Lift slab construction was a re vo l u t i o n a ry idea inthe early 1950s when it was developed by theYoutz-Slick company. Since then it has become abasic method of economical concrete constru c-

tion, especially for office buildings, apart m e n t s, park i n gg a ra g e s, hotels and other stru c t u res chara c t e ri zed byrepetitive framing from floor to floor.

What is lift slab construction?

Ba s i c a l l y, the method entails casting floor and ro o fslabs on or at ground level and jacking them up into po-sition. The traditional lift slab construction sequence isi l l u s t rated in Fi g u re 1. Flat plate floors are commonlyused because they are so well suited to stack-casting, re-q u i ring form w o rk at only the edges of the slab and atfloor openings.

Special lifting collars or shearheads are provided inthe slabs at the columns. Bond breaking compounds areapplied between slabs to separate them. After the slabsh a ve cured long enough to reach a pre s c ribed stre n g t h ,p owe rful hyd raulic jacks mounted on top of the columnslift the slabs into their re s p e c t i ve positions. A console

connected to each hyd raulic jack synchro n i zes the num-ber of turns of the check nuts to assure that the concre t eslab is being raised the same amount at all points.

Lift slab can be used for heights up to about 16 sto-ri e s. Economical column spacing ranges from 22 to 32feet. Columns may be pipe, tubes or wide flange sec-tions; concrete columns may be used in 3- to 4-storybuildings not re q u i ring splices.

The big advantage of erecting concrete buildings us-ing lift slab construction is elimination of most form-w o rk, an especially important factor in areas where laborcosts are high. Co n c rete floor construction at gro u n dl e vel is convenient and re q u i res no shore s, scaffolds orc ra n e s. Slabs can be cast and protected easily duri n gcold weather without expensive heating and enclosure sre q u i red for ord i n a ry construction. Another advantage isreduced handling and hoisting of materials and suppliesthat can simply be placed on top of the slabs and liftedwith them.

Because lift slab uses concre t e, the technique offersgood fire resistance and good acoustic ra t i n g s. Mass de-signed into walls, floors and roofs helps to reduce the

New developments inlift slab construction

BY FRANK A. RANDALL, JR.STRUCTURAL ENGINEER

Figure 1. The lift slab technique reduces costs for multistory buildings by eliminating most formwork. A typical liftingsequence is illustrated above.

effects of daily tempera t u re changes. Co n c rete has at h e rmal storage capacity that delays and reduces tem-p e ra t u re swings in response to solar radiation, outdoort e m p e ra t u re changes and indoor heating. In addition,c o n c rete flat plate floors and roofs reduce buildingc u b a g e, which lowers taxes and heat and wall costs.

An improved lifting procedure

De velopments in the construction field have changedlift slab techniques over its 33-year history. For onething, increased use of pumping and pre s t ressing hasmade cast-in-place flat plate work more efficient. Re c e n ti m p rovements in positioning of the jacks has bro u g h tf u rther adva n c e s. In the conventional system hyd ra u l i cjacks are mounted on top of the columns, limiting theheight of the columns and making it necessary to re-m ove the jacks before splicing on the next upper columnt i e r. The new approach allows columns to be erected astall as 6 stories without field splices. The jack is support-ed off the column by a welded plate that is later used tos u p p o rt the slab shearhead (Fi g u re 2).

Another refinement controls the amount of lift at eachcolumn. A steel tape runs from each column to a centra lsensing device in a console which monitors the re l a t i vem ovements and automatically operates the pumps,switching them off and on as necessary to keep the floorp e rfectly level as it moves upw a rd at approximately oneinch per minute.

Safety is provided by electrically dri ven nuts which fol-l ow the movement of the hyd raulic cylinders. If a gap de-velops above a nut, an alarm first goes off. When the gapg rows to more than 3/8 inch, the associated pump stops,in turn stopping the entire lifting operation. This assure sfall-back protection in case of hyd raulic failure.

Using these improve m e n t s, a firm in Fl o rida has con-s t ructed twenty 2- and 3-story buildings with re p o rt e dtime savings up to five weeks per building and cost sav-ings up to 20 percent, even though the roofs we re onlyf ramed with wood. The elevated concrete floors cost$3.45 per square foot, complete. Second floor slabs arelifted in one day, a job with second and third floor slabsre q u i res 3 days. The same firm has recently constru c t e da 350,000-square-foot, 6-story building for HUD andp roduces about one million square feet of lift slab build-ings per ye a r.

Floors and walls lifted at same time

A new lift slab system has evo l ved in which concre t eb e a ring walls are lifted simultaneously with the slabs. Al-though untried so far in the United St a t e s, this methodhas been employed by a contractor in Latin America forbuilding over 20,000 apartment units totaling 14 millions q u a re feet of space.

Co n c rete bearing walls are cast flat in the same stackwith the slabs and attached to the slab with loops ofplastic ro p e, forming hinges. As the slab is raised, eachwall panel automatically unfolds into position.

Since the walls are load-bearing, there is no need fore x p e n s i ve steel columns or lifting collars as used in con-ventional lift-slab work. The steel columns used for ere c-tion are re m oved and reused elsewhere.

Fi g u re 3 shows the construction pro c e d u re at seve ra ls t a g e s. In the background is a building already lifted; lit-tle finishing work is re q u i red. On the roof of the build-ing in the fore g round are the diesel pumping unit (theconsole is just out of sight on the ro o f) and two “balanceb e a m s” with tra p ezoidal ends show i n g .

The third - s t o ry floor slab is partially raised, lifting hav-ing been tempora rily halted. Note the automatically un-folding walls with their window openings cast in. Thel a b o rer on the second floor is cleaning up the sand andthin concrete coat which had filled the window openingwhen the slab immediately above it was cast at gro u n dl e vel. The only weight on the ground story walls at thistime is that of the second floor slab and half of the we i g h tof the unfolding walls above; the bulk of the weight of thebuilding is in the stack hanging from the jacks above.The crew on the ground, using a bottle jack and shore,lifts the second floor slightly at local points to allow final

Figure 2. Recent changes in lift slab construction includesupporting the hydraulic jack off the column by a weldedplate. The old approach used jacks mounted on top of thecolumns. Columns can now be up to 6 stories tall withoutfield splices.

plumbing of the ground story wall; the man at left is pry-ing the top of the wall panel for final adjustment. Thec a rt frame for relocating some internal walls is in theopening at the ri g h t .

Note that the columns have been stabilized by secur-ing them to the second floor slab. Winches on thecolumns are rigged to the bridge (truss) through pulleyson top of the columns. The bri d g e s, supported on shearpins through the columns, support the jacks; bars fro mthe jacks support the balance beams; each beam is at-tached to the stack of slabs by rods at its ends. Thin, 5-inch-thick, floors are made possible by short spans bothd u ring lifting and when in place.

Walls of only 4-inch thickness are adequate becausethey have a long bearing length. The openings in thewalls will be filled with masonry walls, precast panels orother curtain wall materi a l s.

The perimeter forms have already been stripped anda re probably in use on the next building being cast. Thecolumns and bri d g e s, reusable up to seve ral hundre dt i m e s, can be taken apart for easy tra n s p o rt to the nextj o b. The absence of decking form w o rk shore s, scaffold-ing, hoists and cranes further illustrates the simplicityof the lift slab operation.

Editor’s noteFor a more detailed description of the system for liftingfloors and wall simultaneously, see CONCRETE CON-STRUCTION, September 1981, page 717.

Figure 3. A lift slab system used extensively in LatinAmerica involves casting concrete bearing walls flat in thestack along with the floor slabs. The wall panels are hingedto the floor with plastic rope, allowing them to unfoldautomatically as the stack is raised into position.

P U B L I C AT I O N # C 8 6 0 11 5

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