Construction stage loading on composite floor decking – BS and Eurocode
Iftekhar Waseem, Structural Engineer at Tata Steel, discusses construction stage loading on composite floor decking to BS and Eurocode.
Composite slabs using metal decking during construction must be adequately designed to suit the loading of wet concrete and construction traffic at the limit state.
Both BS and EC have quite similar approach in terms of self-weight but vary slightly in the distribution of the construction load.
BS 5950, Part 4: 1994
Construction load should be considered in addition to the weight of the wet concrete.
The basic construction load on one span of the decking should be taken not less than 1.5 kN/m2. The other spans should be taken as either loaded with wet concrete plus 1/3rd of the basic construction load, or unloaded.
For spans less than 3m, the basic construction load should be increased to 4.5/Lp, where Lp is the effective span of the decking.
The construction load should be in addition to the self-weight of the slab; whichever is the critical for the positive and negative moments as shown in the figures below.
When concrete is poured progressively using the ‘flood technique’ to reach a surface flatness datum level, the excessive deflection as a result from ponding of both deck and primary/ secondary beams must be considered within the total design wet concrete loading. Care must be taken that the assumptions made in respect of the slab thickness are reflected in the calculation of deflections of the slab and the supporting beams.
BS EN 1991-1-6-2005
The following recommended characteristic values of construction loading should be applied:
1) Outside working area – 0.75 kN/m2 generally
2) Inside 3m x 3m ‘working area’ – 10% of the slab self-weight but not less than 0.75 kN/m2 and not greater than 1.5 kN/m2. This area should be treated as a moveable patch load that should be applied to cause maximum effect, and includes Qca and Qcf
3) Actual area – self weight of the deck, load-bearing element (Qcc) and the weight of the fresh concrete for the design thickness (Qcf)
Where, Qca – Personnel, and hand tools
Qcc – Non-permanent equipment in position for use during execution i.e. static and moveable
Qcf – Loads from parts of a structure in a temporary state before the final design actions
EC recommended load application shown in figure below:
However, in accordance with the SCI note AD 346, the above figure implies that the load within the working area would be no greater than outside the working area unless the slab weight exceeds 7.5 kN/m2, which is much greater than in normal composite decks.
The AD note further states that the above EC recommendation implies, there would be no allowance for heaping of concrete in the working area. SCI considers that the omission of an allowance for heaped concrete is unwise, and may not have been intended.
Consequently, SCI recommends the following construction loads during casting of concrete for a composite slab:
A) 0.75 kN/m2 generally
B) An additional load of 10% of the slab self weight or 0.75 kN/m2, whichever is greater, over a 3m x 3m ‘working area’. This area should be treated as a moveable patch load that should be applied to cause maximum effect.
This recommended loading is shown in the figure below.
‘C’ represents the slab self weight.
Wet concrete densities
The following concrete are to be considered. In EC the densities have been increased.
BS – 2400 kg/m3 (NWC) and 1900 kg/m3 (LWC)
EC – 2500 kg/m3 (NWC) and 2000 kg/m3 (LWC)