Deck Slabs - Construction condition.

The decking supports the weight of concrete in the finished slab, excess weight from concrete placement and ponding, the weight of operatives and any impact loads during construction.There is a variety of recommendations for this temporary con- struction load to be considered in the design of the decking: these are expressed in terms of either a uniformly-distributed load or a single line load transverse to the span being checked. In BS 5950: Part 4, the standard used in the design of the decking and composite slab, these construction loads are specified as 1.5kN/m2 or a transverse line load of 2kN/m2  (for spans up to 3m) respectively. Self-weight loads are multiplied by a load factor of 1.41 and construction loads by 1.6.

However, significant loads can be developed before concreting due to storage  of equipment and materials.

Similarly, loads applied to the composite slab before it has gained adequate strength for fully composite action should not exceed  1.5kN/m2. The definition of ‘adequate strength’ is considered to be a cube strength of 75% of the specified value, which is often achieved in 5–7 days after concreting.

The design of single-span decking is usually controlled by deflection and ponding of concrete. Continuous decking is generally designed on strength rather than deflection criteria, and longer unpropped spans can be achieved. The construction load is applied in design as a pattern load even though only one span is likely to be loaded to this extent, in view of the progressive nature of concreting. The 1994 version of BS 5950: Part 4 reduced the construction load to 0.5kN/m2 on deck spans adjacent to the fully loaded one. In BS 59501
a limit on the residual deflection of the soffit of the slab after concreting of the smaller of (span/180) or (slab depth/10) is specified, increased to (span/130) if the effects of ponding are included. Greater deflections are likely to be experienced during concreting.

The design of the decking in bending is dependent on the properties of the profile, and particularly the thin plate elements in compression.Where profiles are stiffened by one or two folds in the compression plate, the section is more efficient in bending.

The design of continuous decking is based, according to code requirements, on an elastic distribution of moment, as a safe lower bound to the collapse strength. Elastic moments are normally greatest at internal supports. For the two equal span case this negative (hogging) moment is equal to that of the positive (sagging) moment of single-span decking.Many profiles with wide troughs are weaker under negative moment than positive, and the effect of the localized reaction at the internal support is to reduce further the negative moment resistance. In design to code requirements, therefore, continuous decking may appear to be weaker than simply- supported decking even though in reality it must be considerably stronger.

In general the load capacity of continuous decking is obtained  from tests. The reduced construction load on adjacent spans has the effect of reducing the design negative moment so that elastic design more closely approximates the results of tests under the full design load (see following section).

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