How to Determine a Building’s Construction Type
August 14, 2010
Determining the type of building construction is dependent on quite a number of things. This article sets forth the various considerations to keep in mind when assessing the building type.
Steps
- Note that all construction types must be classified into the following:
- Frame
- Joisted Masonry
- Light Noncombustible
- Masonry Noncombustible
- Modified Fire Resistive
- Fire Resistive
- Note that there are two major sources identifying these construction types: International Building Code (IBC) and Insurance Services Office (ISO). ISO is traditionally what insurance companies use to denote type; however, IBC is what Architects and Builders use. The following explains what is expected under both:
- International Building Code (IBC): The International Building Code (IBC) is a model building code developed by the International Code Council (ICC). It has been adopted throughout most of the United States. A large portion of the International Building Code deals with fire prevention. It differs from the related International Fire Code in that the IBC handles fire prevention in regards to construction and design and the fire code handles fire prevention in an on-going basis. Parts of the code reference other codes including the International Plumbing Code, the International Mechanical Code, the National Electric Code, and various National Fire Protection Association Standards.
- Insurance Services Office (ISO): The Insurance Services Office, Inc. (ISO) is a provider of data, underwriting, risk management and legal/regulatory services to property-casualty insurers and other clients.
- Be aware of both IBC and ISO classification styles. While one company may use ISO classifications, many submission documents may reference IBC classifications and it is important to be able to convert this to an ISO classification. Why is it important to know this? Often a submittal will indicate an IBC type, not an ISO type, and there have been situations where a frame building has been classified as FR because the submittal was read incorrectly!
Frame Construction (ISO Class I, IBC Type V)
Frame construction is ISO Class 1. ISO Class 1 encompasses IBC Type VA and IBC Type VB. Regardless of whether the IBC classification is A (protected) or B (unprotected) the ISO Class is 1.
- Note the elements of frame building construction:
- Frame buildings are buildings with exterior walls, floors, and roofs with combustible construction — or buildings with exterior walls of noncombustible or slow-burning construction with combustible floors and roofs.
- Frame buildings generally have roof, floor, and supports of combustible material, usually wood, and combustible interior walls.
- Two variations on frame construction don’t change the construction class:
- Masonry veneer (brick veneer)- Masonry veneer is thin layers of brick, stone, or stucco, used for appearance purposes rather than structural support.
- Metal clad – A building with a metal exterior wall may not look like frame construction, but when the metal skin is attached to wood studs and joists, ISO classifies the building as frame.
- Other Conditions That Lead to Classification as Frame Construction include:
- Metal walls or floors sheathed with combustible materials
- Metal floors or roofs with combustible insulation or ceiling material attached to the underside or within 18 inches of horizontal supports
- Composite assemblies of noncombustible materials with combustible materials
- Be familiar with the advantages of frame construction:
- Frame construction offers these advantages:
- easy to erect and alter
- economical
- versatile
- performs well in Earthquake areas – can move
- Be familiar with frame construction disadvantages:
- Frame construction has these disadvantages:
-
- fire can spread rapidly
- highly damageable
- may become unstable in a fire
- may include enclosed spaces where fire can spread undetected
-
Joisted masonry (ISO Class 2, IBC Type III, IBC Type IV)
Joisted Masonry construction is ISO Class 2. ISO Class 2 encompasses IBC Type IIIA and IBC Type IIIB. Regardless of whether the IBC classification is A (protected) or B (unprotected) the ISO Class is 2. IBC Type IV is Heavy Timber construction and is considered ISO Class 2. The reason is that the heavy timbers perform well and do not fail early in a fire.
- Learn the elements of Joisted Masonry construction. Joisted Masonry buildings are buildings with exterior walls of masonry or fire-resistive construction rated for not less than one hour and with combustible floors and roofs. There are several types of masonry used in the exterior bearing walls of joisted masonry buildings:
- brick
- concrete — either reinforced or non reinforced
- hollow concrete masonry units
- tile
- stone
- Note that exterior bearing walls may also be any noncombustible materials with fire-resistance ratings of not less than one hour.
- Be familiar with the variations of Joisted Masonry Construction. There’s one variation on joisted masonry construction that doesn’t change the construction class — heavy timber or mill construction. Heavy timber construction uses wood members much larger than those found in frame (Construction Class 1) or other joisted masonry construction. If the building uses steel columns or beams for walls, the beams must be protected so they have a fire-resistance rating of not less than one hour. Heavy Timber Construction (IBC Type IV); ISO classifies the building as heavy timber construction if it meets these requirements:
- walls of masonry construction
- floors of 3 inch wood plank or 4 inch laminated plank, both surfaced with 1 inch flooring
- roof of 2 inch wood plank, 3 inch laminated plank, or 1-1/8 inch tongue-and-groove plywood deck
- wood column supports not less than 8 inch x 8 inch, wood beams or girders not less than 6 inch x 6 inch, or protected metal
- Understand the advantages of jointed masonry:
- harder to ignite
- consumed more slowly by fire
- more structural stability
- greater salvage value
- lack of concealed spaces (Heavy Timber)
- Be aware that jointed masonry construction has these disadvantages:
- floors and roofs of combustible materials subject to damage by fire
- presence of concealed spaces
Light noncombustible (ISO Class 3, IBC Type IIB)
Light Noncombustible construction is ISO Class 3. ISO Class 3 encompasses IBC Type IIB (unprotected).
- Familiarize yourself with the elements of Light Non Combustible construction. Light Non Combustible buildings are buildings with exterior walls of light metal or other Noncombustible material and with Noncombustible floors and roofs:
- buildings with exterior walls, floors, and roofs of noncombustible or slow-burning materials
- building supports of noncombustible or slow-burning materials
- noncombustible or slow-burning roof decks on noncombustible or slow-burning supports — regardless of the type of insulation on the roof surface
- Be aware of the noncombustible advantages:
- easy to erect
- economical to build
- uses materials that don’t readily burn
- Note the noncombustible disadvantages:
- contains steel, which looses strength at high temperatures
- highly damageable buildings
- unstable buildings under fire conditions
- uses slow-burning materials that do burn — adding fuel to a fire
Masonry Noncombustible (ISO Class 4, IBC Type IIA)
Masonry Noncombustible construction is ISO Class 4. ISO Class 4 encompasses IBC Type Type IIA (protected).
- Learn the elements of masonry non-combustible construction. Masonry non-combustible buildings are buildings with exterior walls of masonry material and with noncombustible or slow burning floors and roofs.
- buildings with exterior walls of masonry — not less than four inches thick, or
- buildings with exterior walls of fire-resistive construction — with a rating of not less than one hour, and
- noncombustible or slow-burning floors and roofs — regardless of the type of insulation on the roof surface
- Note the advantages of noncombustible construction:
- Uses floors and roof supported by superior exterior bearing members that provide for stability and are less likely to collapse during a fire
- Uses materials that don’t readily burn
- Note that masonry noncombustible construction has these disadvantages:
- Uses unprotected steel for interior members of floors and roof, and steel loses strength and becomes less stable and more damageable at high temperatures
- Uses slow-burning materials that do burn — adding fuel to a fire
Modified Fire Resistive (ISO Class 5, IBC Type IB)
Modified Fire Resistive construction is ISO Class 5. ISO Class 5 encompasses IBC Type IB.
- Learn the elements of Modified Fire Resistive Construction. Modified Fire Resistive Buildings are buildings where the exterior bearing walls and load-bearing portions of exterior walls must be of noncombustible materials or of masonry, but exterior nonbearing walls and wall panels may be slow-burning, combustible, or with no fire- resistance rating.
- Buildings with exterior walls, floors, and roofs of masonry materials described in the definition of fire resistive (Construction Class 6) — less thick than required for fire-resistive structures but not less than four inches thick, or
- Fire-resistive materials with a fire-resistance rating less than two hours but not less than one hour
- Note that modified fire-resistive buildings also include structural steel protection techniques — fire-protection material applied to steel. Materials include:
- concrete
- plaster
- clay tile
- brick or other masonry units
- gypsum block
- gypsum wallboard
- mastic coatings
- mineral and fiberboard
- mineral wool
Ceilings Protecting Steel Beams or Joists
What happens when there is no fire-protection material applied to steel beams or joists that support floors or roofs? ISO still considers a building modified fire resistive if it has a suitable ceiling. Ceilings can be plaster or gypsum wallboard or suspended mineral tile. The entire floor-ceiling (a fire-resistive ceiling protecting a floor) or roof-ceiling (a fire-resistive ceiling protecting roof supports) should conform to construction details in a UL-listed or Factory Mutual (FM)-approved design. ISO individually evaluates each approved design.
- Note the advantages of the modified fire resistive construction:
- uses noncombustible materials
- allows greater height and area than other construction classes
- uses load-bearing members or assemblies that resist damage from fire
- Note that modified fire-resistive construction has these disadvantages:
- expensive to construct and repair
- provides a false sense of security
Fire Resistive (ISO Class 6, IBC Type IA)
Fire Resistive construction is ISO Class 6. ISO Class 6 encompasses IBC Type IA.
- Learn the elements of Fire Resistive Construction. The exterior bearing walls and load-bearing portions of exterior walls must be of noncombustible materials or of masonry, but exterior nonbearing walls and wall panels may be slow burning, combustible, or with no fire-resistance rating.
- Walls:
- solid masonry, including reinforced concrete not less than four inches thick
- hollow masonry not less than 12 inches thick
- hollow masonry less than 12 inches thick, but not less than eight inches thick with a listed fire-resistance rating of not less than two hours
- assemblies with not less than a two-hour fire-resistance rating
- Floors and roofs:
- reinforced concrete not less than four inches thick
- assemblies with not less than a two-hour fire-resistance rating
- Structural metal supports:
- Horizontal and vertical load-bearing protected metal supports — including pre stressed and post tensioned concrete units — with not less than a two-hour fire-resistance rating
- Walls:
Pre- and Post tensioned Concrete
Both pre- and post tensioned concrete units have steel cables installed in the concrete to provide tensile strength. With pre stressed concrete units, builders pull the cables tight before pouring the concrete and release them as the concrete cures. With post tensioned concrete units, builders pull one end of the cable tight after pouring the concrete.
- Note the fire resistive advantages:
- uses noncombustible materials
- allows greater height and area than other construction classes
- uses load-bearing members or assemblies that resist damage from fire
- Note that fire-resistive construction has these disadvantages:
- expensive to construct and repair
- provides a false sense of security
Tips
- IBC is more descriptive and also includes A or B types of construction for each class.
- A is protected, meaning that all structural members of a building or structure have an additional fire rated coating or cover by means of sheetrock, spray on, or other approved method. The additional fire rated coating or cover extends the fire resistance of the structural members by at least 1 hour.
- B is unprotected, meaning that all structural members of a building or structure have no additional fire rated coating or cover. Exposed members are only fire resistant according to their natural ability, characteristics, and fire rating.
- How to Classify a Building Class? As stated in the introduction, all buildings must be classified into one of the 6 ISO construction classes. Classification of a building class is based on two factors; Building Elements and Fire Resistance Rating. These factors may not be included in the submission and additional information will need to be requested. First classification is based on the following building elements:
- 1. Structural frame,
- 2. Exterior bearing walls,
- 3. Interior bearing walls,
- 4. Exterior nonbearing walls and partitions,
- 5. Interior nonbearing walls and partitions,
- 6. Floor construction including supporting beams and joists,
- 7. Roof construction including supporting beams and joists are comprised of.
- The building materials used in the construction of the above elements are the foundation for classification, be it wood, steel, or masonry.
- Fire-Resistance Rating is the other factor in determining construction class. The building materials used in the construction of the building elements above will have a fire-resistance rating. Fire-Resistance rating typically means the duration for which a passive fire protection system can withstand a standard fire resistance test. This can be quantified simply as a measure of time, or it may entail a host of other criteria, involving other evidence of functionality or fitness for purpose.
- The building materials and building elements must meet the fire-resistance rating criteria for the construction class selected. Fire-resistance rating can simply be 0 hours, 1 hour, or 2 hour.
- Minimum Rule – It is important to remember when selecting the construction class that the building is only as strong as its weakest element. For example, a masonry building may have an unprotected wood roof. The wood roof is the weakest member such that it has no fire-resistance. Thus the construction class would be Joisted Masonry. Now imagine this same building with a metal deck roof. So long as the supporting members of the building do not contain wood then this building would be Masonry Noncombustible.
- What to ask? To determine the ISO Class of the building we must know the following composition of the building elements:
- Structural frame?
- Bearing walls (interior and exterior)?
- Floor construction?
- Roof Construction?
- What is the fire rating of the materials?
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