A jack post (telepost, adjustable steel column) is a steel post used in the construction trades for temporary support of ceilings, walls and trenches (shoring). The term "jack post" is one of many describing the same system, other names include any assortment of "adjustable", "steel", "jack" and "shoring" with "post", "column" and "prop", depending on their precise construction - the "jack" refers to examples with a jack screw to adjust the precise length of the post. They are also known by any number of trade names, notably "lally column" and "Acrow prop". Jack posts are widely standardized.
Jack posts are telescopic tubular steel props consisting of two primary parts, the main part of the post, and the jack screw or other adjustable fitting on the end. Both ends are normally fit with flat metal plates on the end, providing additional support area. Most jack posts are split in two near the middle, with the upper end carrying the jack designed to slide within the lower portion. Gross adjustment for length is first made by pulling a pin and sliding the two sections within each other until they almost fill the gap, inserting the pin to lock them, then using the screw to close any remaining gap. Other designs used two threaded pipes instead of sliding sections, clamping sections, or other similar concepts to lock the system at a specific length.
Jack posts are mostly used for shoring: temporary supports during building repair or alteration work, rather than scaffolding: access platforms for workers. A typical use is to support an existing horizontal beam while its original masonry supports are removed or repaired. When masonry itself is to be supported, holes are first knocked through the brickwork and a strong 'needle' or 'strongboy' is placed through the hole. A pair of props are then used, one under each end. Existing windows or doorways may also be supported directly, or via needles. As the plates on the end of the posts are typically small, they offer little sideways support. If there is any sideways force, props should be strutted or 'laced' with scaffolding poles.
Acrow props are adjustable for height by a large diameter screw thread, formed on the outside of the outer tube itself. The screw thread provides a fine adjustment over a short range. A loose pin through a series of holes in the inner tube gives a wider range of coarse adjustment. Use of a screw thread also allows the props to be tightened when already in place, to adjust the load that each one bears.
A recent improvement to Acrow props was to shape this baseplate with notches, allowing pallet loads of horizontal props to be stacked neatly, rather than randomly piled.
Although the original Acrow prop was only intended for vertical support, the range has since been extended. Shoring 'push and pull' props with swivelling footplates are available and are used to support concrete formwork. Horizontal strutting props are used for shoring trenchwork. These are similar, but have a 'claw' form to their baseplates.
Acrow props were invented by Swiss-born William de Vigier, who came to London in 1935. At first traditionalism amongst builders preferred the established timber props, cut to fit for each job. The break for Vigier's adjustable prop's came when they were adopted by a few large builders, including Sir Robert McAlpine. By 1939, over 40,000 Acrow props were in use.
Vigier's company was named the Acrow Group, after his solicitor Mr A Crowe. He adopted this name as, like Kodak, it was an easily pronouncable neologism, with the additional advantage of appearing early in alphabetical lists.
Acrow props, their size, strength and construction, are now described by a British Standard BS4074: 1982. A similar lightweight European prop design exists, these have a maximum capacity of only 5 kilonewtons (1,100 lbf).
Props are made in a range of five BS standard sizes:
- size 0 from 3.5 feet (1.1 m) to 6 feet (1.8 m),
- size 1 from 5.75 feet (1.75 m) to 10.25 feet (3.12 m),
- size 2 from 6.5 feet (2.0 m) to 11 feet (3.4 m),
- size 3 from 8 feet (2.4 m) to 13 feet (4.0 m),
- size 4 from 10.5 feet (3.2 m) to 16 feet (4.9 m).
The safe load for each prop is a maximum of 35 kilonewtons (7,900 lbf), reducing slightly when props are fully extended, or by up to half when the load is not perfectly vertical.
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