In this post, Charles J. Tillotson elaborates on Neil A. Lieb’s previous comments, describing the above photo from his archive. The jack rods referred to in the text are the tall, slender steel poles seen throughout the photo.
They often say, a picture is worth a thousand words and this one fits the bill perfectly. The photo is truly an aid to describing the method of slipform construction that was used in grain elevator construction. Neil mentions the one-handed placement of the jack rod, so I’ll start with that.
Slipform construction is made up of many complex disciplines which have to all work together in order to provide the final poured-in-place concrete product.
As mentioned prior to this, the slipping of the formwork used in this type of construction was provided by a series of screw jacks placed apart by an engineered calculation sufficient to lift each jack’s portion of the formwork assembly.
Each screw jack was supported by a wooden, U-shaped yoke, the legs of which were attached to the vertical concrete formwork. Inserted in the top (or horizontal) portion of each yoke was a screw jack (similar to that used in jacking building foundations). A smooth one-inch jack rod was then inserted into the top head of the jack and threaded down through it until stopping at the foundation slab.
A series of horizontal wooden rails at about waist height (looks like a railroad track) were then built directly above the open formwork, the “ties” of which were placed at prescribed intervals and used as a template spacer for inserting the actual vertical reinforcing steel. (See the small, half-inch rebar rods extending vertically out of the open bin forms at each cross tie). The vertical rebar was staggered slightly in an alternating fashion so as to allow the half-inch horizontal rebar to be threaded through the vertical rebar. On the vertical 2x4s that are attached to the exterior side of the formwork and rise above the entire deck assembly, so-called targets placed on their tops were used in leveling the deck in order to provide a final elevator that rose plumb and straight above the foundation.
As the screw jacks were turned (each jack was turned the same amount), the foreman on deck used a leveling instrument and sighted on each target to insure that the formwork was rising true plumb and level. If any of the targets did not align with true level, the portion of the deck out of plumb was corrected by extra turns of the screw jack or jacks as necessary to bring that portion of the deck up level with the rest of the formwork.
Not shown in the photo is the horizontal rebar that was required to form a steel reinforced grid integrally cast in the concrete to form a reinforced concrete structure. Initially, the horizontal steel was wire-tied in place to the vertical rebar prior to one side of the forms being installed. This placement occurred only to the height of the wood bin forms. Once the form-lifting began, the horizontal steel was placed by hand by pushing and threading the rebar horizontally through the vertical rebar. Because of the vertical movement of the formwork, close attention was required as to the spacing between horizontal rebar.
Now, try to imagine: the start gun is fired and the continuous action of the many processes begins, never to stop until the wooden forms and finished structure reaches the prescribed vertical height (some 120 feet) eight days later. Manual labor is involved in each discipline. Personnel changes occur, but each position is filled by a replacement. The gun is fired, cement is mixed and lifted to the deck of the formwork via a Georgia buggy, and the content is dumped into the open form. The pouring of the cement into the formwork is continued in a circular fashion around the entire deck until it reaches a prescribed height in the form.
Once the cement is allowed to solidify in the forms on the foundation slab, the jacking operation begins and the formwork starts its vertical lifting and slipping process. The jacks are turned, the cement is poured, the vertical rebar and jackrods are placed and spliced, and all the while the horizontal rebar is positioned at the proper height and spacing. Pour cement, turn jacks, place rebar, check deck level, and on and on through night and day until the construction reaches final height. The most problematic aspect of this system is the placing of the horizontal steel at the correct spacing, the placement of formed openings in the bins, keeping the hoist in operation, mixing the cement, and obtaining enough set time of the cement mixture so that as the finished concrete walls do not fall apart or slough off.
Also, hanging beneath the formwork structure is the scaffolding for the cement finishers who dutifully serve to patch and smoothly finish the concrete surfaces appearing at the bottom of the vertically slipping formwork.