9.2 Construction Types and Height/Area Limitations (IBC Chapters 5 & 6)

Construction Types and Height/Area Limitations (IBC Chapters 5 & 6)

Principles of Construction Types

The International Building Code (IBC) regulates the materials used in building construction to control fire spread and preserve structural integrity during an emergency. IBC Chapter 6 classifies all buildings into five primary construction types (Types I, II, III, IV, and V). Each type represents a different level of fire safety, with Type I being the most fire-resistive and Type V being the least. Except for Type IV, each type is divided into Subtype A (protected) and Subtype B (unprotected).

"Protected" indicates that the structural frame, floors, roofs, and walls have an added layer of fire-resistance-rated protection (typically gypsum board, spray-applied fireproofing, or concrete encasement), while "unprotected" indicates that the structural members are allowed to remain exposed without a specific fire-resistance rating, subject to other code provisions.

Detailed Breakdown of the Five Construction Types

Type I (Noncombustible, Fire-Resistive)

Type I buildings are constructed entirely of noncombustible materials, such as structural steel and concrete. These structures must maintain their structural integrity during a severe fire.

• Type I-A: Requires the highest fire-resistance ratings in the code. The structural frame, exterior bearing walls, and interior bearing walls must have a 3-hour fire-resistance rating, while floor construction must be 2-hour rated.
• Type I-B: Requires a 2-hour rating for the structural frame and bearing walls, and a 1.5-hour rating for floor construction. Type I structures are typically high-rise buildings, hospitals, and large assembly spaces.

Type II (Noncombustible, Protected or Unprotected)

Type II buildings are also constructed of noncombustible materials (steel, concrete, masonry), but they allow lower fire-resistance ratings than Type I.

• Type II-A: Requires a 1-hour fire-resistance rating for the structural frame, bearing walls, and floor construction.
• Type II-B: Allows the structural frame, bearing walls, and floors to have a 0-hour (unprotected) fire-resistance rating. Type II-B is common for low-rise commercial offices, strip malls, and warehouses.

Type III (Ordinary Construction)

Type III buildings feature noncombustible exterior walls (typically masonry or concrete), while the interior elements (floors, roofs, structural frame) are permitted to be constructed of any materials allowed by the code, which typically means wood frame.

• Type III-A: Requires a 1-hour rating for all interior structural elements, floors, and roofs.
• Type III-B: Allows the interior elements to be unprotected (0-hour rating). The noncombustible exterior walls must still maintain their required rating (typically 2 hours) to prevent fire spread to adjacent properties. This is a common construction type for older downtown commercial buildings.

Type IV (Heavy Timber / HT)

Type IV construction (also known as Heavy Timber) requires exterior walls to be noncombustible or fire-retardant-treated wood, while the interior elements are made of solid or laminated wood without concealed spaces. The structural members must meet minimum nominal size requirements (e.g., columns must be at least 8 inches in their smallest dimension; beams must be at least 6 inches by 10 inches) because large-dimension lumber char-protects itself, slowing the progress of a fire. Recent editions of the IBC have added subtypes IV-A, IV-B, and IV-C to address Mass Timber structures, which utilize cross-laminated timber (CLT) to build taller wood buildings with varying degrees of gypsum board coverage.

Type V (Wood Frame)

Type V construction allows the structural frame, walls, floors, and roofs to be constructed of any materials permitted by the code, which is typically wood framing (light-frame construction).

• Type V-A: Requires a 1-hour fire-resistance rating for the structural frame, bearing walls, and floors. This is typical for multi-family apartment buildings.
• Type V-B: Allows all structural elements to be unprotected (0-hour rating). This is the standard construction type for single-family residential homes.

Building Height and Area Limitations

Once the occupancy group and construction type are selected, the plan reviewer must evaluate the building’s size limits using IBC Chapter 5. The code controls the height in feet, the height in stories, and the allowable area per story to ensure that a building is not too large for its fire protection capabilities and the fire department's reach.

Height in Feet and Stories (IBC Tables 504.3 and 504.4)

The maximum allowable height in feet is determined using Table 504.3, and the maximum allowable height in stories is determined using Table 504.4. Both tables have distinct columns for buildings equipped with automatic sprinkler systems. The code recognizes three sprinkler options:

• NS (Non-Sprinklered): Baseline height and story limits.
• S1 (Sprinklered, Single-Story): Height increases allowed under NFPA 13.
• SM (Sprinklered, Multi-Story): Height increases allowed under NFPA 13, which typically permits an increase of 20 feet in overall height and 1 additional story for most occupancies.

[!IMPORTANT] Buildings equipped with an NFPA 13R sprinkler system (residential up to 4 stories) have separate, slightly lower height increases than those with a full NFPA 13 system.

Allowable Area and Increases (IBC Section 506)

The maximum allowable area per story ($A_a$) is determined based on the tabular area ($A_t$) found in Table 506.2. If a building is equipped with an automatic sprinkler system, the tabular area in the "S1" or "SM" columns is already increased (often three to four times the "NS" baseline).

Frontage Increase (IBC Section 506.3)

If a building has open space (yards, public ways) surrounding its perimeter, the code allows an area increase because the open space provides a buffer against fire spread and gives the fire department better access. The frontage increase ($I_f$) is calculated as follows:

$I_f = (F / P - 0.25) \times (W / 30)$

Where:

• $F$ = The portion of the building perimeter that is open to a public way or yard of at least 20 feet in width.
• $P$ = The total perimeter of the building.
• $W$ = The width of the public way or yard. If the width varies, it is calculated as a weighted average. The value of $W$ must be at least 20 feet, and the maximum value allowed in the equation is 30 feet (unless specific conditions allow up to 75 feet).
• The ratio $F/P$ must be greater than 0.25 (meaning more than 25% of the perimeter must be open).

Total Allowable Building Area (IBC Section 506.2.3 & 506.2.4)

For a single-story building, the maximum area is equal to the allowable area per story ($A_a$). For a multi-story building:

• In a non-sprinklered building, the total area of all stories must not exceed two times the allowable area per story ($2 \times A_a$).
• In a sprinklered building, the total area of all stories must not exceed three times the allowable area per story ($3 \times A_a$). Additionally, no individual story may exceed the single-story allowable area limit.

Special Provisions: Unlimited Area Buildings and Mezzanines

Unlimited Area Buildings (IBC Section 507)

Under specific circumstances, the area of a building is allowed to be unlimited. The most common case is a one-story building of Group B, F, M, or S occupancy that is equipped throughout with an automatic sprinkler system and is surrounded by yards or public ways of not less than 60 feet in width. This open space prevents fire from exposing other buildings and allows firefighting vehicles to access the entire perimeter.

Mezzanines (IBC Section 505)

A mezzanine is an intermediate level between the floor and ceiling of any story. Under the IBC, a mezzanine is not considered a story or part of the building area if:

• Area Limit: The aggregate area of the mezzanine does not exceed one-third of the floor area of the room or space in which it is located. For certain industrial or residential occupancies equipped with voice/alarm systems, this can be increased to one-half.
• Openness: The mezzanine must remain open to the room in which it is located, except for small service areas (restrooms, closets) or if the occupant load of the mezzanine does not exceed 10.
• Egress: The mezzanine must have at least two independent means of egress if the travel distance or occupant load exceeds the thresholds in Chapter 10.