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ARCH 2614/5614 Lecture notes

Jonathan Ochshorn

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Assemblies and egress

Assemblies

Review of compartmentation: requires assemblies to achieve fire-resisting ratings.

Review IBC Table 601 "Fire resistive rating requirements for building elements (hours)"

Examine the footnotes to Table 601:

  1. Roof supports: Fire-resistance ratings of primary structural frame and bearing walls are permitted to be reduced by 1 hour where supporting a roof only.
  2. Except in Group F-1, H, M and S-1 occupancies, fire protection of structural members shall not be required, including protection of roof framing and decking where every part of the roof construction is 20 feet or more above any floor immediately below.
  3. In all occupancies, heavy timber shall be allowed where a 1-hour or less fire-resistance rating is required.
  4. An approved automatic sprinkler system in accordance with Section 903.3.1.1 shall be allowed to be substituted for 1-hour fire-resistance-rated construction, provided such system is not otherwise required by other provisions of the code or used for an allowable area increase in accordance with Section 506.3 or an allowable height increase in accordance with Section 504.2. The 1-hour substitution for the fire resistance of exterior walls shall not be permitted.

    For example, compare Type V-A (using sprinklers to avoid 1-hr construction) versus Type V-B (using sprinklers for bonus area and height): Assume Group B occupancy and no frontage increase:

    1. Type V-A gets 3 stories (50 ft) and 18,000 sq.ft. allowable area per floor.
    2. Type V-B gets (2+1) stories (40+20 ft) and 9,000 x (1 + 2 + 0) = 27,000 sq.ft. allowable area per floor.

Note that heavy timber consists of big wooden cross-sections as follows:

Mass timber buildings, e.g., made with cross-laminated timber, offer another possibility for wood-framed tall buildings, typically governed by heavy timber rules.

Examples of various assemblies are given: see chapter 7, IBC, or product literature for gypsum board, masonry units, etc. Typical 1-hour partition assemblies can be fabricated with normal wood or steel studs and 1 layer each side of 5/8" thick "firecode" gypsum board fastened according to the specifications. For a 2-hour rating, one typically uses 2 layers of gypsum board each side, etc. Masonry units (or concrete) is also commonly used for 1-, 2-, 3-, or 4-hour-rated assemblies. For hollow block, manufacturers provide "equivalent" thicknesses so that their ratings can be computed. Sprayed-on or troweled-on cementitious fireproofing is commonly used for steel elements. Shaftwall, where gypsum board can be applied from one side of a shaft (the side with a floor to stand on...) was illustrated. Some images follow:

fireproofing for steel elements


1-hour assemblies for wood or steel partitions

Changes to the requirements for shaft enclosures (exit stairs, etc.) as a result of recommendations made in the aftermath of the World trade Center collapse:
In the latest versions of the IBC (Chapter 4), enclosures for interior exit stairways and elevator hoistway enclosures shall have impact-resistant construction board, concrete, or masonry materials. This applies to high-rise buildings (buildings with an occupied floor located more than 75 feet above the lowest level of fire department vehicle access) of Risk Category III or IV (see Section 1604.5 of the 2015 IBC), and to all high-rise buildings that are more than 420 feet in building height.


Fire-rated glass types

  1. Wired glass:
    • The grid of wires holds the glass together under high heat, but is not great for impact loads unless a surface-applied film is added.
    • Not permitted in hazardous (high-impact) locations like doors as of 2003 IBC
    • Typically good up to 45 minutes (or 90 minutes with films applied)
    • Generally cheaper than other forms of fire-rated glass
  2. Ceramics:
    • Looks like regular glass, but it's molecular structure — crystalline — is different, and better able to stay together under high heat
    • 20 - 90 minute ratings possible, or up to 180 minutes (3 hours) with laminated or filmed versions
    • Sizes up to 35 x 95 inches (about 3 x 8 feet) with thicknesses of about 3/16 in. (or 5/16" when laminated)
    • Can be configured into insulated glass (IG) units
    • Can be etched or sandblasted
  3. Glass firewalls:
    • Contain multiple layers with intumescent interlayers which foam up under high heat
    • Thus, useful when subject to radiant or conductive heat, rather than just flame and smoke
    • Up to 120 minute (2 hour) fire ratings possible.

Fire safing
Firestopping (fire safing) is an increasingly important component of assemblies, to close of gaps and holes, especially at penetrations of conduit, pipes, etc., and at floor-curtain wall intersections.
fire safing image

Exterior wall assemblies

There have been many disastrous fires that have spread over exterior wall assemblies constructed with combustible materials — mainly outside the U.S. — where combustible material, like foamed plastic insulation and /or certain water-resistive barriers have not been designed or tested to limit combustion and flame spread. In the U.S., such fires have been generally prevented through compliance with NFPA 285 and its predecessors.

exterior cladding fires in London and Beijing
Grenfell Tower fire, London: 2017 (left); and China Central Television (CCTV) Hotel, OMA: 2009 (right)

Reasons for exterior cladding fires and their prevention:

References (pdf): John Valiulis, Building Exterior Wall Assembly Flammability: Have We Forgotten the Past 40 Years? | Owens-Corning Enclosure Solutions | Joseph Lstiburek, BSI-098: Great Fire of London

Some notes from the John Valiulis reference:

Egress

Refer to Chapter 10, IBC 2012 and "Egress" chapter of The Architect's Studio Companion, by Allen and Iano (course reserve).

A comprehensive discussion of exit stairs as defined in the IBC can be found here.

Means of egress is continuous and unobstructed path of travel from any point in a building to a public way. It consists of 3 basic parts:

Review:

Notes on egress:

Here's a short (2 minute) video showing some egress and guard rail issues in E. Sibley Hall, Cornell Univeristy: