Long term deflection and Cracking design - Preliminary

The long term deflection and crack calculation is an extension to the FE slab design module, which enhances the capabilities of the FE slab design to utilise the defined reinforcement, as part of a non-linear analysis, to account for the time-dependent properties of the concrete and the time-dependent loading on the slab, to calculate the long term slab deflections and crack widths, in accordance with BS EN 1992-1-1:2004 and the Concrete Society Technical Report TR58 "Deflections in concrete slabs and beams"

The first stage of the process is to carry out an analysis and slab design. This defines the reinforcement in the slab which is required to determine the stress and strain distribution in the slab, which are required to determine whether or not a cross section is cracked and to determine the service stress in the reinforcement. However, to carry out a long term deflection design, it is necessary to modify the model so that it is suitable for a long term deflection analysis.

The analysis for long term deflection and cracking is based on the construction sequence for each slab, looking at the variation of loading from construction across the anticipated service life of the structure. Whereas for the ultimate limit state design certain loads may be grouped together in a single load group for convenience, for example using the same load group for both services and finishes, when it comes to a long term deflection and cracking analysis, the time at which a load is applied is of considerable importance and so loads should be separated as far as possible, so that the long term deflection and cracking analysis can deal with each load being applied at the appropriate time in the analysis. In terms of determining the long term deflection of the slab, the age of the concrete at the time of loading and the stresses induced by the loading are crucial and it is desirable to separate the loads out as far as possible to give the most accurate prediction of long term deflection and cracking.

In addition to the loads applied to the slab, at the construction stage the slab may be subjected to loads from the construction of the structure above, due to loads from propping associated with the pouring of the concrete slabs above. Even where propping extends below the level being considered, the elastic shortening of the props means that load is still exerted on the slab under consideration. The distribution of loads based on the numbers of floors being propped is explained in various Technical Publications. For an introduction, see the Structural Engineer, December 2016, Temporary Works Tool kit "An introduction to back propping of flat slabs"

Note: The long term deflection and cracking check only assesses the serviceability limit state implications on the structure. It may also be necessary to consider the implications of the construction stage loading on slabs in the ultimate limit state. Reference should be made to a relevant publication or design guidance.

When carrying out a long term deflection and cracking analysis, it may also be necessary to consider the extent of the model used for checking the slab. In a complete structural model, while it is possible to exclude all the loads above the level being considered by assigning all the loads on any level to a specific load group number and then excluding the loads associated with the slabs above, this does not exclude the stiffnesses of the structure. It may, therefore, in certain circumstances, be desirable to reduce the extend of the structure modelled, or even to carry out the long term deflection on models of each floor under consideration as separate models. There is no specific guidance on this aspect of the long term deflection and cracking analysis in the published documentation.

It is important to note that the assessment of long term deflection and cracking is not a construction sequence analysis and does not analyse the structure at phases of construction. The module is intended to estimate the deflections in slabs and not the deflected shape that may occur due to the construction phasing or sequence.