Reduction in maximum spans for lumber beams or in maximum supported length of floor joists used to support floors with a concrete topping not more than 51 mm thick, in residential applications or where the live load on any floor does not exceed 1.9 kPa, in wood-frame constructions to which Section 9.23. applies [see Sentence 9.23.1.1.(1) for application of Section 9.23.].

To limit the probability of excessively long spans of beams or joists, which could lead to beams being unable to resist the additional weight of a concrete topping, which could lead to excessive deflection or failure.

This is to limit the probability of:

• compromised structural integrity,
• where elements support or are part of an environmental separator, the failure of required environmental separation elements, which could lead to deterioration, or
• an inability to resist expected loads, which could lead to
  • the excessive deflection or vibration of floors, or
  • the excessive movement or deformation of walls.

This is to limit the probability of:

• for floors and elements that support floors, persons losing their balance, tripping or falling, or
• the compromised operation of doors or windows required for egress in an emergency.

This is to limit the probability of harm to persons.