When do you know when to use 508.4 vs 706.4?
Here is a note from the 2018 code commentary:
The minimum fire-resistance ratings required in Table 706.4 generally correlate with the fire-resistance ratings required by Table 508.4. However, Table 706.4 contains provisions that acknowledge the need for more conservative ratings for fire walls due to their unique role in creating separate buildings.
Here is my take:
Table 508.4 gives the required ratings for Fire Barriers separating occupancy groups in mixed occupancy buildings (see 707.3.9).
For example, if you have a 200-person lecture hall (Group A-3) in an office building (Group B), 508.4 shows that you would need a 1-hr Fire Barrier separating the occupancies if the building has automatic sprinklers, and a 2-hr Fire Barrier if the building is not sprinklered. I believe this would apply no matter what construction type is used and no matter the size of the building.
Table 706.4 refers to the fire-resistance ratings for Fire Walls, which can be used to separate buildings into different fire areas or construction types, or for separating a building which would otherwise exceed the allowable area limitations from 506.2.
We would need to use a Fire Wall if we wanted to design a 100,000 sf office building using Type IV Heavy Timber construction, but we wanted to avoid adding automatic sprinklers for some reason. This situation would require that our building is designed as if it were two separate buildings, which can be accomplished by using a fire wall. Referring to 706.4, the fire wall needs to have a 3-hr rating.
I’m having trouble thinking of better or more comparable examples. Hopefully this helps, and if not, let me know your thoughts.
Thank you for your response. I appreciate the example.
@Smith.Marks thanks for the great reference!