Types Of Construction PDF

6 Types of Construction The classification of buildings by types of construction has been a part of most model codes from their inception. The Uniform Building Code contained these classifications in the first, 1927 edition. Those familiar with construction will understand the basic distinctions be...

6 Types of Construction The classification of buildings by types of construction has been a part of most model codes from their inception. The Uniform Building Code contained these classifications in the first, 1927 edition. Those familiar with construction will understand the basic distinctions between construction-type classifications. Definitions of building construction in the older model codes went into greater detail about construction materials for such elements as structural framework and stair treads. The new criteria define the subdivision of materials between “combustible” and “noncombustible” by their test performance under given conditions. This is a more precise definition, but more obscure for the casual code user who understands intuitively that steel or concrete are noncombustible materials versus how they perform in testing per ASTM E 119, Standard Test Methods for Fire Tests of Building Construction and Materials. TYPES OF CONSTRUCTION The IBC classifies all buildings into five broad categories based on the fire-resistance capabilities of the predominant materials used for their construction. The five types of construction are given Roman-numeral designations, and progress downward in fire resistance from Type I, the most fire-resistive construction, to Type V, the least fire resistive. The five types of construction classes are subdivided into two broad categories, A and B, based on the inherent fire resistance of the materials and the degree of fire protection applied over the structural members. “A” denotes “protected” construction and “B” denotes less protected construction. These are further subdivided according to the fire resistance gained by the application of protection to major elements of the construction systems. This fire-resistance rating is predicated on the protection of the elements from exposure to fires both from within the building and from adjacent structures. Height and Area Tables from § 504 and § 506 Construction Type (Most fire resistive Type I A B Type II A B Least fire resistive) Type III A B Type IV A B C HT Type V A B Group (Occupancy) The tables in § 504 and § 506 correlate the five types of construction with the allowable heights and areas for buildings, based on their occupancy. The designer must work with three sets of dependent variables when making the initial code analysis. The desired occupancy and the desired building height and area will determine the type of construction allowable under the code. Typically, for most projects, the use (occupancy) of the building is a given and the design choices are made about which construction types to use based on economics and design aesthetics. Table 601 defines the required fire resistance of major building elements for each type of construction. Note that Chapter 7 governs the actual materials and assemblies used in fire-resistancerated construction. § 602.1 refers code users to § 703.2, which contains the criteria for fireresistance ratings and fire tests. There is a direct relationship between occupancy type and number of occupants to the construction classification. Higher occupant quantities, more hazardous occupancies, and occupants with special needs, such as children and the infirm, all require more inherent structural fire resistance or additional levels of active fire-suppression systems. 84 / BUILDING CODES ILLUSTRATED—THE BASICS A B E F H I M R S U • Designer will usually enter from the occupancy-group side of the tables in Chapter 5. • Intended occupancy and desired building height and area will determine the type of construction allowable under the code. For example, a Group B occupancy housed in a three-story building with 19,000 sf (1765 m2) in area per floor, assuming no increases for sprinklers or frontage, could be of Type III-B construction per basic height and area allowances. The building may always be built of “more fireresistive” construction types to the left of Type III-B in the table. With the provision of sprinklers and the use of frontage increases, it may also be possible to use a “less fire-resistive” construction type to the right of Type III-B in the table. This is an iterative process. CONSTRUCTION CLASSIFICATION The broadest distinction between the various types of construction can be summarized in this table. Protected Elements Less Protected Elements Unprotected Elements Noncombustible Type I-A, II-A Type II-B Combustible — Type I-B (vis-à-vis Type I-A; still > Type II-A) — — Mixed Systems Type III-A Type III-B (Still has some fire resistance) — Mass Timber Heavy Timber Type IV-A — Type IV-C — Any Materials Type V-A Type IV-B Type IV — — • Decreased fire resistance Materials Type V-B • Decreased fire resistance • Note that levels of fire resistance decrease from left to right and top to bottom of this table. Noncombustible Materials The principal elements of construction Types I and II are made of noncombustible materials. The Uniform Building Code defined “noncombustible” as “material of which no part will ignite and burn when subjected to fire” (1997 UBC § 215). The IBC definition of noncombustibility is contained in § 703.3.1, and states that materials required to be noncombustible must meet the test criteria prescribed in the American Society for Testing and Materials (ASTM) Standard E136. Combustible Materials The elements of Types III, IV, and V construction allow the use of combustible materials in varying degrees. Additional levels of fire protection can increase the fire-resistance rating of these three types of construction. Note that construction types with mixed elements of noncombustible and combustible construction are considered combustible and are of Types III, IV, or V. • Noncombustible materials include masonry, concrete, and steel. • Note that application of additional fire protection materials to the noncombustible elements of Types I and II construction yields higher Type I-A, I-B, and II-A ratings, above the basic II-B classification for unprotected noncombustible construction. • There is no definition of “combustible construction” contained in the IBC. Technical dictionaries of construction or mining terms define it as “capable of undergoing combustion or of burning. Used esp. for materials that catch fire and burn when subjected to fire.” By inference these materials, such as wood and plastics, would be those that do not comply with the requirements for noncombustibility contained in ASTM E136. TYPES OF CONSTRUCTION / 85 PASSIVE FIRE PROTECTION • The principle of fire resistance is based on the desire to limit the spread of fire from: • one building to another, • one area to another, • one floor to another, to prevent structural collapse. • The requirements for the fire resistance of internal building elements are based on the theory of compartmentalization. The idea is to divide a building into distinct zones with barriers to retard the propagation and spread of fire. • Table 601 specifies the required fireresistance ratings of major building elements for the five types of construction. The fire resistance of building elements gives occupants time to leave the building in the event of an emergency and allows firefighters time to fight the fire by retarding structural failure for a designated time period. The categories of building elements are: • Structural frame • Bearing walls • Nonbearing wall requirements are noted in § 602, Table 601, and Table 705.5. Interior nonbearing wall and partition requirements are per Table 601. Exterior nonbearing wall requirements are per Tables 601 and 705.5. • Floor construction • Roof construction 86 / BUILDING CODES ILLUSTRATED—THE BASICS PASSIVE FIRE PROTECTION The classification by types of construction determines the level of passive fire resistance that is inherent in the building’s structure and envelope. This is distinct from active fire-suppression systems such as sprinklers. As we will discuss in Chapters 7 and 9, there can be trade-offs under the code between passive and active systems. The idea is to look at the building as a whole and provide a balance of fire protection to achieve a predetermined level of structural protection and occupant safety. It is important to remember that most building owners will opt for the lowest fire-resistance rating possible, as there is a direct relationship between providing fire resistance and cost of construction. The determination of construction type usually entails an analysis of both the desired occupancy and the requirements for fire separation distance between buildings or parts of buildings. This helps determine the necessary minimum fire resistance of the building elements. The building official will examine the classification assigned by the designer and make the final determination of classification. The building is to be looked at as a whole system and considered in the aggregate. § 602.1.1 notes that although portions of the building may exceed the requirements for the type and for the building occupancy, the whole building need not meet requirements higher than those necessary for the intended occupancy. • Passive fire resistance results from the use of construction materials and assemblies that can be expected to withstand exposure to fire without collapsing or exceeding a certain temperature on the side facing away from a fire. Examining the Chapter 5 height and area tables discloses that the application of fireresistive materials to construction materials can allow increases in the allowable heights and areas. This recognizes that combustible materials may be made more fire-resistive by the application of fire-retardant coverings to accomplish the goal of slowing the spread of fire and to extend the durability of the building structure in a fire. The application of passive fire-resistive materials can increase the durability of even noncombustible materials in a fire. Even steel or concrete, if unprotected, can lose strength under fire exposure. • Active fire protection consists of fire-detection and extinguishing systems—piping connected to a suitable water supply and supplied with valves or sprinkler heads made to open automatically at a certain temperature. • A study of the allowable height and area tables in Chapter 5 reveals that the provision of sprinklers typically allows for area and/or height increases for most construction types, independent of whether the construction materials are combustible or not, or are provided with passive fire-protection measures. TYPES OF CONSTRUCTION / 87 TABLE 601 Even if some building elements satisfy the fireresistance rating requirements for a higher type of construction, the building as a whole should be classified based on those elements that conform to the lowest type of construction that meets the minimum requirements of the code based on occupancy. Below is an abbreviated version of Table 601. Table 601 specifies the fire-resistance rating requirements for the major building elements, based on a specified type of construction or construction classification. A building may only be classified as a single type of construction unless a fire wall divides it into separate structures, or it falls into the special provisions for mixed construction permitted under § 510. Required Fire-Resistance Ratings in Hours Construction Type Type I A Type II Type III Type IV Type V B A B A B A B C HT A B 2a, b 1b 0 1b 0 3a 2a 2a HT 1b 0 2 2a 1 1 0 0 2 1 2 0 3 3 2 2 2 2 2 1/HT 1 1 0 0 Building Element* Primary Structural Frame** 3a, b (See Table 601, Footnote f) Bearing Walls** (See Table 601, Footnotes e and f) Exterior (see Table 705.5 & § 704.10) 3 Interior 3a Nonbearing Walls Requirements for nonbearing exterior walls and interior partitions are noted in § 602, Table 705.5, and § 2304.11.2. § 602 allows the use of fire-retardant-treated wood framing and sheathing in exterior wall assemblies of Type III or IV construction with a fire rating of 2 hours or less. § 2304.11.2 allows the use of cross-laminated timber in exterior walls under certain conditions. Floor Construction & Secondary Members (see § 202) Roof Construction & Secondary Members (see § 202) 2 2 1 0 1 0 2 2 2 HT 1 0 11/2b 1b,c 1b,c 0c 1b,c 0 11/2 1 1 HT 1b,c 0 * Under the previous model codes, elements such as stairways were also defined in terms of construction materials under the sections addressing types of construction. The new test-based criteria called out in § 1011.7 state that stairways be “built of materials consistent with the types permitted for the type of construction of the building.” This new definition may lead to confusion when applied in conjunction with § 603, which allows the use of combustible materials in Type I and Type II construction. ** The footnotes to Table 601 contain essential information for the designer to consider when making design decisions and when reviewing Table 601 in conjunction with the height and area tables in Chapter 5, which correlate allowable height and building areas with types of construction and occupancies. The designer should read the footnotes carefully, as they contain notable exceptions and trade-offs. While it is important to understand the footnotes to Table 601, the code user must use great caution when “reading up” from the footnotes to be certain that the provisions of the footnote apply to the specific building in question. The table should typically be read from top to bottom in the column that applies to the construction being analyzed. 88 / BUILDING CODES ILLUSTRATED—THE BASICS Footnotes a and b are worthy of note: Footnote a: 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. Footnote b: Except in Group F-1, H, M, and S-1 occupancies, fire protection of structural members in roof construction is not required, including protection of primary structural frame members, roof framing, and decking where every part of the roof construction is 20' (6096) or more above any floor immediately below. Fire-retardant-treated wood members are allowed to be used for such unprotected members. TABLE 601 Primary Structural Frame • The “primary structural frame” [P] is defined in § 202 as including all of the following structural members: 1. The columns 2. Structural members having direct connection to the columns, including girders, beams, trusses, and spandrels 3. Members of the floor construction and roof construction having direct connections to the columns 4. Members that are essential to the vertical stability of the primary structural frame under gravity loading (emphasis added) P P P P P Lateral only P P P P P We construe description 4 to mean that diagonal bracing that does not contribute to the vertical stability of the rest of the primary frame under gravity loading is not to be considered part of the primary framing system. Thus, such items as diagonal braces designed to carry only lateral loads would likely not require the same fire-resistance rating as the primary structural frame, even if connected to the columns. This is a subject to discuss in detail with the project structural engineer and with the AHJ early in the code-analysis process. • The other elements not making up the structural frame are covered by the requirements for floor and roof construction. TYPES OF CONSTRUCTION / 89 TABLE 601 ≥20' (6096) • Footnote a: This footnote recognizes that roof framing at interior bearing walls in Type I construction has a lighter fire load than does the primary structure for floors. • Footnote b: The conditions in this footnote refer to special provisions related to fire protection and building element protection in roof construction. Except in Group F-1, H, M, and S-1 occupancies, fire protection of structural members in roof construction shall not be required, including protection of primary structural frame members, roof framing, and decking where every part of the roof construction is 20' (6096) or more above any floor immediately below. Fire-retardant-treated wood members shall be allowed to be used for such unprotected members. This footnote has been revised to make clear that fire protection may be omitted from the primary structural frame members as well as from roof framing and decking. • Footnote c: Allows the use of heavy timbers where a 1-hour or less fire-resistance rating is required. This can even apply in buildings of “noncombustible” Type I or II construction. • Footnote d: This footnote is meant to remind the code user that other issues, such as corridor-rating requirements, may affect rating requirements for nonbearing walls. 90 / BUILDING CODES ILLUSTRATED—THE BASICS TABLE 601 • Footnote e: This footnote advises the designer to compare the requirements of both Tables 601 and 705.5 to determine the fire-resistive requirements for exterior bearing walls. It is possible that the requirements of Table 705.5 will require a higher fire-resistance rating for an exterior wall than Table 601; in the event of overlapping requirements, the most restrictive is to govern per § 102.1. For example, a Group M occupancy in a Type V-A building located within 5' (1524) of a property line would require a 2-hour wall rating per Table 705.5 and only a 1-hour rating per Table 601. In this case the 2-hour requirement would govern, as the code requires the most restrictive provision to govern per § 102.1. We also recommend examining § 705 for additional provisions that apply to exterior walls. • Footnote f: This footnote refers the user to § 704.10 for the fire resistance of the primary structural frame. • Footnote g to Table 705.5 correlates the requirements for fire-resistance ratings in nonbearing walls with the allowances for the amount of wall openings as described in Table 705.8. Where unlimited openings are allowed per Table 705.8, the wall rating may be 0 hours. This typically occurs at a fire separation distance of 20' (6096) in sprinklered buildings. • Footnote i to Table 705.5 allows unprotected exterior walls in R-3 occupancies of Type II-B or V-B construction. TYPES OF CONSTRUCTION / 91 TYPES I AND II CONSTRUCTION The building elements of Type I and Type II construction are of noncombustible materials. As noted previously, the definition of noncombustible is contained in § 703.3 and requires meeting the criteria of ASTM E 136. § 603 contains a list of notes specifying which combustible materials can be used in buildings of Types I and II construction. • Type I-A construction, providing the highest level of fire-resistance-rated construction, requires passive protection for all elements of the structure. • Type I-B construction is similar to Type I-A construction but permits a 1-hour reduction in fire-resistance rating for the structural frame and bearing walls, and a 1/2-hour reduction for roof construction. • Type II-A construction requires passive protection of all elements of the structure. • Type II-B construction allows unprotected noncombustible building elements. This type of construction was described as “Type II-nonrated” in some previous model codes. ≤6' (1829) Combustible Materials in Types I and II Construction The key to the use of combustible materials in a noncombustible construction type is understanding that these uses are ancillary to the primary structure of the building. The premise for allowing the use of these combustible materials is that they will be of limited quantity and used under defined conditions where they will not contribute in any large measure to compromising the desired level of fire resistance in the structure. As one can see, the rules for Construction Types I and II, while calling for noncombustible materials, allow for a number of exceptions when all the requirements are taken together. 92 / BUILDING CODES ILLUSTRATED—THE BASICS 1. Note 1 calls out allowances for the use of fire-retardant-treated wood for nonbearing walls and partitions, and for roof construction. 2. Thermal and acoustical insulation materials with a flame spread of less than 25 have ratings corresponding to Class A interior finishes per ASTM E 84. These indices may be higher, as noted in the exception, where insulation is encapsulated between layers of noncombustible materials without an air space. 3. Foam plastics are allowable if compliant with the provisions of Chapter 26 of the code. Note especially the provisions of § 2603.5 regarding the use of foam plastics on the exterior of Type I, II, III, and IV buildings. 4. Most roof coverings have a classification of A, B, or C, so this should almost always be usable. 5. The code recognizes that combustible decorative and utilitarian interior finishes, such as wood floors, will be applied over the noncombustible structural elements if installed per § 804. 6. Millwork such as doors, door frames, window sashes, and window frames are acceptable. 7. Interior wall and ceiling finishes are acceptable if installed per § 801 and 803. 8. Trim installed per § 806. 9. Wood trim at or near grade level is acceptable where not beyond ready firefighting access up to a level of 15' (4572) above grade. 10. This note requires fire-stopping in wood floors in Type I and II construction per § 805. This is similar to Note 5. 11. The key item in this section is that the area in question be occupied and controlled by a single tenant. These provisions do not apply for multi-tenant spaces. Also, these partitions must not define exit access passages that could be construed as corridors for the use of more than 30 occupants. This note also recognizes that single-tenant floors can have partitions of wood or similar light construction if they are lower than 6' (1829) and allow standing occupants to generally survey the occupied space in the event of an emergency. TYPES I AND II CONSTRUCTION 12. Stages and platforms are defined in § 410 as raised areas used for worship, lectures, music, plays, or other entertainment. 13. See specifically the provisions of Chapter 14 and § 705.2.3.1 regarding the relationship of fire separation to the fire-resistive properties of combustible wall coverings, bays, and balconies. Note also that plastic veneers are to comply with Chapter 26 of the code. Projections from exterior walls are specified in § 705, and these too are related to the type of construction. § 705 in turn refers also to § 1405 regarding the use of combustible materials on the exterior of buildings. 14. This note acknowledges the necessity for combustible blocking and nailers for installation of finishes and cabinetry as being incidental to the structural fire resistance of the building as a whole. 15. Plastic glazing materials are permissible in conformance with the requirements of Chapter 26 of the code. 16. This is similar to the allowance for use of combustible materials per Note 14. 17. Decorative exterior plastic veneer is allowable if installed in accordance with § 2605.2. 18. This is similar to the allowance for use of combustible materials per Note 14. 19. This is a cross-reference to Footnote c of Table 601 and is redundant. 20. These aggregates are allowed to be combustible by § 703.2.2 with the proviso that the assembly meets the code’s fire-resistance test criteria. 21. Fire-resistance materials may not strictly be considered noncombustible and must be applied under special inspection per § 1705.15 and § 1705.16. 22–25. These exceptions are similar to Note 20 and allow for combustible materials to be used if they meet specific criteria spelled out in other sections of the IBC or in other I codes. 26. Allows combustible materials in wall construction of freezers and coolers that are less than 1,000 sf (92.9 m2) when the walls are lined on both sides with noncombustible materials and the building is protected throughout with an automatic sprinkler system. 27. This item allows for the use of wood nailers for parapet flashing and roof cants, recognizing that these elements are needed for proper fastening of roofing materials. TYPES OF CONSTRUCTION / 93 TYPE III CONSTRUCTION Type III buildings are a mix of noncombustible and combustible elements, having noncombustible exterior walls and combustible interior construction. These building types arose in the U.S. at the end of the 19th century out of a desire to end the kind of conflagrations that struck congested business districts such as in Chicago. The buildings were designed to prevent a fire from spreading from building to building by requiring noncombustible exterior walls of buildings. • The construction materials on the exterior of a Type III building are required to be of noncombustible materials. Fire-retardanttreated wood is allowable in exterior walls where the required fire-resistance rating is 2 hours or less. Table 601 requires 2-hour walls for Type III-A buildings; thus fire-retardanttreated wood is acceptable except where this is superseded by the requirements of Table 705.5 based on fire separation distance. This would only occur in Group H occupancies with a fire separation distance of less than 5' (1524). • Type III buildings are considered combustible since the code allows their interior building elements to be of combustible materials and also to be of unprotected construction if allowed by the building height and area allowances based on occupancy. • A very common example of a Type III-B building is the tilt-up warehouse. This building type has concrete exterior walls, with a wood-framed roof supported on unprotected steel pipe columns at the interior of the building. 94 / BUILDING CODES ILLUSTRATED—THE BASICS TYPE IV CONSTRUCTION There is a new set of construction types in the 2021 IBC. Recognizing the development of new “mass timber” production techniques, the category of Type IV buildings has been greatly expanded. Per IBC § 602.4, Type IV is that type of construction in which the building elements are mass timber or noncombustible materials and have fire-resistance ratings in accordance with Table 601. Mass timber elements are to meet the fire-resistance-rating requirements of this section based on either the fire-resistance rating of the noncombustible protection, the mass timber, or a combination of both. Mass timber elements of Types IV-A, IV-B, and IV-C construction are to be protected with noncombustible elements applied directly to the mass timber. The old Type IV construction is now labeled as IV-HT for “heavy timber.” The requirements and code criteria for those previous Type IV buildings remain unchanged from previous code editions. We have chosen to introduce the new Type IV classifications in reverse order from how they occur in the code—Type IV-C is introduced first, then Type IV-B, and Type IV-A last. Because mass timber elements in Type IV-C construction may be unprotected, they can be introduced and illustrated more clearly without the partial fire-protective coverings required by Type IV-B and the full fire-protective coverings required for Type IV-A. In this way, the graphics for Types IV-B and IV-A construction can have more of their mass timber elements partially or fully covered with fire-protective materials. As noted, the size of buildings in terms of their height and area is related to the fire resistance of each level of Type IV construction. More fire resistance means a larger building in both height and area. The code defines mass timber as structural elements “primarily of solid, built-up, panelized, or engineered wood products that meet minimum cross-section dimensions of Type IV construction.” Products in the mass timber family include: • Cross-laminated timber (CLT) is a prefabricated, engineered wood product consisting of three, five, or seven layers of dimension lumber oriented at right angles to one another and bonded under pressure with adhesive to form structural panels. • Nail-laminated timber (NLT) is created by layering 2× dimension lumber on edge and fastening the assembly with nails or screws to produce a larger structural element. • Dowel-laminated timber (DLT) is made by placing 2× dimension lumber on edge (like NLT) and friction-fitting together with wood dowels. • Glue-laminated timber (glulam; GLT) is made by laminating stress-grade lumber with adhesive under controlled conditions, usually with the grain of all plies being parallel. • Parallel strand lumber (PSL) is a structural lumber product made by bonding long, narrow wood strands together under heat and pressure using a waterproof adhesive. • Laminated veneer lumber (LVL) is a structural lumber product made by bonding layers of wood veneers together under heat and pressure using a waterproof adhesive. TYPES OF CONSTRUCTION / 95 TYPE IV CONSTRUCTION Type IV-A, IV-B, and IV-C Comparison Table Example for Group B occupancy, assuming no frontage/open space increase: 9 stories 12 stories 18 stories Type IV-C Up to 9 stories Max. height: 85' (25 908) Max. area: 405,000 sf Type IV-B Up to 12 stories Max. height: 180' (54 864) Max. area: 648,000 sf Type IV-A Up to 18 stories Max. height: 270' (82 296) Max. area: 972,000 sf General description § 602.4.3 Type IV-C Building elements are generally unprotected with mass timber construction elements exposed. § 602.4.2 Type IV-B Building elements are generally protected with certain areas of mass timber exposed. § 602.4.1 Type IV-A Building elements are fully protected with mass timber elements not visible inside or outside of the building. Exterior protection on outside face of exterior walls § 602.4.3.1. Exterior side of walls of combustible construction is to be protected with noncombustible protection having a minimum assigned time of 40 minutes, except for waterresistive barriers having a specified fire resistance, flame spread, and smoke generation. § 602.4.2.1. Outside face of exterior walls of mass timber construction is to be protected with noncombustible protection having a minimum assigned time of 40 minutes, except for waterresistive barriers having a specified fire resistance, flame spread, and smoke generation. § 602.4.1.1. Outside face of exterior walls of mass timber construction is to be protected with noncombustible protection having a minimum assigned time of 40 minutes, except for waterresistive barriers having a specified fire resistance, flame spread, and smoke generation. Interior fire protection § 602.4.3.2. Mass timber elements coverings on mass may be unprotected. timber structure § 602.4.2.2. Interior faces of all mass timber elements, including the inside face of exterior mass timber walls and mass timber roofs, are to be protected, as required by this section. See exceptions to § 602.4.2.2 for allowances for exposed mass timber. § 602.4.1.2. Interior faces of all mass timber elements, including the inside faces of exterior mass timber walls and mass timber roofs, are to be protected. Protection time for None; protection not required. interior protection § 602.4.2.2.1. Noncombustible protection is to contribute not less than 80 minutes. § 602.4.1.2.1. Noncombustible protection is to contribute not less than 80 minutes. Protection time and area None; protection not required. for interior protection § 602.4.2.2.2. Faces of all mass timber elements, including the inside face of exterior mass timber walls and mass timber roofs, are to be protected. See exceptions to § 602.4.2.2.2 for allowances for exposed mass timber. Not applicable to Type IV-A. Full protection is required. C 96 / BUILDING CODES ILLUSTRATED—THE BASICS B A TYPE IV CONSTRUCTION Type IV-A, IV-B, and IV-C Comparison Table C Type IV-C B Type IV-B A Type IV-A Exceptions for percentages None; protection not required. of unprotected mass timber. § 602.4.2.2.2. See exceptions for allowances for exposed mass timber. Not applicable to Type IV-A. Full protection is required. Floor protection on top of mass timber floor structure § 602.4.3.3. Floor finishes in accordance with § 804 shall be permitted on top of the floor. Construction is the same as having the floor unprotected. § 602.4.2.3. The floor assembly shall contain a noncombustible material not less than 1" (25.4) in thickness above the mass timber floor. Floor finishes in accordance with § 804 shall be permitted on top of the noncombustible material. § 602.4.1.3. The floor assembly shall contain a noncombustible material not less than 1" (25.4) in thickness above the mass timber floor. Floor finishes in accordance with § 804 shall be permitted on top of the noncombustible material. Roofs and roof § 602.4.3.4. Roof coverings in covering protection on accordance with Chapter 15 shall top of mass timber be permitted on the outside surface roof structure of the roof assembly. § 602.4.2.4. The interior surfaces of roof assemblies shall be protected in accordance with § 602.4.2.2, except in occupiable spaces, they shall be treated as a concealed space with no portion left unprotected. Roof coverings in accordance with Chapter 15 are permitted on the outside surface of the roof assembly. § 602.4.1.4. The interior surfaces of roof assemblies are to be protected. Roof coverings in accordance with Chapter 15 are permitted on the outside surface of the roof assembly. Concealed spaces may contain limited amounts of noncombustible materials. § 602.4.3.5. Concealed spaces may not contain combustibles other than electrical, mechanical, fire protection, or plumbing materials, and equipment permitted in plenums in accordance with § 602 of the International Plumbing Code. Combustible construction forming concealed spaces shall be protected with noncombustible protection having a minimum assigned time of 40 minutes. § 602.4.2.5. Concealed spaces may not contain combustibles other than electrical, mechanical, fire protection, or plumbing materials, and equipment permitted in plenums in accordance with § 602 of the International Plumbing Code. Combustible construction forming concealed spaces shall be protected in accordance with § 604.4.1.2. § 602.4.1.5. Concealed spaces may not contain combustibles other than electrical, mechanical, fire protection, or plumbing materials, and equipment permitted in plenums in accordance with § 602 of the International Plumbing Code. Combustible construction forming concealed spaces shall be protected in accordance with § 604.4.1.2. Shaft protection for stairs and elevator hoistways § 602.4.3.6. Shafts shall be permitted in accordance with § 713 and § 718. Shafts and elevator hoistway and interior exit stairway enclosures shall be protected with noncombustible protection having a minimum assigned time of 40 minutes on both the inside and the outside of the shafts. § 602.4.2.6. Shafts shall be permitted in accordance with § 713 and § 718. Both the shaft side and room side of mass timber elements shall be protected in accordance with § 602.4.1.2. § 602.4.1.6. Shafts shall be permitted in accordance with § 713 and § 718. Both the shaft side and room side of mass timber elements shall be protected in accordance with § 602.4.1.2. TYPES OF CONSTRUCTION / 97 TYPE IV-C CONSTRUCTION Mass timber elements in buildings of Type IV-C construction may typically be unprotected, except for the exterior side of walls of combustible construction or of mass timber, which must have additional exterior protection with a minimum assigned resistance time of 40 minutes. § 602.4.3.1 prohibits all combustible materials on the exterior side of mass timber walls, except for code-compliant water-resistive barriers. • For roofs, any materials complying with IBC Chapter 15 are acceptable. Additional protection is not required. • For flooring, any materials complying with § 804 are acceptable. Additional protection is not required. • Exterior finish over water-resistant barrier. • Exterior of exterior wall is to have a 40-minute noncombustible barrier [ 5/8” (16) Type-X gypsum board or equal]. • Exterior mass timber wall or other combustible wall material. • Interior face of mass timber wall may be exposed. • Mass timber beams, columns, and floors may be fully unprotected in Type IV-C construction. 98 / BUILDING CODES ILLUSTRATED—THE BASICS TYPE IV-B CONSTRUCTION Mass timber elements in buildings of Type IV-B construction must typically be protected, except that a certain percentage of mass timber may be exposed per § 6024.2.2.2. Also, the outside face of exterior walls of combustible construction or of mass timber must have additional exterior protection with a minimum assigned resistance time of 40 minutes. § 602.4.2 prohibits all combustible materials on the exterior side of mass timber walls, except for code-compliant waterresistive barriers. • Interior faces of mass timber roofs at occupied spaces are to be protected, with some percent exposed as permitted per Equation 6-1. Roof covering is to comply with IBC Chapter 15. • The floor assembly is to have a noncombustible material not less than 1" (25 mm) in thickness above the mass timber. Floor finishes in accordance with § 804 shall be permitted on top of the noncombustible material. The interior surfaces that face occupied spaces below the mass timber are to be fire protected. • Exterior finish over water-resistant barrier. • Exterior of exterior wall is to have a 40-minute noncombustible barrier [ 5/8” (16) Type-X gypsum board or equal]. • Exterior mass timber wall. • Interior face of mass timber wall is to be fire protected. Portions may be unprotected per Equation 6-1. • Mass timber beams, attached columns, and ceilings may be partially unprotected in Type IV-B construction per Equation 6-1. • Isolated mass timber columns may be unprotected in Type IV-B construction. TYPES OF CONSTRUCTION / 99 TYPE IV-A CONSTRUCTION Mass timber elements in buildings of Type IV-A construction must be fully protected. Also, the outside face of exterior walls of combustible construction or of mass timber must have additional exterior protection with a minimum assigned resistance time of 40 minutes. § 602.4.1.1 prohibits all combustible materials on the exterior side of mass timber walls, except for code-compliant water-resistive barriers. • Interior faces of mass timber roofs are to be protected. Roof covering is to comply with IBC Chapter 15. • Floor assembly is to have a noncombustible material not less than 1" (25 mm) in thickness above the mass timber. Floor finishes in accordance with § 804 shall be permitted on top of the noncombustible material. The interior surface of the mass timber that faces the occupied space below is to be fire protected. • Exterior finish over water-resistant barrier. • Exterior of exterior wall is to have a 40-minute noncombustible barrier [ 5/8” (16) Type-X gypsum board or equal]. • Exterior mass timber wall. • Interior faces of all mass timber elements, including inside faces of exterior mass timber walls, are to be fire-protected. • Mass timber beams, columns, and floors are to be fully protected in Type IV-A construction. 100 / BUILDING CODES ILLUSTRATED—THE BASICS TYPE IV-HT CONSTRUCTION § 602.4.4 Type IV-HT Type IV-HT (Heavy Timber) is that type of construction in which the exterior walls are of noncombustible materials and the interior building elements are of solid wood, laminated heavy timber, or structural composite lumber (SCL), without concealed spaces or with concealed spaces complying with § 602.4.4.3. The minimum dimensions for permitted materials, including solid timber, glued-laminated timber, SCL, and cross-laminated timber (CLT), and the details of Type IV construction shall comply with the provisions of this section and § 2304.11. Exterior walls complying with § 602.4.4.1 or 602.4.4.2 shall be permitted. Interior walls and partitions not less than 1-hour fire-resistance rated or heavy timber conforming with § 2304.11.2.2 shall be permitted. • This type of construction utilizes heavy-sawn timbers in older buildings but will likely use glued-laminated structural wood members in modern construction. Type IV-HT buildings came about to address fire-safety conditions for traditional methods of building manufacturing and storage buildings, as Type III buildings did for office and residential occupancies. The type of construction used in Type IV buildings is known as “mill construction” or “heavy timber” construction. These buildings utilize heavy timber structural members and heavy wood floor decking inside exterior walls of noncombustible construction. Many of these buildings also have movable heavy metal shutters to close off exterior openings to prevent a fire outside the building from propagating into the building through unprotected openings. The criteria for “heavy timber” (HT) construction are based on the past performance of historical construction, not on the scientific rationales contained in ASTM E 136 and E 119. These buildings have a good empirical performance record in fires. The insurance industry promoted construction of these types of buildings during the late 1800s and into the 1900s to limit their fire losses. • Exterior walls of noncombustible construction Type IV buildings generally burn slowly under fire conditions. The heavy timber members begin to flame and char at about 400°F (204.4°C). As the charring continues, it retards further deterioration of the wood members by insulating the interior of the wood members from the fire. TYPES OF CONSTRUCTION / 101 TYPE IV-HT CONSTRUCTION • Per changes adopted in the 2018 IBC, cross-laminated timber may be used in Type IV-HT construction. The material is “crosslaminated timber” (CLT). It is defined as: “A prefabricated engineered wood product consisting of not less than three layers of solid-sawn lumber or structural composite lumber where the adjacent layers are cross oriented and bonded with structural adhesive to form a solid wood element.” • Cross-laminated timber (CLT) not less than 4" (102) in thickness complying with § 2303.1.4 is permitted within exterior wall assemblies with a 2-hour rating or less. Heavy timber structural members appurtenant to the CLT exterior wall shall meet the requirements of Table 2304.11 and be fire-resistance rated as required for the exterior wall. • The exterior surface of the cross-laminated timber and heavy timber elements is to be protected by one the following: • Fire-retardant-treated wood siding at least 15/32" (12) thick, or • 1/2" (12.7) minimum gypsum board, or • A noncombustible material or or Siding 102 / BUILDING CODES ILLUSTRATED—THE BASICS CLT [Min. 4" (102) thick] TYPE IV-HT CONSTRUCTION • A key element of heavy timber construction is that there are no concealed spaces in the building elements. This prevents fire from smoldering inside building elements and propagating unobserved. This is part of the system of this type of construction to prevent sudden structural collapse. • Detailed requirements for heavy timber construction that used to appear in § 602.4 have been moved and now appear in § 2304.11 with other wood construction requirements. ≥ 20' (6096) § 602.4.4.3. Concealed Spaces Concealed spaces have historically been prohibited in heavy timber buildings. They are now allowed if they meet certain conditions. They may not contain combustible materials other than building elements and electrical, mechanical, fire protection, or plumbing materials and equipment permitted in plenums in accordance with § 602 of the International Mechanical Code. Concealed spaces shall comply with applicable provisions of § 718, which addresses fireblocking and draftstopping requirements in combustible concealed locations. Concealed spaces are to be protected in accordance with one or more of the following: 1. The building must be sprinklered throughout and to be provided also in the concealed space. 2. The concealed space is to be completely filled with noncombustible insulation. • Note that some fire separation criteria for heavy timber buildings are noted in § 602.4, not in Table 705.5 as for other types of construction. • Per § 602.4.4.4, where a horizontal separation of 20' (6096) or more is provided, wood columns and arches conforming to heavy timber sizes complying with § 2304.11 are permitted to be used externally. Items inside concealed spaces must be noncombustible, except for building elements and electrical, mechanical, fire protection, or plumbing materials and equipment permitted in plenums in accordance with the International Mechanical Code. Concealed spaces shall be protected in accordance with one of the following: • Option 1: Sprinkler the building and sprinkler concealed spaces. (Not required within interior walls and partitions if 1-hour or greater protection is provided per the Exception to § 602.4.4.3.) • Option 2: Fill concealed spaces fully with noncombustible insulation (for the entire length of the concealed elements inside the space). • Option 3: Apply minimum of 5/8" (16) Type-X gypsum board on the inside surfaces of the concealed spaces. 3. Surfaces within the concealed space area to be fully sheathed with not less than 5/8" (16) Type-X gypsum board. Exception: Concealed spaces within interior walls and partitions having a 1-hour or greater fireresistance rating complying with § 2304.11.2.2 do not require additional protection. TYPES OF CONSTRUCTION / 103 TYPE V CONSTRUCTION Type V construction is the least restrictive construction type. It allows the use of any materials permitted by the code. A typical example of Type V construction is the conventional light-woodframed single-family residence. • Because any element of Type V construction may be combustible, the fire resistance of building elements is typically provided by the application of fire-resistant materials, such as gypsum board, to the building parts. • Type V-A construction is protected construction and all major building elements must therefore have a 1-hour fire-resistance rating. The only exception to this is for nonbearing interior walls and partitions contained in Table 601. • Type V-B construction is unprotected and requires no fire-resistance ratings, except Table 705.5 requires exterior-wall protection. 104 / BUILDING CODES ILLUSTRATED—THE BASICS