Loading Sequence

The loading applied to the slab and the age of the concrete at the time of application play a very significant role in determining the long term serviceability behaviour of the structure. In particular, those loads applied to the concrete at an early age as part of the construction process are of considerable importance. A key stage in the calculation of long term deflections and crack widths is the definition of the loading events on the slab and the relevant time periods associated with each load. It is also important to consider what proportion of load is considered permanent when considering the variable actions (imposed loads) on the slab.

Proportion of imposed loading considered to be permanent

The assessment of long term deflections accounting for the effects of creep and shrinkage are assessed in the Eurocode using the quasi-permanent combination. The long term deflection is, therefore, based on permanent actions and a proportion of the imposed load that is determined to be permanent. The quasi-permanent combination is given in BS EN 1990:2002+A1:2005 as Equation (6.16b) and is

where

                    Characteristic value of the permanent action j

Characteristic value of variable action i

                    Factor for the quasi-permanent value of the variable action i

The value of  is set by the UK National Annex and is based on the category of use for the imposed load. The values of  are given in Table NA.A1.1.

The long term deflection module also includes the option to check deflections using the Frequent load case. This uses a  on the leading imposed variable. The module provides a drop down option to define which load is taken as the  leading variable.

Pattern Loading

While BS 8110 requires that the effects of pattern loading be explicitly accounted for when assessing deflections, long term deflections calculated in accordance with the Eurocode only require checking with the quasi-permanent load case, with no mention made in regard to loading patterns. According to Section 7.5.3 of TR58, it is reasonable to ignore the effects of pattern loading except in the case of members which will undergo little or no cracking under bending. This then suggests that there is no requirement to consider the effects of pattern loading on slabs when considering long term deflections using the Eurocode. In the case of continuous suspended slabs, it is likely that cracking at the supports would nullify the effect of the pattern loading.

Should it be felt necessary to consider the effects of pattern loading, it would be necessary to modify the loading on the FE surfaces to assign different load group numbers to the loads to form a loading pattern. Once done, separate slab deflection briefs would then be required to look at the long term deflection and cracking utilising the selected loads that form the required pattern.

Early age loading

Early striking times for cast in situ concrete mean that concrete slabs are required to support their own self-weight at much earlier times, as part of the process of striking the formwork associated with the slab. Furthermore, as part of the construction process when using in situ concrete, is the requirement for a floor to support the formwork and falsework associated with the construction of slabs above, where testing has shown that 70% of the loads from a newly cast slab are supported by the floor immediately below with only 30% carried by the level below that. This uneven split in the load is due to the elastic shortening of the props. These loads applied at an early age, combined with the fact that the concrete is still in the process of curing mean that there may be a temporary overload in the structure, particularly for structures where the imposed variable load will be quite low (when compared to the self-weight). The effect of early age loading may be to increase the long term deflections of the slab due to cracking of the slab, with cracking occurring over more of the slab than would happen under the serviceability limit state. Creep will also be higher for early age loading.

The effects of early age loading can be accounted for directly in the Long Term Deflections and Cracking module. This is done by including loading events for the early age loading of the slab. These loading events are then included in the analysis, accounting for the early age concrete properties and an assessment of cracking in the slab, with the resulting cracking being accounted for in subsequent iterations over time.

Creating Time Dependent Loading Sequences

Time dependent load events are input in the Slab Deflection Analysis load events area. This is accessed by selecting the  icon from the Edit the slab load events input area. This will open the Slab Load Events input area.

Each load event can be labelled using the Event Name input line. The time period associated with the loading event is input with a start and end time, input in days. The load group which represents the loading event is then selected and the appropriate quasi-permanent load factor input along with the relevant beta factor. Additional loading events are added by selecting  to add a new input line in the table.

The default End time is 18250 days, which corresponds to a 50 year period. This value can be amended in the default inputs. For load events that occur over a shorter time period, both the start and end time need to be defined. It is not necessary for load events to be input in strictly chronological order, though it may be desirable to assist reviewing the loading events as displayed in the outputs.

The Concrete Centre publication "How to design concrete structures using Eurocode 2 Part 8. Deflection Calculations" gives the following example of a loading history for a slab.

Figure 1 - example of loading time history for a slab (Concrete Centre)

The aim of the Slab Load Events input area is to reproduce the loads that the slab will be exposed to throughout the construction and service life of the structure. Each time step representing a change in loading then becomes a stage for the calculation of the deflection and cracking of the slab.