405.1 General. The provisions of this section apply to building spaces having a floor level used for human occupancy more than 30 feet (9144 mm) below the lowest level of exit discharge.

Exceptions:

1.One- and two-family dwellings, sprinklered in accordance with Section 903.3.1.3.

2.Parking garages with automatic sprinkler systems in compliance with Section 405.3.

3.Fixed guideway transit systems.

4.Grandstands, bleachers, stadiums, arenas and similar facilities.

5.Where the lowest story is the only story that would qualify the building as an underground building and has an area not exceeding 1,500 square feet (139 m²') and has an occupant load less than 10.

405.2 Construction requirements. The underground portion of the building shall be of Type I construction .

[F] 405.3 Automatic sprinkler system. The highest level of exit discharge serving the underground portions of the building and all levels below shall be equipped with an automatic sprin­kler system installed in accordance with Section 903.3.1.1. Water-flow switches and control valves shall be supervised in accordance with Section 903.4.

405.4 Compartmentation.  Compartmentation shall be in accordance with Sections 405.4.1 through 405.4.3.

405.4.1 Number of compartments. A building having a floor level more than 60 feet (18 288 mm) below the lowest level of exit discharge shall be divided into a minimum of two compartments of approximately equal size. Such compartmentation shall extend through the highest level of exit discharge serving the underground portions of the building and all levels below.

Exception: The lowest story need not be compartmented where the area does not exceed 1,500 square feet (139 m²) and has an occupant load of less than 10.

405.4.2 Smoke barrier penetration. The compartments I shall be separated from each other by a smoke barrier in accordance with Section 709. Penetrations between the two compartments shall be limited to plumbing and electrical piping and conduit that are fire stopped in accordance with Section 712. Doorways shall be protected by fire door assemblies that are automatic closing by smoke detection in I accordance with Section 715.4.7.3 and are installed in accordance with NFPA 105 and Section 715.4.3. Where provided, each compartment shall have an air supply and an exhaust system independent of the other compartments.

405.4.3 Elevators. Where elevators are provided, each compartment shall have direct access to an elevator. Where an elevator serves more than one compartment, an elevator lobby shall be provided and shall be separated from each compartment by a smoke barrier in accordance with Section  709. Doors shall be gasketed, have a drop sill and be auto­matic closing by smoke detection in accordance with Section 715.4.7.3.

[F] 405.5 Smoke control system. A smoke control system shall be provided in accordance with Sections 405.5.1 and 405.5.2.

[F] 405.5.1 Control system. A smoke control system is required to control the migration of products of combustion in accordance with Section 909 and the provisions of this section. Smoke control shall restrict movement of smoke to the general area of fire origin and maintain means of egress in a usable condition.

[F] 405.5.2 Compartment smoke control system. Where compartmentation is required, each compartment shall have an independent smoke control system. The system shall be automatically activated and capable of manual operation in accordance with Section 907.2.18.

[F] 405.6 Fire alarm systems. A fire alarm system shall be provided where required by Section 907.2.19.

[F] 405.7 Public address. A public address system shall be provided where required by Section 907.2.19.1

405.8 Means of egress. Means of egress shall be in accordance with Sections 405.8.1 and 405.8.2.

405.8.1 Number of exits. Each floor level shall be provided with a minimum of two exits. Where compartmentation is required by Section 405.4, each compartment shall have a minimum of one exit and shall also have an exit access door­way into the adjoining compartment.

405.8.2 Smokeproof enclosure. Every required stairway serving floor levels more than 30 feet (9144 mm) below its level of exit discharge shall comply with the requirements for a smokeproof enclosure as provided in Section 1020.1.7.

[F] 405.9 Standby power. A standby power system complying with Section 2702 shall be provided standby power loads specified in Section 405.9.1.

405.9.1 Standby power loads. The following loads are classified as standby power loads.

1. Smoke control system.

2. Ventilation and automatic fire detection equipment for smokeproof enclosures.

3. Fire pumps. Standby power shall be provided for elevators in accor­dance with Section 3003.

405.9.2 Pick-up time. The standby power system shall pick up its connected loads within 60 seconds of failure of the normal power supply.

[F] 405.10 Emergency power. An emergency power system complying with Section 2702 shall be provided for emergency power loads specified in Section 405.10.1.

[F] 405.10.1 Emergency power loads. The following loads are classified as emergency power loads:

1. Emergency voice alarm communications systems.

2. Fire alarm systems.

3. Automatic fire detection systems.

4. Elevator car lighting.

5. Means of egress and exit sign illumination as required by Chapter 10.

[F] 405.11 Standpipe system. The underground building shall be equipped throughout with a standpipe system in accordance with Section 905.