Table 20 and 21 on pages 70-71 of DD9999 set out minimum periods of fire resistance for elements of structure, and the document allows table 21 to be used instead of table 20 where the building design incorporates ventilation conditions which would reduce the fire loading on the elements of stucture. These ventilation conditions are set out in table 22 on page 72.
What I cannot understand is that the requirements of table 21 are often much more onerous than those of table 20. Take a block of flats of 17m height. Table 20 allows 60 minutes fire resistance, Table 21 105 minutes.
Industrial high hazard 27m high table 20 says 150 minutes, table 21 says 240 minutes.
Has table 20 been based on the reduced periods in the approved document B whilst table 21 is still based on earlier research?
