Topic : Common Structural Forms for High-rise Buildings in Hong Kong

Topic :Common Structural Forms forHigh-rise Buildings in Hong KongPrepared by Raymond WongwindBuilding bends or deflectsunder lateral forceThe more rigid of a buildingstructure, the less it deflectsDeflection is a comfortconsideration for buildingusers. Building regulationhas a basic criteria ondeflection.The structure of a buildingshould be so designed tohave a deflection less than1/500 of its heightThe problems of talland slim building1

Various deflection modes under wind tunnel testStructural forms suitable for buildings in HK1. Column and beam (framed) structure in RCsometimes called framed buildings which are structuresformed by the framed elements usually in the form ofcolumns and beams, as well as further strengthened asnecessary by the introduction of rigid floor membranes andexternal 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 oftall and slim buildings- more suitable to construction low-rise or horizontalspreadbuildings such as schools, podium or carparkstructures2

Example oflayout designevolution from a5 x 3 grid systemExample of In-situ RC frame building3

Structural forms suitable for buildings in HK2. Load bearing wall (shear wall) structure in RC- More complicated formwork is required- cost of construction may increase especially forbuilding with more internal walls or in complicatedshape- the load bearing walls can strengthen the rigidity of thebuilding and able to resist building against deflection- building examples: public buildings such as theHarmony BlocksExample of load-bearingwall structures4

Structural forms suitable for buildings in HK3. Load bearing wall plus core wall structure- Complicated formwork is often required especially informing the core- cost of construction is higher, especially for buildingswith complicated shape (e.g. lots of set-back or lightwells- has very strong rigidity to resist deflection- this is the most common structural form for use in HK’sresidential buildings due to:. walls between residential flats can be best used as aload bearing wall (fire resistance is one reason). To fulfill building regulation for prescribed windowareas. To satisfy the rigidity of high-rise buildingsExample of load-bearingwall structures with buildingarea below 500 sq m5

Example of load-bearing wall structures ofbuilding area around 800 – 1000 sq mExample of load-bearingwall structures withseveral blocks linkedtogether to form aconnected structure6

Structures inload-bearing wallwith a RC coreStructural forms suitable for buildings in HK4. RC Centre core plus a RC frame structure- Confine to more regularly shaped or buildingwith simpler layout, such as usual tower typebuildings- fair rigidity performance but may not besufficient for building higher than, say 50 storeys.- rigidity can be improved by the adding in ofother stiffening members- plenty of window areas (it is only a frame in theexternal face the building)- The internal core may have difficulty in gettingnatural ventilation or sunlight.7

Example ofcored structurewith an externalframe for towertypeofficebuildingVarious layout arrangementfor building using core andouter frame structure8

Various layout arrangementfor building using core andouter frame structureExample of cored structurewith an external frame –Central Plaza in Wanchai9

Example of cored structurewith an external frame –Langham Place in Mong KokIt is sometimes quitedifficult and costly toconstruct the RC core10

Structural forms suitable for buildings in HK5. Center core plus a outer core structure (tube in tube)- Formwork usually very complicated with the flooradded in at a second stage- building shape confined to tubular forms- area of windows limited due to the provision of astructurally strong external tube (wall)- quite strong in rigidity and be able to construct tovery tall structure- building examples – Hopewell Center (HK) or theprevious World Trade Center in New YorkExamples of tube-in-tube structures – Jardin House in Central11

Hopewell Centre in WanchaiExamples of a framing plan showing the location/layout ofwalls, columns and beams12

Structural forms suitable for buildings in HK6. Steel frame structure- building is lighter than usual RC structure due to therelative higher weight/strength ration of steel- construction cost is lower especially for buildingsadopting high level of standardization- building is weak in rigidity due to steel is a moreflexible (elastic) material when compare to RC- additional stiffening elements in the form ofinternal/external trusses, out-rigger or bracing membersare often required to make the building structure morerigid- addition work is required to fire-protect the steelstructure- building examples: The Center, China Bank BuildingExamples of building constructed in pure structural steel frame13

Construction of Structural steel frame buildingStructural forms suitable for buildings in HK7. Composite structure (RC Core plus steel frame, hybridstructure)- building can be lighter than usual RC structure butwith a more acceptable rigidity than pure steelbuildings- construction cost is higher for the adding in of a RCcore which requires a complicated formwork system- it is time consuming and more complicated to makeconnections between the RC and steel elements- It is even more difficult and costly if stiffeningmembers are introduced into the building in order tomake it more rigid to resist deflection- Building examples: Time square, Manual Life Tower,Cheung Kong Center, IFC 1 and 214

Examples of composite structures15

More examples of composite structuresMore examples of composite structures16

The IFC 2Complicated stiffeningelements incorporated insteel or compositebuilding to improve therigidity of the structureCheung KongCenterOutrigger system usedin Cheung Kong Center17

The Cheung Kong CenterComplicated stiffening elementsincorporated in steel or compositebuilding to improve the rigidityof the structure18

Complicated stiffeningelements incorporatedin steel or compositebuilding to improve therigidity of the structureCost idea of using RC, Steel and compositemethod in constructing tall buildings19

How to construct various forms of BuildingsBuildings can be constructed or formed in a numberof ways, such as:1. In-situ manner using formwork – for allreinforced concrete buildings2. Prefabricated manner – for using part or all inprecast concrete elements3. Prefabricated manner – using structural steelelements4. Hybrid manner – making combination of any ofthe above systemsConstruction of in-situ RC buildingsForming the wall structureusing aluminum mouldsForming the wall structure usingmanual-type timber formwork20

Construction of in-situ RC buildingsForming the structure using largepanel-type forms for walls or columnsConstruction of in-situ RC buildingsForming the wall structure usingmechanical-operated formwork systems21

Construction of in-situ RC buildingsForming the floor structure usingtimber formwork systemsConstruction of in-situ RC buildingsForming the floor structureusing table formwork – mostsuitable for office buildings22

Construction of in-situ RC buildingsForming the floor structure usingtable formwork – floor beingstrengthened by tensioning techniqueConstruction of prefabricated frame buildingHeadquarters building ofHong Kong Jockey ClubPodium – KCRKowloon Station23

Construction of prefabricated frame buildingUsing precast elements as part of theexternal walls as well as the permanentformwork for load-bearing wallsAn external view of building –majority of external walls areconstructed in precast mannerAdvantageous features of framed structure1. Speedy construction due to simplicity in geometry –consist of only columns and beams (or partially by thefloor slab) as the main structural elements2. 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 beingfinished becomes an cover to protect the lower floors fromsun and rain5. Offer large unobstructed floor areas – without obstaclebetween columns6. Flexible utilization of space24

Advan. . features of framed structure (continued)7. Adaptable to almost any shape8. Easily altered within limits of frame– regular or non-regular grid system is very adaptable inspatial arrangement9. Offsite preparation possible – especially for prefabricatedconstruction using precast concrete or structural steelelements10. May be designed to accommodate movement11. Acceptable distribution of natural light – window openingscan be provided easily on eternal walls12. Easy to design structurally including computer design – again,due to simple geometryPrinciple factors affecting choice of frameProduction consideration• Availability of materials, labour and plant• Speed of construction• Availability of mechanical plantsArchitectural Design consideration• Size and shape of site• Conditions of site• Integration of mechanical elements• Architectural or aesthetic requirements25

Principle factors affecting choice of frameStructural Design consideration• Foundation consideration• Standardization of members or design• Span and floor loads• Fire resistance requirement• Maintenance consideration• Integration of architectural & structural elementsFeatures of In-situ concrete frame constructionConstruction aspects1. Insitu concrete is inconsistent in quality in performance –mixing, delivery, placing and compacting of concreterequire very tight quality control2. Overlapping formwork, steel fixing and concretingprocess make site operations more difficult – these worksare to be done almost at the same time at the samelocation3. Formwork erection is often timely, expensive,environmental unfriendly and labour intensive4. Increase a lot of wet-work – difficult to maintain site in aclean and tidy environment26

Features of In-situ concrete frame constructionConstruction aspects – examples of problemImproper placing and compaction willserious lower the strength of concreteFeatures of In-situ concrete frame constructionConstruction aspects – examples of problemFinishing the concrete surface afterplacing – improper finish will affectthe size and dimension of member27

Features of In-situ concrete frame constructionConstruction aspects – examples of problemExterior environmentInterior environmentIt is difficult to maintain a clean and tidy site using in-situconcrete structure for the casting process is wet and dirtyFeatures of In-situ RC frame constructionConstruction aspects (continue)5. Easier to allow for dimensional discrepancy – formworkcan relatively easy to provide any ready dimension andshape6. Concrete is cheap, easy available and can be cast intocomplicated shape7. Frame erection affected seriously by climatic condition – inwindy day it is difficult to erect formwork nor to placeconcrete8. Durability affect by heat, moisture or environmentalfactors – concrete is not so durable as most person think28

Features of in-situ RC frame construction (cont.)Structural characteristics and design aspects1. Concrete design more flexible and accurate than steelbecause of non dependence on standard sections2. Concrete frame easily damaged by building movements3. Problems of creep & shrinkage (cause damage to finishes)4. Higher dead load and bigger structural elements5. Better fire resistance6. Durability sensitive to workmanship7. Overloading cause irreversible structural damages8. Flexible in layout and shapeTypical case using traditionaltimber formwork for framestructure in RC with loadbearingwallsThe CCCU Building of CityU29

Features of structural steel constructionStructural characteristics1. Steel has lower dead load (unless encased in concrete)2. Steel frame can easily to have the layout changed to suitnew design than concrete, especially requiring alterationafter completion of the building.3. Easily accommodate building movements for steel is arelatively flexible material and have greater adaptability totake up strain and deformation4. However, it is weaker in fire resistance for steel will losestrength and deform serious at higher temperature (at 600oCor above). This can be improved by applying fire resistantplaster30

Features of structural steel constructionStructural characteristics (continue)5. Beams usually shallower in size than for concrete Beams6. Sections in various convenient type and size thuseconomical in design and uses – refer to standard steelsections7. Produce larger deflection & deformation (Bend) underwind – this will cause uncomfort to users of building8. Steel has higher thermal expansion value9. Rusting or corrosion problems – very strict anti-corrosiontreatment has to be appliedFeatures of structural steel construction (cont.)Construction aspects1. Quicker to erect and can be prepared offsite – inworkshop/factory environment with good production andquality control2. Erection on site less affected by climatic conditions –without placing of concrete3. Longer and highly coordinated planning – require long timeto make structural design, fabrication and delivery4. Higher requirement in the control of dimensional accuracybefore and during construction (in fabrication and inerection31

Features of structural steel construction (cont.)Construction aspects5. Demand greater transportation or cranages provision6. Complicated quality assurance procedures – especially inthe control of welding connection7. Difficult to make connection with rigid structures –especially in composite structure in which RC core structureis involved8. More complex when building become more gigantic andtaller – due to the huge number of structural members, somemay be of very heavy section9. Induce more works at higher altitudeFeatures of structural steel constructionConstruction aspects – highlight of some problemsDelivery of structural steelmember by shipLarge member delivery to sitewaiting to hoisting up for erection32

Features of structural steel constructionConstruction aspects – highlight of some problemsHoisting of the structural steelmember to the upper floorFeatures of structural steel constructionConstruction aspects – highlight of some problemsConnecting steel members with RCelement – an anchor frame may berequired for making strong connection33

Features of structural steel constructionConstruction aspects – highlight of some problemsConstructing the composite floorin steel/RC composite structureTo connect floor to core wall –starter bars are to be providedwithin the core structureFeatures of structural steel constructionConstruction aspects – highlight of some problemsVery large component often incomplicated shape34

Features of structural steel constructionConstruction aspects – highlight of some problemsHuge amount of structural membersmake erection very complicatedFeatures of structural steel constructionConstruction aspects – highlight of some problemsVery complex and gigantic framework as stiffening membersto the overall structure – Belt truss system, the IFC II35

Features of structural steel constructionConstruction aspects – highlight of some problemsVery complex and gigantic framework asstiffening members to the overall structure –Outrigger, Cheung Kong CenterFeatures of structural steel constructionConstruction aspects – highlight of some problemsThe existence of very complicated jointingin steel structure36

Features of structural steel constructionConstruction aspects– highlight of some problemsRequired to perform works atvery high altitudeFeatures of structural steel constructionConstruction aspects – highlight of some problemsWorkers work at high altitude37

Features of structural steel constructionConstruction aspects – highlight of some problemsConnecting large and complicate shapedcomponents can be very difficultFeatures of structural steel constructionConstruction aspects – highlight of some problemsConcrete has to be trimmed to allowsteel to pass throughExamples of location where unmatched dimension causing problem38

Features of structural steel constructionConstruction aspects – highlight of some problemsRoof truss andthe decking noton the same levelExamples of location where unmatched dimension causing problem39