August_1966 — Page 65

Collars, reinforcing and conduits in place prior to slab pouring

slabs are lifted, thus facilitating lay- projecting, are called "pour strips" out work.

and are cast in place after all slab sections have been raised and fixed in place.

After all the slabs are cast and the roof slab allowed to attain requir- ed strength, the machinery for the lifting operation is set up. Hydraulic jacks are placed at the top of each column. These jacks are connected to the slabs by threaded rods which pass down along the columns into slots in the collars.

They are also connected to a con- trol console by hydraulic lines. The equipment is designed so that all column jacks are simultaneously and uniformly controlled from this single console.

Slowly the slabs are raised into position, the normal rate being about 4 to 10 ft. per hour. The cor- ner jacks operate alone at first so that the slab is raised at these points to allow air in between to break the bond with the adjoining slab. When the slab has reached its proper posi- tion, it is locked into place, and after plumbing and levelling the structure the lifting collar is welded to the column.

The lifting sequence which in- volves the height to which the first and then subsequent slabs should be lifted, as well as temporary holding points is determined in advance. This sequence is individually adapted to each job, depending on the num- ber of slabs to be raised. the height at which column loads are best dealt with, and even cost considerations.

In general, with a two-storey build- ing the roof and second floor are lift- ed together, the second floor fixed in position, and then the roof position- ed. With a multi-storey building, the roof is usually moved into position first, and then the remaining slabs are lifted singly, in pairs, or even three at a time.

In very large buildings calling for particularly large floor slabs. the slabs are divided into sections with gaps of 1 to 4 ft. between them. These gaps, with reinforcing steel left

Far East Architect & Builder August, 1966

Materials to be used on floors can be placed on slabs and lifted with them. These include masonry and plastering materials. plumbing fix- tures and ducts. This eliminates the need of a tower hoist for bulky or heavier items.

Evaluation

In considering the lift-slab techni- que for any given construction job. the first thing to take into account is the size of the job. Since there is a basic cost for transporting and setting up the elaborate lifting equipment re- quired, it may not be economical for smaller structures i.e., slabs with fewer then eight columns. The cost

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of lightweight concrete is another fac- tor which must be considered. In evaluating the economy of lift slab its cost should be compared with that of other methods.

Although cost would probably be a major factor in any decision, the following are distinct advantages of lift slab:

1. Speed slabs can be rein-

forced, cast, finished, cured and erected into final position far more quickly at ground level than when men and materials have to be lifted.

2. Reduction of formwork

this

not only saves money, time and effort, but leaves the con- struction site free and clear for continuing operations.

3. Quality of concreting

since

all slabs are cast and finished on the ground and the smooth finish of one floor provides the form for the ceiling of the next, the finished surface of both floors and ceilings of lift- slab buildings are of high quality.

4. Convenience

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it is easier, fas- ter and more economical to carry out the major construc- tion procedures on the ground, such as the concreting opera- tions, installing of plumbing and electrical devices.

No evaluation would be complete without consideration of possible dis- advantages. One might be consider- ed the span limit for ordinarily rein- forced slabs. However, the prestress- ing of slabs can overcome this limita- tion to a great extent and eliminate the need for separate sectional slabs.

Since the structure is loosely con- nected until all welding and bracing is completed special care must be taken to temporarily brace against the toppling effect after one or two slabs have been lifted.

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Lifting starts after slabs have been placed and cured at ground level

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