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LIFT-SLAB SYSTEMPrepared by: Dorothy P. CastroLift-slab construction was a revolutionary idea invented and developed in the early 1950s by a collaboration of Philip N. Youtz and Thomas B. Slick, resulting in what came to be known as the Youtz-Slick Lift-Slab Method of Construction.

Basically, the method entails casting floor and roof slabs on or at ground level and jacking them up into position. Flat plate floors are commonly used because they are so well suited to stack-casting, requiring for work at only the edges of the slab and at floor openings.

The HistoryOriginally , lift-slabs were reinforced with mild steel reinforcing, which limited the column spacing or required very thick slabs.With the advent of post-tensioning , however the column spacing was increased and the thickness of the slabs were reduced. Contemporarily, all lift-slabs are post-tensioned.Developments in the construction field have changed lift slab techniques over its 33 year history, increase use of pumping and prestressing has made cast-in-place flat plate work more efficient.

Photographs of what is believed to be one of the earliest, if not the first, lift slab structure constructed in the US.The traditional lift slab construction sequence as illustrated in the figure.Special lifting collars or share heads are provided in the slabs at the columns. Bond breaking compounds are applied between slabs to separate them. After the slabs have cured long enough to reach a prescribed strength powerful hydraulic jacks mounted on top of the columns lift the slabs into their respective positions. A console connected to each hydraulic jack synchronizes the number of turns of the check nuts to assure that the concrete slabs is being raised the same amount at all points.

AdvantagesThe big advantage of erecting concrete buildings using lift slab construction is elimination of most form work; only the sides need to be formed , an important factor in areas where labor cost are high.Lift slab can be used for heights upto about 16 stories. Economical column spacing ranges from 22 to 32 feet. Columns may be pipe, tubes or wide flange sections; concrete building columns may be used in 3 to 4 story buildings not requiring splices.Another advantage is reduced handling and hoisting of materials and supplies that can simply be placed on top of the slabs and lifted with them.There is little need for finishing the bottom of the slabs, since they will be as smooth as the floor finish of the slab below and thus the bottom of the slab can be used directly as a ceiling.The technique offers good fire resistance and good acoustic ratings. Mass designed into walls, floors and roofs helps to reduce the effects of daily temperature changes.

Limitations

The method has limitations too, the principal one being that buildings must be specifically planned for the same , or it will not have any economic advantages over conventional construction.

General ConsiderationsThe lift slab method of construction presents certain unique engineering considerations, during both the design phase and the construction phase of a project.

These considerations must be recognized and adequately addressed during the structural design, during the planning of the lift-slab operation, during the preparation of the shop drawings, and during the construction.

Structural engineering is required in all of these phases by various engineers employed by different organizations and with different responsibilities.

The principle of lifting the load

Main components of machine are the cylinders and two threaded winches between two steel beams. Winches are connected with screw bundle concreted into the ceilings, lower bridge of the machines is underpinned by steel pipes. When piston reaches the clearence, weight of lifted bundle is loaded over from upper bridge to lower bridge and upper bridge is let back to zero setting. (Nuts on winches are driven by cog-wheels, so that the winch gets into lifting position again.) Lifting is carried out in 10 cm steps so, that this way is also done in controlled parts.

Lifting jack

Side elevation of a building being constructed in accordance with with this technology

Side elevation showing an additional slab being constructed in preparation for raising.

The lifting process..Various Lift slab Systems of ConstructionThe Youtz-Slick System

The foundations are constructed and backfilled and then the slab on grade is constructed.

Openings are left in the slab on grade to permit the erection of steel columns which are then erected and plumbed.

In the Youtz-slick system a lifting collar is cast into each slab at each column. This collar provides a method to hook up lifting rods at each collar, so that the slab can be lifted, and a method to secure the lifting collar to the supporting column, either permanently or temporarily.

When the first tier of columns iserected, all the lifting collars for all the slabs to be lifted are installed over thecolumns of this tier and are temporarily suspended above the ground . The Lift-Plate System

The Lift-plate system differs from the Youtz-slick system in that a pair of jacks are mounted on each column, one on each side.

This feature makes it possible to have high tiers of many stories, up to about six stories.

The length of column above the jacks is unloaded and does not affect the stability of the column. Whereas a Youtz-slick lifting collar is one piece and needs to be threaded over the columns of the first tier, a Lift-plate lifting collars comes in two pieces which are bolted together after being put in place.

The lift-plate system is the same as the Youtz-slick system in regard to the construction of the foundations and slab on grade, and the casting and post-tendering of thee slabs to be lifted.

Floors and walls lifted at the same time. A new lift slab system has evolved in which concrete bearing walls are lifted simultaneously with the slabs. Concrete bearing walls are cast flat in the same stack with the slabs and attached to the slab with loops of plastic rope, forming hinges. As the slab is raised, each wall panel automatically unfolds into position. Since the walls are load bearing, there is no need for expensive steel columns or lifting collars as used in conventional lift-slab work. The steel columns used for erection are removed and reused elsewhere.

Walls of only 4-inch thickness are adequate becausethey have a long bearing length. The openings in the walls will be filled with masonry walls, precast panels or other curtain wall materials.The columns and bridges, reusable up to several hundredtimes, can be taken apart for easy transport to the nextjob. The absence of decking formwork shores, scaffolding,hoists and cranes further illustrates the simplicityof the lift slab operation.

Jacking equipment must be marked with themanufacturers rated capacity and must be capable ofsupporting at least two and one-half times the load being lifted during jacking operations and the equipment must not be overloaded. Such equipment includes, but is not limited to, the following:threaded rods, lifting attachments, lifting nuts, hook-upcollars, T-caps, shearheads, columns, and footings. Lift-slab operations must be designed and planned by aregistered professional engineer who has experience inlift-slab construction. Such plans and designs must beimplemented by the employer and must include detailedinstructions and sketches indicating the prescribed methodof erection. The plans and designs must also includeprovisions for ensuring lateral stability of the building/structure during construction.The Occupational Safety and Health Administrations standard for concrete and masonry construction Subpart Q,Concrete and Masonry Construction, Title 29 of the Code of Federal Regulations (CFR), Part 1926.700 through 706Sets forth requirements with which construction employers must comply to protect construction workers from accidents and injuries resulting from the premature removal of formwork, the failure to brace masonry walls, the failure to support precast panels, the inadvertent operation of equipment, and the failure to guard reinforcing steel.

Rules after collapse Under no circumstances shall any employee who is notessential to the jacking operation be permitted immediately beneath a slab while it is being lifted.Jacks/lifting units must be designed and installed so thatthey will neither lift nor continue to lift when loaded inexcess of their rated capacity; and jacks/lifting units musthave a safety device which will cause the jacks/liftingunits to support the load at any position in the event oftheir malfunction or loss of ability to continue to lift.No employee, except those essential to the jacking operation, shall be permitted in the building/structure while any jacking operation is taking place unless the building/structure has been reinforced sufficiently to ensure its integrity during erection. The phrase reinforced sufficiently to ensure its integrity as used in this paragraph means that a registered professional engineer, independent of the engineer who designed and planned the lifting operation, has determined from the plans that if there is a loss of support at any jack location, that loss will be confined to that location and the structure as a whole will remain stable.

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