Typical collar used in lift-slab construc- tion, with column in place
cleared and graded earth or the isol ated concrete footings that support the columns. In either case, the foundation is designed to meet the
construction conventional ments for load bearing.
are
are
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essentially free-standing. being anchored only at their base. Moreover, the jack placed at the top for lifting the slabs up the column is applying its own additional load.
Under certain conditions therefore the columns are braced at the top be- fore the roof is lifted.
In some lift-slab construction the columns may be only temporary, and are removed after the bearing walls are erected for support.
Collars. The lifting collar is a connecting device that fits around the column and secures it to the concrete slab in which it is embedded. It is made of cast or fabricated steel and has slots through which the lifting rods of the hydraulic jack are con- nected.
There is a lifting collar at each column for each slab to be raised. The collars require-
are threaded
the columns at the time the latter are erected.
If separate footings are to serve as the foundation, with no connecting floor slab, these
checked for stability against overturning, since the lift slabs ride up along the columns these footings support.
Where the ground floor slab is to serve as the foundation as in garages. it is essentially a form for the lift slabs and so must be carefully finish- ed and cured to the degree of smooth- ness desired for the ceiling. Even in office and apartment buildings which usually have suspended ceil- ings a smooth slab is needed for final flooring material.
Columns. Columns may be either of precast or site-cast concrete or of steel. Their number and location will depend on the architect's design for the floor plan, The outside columns may be placed at the peri- meters of the structure or, preferably, set in for a cantilevered effect since this reduces maximum bending mo- ments by producing a continuous beam effect in the slabs,
Since the columns-along with hydraulic jacks mounted on top supply the means by which the entire structure is raised, special attention must be given to their anchorage, stiffness, lifting capacity, ability to withstand wind stresses and, above all, the stresses of the lifting operation itself. Usually if the column is adequately designed for final condi- tions it is capable of handling the lifting loads.
As for the column itself, the main purpose is to obtain a design that will adequately fulfil the load-bearing re- quirements of the completed building, as well as temporary stresses induced by the lifting operation.
This may call for steel or for reinforced or prestressed concrete. Once the pro- per column design is obtained, with the need for strength and stiffness to withstand lateral as well as vertical stresses taken into account, the columns are then cast, set up with special lifting collars and plumbed.
The most critical stage for the columns during the lifting_cycle comes at the beginning when the roof slab is being raised. At this point
on
Care must be taken during con- struction of the slabs that the space between collars and columns and the open slots of the collars are kept free of concrete, which could inter- fere with the lifting equipment. To keep these spaces free from clogging. a caulking compound, rags, paper or any other suitable material is used as a temporary filler. This material is cleaned out of the collar openings just before the lifting equipment is to be installed.
After the floor slabs are in place, and the structure plumbed and levell- ed, the collar devices are welded to the columns. Until this is done the slabs are supported temporarily with wedges and cotters.
Slabs In slab construction one of the chief advantages of the lift-slab method is realised: all reinforcing, installing of embedded items such as the connecting collars and electrical and plumbing sleeves and inserts, and the concreting operation itself is done at ground level. This means that ready-mix trucks can roll right up to the edge of the slab area and place
the concrete for the entire structure directly from the chute.
The first essential step in ordinary
Console for controlling jacks and timing the lifting sequence
slab construction (to follow the positioning of lifting collars around the columns) is the placing of the extra reinforcement to help withstand the stresses of lifting. The rest of the reinforcement is arranged in about the same way it would be for conventional slab work.
Since each slab serves as a form for the one placed directly on top of it, the simple flat plate is most often used. Variations have been success- fully introduced, however, in the form of waffle or ribbed patterns.
Minimum slab thickness is usually 6 in. and maximum span length about 28 ft. Prestressing of the slab can in- crease the span length somewhat.
Slabs are generally divided into several sections because the slab length is limited. This permits the construction crew to divide its opera- tions and work can continue with reinforcing or placing on some sec- tions while others are curing.
Careful finishing of lift slabs with a machine trowel is particularly im- The portant, as is proper curing. time period allowed for curing each slab need be no longer than for ordinary slab work.
For speed of operation a curing compound is usually applied. These resinous type compounds will also act as bondbreakers and hardeners. If a non-curing type separator is used, the slab is properly cured before the separator is applied. This compound may be brushed, sprayed or rolled on to the concrete surface- with spraying preferred. It usually dries in from 20 to 40 minutes so that work may proceed on the next slab as soon as a minimum strength of 1,000 p.s.i. is attained usually in about 24 hours.
Lifting Sequence
Concrete for the new slab is placed directly over the concrete just treated with no danger of adhering where the separating compound has been pro- perly applied. Chalk line marking of partitions, doors and other locations the separating compound will appear on floor and ceiling when
on
Typical lifting jack which is placed on top of each column
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Far East Architect & Builder August, 1966
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