Example using Method One and Three of backpropping load calculations 175mm thick RC Slab - Five floors - One set of Formwork
- Two sets of backprops
DESIGN BRIEF Output
To carry out the backpropping calculations on a FIVE storey insitu reinforced concrete thin flat slab building with a regular column grid.
The permanent works drawing details a solid 175mm 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.
Assumptions
1) Assume a concrete density of 24 kN/m3. (Section 4.3.1) 2) The self weight of the formwork/falsework is 0.50 kN/m2. 3) Ignore the self weight of any backpropping.
4) When concreting a slab, the working area load (w.a.l.) of 0.75 kN/m2 is applied on the falsework.
5) Construction Operation Loads (c.o.l.) on floor slabs are ignored in Part Two of the backpropping calculations.
6) Ignore the additional transient insitu concrete load in the backpropping calculations.
{Comment: It will be included in the falsework design.}
7) Any screeds to be added to the concrete floor are for future work and not included in these calculations.
8) Use only ONE set of formwork/falsework for slab construction.
9) A maximum of TWO sets of backprops could be made available.
10) Where a backprop is pretensioned in position in Part Two, it is assumed to be pre-loaded to 0.50 kN/m2.
11) The slab that carries the falsework is known as “supporting slab”.
12) The completed slab behaves elastically under applied load.
13) 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 (M1) 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 in Example 5 on 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.
Output
Loadings kN/m2 A. Permanent Works Weight of slab 24 x 0.175 = 4.20 Weight of finishes = 1.00 Super imposed floor load = 2.00 Total design service load = 7.20
Design Service
load 7.20 kN/m2
B. Temporary Works Weight of formwork / falsework = 0.50 Working area load (Section 4.3.2.2) = 0.75 Transient insitu concrete load = 0.00 (Would be 0.75 for 175mm slab thickness) Minimum imposed construction load on completed slab = 0.75
w.a.l 0.75 kN/m2 Concrete strength
The permanent works designer has completed the design based on an applied service load of 7.20 kN/m2 and a specified concrete strength.
Striking Criteria
If a distributed load on a slab is less than the specified, then the required concrete strength for striking the slab at that stage may be reduced prorate to the specified strength. (See Section 5.3.6.1) Backpropping Calculations and Notation
Storey heights are diagrammatical.
(M1) is Method One.
(M3) is Method Three.
concreting a slab 6.50 Load in a prop or in falsework
10.08 Load in a slab above the design service load
PART ONE
- Calculations with hand tight backprops and including the construction operation load on floors.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
6.1 Erect forms &
Slab 1 loaded 22% ABOVE design load
6.4
Hence start from 6.4 by removing falsework before disturbing the backprops.
6.5
1 The 76% is ratio of loads as 5.45/7.20 = 76% of specified 28 day design strength.
2 The assembly now has to carry the removed 1.40kN/m² between the two floors. As all the floors
Part One (c.o.l. and hand tight backprops) continued.
What happens if lower backprops 0 to 1 were accidentally removed first?
6.8 Stage Operation Slab
Level
Loads (kN) per square metre Load in
props
Load in floor slab(s) Existing Added TOTAL
Part One (c.o.l. and hand tight backprops) continued.
5 The falsework weight was taken on the slab 3 before it was backpropped, and is included in slab 3 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 One (c.o.l. and hand tight backprops) continued.
Slab 4 loaded 21% ABOVE design load
6.13
Slab 3 loaded 14% ABOVE design load
Remove falsework to slab 5 before removing any backprops.
Notes to Part One calculations:
1) Several “What If?“ scenarios have been included as explanation of the complexity of backpropping and the use of correct procedures.
2) Stages 6.3, 6.4, 6.6(b) and 6.7 use Method Three equations which are reproduced in Example 5 page 48.
3) The calculations show that during construction ALL the floors will be stressed to above their design service load :
Slab ONE 22% (6.3) Slab TWO 26% (6.6(b) )
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
- 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
6.15 Erect forms &
6.17 Insert preloaded backprops
Slab 1 loaded ABOVE design load
6.19
1 The 65% is ratio of loads as (0.50+4.20)/7.20 = 65% of specified 28 day design strength.
Part Two (no c.o.l. and preloaded backprops) continued.
Stage Operation Slab Level
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 2 loaded slightly ABOVE design load
What happens if lower backprops 0 to 1 were removed first ? Stage Operation Slab
Level
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
Continues
Stage Operation Slab Level
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 ABOVE its design load
Stage Operation Slab
Level
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.
Hence slab 4 is slightly loaded to above its service design load – caused by the inclusion of the working area load. See 6.33 after pouring slab 5 is finished.
6.33
After pouring the slab the working area load becomes
nil.
(The backprops remain preloaded.)
5 (M1) 0 0 0
4.20
4 65% 1.47 4.20 +2.73 6.93
+0.50 1.97
3 23% 0.50 4.20 +0.97 5.17
+0.50 1.00
2 12% 4.70 +0.50 5.20
Now allow the new slab to take up its deflected shape first by removing the falsework before removing the backprops.
6.34 backprops. Remove Completed.
5 0 +4.20 4.20
4 6.93 -0.50
-2.73 4.20
3 5.17 -0.97 4.20
2 5.20 -0.50
-0.50 4.20
1 4.20 0 4.20
Notes to Part Two - calculations:
4) Several “What If?“ scenarios have been included as explanation of the complexity of backpropping and the use of correct procedures.
5) Stages 6.18, 6.19, 6.24 and 6.25 use Method Three equations reproduced at Worked Example 5 page 48.
6) The calculations show that during construction ALL the floors will become slightly stressed to above their stated design service load during the placing of concrete:-
Slab ONE 4% (6.18) Slab TWO 8% (6.24) Slab THREE 3% (6.28) and Slab FOUR 3% (6.32)
Output Maximum
loads Falsework
6.20 kN/m2 Backprops
kN/m2.232 General notes applicable to Part One and Part Two on next page.
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
General Notes Applicable to Parts One and Two 7) Not all the calculation stages shown are required.
8) What happens to the transfer of loads in slabs if a lower prop in compression is removed? See Stages 6.5(a), 6.8(a), 6.20(a) and 6.26(a).
It is equivalent to applying a tension load downwards on the lowest slab. As the slabs and props are all considered to be elastic, it is realistic to assume that Method One also works in reverse, so that the effect of a downwards pull on the lower floor is distributed in reverse in order of application. For one level of backprops 70/30 with lowest floor now carrying 70% of the removed load.
For two levels of backprops 65/23/12 with lowest floor now carrying 65% of the removed load.
9) Where a slab is overloaded to above its design service load, this fact should be made known to the Permanent Works Designer.
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”.)
Output
Comments
(a) Each site will be different - this is one example only.
(b) Always liaise with the Permanent Works designer and the Temporary Works designer for backpropping calculations.
(c) The newly cast slab will nearly always have to be designed to support the weight of the formwork and falsework very shortly after being struck. This avoids having to lower the formwork / falsework to the ground, and allows it to be moved directly onto the new slab.
(d) The backpropping calculations confirm that for the PWD design loading given, each slab may become loaded to above design load if working area load and/or construction operations loads are considered. Engineering judgement is required.
The management of this risk is outside the scope of the Formwork Guide.
(e) The structure can only be safely constructed using two sets of backprops.