Back Propping Annex M_BS5975

BS 5975 : 1996

Annex M (informative) Selection of propping and repropping procedures for multi-storey buildings

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M.2 Procedures Three possible site procedures, with their structural implications, can be identified as follows. a) Removal of props or other supports 1) where slabs are spanning between beams, the slabs both sides of a beam should be released before props under the beam are disturbed; 2) the removal of support to any single slab or beam should be undertaken in the following two stages, to avoid the risk of overloading any props: i)ease all props by about the same amount (e.g. one or two turns on an adjustable prop); ii)starting at mid span (or cantilever tip) and working towards the supports, remove the props. b) Repropping. After all the props and formwork at one level have been removed, an appropriate number of props should be reintroduced and adjusted to give a solid contact with the concrete above and below; they should not, however, be deliberately extended enough to put a significant load into them. Provided the props are carefully erected, and the end plates are in direct contact with the slabs, it may be possible for them to carry higher loads than those given in 3.9.6. However, it is recommended that the loads imposed on props complying with the requirements of BS 4074 should not exceed 35 kN. The load reduction due to the removal of formwork is normally disregarded in propping calculations. Repropping should, where possible, be in alignment with the props above. Where a prop at the level above is not directly over a reprop, the effect on the slab should be considered. c) `Quick-strip method'. This method may be offered as an optional facility with proprietary support equipment. With this method, the structural props need to remain undisturbed (and hence loaded) while the forms and their immediate supports are removed. It is necessary to check that the slab can span between the structural props before the formwork is removed.

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M.1 General In multi-storey reinforced concrete construction, it is usually necessary for several slabs to contribute to the support of the last one to be cast. The supporting levels will generally not have reached their twenty-eight day design concrete strengths, and will be of differing ages (and therefore stiffness). Furthermore, the deflections of maturing reinforced concrete members under a sustained load at an early age are not easy to predict. With the tendency towards both faster construction cycles and the use of larger spans (in which the design live load is decreased relative to the dead load), the importance of the selection and control of propping procedures increases. In the case of post-tensioned prestressed concrete construction, the tensioning operations will affect load distribution, and special consideration needs to be given to the propping procedure. In simple cases where a single floor does not have sufficient strength to support a further slab of fresh concrete and progress is not very rapid, stripping and repropping may be undertaken so that two floors support the new floor. Table M.1 shows a method of analysing this approach. For cases where props are left undisturbed table M.2 gives an analysis. Table M.3 is appropriate when a further level of repropping is in use. Where it is possible to strip props one at a time and replace them without allowing any deflection of the slab above to occur, they may be considered as undisturbed (see tables M.2 and M.3). These analyses are based on the following simplified assumptions: a) creep and shrinkage of the concrete can be disregarded; b) the footings beneath the props to the first suspended level are taken as rigid; c) all props are rigid (i.e. all levels connected by props are subject to the same deflections); d) all suspended slabs are equally stiff. Some confirmation of figures calculated in this way has been obtained from site measurements, but both assumptions b) and c) tend to underestimate the load carried by upper slabs.

Annex M

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Annex M

M.3 Examples Three examples of the calculation of propping loads are given in tables M.1, M.2 and M.3. In each case, the load W represents the weight of the reinforced concrete plus the formwork and falsework, which is sometimes taken as 10 % of the concrete weight. An additional allowance may be made for any known imposed loads (see 4.4) but for simplicity, the calculation of these is not shown in these examples. Two useful arithmetic checks that can be made on calculations in these and similar examples are: a) the total slab loads (in the right hand columns) at each stage should equal the total weight of slabs being considered; b) at each slab level, the prop load acting from above plus slab weight should equal the load transmitted by the slab to the support plus the prop support load from beneath.

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BS 5975 : 1996

It is important that the first lift of props on a non-suspended slab is not left in too long, as the total weight of all the slabs constructed will act on them. Once they are removed, slabs can deflect and hence contribute to the support of loads. Where a particular system of propping and repropping is considered for the construction of a large number of similar slabs, the loads on the slabs converge on specific values, and some general conclusions may be drawn. 1) The most heavily loaded slab is the one under the lowest set of props (not reprops) and its load will not exceed 2.5W. 2) The effect of increasing the number of propped levels is to allow a slab more time before it carries its maximum construction load, but not to modify that load significantly.

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Table M.1 Two lifts of propping with repropping

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Annex M

BS 5975 : 1996

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Table M.2 Two lifts of propping without repropping

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Table M.3 Two lifts of propping plus one lift of repropping

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NOTE. To this stage, propping procedure repeats stages I to V of table M.2.

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BS 5975 : 1996

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Table M.3 Two lifts of propping plus one lift of repropping (concluded)

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