Example using Method One and Three of backpropping load calculations 205mm thick RC Slab - Four floors - One set of Formwork
One / Two sets of backprops
DESIGN BRIEF Output
To carry out the backpropping calculations only, on a FOUR storey insitu reinforced concrete flat slab building with columns on a regular grid.
The permanent works drawing details a solid 205mm thick RC slab.
Each slab will be cast and allowed to take up its deflected shape and become self-supporting prior to further construction of subsequent floors. The effects of not doing this are highlighted in the calculations.
Assumptions
1) Assume a concrete density of 24 kN/m3. (Section 4.3.1) 2) Ignore the self weight of the formwork.
3) Any screeds to be added to the concrete floor are for future work and not included in these calculations.
4) Use only ONE set of formwork/falsework for slab construction.
5) A maximum of TWO sets of backprops could be made available.
6) Where a backprop is preloaded in position, it is assumed to be pre-loaded so that it imposes an upwards and downwards load of 0.50 kN/m2 to the slab above and below, respectively.
7) The slab that carries the falsework is known as “supporting slab”.
8) The completed slab behaves elastically under applied load.
9) An agreed procedure for striking and backpropping will be established following the outcome of these calculations.
Method
The calculation method used is the Method One tabular method of calculating backpropping loads given Table 34 at Section 5.4.2.3. The output of the calculations is expressed in load per unit area (kN/m2).
Certain parts of the calculation give additional information using the equations from Method Three, reproduced at page 48.
This example is in two parts so that the effects of applying
construction operation loading on floors and preloading backprops are fully appreciated. The two parts are:-
PART ONE: Assumes that there will be a construction operation load on every floor at all stages of construction, and that the backprops are inserted hand tight.
PART TWO: Assumes that there are no construction operation loads on general floors, but allowance is made for loads during placing of concrete. Backprops are inserted preloaded.
LOADINGS {NOTE: the additional transient concrete load (Section 4.3.2.4) of 0.75kN/m2 is not considered in the backpropping calcs.}
Construction operation load (c.o.l) on any floor (Service Class 1 ) = 0.75
w.a.l.
0.75 kN/m2
Hence although the c.o.l. design load for the falsework will be the working area load plus transient concrete load (1.50 kN/m2),for
backpropping calculations it can be reduced to either 0.75 kN/m2 or be regarded as nil. See research and CS 140 Flat Slab Guide.
c.o.l 0.75 kN/m2 Concrete strength
The permanent works designer has completed the design based on an applied service load of 8.00 kN/m2 and a specified concrete strength.
Striking Criteria
If a distributed load on a slab is less than the specified design load, then the required concrete strength for striking the slab at that stage may be reduced from the specified strength by a corresponding amount.
See Section 5.3.6.1
PART ONE
- Calculations including c.o.l. and hand tight backprops.Calculations and Notation
Storey heights are diagrammatical. (M1) is Method One. (M3) is Method Three.
6.50 Load in a prop or in falsework. concreting a slab 10.08 Load in a slab above the design load.
Stage Operation Slab Level
Loads (kN) per square metre Load in
props
Load in floor slab(s) Existing Added TOTAL
5.1 Erect falsework and pour slab
Part One (c.o.l. and hand tight backprops) continued.
Stage Operation Slab
Level Sketch of arrangement
Assumed
% of wp
transfer
Loads (kN) per square metre Load in
props
Load in floor slab(s) Existing Added TOTAL
5.3
Slab 1 loaded 20% ABOVE design load
5.4
2 Removing the backprops reverts the total load in ground slab to 0.00kN/m2, i.e.a reduction of
Part One (c.o.l. and hand tight backprops) continued.
3 The assembly now has to carry the removed 0.52kN/m² between the three floors. As all the Stage Operation Slab
Level
Loads (kN) per square metre Load in
props Load in floor slab(s)
Part One (c.o.l. and hand tight backprops) continued and Part Two.
Notes to Part One calculations:
1) Several “What If?“ scenarios have been included at stages 5.4(a), 5.5(a) and 5.6(a) as explanation of the complexity of backpropping and to highlight the use of correct procedures.
2) The likely maximum load on the falsework supports is 5.75kN/m2. For design, this load should be increased by the application of the transient insitu concrete load of 0.75 kN/m2.
3) The maximum total construction load on a slab occurs at stage 5.5(b) of 9.95 kN/m².
4) The lowest supporting slab is often overloaded to above its design service load, and this fact should be made known to the Permanent Works Designer.
PART TWO
- Calculations with preloaded backprops, including working area loads (w.a.l.) but ignoring construction operation loads on floors.Calculations and Notation (Storey heights are diagrammatical)
(M1) is Method One (M3) is Method Three 6.50 Load in a prop or in falsework concreting a slab 10.08 Load in a slab above the design load
Stage Operation Slab Level
Loads (kN) per square metre Load in
props
Load in floor slab(s) Existing Added TOTAL
5.10 Erect props and
Part Two (no c.o.l. and preloaded backprops) continued.
Stage Operation Slab
Level Sketch of arrangement
Assumed
% of wp
transfer
Loads (kN) per square metre Load in
props
Load in floor slab(s) Existing Added TOTAL
5.13
Slab 1 loaded ABOVE design load
5.14
5 When c.o.l. is ignored on the completed floors, it is reasonable to allow at least the working area load of 0.75 kN/m2 on the formwork/falsework as the transient insitu concrete load is not present.
Part Two (no c.o.l. and preloaded backprops) continued.
Slab 2 loaded 2% ABOVE design load
5.18
5.19 Move backprops up from O-1 to Stage Operation Slab
Level Sketch of arrangement
Assumed
% of wp
transfer
Loads (kN) per square metre Load in
props
Load in floor slab(s) Existing Added TOTAL
Part Two (no c.o.l. and preloaded backprops) continued.
Slab 3 loaded 3% ABOVE design load
5.21
Stage Operation Slab
Level Sketch of arrangement
Assumed
% of wp
transfer
Loads (kN) per square metre Load in
props
Load in floor slab(s) Existing Added TOTAL
Notes to Part Two calculations:
5) Several “What If?“ scenarios have been included at stages 5.15 (a) and 5.18(a) as explanation of the complexity of backpropping and to highlight the use of correct procedures.
6) The likely maximum load on the falsework supports is 5.75kN/m2. For design, this load should be increased by the application of the transient insitu concrete load of 0.75 kN/m2.
7) The maximum total construction load on a slab occurs at stage 5.16 of 8.70 kN/m2.
8) The justification to ignore the construction operations load (c.o.l.) is based on the research on an actual building (European Concrete Building Project). This demonstrated that when calculating
backpropping loads, the construction operation loads on the existing slabs need not be included. They were not measured in practice.
(See Section 7 and Figure 27 in CS 140 “Guide to Flat Slab Formwork and Falsework”.)
9) In Part Two, the supporting slab is often overloaded to above its design service load, and this fact should be made known to the Permanent Works Designer.
Output Falsework
design 6.50 kN/m2
Maximum Floor load
8.70 kN/m2
General Notes Applicable to all calculations:
10) What happens to the transfer of loads in slabs if a lower prop in compression is removed?
As the floors are elastic, the lowest floor reverts to its unpropped state, and the reduction of load on that floor is distributed to the remaining floors. It is realistic to assume that Method One also works in reverse, so that the effect of the reduction on the lowest floor is distributed to the remaining floors in order of application.
Hence if one level of propping remained, the ratio of 30/70 would apply, split with the lower floor carrying 70% of the removed load.
11) Where a slab is loaded to above its design service load, the approval of the Permanent Works Designer should be sought. Note that loading a slab to above its design service load may be acceptable – See Annex E in CS 140 “Guide to Flat Slab Formwork and
Falsework”.)
continues
Output
General Notes –
Method Three
12) Method Three at Section 5.4.2.3 uses simple equations, reproduced below, to generate loads in floors and backprops based on relative stiffness of floors. In this example the ground / foundations at level 0 are assumed rigid such that Slevel 0 = ∞.
One level of backprops
Load in backprops p p
Two Levels of backprops
Load in upper backprops, where Ss3 = ∞ and Ss1 = Ss2 is given by
13) Stages 5.3(a), 5.5(b), 5.13, 5.14, 5.16 and 5.17 use Method Three equations, see 12) above, assuming that the foundation / ground slab is infinitely stiff and Sground = ∞. See Section 5.4.2.5.
Comments
(a) Each site will be different - this is one example only.
(b) Always liaise with the Permanent Works Designer (PWD) and the Temporary Works Designer(TWD) for backpropping calculations.
{The UK CDM Regulations place a duty on CDM co-ordinators to ensure this liaison takes place.}
(c) Is the additional transient concrete load relevant in
backpropping calculations? In this example it represents a total load of 3m x 3m x 0.75 = 6.75 kN applied to the falsework.
Assuming the columns are on, say, a 7m grid, this load
effectively only applies 0.14 kN/m2 to the structure. As it is transient, it is reasonable to assume that it could be omitted in backpropping calculations for the slabs, but of course is relevant in the falsework design.
(d) The first, second and third floor slabs are loaded to above their design capacity when backprops are inserted hand-tight. When the same arrangement is loaded with pretensioned backprops and the c.o.l. on completed floors ignored, the “above design
capacity” is reduced, but not eliminated.
(e) This example shows that during construction several floors will be overloaded to above the service design load. Again, See both (d) and Notes 3 and 7.
Output