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COLLEGE OF ARCHITECTURE AND PLANNINGDepartment of Architecture and Building Sciences

Dr. Mohammed Ghonim

5. LIFT-SLAB METHOD

ARCH 436Contemporary Building Construction Methods

Lecture ObjectivesUpon completion of this lecture, the student will be able to:

1. Explain the concept of lift-slab construction method.

2. Discuss the advantages of lift-slab method.

3. Determine the tools & materials required for this system.

4. Describe the process of lifting the slabs.

4. Draw sketches for lift-slab details.

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Lecture Content

• Introduction.

• Historical Brief.

• Advantages of lift-slab Construction Method.

• Sequence of Lifting Slabs.

• Construction Sequence.

• Tools & Materials: Hydraulic Jacks.

• Tools & Materials: Lifting Collars.

• Tools & Materials: Shear Blocks.

• Tools & Materials: Bond Breakers.

• Details.

• New Developments in Lift-slab Construction.

IntroductionLift slab construction is a method of constructing concrete buildings by casting the floor orroof slab on top of the previous slab and then raising the slab up with hydraulic jacks, sobeing cheaper and faster as it does not need forms & shores as it is needed for cast-in-placeslabs.

Lift-slab construction can be more economicthan conventional construction when thebuilding is vertically uniform, such as forhotels, apartment buildings, and dormitories,and where the slab designs are repetitive.However, lift-slab buildings have certainlimitations that need to be recognized earlyin the planning process, thus, for thetraditional organization of a constructionproject. where the owner retains an architectto design a building.The architect should retain his structuralengineer early in the design process andboth should contact a lift-slab contractor fordesign advice at that time. When the ownercontracts with a firm to both design andconstruct a project, the firm should alsocontact a lift-slab contractor for early advice.

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Historical Background

Johnstone Hall, a Clemson University dormitory in Clemson, South Carolina, was erectedusing this method in 1954. Lift slab construction was also involved in the L'AmbiancePlaza collapse in Bridgeport, Connecticut, in 1987, and resulted in a nationwide federalinvestigation into this construction technique in the United States, and Connecticut imposeda temporary delay on lift slab construction.

L'Ambiance Plaza, Bridgeport.

Lift-slab construction is a method ofconstruction where the concrete floorsand roof slabs are cast atop one anotherat ground level and then are lifted intotheir final position by means of jacks.The method was invented and developedin the early 1950s by a collaboration ofPhilip N. Youtz and Thomas B. Slick.resulting Youtz-Slick Lift-Slab Method ofConstruction.

The Advantages of Prestressed Concrete

• Lift-slab construction method becomes more advantageous in buildings with similar floorplans throughout the height of the building and where flush slab may be desired.

• This method eliminates the need for redundant formwork as only shuttering required onthe edges, therefore casting concrete slabs is the simplest stage in whole constrictionprocess of lift-slab method.

• Lift-slab method may be employed with ribbed slabs not only flat slabs with somecompromise of the ease of casting.

Cast-in-place concrete slabs requireextensive formwork unlike lift-slab method

The advantage of this system is that the onlyformwork required is to the edges of the slabs andno centering is required to the soffit of the slabs.Also, the slabs are cast monolithically and can bedesigned to span continuously between the points ofsupport and so employ the least thickness of slab.Where it is convenient to cantilever slabs beyond theedge columns and where cantilevers for balconies.This system produces a structure where a flush slabfloor is most appropriate and for this type ofstructure the lift slab construction may well becheaper than any other form of structure.It can be employed for beam and slab or waffle slabfloors, but the forms necessary between each takemuch of the advantage of simple repetitive casting atground level.

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Sequence of Lifting Slabs1. The sequence of lifting slabs is influenced by the following

factors:• Weight of the slabs.• Height of the building.• Lifting capacity of jacks.• Cross sectional area of columns during initial lifting.

Sequence of lifting the slabs of a three-storey building.

Sequence of Lifting Slabs

Sequence of lifting the slabs of a four-storey building.

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Sequence of Lifting Slabs

Sequence of lifting the slabs of a five-storey building.

Construction Sequence1. The steel or concrete columns are first put position and rigidly connected to the

foundation then the ground floor slab is cast.2. When it has matured it is sprayed with two or three coats of a separating medium

consisting or what is called bond breaker.

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Construction Sequence3. Lifting collars are cast into each slab around each column.4. The first floor slab is cast inside edge formwork on top of the ground floor slab and when

it is mature it is in turn coated or covered with the separating medium and next floor slabin cast on top of it.

5. The casting of the other slabs continues until all the floors and roof have been cast oneon the other on the ground.

Construction Sequence5. The slabs are lifted by jacks , operating on the top of each column, which lift a pair of

steel rods attached to each lifting collar in the slab being raised.6. A central control synchronizes the process for a uniform lift from all directions.

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Tools & Materials

Hydraulic JacksThis jack is a hydraulic piece of equipment which has positive safety devices on it. The jackcan lift slabs on columns loaded up to 100,000 pounds at speeds of up to 14 feet an hour.

Tools & Materials

Lifting CollarsLifting collars are cast into each slab around eachcolumn providing a means to lift the slab and alsoproviding shear reinforcement, they are fixed tocolumns by welding shear blocks to plates weldedcolumn flanges and to the collar after the slab hasbeen raised in position.

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Tools & Materials

Shear BlocksShear blocks is a steel component used to hold the lifted slab in its final elevation.

Tools & Materials

Bond BreakersThe main function of bond breakers is to minimize dynamic loads during lifting or stripping ofprecast members and permit their complete, clean separation from casting slabs or molds.In lift-slab construction bond-breaking compounds permit the slabs to be separated cleanlyand easily from one another.Bond breakers include wax dissolved in a volatile spirit, polythene sheet or building papermay also be used as an alternative.

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Details

Lift-slab construction method used in acircular building

Slabs may be prestressed

Cross bracing may be applied to overcome the systemweakness against lateral forces.

Slabs may be divided into parts to facilitate lifting control.

New Developments in Lift-slab Construction

A lift slab system used extensively in Latin Americainvolves casting concrete bearing walls flat in thestack along with floor slabs. The wall panels arehinged to the floor, allowing them to unfoldautomatically as the stack is raised into position.

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Summary• Lift slab construction is a method of constructing concrete

buildings by casting the floor or roof slab on top of theprevious slab and then raising (jacking) the slab up withhydraulic jacks, so being cheaper and faster as it does notneed forms & shores as it is needed for cast-in-place slabs.

• This method eliminates the need for redundant formworkas only shuttering required on the edges, therefore castingconcrete slabs is the simplest stage in whole constrictionprocess of lift-slab method.

• Lift-slab construction method becomes moreadvantageous in buildings with similar floor plansthroughout the height of the building and where flush slabmay be desired.

• The sequence of lifting slabs is influenced by the followingfactors: weight of the slabs; height of the building, liftingcapacity of jacks, cross sectional area of columns duringinitial lifting.

• Slabs are lifted by jacks, operating on the top of thebuilding columns, this jacks lift a pair of steel rodsattached to each lifting collar in the slab being raised.

• This system requires a central and accurate control toprovide a uniform lift from all directions.

Lecture ActivityEach student is required to prepare a brief but informative internet-based research about one of the topicsrelated to this lecture (Lift-slab Construction); i.e., new developments, successive and local case studies orimportant details. The research should be presented in only one A4 sheet.

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References

Allen, EA. (2009) Fundamentals of building construction materials and methods, John Wiley & Sons.

Ching, Francis (2008) Building Construction Illustrated, John Wiley & Sons. Emmitt, S.& Gorse, C. (2006) “Barry’s Advanced Construction of Buildings”, Blackwell Publishing.

Mehta, Madan et. al. (2013) “Building Construction”, Pearson, USA. Randall, Frank (1986) “New developments in lift slab construction” http://en.wikipedia.org.

http://nptel.ac.in/courses/105106117/pdf/1_Introduction/1.2_Advantages_Types_of_Prestressing.pdf http://www.britannica.com http://youtube.com