure steel buildings - construction cost is higher for the adding in of a RC core which requires a complicated formwork system - it is time consuming and more complicated to make connections between the RC and steel elements - It is even more difficult and costly if stiffening members are introduced into the building in order to make it more rigid to resist deflection - Building examples: Time square, Manual Life Tower, Cheung Kong Center, IFC 1 and 2 14

15 Examples of composite structures 15

16 More examples of composite structures More examples of composite structures 16

17 The IFC 2 Complicated stiffening elements incorporated in steel or composite building to improve the rigidity of the structure Cheung Kong Center Outrigger system used in Cheung Kong Center 17

18 The Cheung Kong Center Complicated stiffening elements incorporated in steel or composite building to improve the rigidity of the structure 18

19 Complicated stiffening elements incorporated in steel or composite building to improve the rigidity of the structure Cost idea of using RC, Steel and composite method in constructing tall buildings 19

20 How to construct various forms of Buildings Buildings can be constructed or formed in a number of ways, such as: 1. In-situ manner using formwork for all reinforced concrete buildings 2. Prefabricated manner for using part or all in precast concrete elements 3. Prefabricated manner using structural steel elements 4. Hybrid manner making combination of any of the above systems Construction of in-situ RC buildings Forming the wall structure using aluminum moulds Forming the wall structure using manual-type timber formwork 20

21 Construction of in-situ RC buildings Forming the structure using large panel-type forms for walls or columns Construction of in-situ RC buildings Forming the wall structure using mechanical-operated formwork systems 21

22 Construction of in-situ RC buildings Forming the floor structure using timber formwork systems Construction of in-situ RC buildings Forming the floor structure using table formwork most suitable for office buildings 22

23 Construction of in-situ RC buildings Forming the floor structure using table formwork floor being strengthened by tensioning technique Construction of prefabricated frame building Headquarters building of Hong Kong Jockey Club Podium KCR Kowloon Station 23

24 Construction of prefabricated frame building Using precast elements as part of the external walls as well as the permanent formwork for load-bearing walls An external view of building majority of external walls are constructed in precast manner Advantageous features of framed structure 1. Speedy construction due to simplicity in geometry consist of only columns and beams (or partially by the floor slab) as the main structural elements 2. Very rigid and stable able to resist tremendous vertical (dead load) and lateral loads (wind) 3. Reduced dead load absent of thick shear wall etc. 4. Roofed over at an earlier stage every floor slab being finished becomes an cover to protect the lower floors from sun and rain 5. Offer large unobstructed floor areas without obstacle between columns 6. Flexible utilization of space 24

25 Advan.. features of framed structure (continued) 7. Adaptable to almost any shape 8. Easily altered within limits of frame regular or non-regular grid system is very adaptable in spatial arrangement 9. Offsite preparation possible especially for prefabricated construction using precast concrete or structural steel elements 10. May be designed to accommodate movement 11. Acceptable distribution of natural light window openings can be provided easily on eternal walls 12. Easy to design structurally including computer design again, due to simple geometry Principle factors affecting choice of frame Production consideration Availability of materials, labour and plant Speed of construction Availability of mechanical plants Architectural Design consideration Size and shape of site Conditions of site Integration of mechanical elements Architectural or aesthetic requirements 25

26 Principle factors affecting choice of frame Structural Design consideration Foundation consideration Standardization of members or design Span and floor loads Fire resistance requirement Maintenance consideration Integration of architectural & structural elements Features of In-situ concrete frame construction Construction aspects 1. Insitu concrete is inconsistent in quality in performance mixing, delivery, placing and compacting of concrete require very tight quality control 2. Overlapping formwork, steel fixing and concreting process make site operations more difficult these works are to be done almost at the same time at the same location 3. Formwork erection is often timely, expensive, environmental unfriendly and labour intensive 4. Increase a lot of wet-work difficult to maintain site in a clean and tidy environment 26

27 Features of In-situ concrete frame construction Construction aspects examples of problem Improper placing and compaction will serious lower the strength of concrete Features of In-situ concrete frame construction Construction aspects examples of problem Finishing the concrete surface after placing improper finish will affect the size and dimension of member 27

28 Features of In-situ concrete frame construction Construction aspects examples of problem Exterior environment Interior environment It is difficult to maintain a clean and tidy site using in-situ concrete structure for the casting process is wet and dirty Features of In-situ RC frame construction Construction aspects (continue) 5. Easier to allow for dimensional discrepancy formwork can relatively easy to provide any ready dimension and shape 6. Concrete is cheap, easy available and can be cast into complicated shape 7. Frame erection affected seriously by climatic condition in windy day it is difficult to erect formwork nor to place concrete 8. Durability affect by heat, moisture or environmental factors concrete is not so durable as most person think 28

29 Features of in-situ RC frame construction (cont.) Structural characteristics and design aspects 1. Concrete design more flexible and accurate than steel because of non dependence on standard sections 2. Concrete frame easily damaged by building movements 3. Problems of creep & shrinkage (cause damage to finishes) 4. Higher dead load and bigger structural elements 5. Better fire resistance 6. Durability sensitive to workmanship 7. Overloading cause irreversible structural damages 8. Flexible in layout and shape Typical case using traditional timber formwork for frame structure in RC with loadbearing walls The CCCU Building of CityU 29

30 Features of structural steel construction Structural characteristics 1. Steel has lower dead load (unless encased in concrete) 2. Steel frame can easily to have the layout changed to suit new design than concrete, especially requiring alteration after completion of the building. 3. Easily accommodate building movements for steel is a relatively flexible material and have greater adaptability to take up strain and deformation 4. However, it is weaker in fire resistance for steel will lose strength and deform serious at higher temperature (at 600oC or above). This can be improved by applying fire resistant plaster 30

31 Features of structural steel construction Structural characteristics (continue) 5. Beams usually shallower in size than for concrete Beams 6. Sections in various convenient type and size thus economical in design and uses refer to standard steel sections 7. Produce larger deflection & deformation (Bend) under wind this will cause uncomfort to users of building 8. Steel has higher thermal expansion value 9. Rusting or corrosion problems very strict anti-corrosion treatment has to be applied Features of structural steel construction (cont.) Construction aspects 1. Quicker to erect and can be prepared offsite in workshop/factory environment with good production and quality control 2. Erection on site less affected by climatic conditions without placing of concrete 3. Longer and highly coordinated planning require long time to make structural design, fabrication and delivery 4. Higher requirement in the control of dimensional accuracy before and during construction (in fabrication and in erection 31

32 Features of structural steel construction (cont.) Construction aspects 5. Demand greater transportation or cranages provision 6. Complicated quality assurance procedures especially in the control of welding connection 7. Difficult to make connection with rigid structures especially in composite structure in which RC core structure is involved 8. More complex when building become more gigantic and taller due to the huge number of structural members, some may be of very heavy section 9. Induce more works at higher altitude Features of structural steel construction Construction aspects highlight of some problems Delivery of structural steel member by ship Large member delivery to site waiting to hoisting up for erection 32

33 Features of structural steel construction Construction aspects highlight of some problems Hoisting of the structural steel member to the upper floor Features of structural steel construction Construction aspects highlight of some problems Connecting steel members with RC element an anchor frame may be required for making strong connection 33

34 Features of structural steel construction Construction aspects highlight of some problems Constructing the composite floor in steel/rc composite structure To connect floor to core wall starter bars are to be provided within the core structure Features of structural steel construction Construction aspects highlight of some problems Very large component often in complicated shape 34

35 Features of structural steel construction Construction aspects highlight of some problems Huge amount of structural members make erection very complicated Features of structural steel construction Construction aspects highlight of some problems Very complex and gigantic framework as stiffening members to the overall structure Belt truss system, the IFC II 35

36 Features of structural steel construction Construction aspects highlight of some problems Very complex and gigantic framework as stiffening members to the overall structure Outrigger, Cheung Kong Center Features of structural steel construction Construction aspects highlight of some problems The existence of very complicated jointing in steel structure 36

37 Features of structural steel construction Construction aspects highlight of some problems Required to perform works at very high altitude Features of structural steel construction Construction aspects highlight of some problems Workers work at high altitude 37

38 Features of structural steel construction Construction aspects highlight of some problems Connecting large and complicate shaped components can be very difficult Features of structural steel construction Construction aspects highlight of some problems Concrete has to be trimmed to allow steel to pass through Examples of location where unmatched dimension causing problem 38

39 Features of structural steel construction Construction aspects highlight of some problems Roof truss and the decking not on the same level Examples of location where unmatched dimension causing problem 39