16.9Three-Sided Fire Exposure Condition

Members subject to a three-sided fire exposure condition shall be considered in separate groups unless the following conditions are satisfied:

• a) The characteristics of the members of a group as given below, shall not vary from one another by more than

1. Concrete density: $\frac{\text { highest in group }}{\text { lowest in group }} \leq 1.25$, and
2. Effective thickness (h_e): \frac{largest in group}{smallest in group} \leq 1.25.

where the effective thickness $\left(h_{e}\right)$ is equal to the cross-sectional area excluding voids per unit width, as shown in Fig. 25A.

• b) Rib voids shall either be:

1. all open; or
2. all blocked as shown in Fig. 25B.

• c) Concrete slabs may incorporate permanent steel deck formwork.