9.1a use of modular ratio in designs

A number of points arise with regard to the use of modular ratios:

(i) In comparisons to normal deflection analysis for reinforced concrete design  long term loading can produce deflections are three  times the values  of that for short term loading. This correspond to a modular ratio value of 26 and is  significantly larger than that specified in the 1982 British Standard. It is unlikely that any difference should be attached to the fact that  the slab is externally reinforced compared to normal RC slabs as the creep is associated with the concrete and not the steel reinforcing.

This being the case, previous tables utilizing values of 15 and 18 are likely to underestimate the value of long term deflections

(ii) In fact, for any given  slab thickness, the allowable superimposed load decreases with increasing span and therefore the ratio of dead load to superimposed load increases as  span increases. Therefore as span increases the value of   increases, with the result that member properties decrease.
                       
Unless a value of modular ratio associated with the max specified span for any given depth is used for the complete range of spans, then the deflections  calculated in previous tables for the larger spans is likely to be underestimated.

(iii) The use of calculating an effective overall modular ratio by factoring modular ratios for dead load and live loads by their respective proportion of overall loading, while giving a better estimation, is incorrect. A more accurate means is to calculate the member properties associated with each part of the loading and then factor the member properties by their proportion of loading to obtain effective member properties. The overall effective modular ratio can be obtained by back- figuring the values of  n on an iterative basis to find an equivalent value of n which equates the earlier member property values.

For concrete creep effects to be more accurately incorporated, the items raised  are required to be addressed and included within the calculation of span tables.