Design Requirements for slip forms

Number one on the requirement list is a layout that remains typical from floor to floor. This will enable a form to be built not have to be modified to any great extent as the slips progress. If possible, the wall thicknesses should remain constant throughout the full height of the walls, although walls can be narrowed down by the use of filler panels inserted in the form. Sometimes the saving in concrete is more than offset by the additional labor spent in modifying the form, not to mention the lime lost. Minimum wall thicknesses may vary depending on whether lightweight or hard-rock concrete is used, but generally they should be at least 7 inches to avoid “pull ups” resulting from the friction between the form face and fresh concrete.

The reinforcing-steel design is the number-two item that will govern the efficiency of a slip form operation. Large concentrations of steel, such as found in spandrel beams and interior beams, should be kept to an absolute minimum, for these concentrations make it difficult, if not impossible, to place the steel while the slip is in progress. In some cases horizontal construction joints have to be located with this in mind. This also causes the workload of the iron workers to vary greatly, thus making it difficult to maintain an efficient crew size. If the slip is not to be a continuous one, then the vertical steel should be detailed so that it can be placed while the form is at rest. If continuous, a pattern of splice points satisfactory to the engineer should be worked out so that the vertical steel can be placed while the form is moving. Again, consideration should be given to providing as even a work load as possible for the iron-workers by establishing the pattern of splices so that only part of the steel has to be spliced at any one elevation. Horizontal steel for slab doweling. No. 5 bars or smaller, may be bent to lay along the face of the form and straightened out after the form has slipped by; or, if large in diameter, the bars can be welded. The ends of the bars to be welded are normally encased in a block-out that is set in the form and therefore exposed after the slip. If bars are to be welded, only weldable steel should be used. This should be settled before construction starts.

Where a slab occurs on both sides of the slip formed wall, slots through the wall are a cost-effective way to accommodate horizontal slab reinforcing steel that penetrates the wall. The slots are most easily formed by tying styro-foam blocks to the vertical reinforcing steel at block-out locations. The styro-foam is then easily removed from the wall by a combination of chipping it out and sandblasting. Placing a sand layer in the slip-form is an economical manner of achieving a continuous horizontal slot in the wall when such a configuration is required In order to achieve a satisfactory finished product, in this case a completed structure incorporating all required architectural details, care must be taken to ensure that all the items to be installed within or connecting to the slip-formed walls have designed in tolerances to ensure proper fit. This is no different from conventional construction, but the difficulty of setting work to an exact position from a moving deck accents this importance.

ACI Committee 1171 specifies that translation and rotation from a fixed point at the base of the structure for heights of 100 ft or less not exceed 2 inches and for heights greater than 100 ft, times the height but not more than 8 in. From this it can be seen that on tall buildings where the elevator shafts are of slip-formed concrete construction and the elevator door frame and silt remain in constant vertical alignment, the profile of the shafts in relation to plumb should be plotted so that the rail line can be set and fit properly with¬in the shaft. In this manner, the proper architectural detail is maintained at each floor. Using a plaster skim coat on the lobby side of the elevator walls allows more flexibility in setting the elevator frames to the exactness required. Differences in projection of the frames from the concrete wall can be adjusted for appearance by varying the thickness of the skim coal.

When framed openings are required in slip-formed walls, they can be handled in several ways. The first method, which can be used in the case of pressed steel or channel frames, is to set the frame in its final position similar to the way it would be in conventional construction, then place the concrete around it as the slip progresses. If this method is used, it is necessary to position the frame firmly by welding or some other means to make sure that it does not shift while the form is sliding. The second method is to provide a block out in the wall slightly greater than the out-to-out frame dimension and install the frame after the slip has moved out of the way. The third method is to provide over-sized block-outs in the walls and then set the frames after the slip has passed, and either grout them in or plaster around them. Any of the above-mentioned methods is satisfactory, but the method selected should be dependent on the architectural requirements of the Job. The cast-in-place frame method is least desirable.

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