7.2 Prefabricated Construction Systems adopted in Hong Kong

Prefabricated Construction Systems adoptedin Hong KongPresented byRaymond W M WongDivision of Building Science & TechnologyCity University of Hong Kong

Development and Application of PrefabricatedTechnology in Building Construction in HK1. Had been used once in early 1970s in a pilot project toconstruct a series of 17-storey public houses2. Used in a broader scale in the mid/late 1980s in theconstruction of public houses where all the façades of theflats were constructed in precast in a after-fixed manner3. Improvement to public houses by introducing in-situ joinedfaçade and broader use of other precast elements inearly/mid 1990s.4. A few private developments pioneered in the use ofprefabricated system in the construction of buildings ofvarious nature since early 1990s

Development and Application (continue)5. Packaged projects introduced from late 1990 onward by theHong Kong government for the construction of a series ofschools using semi-prefabricated method6. A series of government quarter buildings using similarconstruction techniques were introduced in 20007. A few special buildings such as bus depot or train stationpodium employed large amount of prefabricated elements ofwhich some were primary structural members (1997 onward)8. Popular use of prefabricated external wall from 2002 onwardin residential buildings due to the introduction of a newguideline encouraging the use of precast façades as a formof green elements in construction (exemption of Gross FloorArea)

Prefabricated ConstructionPrefabricated construction refers to constructionmaking use of readily-assemble componentswhich are prefabricated off site and to connectthem on site into a permanent structure in aconvenient and structurally strong manner.Materials suitable for prefabricated constructionmainly include concrete, steel and timber. Outof the three named, prefabrication using precastconcrete is the most popular in Hong Kong.

Prefabricated ConstructionThe basic principle of using prefabrication inconstruction is to take the advantage of allowingsome of the major building elements beingproduced off-site in a prefabrication yard orfactory where productivity, quality and cost ofworks can be better controlled. The finishedelements can then be transported to the site forassembly and connection.

Prefabricated ConstructionThe Construction using precast concrete has thefollowing advantages.a) Precast elements produced in factory environmentcan be worked to a very high structural orarchitectural standards.b) Precast elements is more durable when compareto structure constructed in in-situ concrete.c) Elements can be stressed to produce structures ofhigher performance.

High quality precastelements can be producedin well-equipped castingyard/factory

Stressing work applied during thecasting process

Prefabricated ConstructionContinue - advantages of precast concreted) Productivity is much higher than cast in-situ worksespecially when standardized design is adopted.e) Assembly & installation process carried out on sitecan save up significant amount of formwork &falsework especially when the structure is of veryhigh headroom.f) Construction can normally be done in higher speedwith easier supervision and reduced cost.

Falsework involvedwhen casting a floorwith high headroom

Limitation and drawback in using precastconstruction in Hong KongThough prefabricated construction exhibits muchbenefits over traditional construction methods, itis still not popularly received in the construction ofbuildings in Hong Kong. One very major reasonis that prefabrication construction using precastconcrete components cannot produce a very rigidstructure. This drawback becomes moresignificant when used in high-rise buildings wherehuge lateral wind load has to be taken into account.

The insufficient rigidity mainly originatedfrom the joints of the major elements, such asin joints between columns and columns, beamsand columns, or beams to paneled walls etc.Sometimes, rigid joint design can be made toimprove the performance, but this will, again,complicate the connection and increase thetime and cost of work. Furthermore,prefabricated construction also inherits withthe following drawbacks.

Provision for the forming of arigid joint between members

Drawback:TTighter and longer period of coordination are required toallow for structural design, construction planning,procurements and approval proceduresMMore time required in advance for production of precastelements until they can be accumulated to sufficientnumbers for delivery to site to start installation.Huge working space is required for the carrying out offabrication works. This is particularly critical under HongKong situation. Though most of the fabrication works arerecently carried out in casting yards in China, this willincrease the transportation cost by sea or on land.

Precast elementsrequired huge amount ofspace as handling andstorage area

d) SStorage and pre-installation handling, again, requires extraworking space.e) IInstallation of precast elements on site requires carefulplanning and extra cranage provisions.f) HHandling and assembly of heavy precast members, posescertain safety problems and increase risk both to humanoperatives or to the semi-completed structure itself.g) TThe structure under assembly in its semi-completed statemay cause difficulties in making access into the work spot.h) DDefective connections may result to cracks and waterleakage which will create further maintenance problems.

Transportation, access andlifting problems in theusing of precast

Leakage problem due todefective jointing during theconnection process

Connecting precast elements1. Connecting column to foundation base(similar for connecting column to column)Precast column usually has a cast-in steel plate atthe bottom. This plate is fastened to anchor boltsset into a concrete footing or base

Connecting columnto footing baseusing anchor bolts

Connecting columnto base (similar forcolumn to columnconnection)

2. Connecting beam to columnPrecast beam is connected to column in a variety ofways. For non-rigid jointing, corbel is usually formedon the column to support the precast beam. However,anchorage provision should be provided to secure themembers from seismic or wind action.Rigid joint (moment resisting) is common for buildingrequiring stronger performance. In this case, haunchingarrangement by inserting link bars between the columnand beam members and infill the joint afterward withconcrete can often produce an acceptable joining result.It should be noted that suitable propping support shouldbe provided before the final setting of the joint.

Beams rest oncorbel of column(with tie bolt tosecure beams frommovement)

Connectingbeam to column(non-momentresisting)

Haunchingarrangement toconnect beamto column

3. Topping to form the deck/slabDeck or slab of precast structure often appears in theform of a RC topping, placed on top of the precastbeam and semi-slab (precast floor plank).Semi-slab (rebarwill be fixed ontop)

Case Examples

Generations of Public Houses(Harmony Block) constructedusing semi-prefabricated concept

First generation of HarmonyBlock using after-fix façadeLater version of Harmony Block using inisitufaçade cast at the same time with structural walls

Precasting yard was set up toproduce precast elements for publichousing projects in early to mid 1990

After-fix façade system requiredthe formation of small amount ofin-situ joint and grouting thateasily caused leaking problem

Detail of façade installationusing in-situ jointingarrangement in the later versionof Harmony Block Construction

Forming the floor slab in theHarmony Block constructionusing semi-slab (right) andaluminium formwork (above)

Introduction of mechanicalformwork systems incorporatingthe standard precast elements inthe construction of latest versionof public houses in the late 1990

Public houses using Concorddesign with special emphasison the use of mechanicalisedconstruction system

Layout of the ConcordBlock showing theposition of the cast-insitucore wall, shearwalls and precast façade

Other pioneered projects in theearly 1990 constructed usingsemi-prefabricated concept

The construction of the Headquarter and MembersFacilities Complex for the Hong Kong Jockey Club

Forming the floor slab using precastsecondary beams and semi-slab for the mainbuilding (above) and members’ complex (right)

Detail of the floor system with thecast in-situ main beams and theplace-in precast secondary beams

Introduction of prefabricatedconcept to construct governmentquarters in 2000

Government quartersin West Kowloon

Layout of the quarter building showing typcial phasing arrangement of work

Features in the construction –extensive use of precastfaçade and lintel beams,precast cast-in-situ internalpartitions are also introduced

Detail of the façade andthe lintel beam installation

Installation of the cast-in-situprecast internal partition

Construction of special buildingsusing high proportion of precastsince 1998

Construction of the KowloonCanton Rail podium deck –cargo handling bay on theground level and on top for alarge-scale residentialdevelopment afterwardKowloon Canton RailwayStation podium/deck

Forming of the castin-situmain beams

Jointing detail of themain beam and precastsecondary beams

Semi-slab with RCtopping to form thecomposite podium deck

Kowloon Motor BusMaintenance Depot- 3-level depot buildingwith columns, main andsecondary beams all inprecast

Erecting of the building frameusing precast columns and beams

Connecting the precast column to thefoundation cap with a base plate

Placing the secondary beamsonto the main beam

Connecting beams to columns bystitching, a means to provide anin-situ joints with rigid RC links

Detail of the beam/columnand beam/beam joints

Recent Residential/CommercialDevelopments using prefabricatedconcept to construct

Residential developmentat Cyber Port

Façade and general installation details

Features in the construction –large amount of external wallsare constructed in precastfaçade untis, with majorstructural links tie back to theinner shear walls and floorslab

Incorporation of other sophisticated formwork in construction –steel gang form for walls and table form for floor

Residential Developmentat Taikoo ValleyConstruction features –the use of large amountof precast units both ofstructural or architecturalnature. This includes thebalcony, façade units,loss-form for externalwalls and some shadingfins.

Detail of the precast balcony unit

Detail of the loss-form

Residential developmentat Ma Tau Wai

Overview of the precastinstallation arrangement

Façade installation detailas seen on the floor deck

Cambridge House – a 42-storeyOffice Building at Quarry Bay

Features in construction –precast beam trough andprecast sub-slab with RCtopping forming the floor system

Placing of the precast beam troughto form the floor layout

Recent Achievements of Hong Kong’sPrefabricated Technology• Previous defects such as leakage problems have beenimproved• Application diversified to other forms of construction• Capable to apply to rather complicate-shaped buildings• Incorporation of other advanced technology in theconstruction (such as use with mechanical form/tensioned elements)• Generally mastering of skill and growing popular

Future Trend in the application ofPrefabricated Technology in Hong Kong1. To increase the use of prefabrication for more structuralelements such as for beam, column or load bearing wall2. Use more modulated elements or plug-in units3. Use more for architectural or decorative elements4. Incorporation of other composite elements in the designsuch as using at the same time with structural steel5. Apply prefabrication more to medium-rise buildings withmore standardized structural items6. Improve the supply chain management to makeproduction more efficient and economical

Practical Constraints1. Tighter coordination to allow for structural design,construction planning, procurement & approvalprocedure2. Required a critical amount of precast items before workcan be economically used3. Huge work space for handling precast elementsespecially in congested urban environment in HongKong4. Congested site environment makes access anddelivery of heavy precast units to the work spotbecomes difficult5. Quality assurance become critical especially wherelarge amount of precast components are used asstructural nature

Examples of very congestedconstruction environmentwhere prefabrication approachis adopted

Example of Innovative Project (Local)The Integer

The Integer Houseat Admiralty

Floor plan of the experimental house

The working concept

Typical connection details

Assembly of the 3-D modules

Possible future development for high-rise structures using similar concept

Example of Innovative Project (overseas)The Habitat 67 in Montreal, Canada

Habitat 67 as seen along the bank of St Lawrence RiverBuildingSection

Habitat 67 as seen along the bank of St Lawrence River

Close up look of themodulated dwelling units

Close up look of the modulated dwellingunits and the linking structure in theform of a access corridor

View as seen onthe deck level

Forming and Installation ofthe modulated units

Installation of the modulated units and the associated structure

BuildingSection

Floor plan

Connecting the modules

Prefabricated ConstructionApplication in Civil Works

Examples of application of prefabricatedtechniques in civil construction works1. Used in highway projects such as cantilevered deckof elevated bridge, parapets of expressway and roadcurb2. Precast girder units or beams for elevated roadway(viaduct)3. Precast beams or decks for elevated pedestrianfootbridges4. Tunnel lining especially for tunnel formed by tunnelboring machine5. Decks for long span bridges

Examples of application (continue)6. Marine work such as seawall and deck of terminalberth7. Soil retaining element such as for earth-balancedtype retaining structure8. Platform for railway station9. Parapets and noise barrier panels10. Overhanging ducts or services channels forunderground facilities11. Nullah section for storm water discharge

Elevated pedestrianfootbridge in Mong Kok

Piers formed usingprecast column

Precast girder erectedon top of the piers toform the main span ofthe footbridge

Extending precast plank toform the deck of the footbridge

Structural arrangementof the footbridge

Precast V-shape beambeing lifted to pier headby launching gantryPrecast V-shape beamfor the Route 3elevated expresswayat Kwai Chung

Precast box-girder for the RamblerStrait Bridge of Route 3

Forming the elevated bridge of the West Kowloon Expressway

West Kowloon Expressway –installing the precast box girder usingbalanced cantilever method with thehelp of track mounted gantry crane

Forming the elevated trackof the West Rail using theslung launching girder

Forming the elevated trackof the West Rail using thelongitudinal steel supportbeam

Forming the elevated track ofthe KCR Ma On Shan Lineby precast girder and lifted todeck level for erection usingmobile crane

Placing the precastgirder in position

Project example – Road T7 in Ma On Shan (TTD project)Technical Features1. Total length of viaduct – 2200m (approx.)2. Span length – Average 40m3. Total nos. of segment used – 790 nos.4. No. of segment per span – average 14 nos.5. Weight of segment – ranging from 50 to 98 tons6. Weight of the launching machine – 310 tons7. Max. weight of machine with segments – 1300 tons8. Length of launching machine – 85m9. Casting of the box-girder segment – match-cast

Construction of highway bridge using precast box-girder (viaduct) –Road T7 in Ma On Shan

Early stage of the project –formation and forming thepiers for the viaduct

Overall layout of the precasting yard

Forming the box-girder units

Formwork to form the innervoid of the box-girder units

Overview of thelaunching machine

Drawing provided byChina Harbour Engg.Co. (Group)Launching sequence of the box-girder units using launching machine

Installation of the box-girders

Installation of the box-girders

Other example of applicationin civil engineering works

Precast parapet unit for the highwayand railway road/track way

Precast floating slab forthe laying of railway track

Tunnel lining formedby precast segments

Detail of the lining asseen inside the tunnel tube

Forming the station platformusing precast curb and plank(KCR Tsim Sha Tsui Station)

Forming the cable hood

Delivery of the precast units

Forming the deck of the Ting KauBridge using precast concrete plankForming the deck of the berth forthe Container Terminal No. 9

Precast concrete plankas cover to podium deck

Precast vertical panelforming an earth-reinforcedtype retaining structure

Forming storm water discharge culvert – Tseung Kwan O Reclamation

Prepare for the foundationto support the culvertDelivery and storage alongthe temporary seawall

Precast units to form the 4mhigh 5-cell culvert system inTseung Kwan O

Positioning the Precast unitsonto the culvert alignment

Precast culvert in the formof semi-submerged tube –Tsuen Wan WestReclamation as advancedwork for the West RailTsuen Wan Station

Sinking of the precastculvert section

Positioning of the culvertsection onto to formed bed

Connecting the culvertfrom the precast sectionto the in-situ section

Precast box as footing to bridgetower – Tsing Ma Bridge toweron Ma Wan side

Precast box as pump housefor water cooling system inthe Central Reclamation

End of PresentationandThank youIf you require more information about other relatedconstruction technology topics, you can enterthe homepage of Raymond Wong athttp://personal.cityu.edu.hk/~bswmwong/