9.1 incorporation of concrete creep effects

Previously, creep effects have been included in span tables by utilizing different values of  modular ratio  depending on the Live Load, duration period. In this manner, long term deflection  and strength effects  were catered for by correspondingly lower values of second moment of inertia and section modulus respectively. The 1982 version  of BS5950:Part 4 contained  clauses which allowed the use of  an average value  of 15 for the modular ratio, but also permitted a separation of modular ratio values depending on the load duration. For that part of the total loading which was considered to be long term loading, a value of 20 was considered appropriate, while for short term loading, the normal value of 8 was specified.

The 1994  and current version of the British Standard  has removed the specification of values of modular ratio from the code and instead has referenced  an associated Standard.To date manufacturers have either used a value of 15 as the main basis for span tables (with member properties for n=20 included), or have utilized  a straight ratio of short term and long term loading to determine an arbitrary value of modular ratio. In either case a set value of n has been used in a tables preparation.