In Caves of Steel: Fire Protection and Life Safety for Underground Structures

From "Robbies World Blog"
In his serial, The Caves of Steel, Isaac Asimov writes of a future crowded nation where underground living is commonplace. However, in Beijing this is a current reality.  Nearly one million people live beneath the city. These people are referred to as the shuzu, or “rat tribe”, and they take advantage of the bunkers and fallout shelter systems that were dug in the late 1960’s to 1980’s. These subterranean dwellings present multiple issue to the fire protection and life safety of occupants. NFPA 101, Life Safety Code,  Chapter 11, Section 7, provides the basic requirements for life safety within underground and limited access structures.
Underground structures are defined as, “a structure or portions of a structure in which the floor level is below the level of exit discharge”.  A limited access structure is, “a structure or portion of a structure lacking emergency access openings”. To qualify as an emergency access opening the window, panel, or similar opening must meet the following criteria:
  • minimum dimensions of 22" width x 24" high
  • bottom of opening  less than 44" AFF
  • unobstructed, identifiable, and accessible from both the interior and the exterior

It is important to clearly define whether the space is considered an underground or limited access structure. Each of these have their own, and overlapping, fire protection and life safety requirements. Both structure types must meet all of the following requirements:

  • Fire sprinkler system is required (with some exceptions, noted in 101:
    • Sprinklers may be omitted if the following applies:
      • Occupant load is 50 or fewer for new structures, 100 or less for existing
      • If the structure is permitted to have a single exit (per occupancy use), and the common path of travel does not exceed 50 feet.
  • Emergency lighting is required, throughout

In addition to these requirements, structures that expressly meet the definition of underground structure must comply with these additional requirements.

  • Automatic smoke venting systems (required, if following applies):
    • Occupant load is greater than 100 persons
    • Occupiable floors are greater than 30 feet below exit discharge
    • Space contains combustible contents, interior finishes, or construction
  • Exit stair enclosures are required to have code compliant signage that points to the direction of exit discharge

This is only an encapsulation of the fire protection requirements as outlined in the Life Safety Code. There are many other factors to be considered for safe underground dwelling. Aside from the building code, loads, and material requirements, there are very real emergency response issues. These challenges include ventilation, communication, orientation, access and egress, and patient removal.  In areas of occupied subterranean environments departments should be aware of their existence and have robust plans for the maintenance and enforcement of codes and standards, and emergency response.