112 BSI 1996

BS 5975 : 1996 Annex M

Annex M (informative)

Selection of propping and reproppingprocedures for multi-storey buildings

M.1 General

In multi-storey reinforced concrete construction, it isusually necessary for several slabs to contribute to thesupport of the last one to be cast. The supportinglevels will generally not have reached theirtwenty-eight day design concrete strengths, and will beof differing ages (and therefore stiffness). Furthermore,the deflections of maturing reinforced concretemembers under a sustained load at an early age arenot easy to predict.

With the tendency towards both faster constructioncycles and the use of larger spans (in which the designlive load is decreased relative to the dead load), theimportance of the selection and control of proppingprocedures increases.

In the case of post-tensioned prestressed concreteconstruction, the tensioning operations will affect loaddistribution, and special consideration needs to begiven to the propping procedure.

In simple cases where a single floor does not havesufficient strength to support a further slab of freshconcrete and progress is not very rapid, stripping andrepropping may be undertaken so that two floorssupport the new floor. Table M.1 shows a method ofanalysing this approach. For cases where props are leftundisturbed table M.2 gives an analysis. Table M.3 isappropriate when a further level of repropping is inuse. Where it is possible to strip props one at a timeand replace them without allowing any deflection ofthe slab above to occur, they may be considered asundisturbed (see tables M.2 and M.3). These analysesare based on the following simplified assumptions:

a) creep and shrinkage of the concrete can bedisregarded;

b) the footings beneath the props to the firstsuspended level are taken as rigid;

c) all props are rigid (i.e. all levels connected byprops are subject to the same deflections);

d) all suspended slabs are equally stiff.

Some confirmation of figures calculated in this wayhas been obtained from site measurements, but bothassumptions b) and c) tend to underestimate the loadcarried by upper slabs.

M.2 Procedures

Three possible site procedures, with their structuralimplications, can be identified as follows.

a) Removal of props or other supports

1) where slabs are spanning between beams, theslabs both sides of a beam should be releasedbefore props under the beam are disturbed;

2) the removal of support to any single slab orbeam should be undertaken in the following twostages, 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) andworking towards the supports, remove theprops.

b) Repropping. After all the props and formwork atone level have been removed, an appropriatenumber of props should be reintroduced andadjusted to give a solid contact with the concreteabove and below; they should not, however, bedeliberately extended enough to put a significantload into them.

Provided the props are carefully erected, and theend plates are in direct contact with the slabs, itmay be possible for them to carry higher loads thanthose given in 3.9.6. However, it is recommendedthat the loads imposed on props complying with therequirements of BS 4074 should not exceed 35 kN.

The load reduction due to the removal of formworkis normally disregarded in propping calculations.

Repropping should, where possible, be in alignmentwith the props above. Where a prop at the levelabove is not directly over a reprop, the effect on theslab should be considered.

c) `Quick-strip method'. This method may be offeredas an optional facility with proprietary supportequipment.

With this method, the structural props need toremain undisturbed (and hence loaded) while theforms and their immediate supports are removed.

It is necessary to check that the slab can spanbetween the structural props before the formwork isremoved.

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M.3 Examples

Three examples of the calculation of propping loadsare given in tables M.1, M.2 and M.3. In each case, theload W represents the weight of the reinforcedconcrete plus the formwork and falsework, which issometimes taken as 10 % of the concrete weight.

An additional allowance may be made for any knownimposed loads (see 4.4) but for simplicity, thecalculation of these is not shown in these examples.

Two useful arithmetic checks that can be made oncalculations in these and similar examples are:

a) the total slab loads (in the right hand columns) ateach stage should equal the total weight of slabsbeing considered;

b) at each slab level, the prop load acting fromabove plus slab weight should equal the loadtransmitted by the slab to the support plus the propsupport load from beneath.

It is important that the first lift of props on anon-suspended slab is not left in too long, as the totalweight of all the slabs constructed will act on them.Once they are removed, slabs can deflect and hencecontribute to the support of loads.

Where a particular system of propping and reproppingis considered for the construction of a large number ofsimilar slabs, the loads on the slabs converge onspecific values, and some general conclusions may bedrawn.

1) The most heavily loaded slab is the one underthe lowest set of props (not reprops) and its loadwill not exceed 2.5W.

2) The effect of increasing the number of proppedlevels is to allow a slab more time before it carriesits maximum construction load, but not to modifythat load significantly.

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BS 5975 : 1996 Annex M

Table M.1 Two lifts of propping with repropping

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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

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

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

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