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Types of Building Construction
Key NFPA 220 Definitions
• Fire Resistance Rating. The time, in minutes or hours, that materials or assemblies have withstood a fire exposure as established in accordance with the test procedures of NFPA 251.
• Flame Spread Index. A number obtained according to ASTM E 84 or UL 723.
• Limited-Combustible Material. Refers to a building construction material not complying with the definition of noncombustible that, in the form in which it is used, has a potential heat value not exceeding 8141 kJ/kg (3500 Btu/lb), where tested in accordance with NFPA 259 and includes either (1) materials having a structural base of noncombustible material, with a surfacing not exceeding a thickness of 3.2 mm ( in.) that has a flame spread index not greater than 50, or (2) materials, in the form and thickness used having neither a flame spread index greater than 25 nor evidence of continued progressive combustion, and of such composition that surfaces that would be exposed by cutting through the material on any plane would have neither a flame spread index greater than 25 nor evidence of continued progressive combustion, when tested in accordance with UL 723 or ASTM E 84.
• Noncombustible Material. A material that, in the form in which it is used and under the conditions anticipated, will not ignite, burn, support combustion, or release flammable vapors when subjected to fire or heat. Materials that are reported as passing ASTM E 136 are considered noncombustible materials.
There are five basic classifications of building construction;
Type I Fire Resistive
Type I fire-resistive construction is a type in which structural elements are of an approved noncombustible or limited combustible material with sufficient fire-resistive rating to withstand the effects of fire and prevent its spread from story to story. Concrete-encased steel, monolithic-poured cement, and steel with spray-on fire protection coatings are typical of Type I, Generally, the fire-resistive rating must be three to four hours depending on the specific structural element. Fire-resistive construction is used for high-rises, large sporting arenas, and other buildings where a high volume of people are expected to occupy the building.
Most Type I buildings are typically large, multistoried structures with multiple exit points. Fires are difficult to fight due to the large size of the building and the subsequent high fire load. Type I buildings rely on protective systems to rapidly detect and extinguish fires. If these systems do not contain the fire, a difficult firefight will be required. Fire can spread from floor to floor on high-rises as windows break and the next floor windows fail, allowing the fire to jump. Fire can also make vertical runs through utility and elevator shafts. Regardless, firefighters are relying on the fire-resistive methods to protect the structure from collapsing.
• These buildings can be recognized by having a skeletal frame work including either poured concrete or heavy steel I beam construction with fire protective coatings.
• These buildings will generally have localized collapse and possible spalling of the concrete and have a potential for beam sagging prior to the total collapse.
• Generally the load will be supported and the load will be in place.
• There have been cases of areas totally consumed by fire that did not produce threat of building or area collapse.
• Care must be taken in these buildings that are under construction where the fire involves formwork or unprotected steal I beams.
Collapse Hazards• There are two basic types of fire-resistive construction: reinforced concrete buildings and structural steel buildings.
• Both are designed to resist fire which burns out an entire floor without spreading flames to other floors or collapsing the structure.
• However, during serious fires a collapse danger does exist with both types of construction. In reinforced concrete buildings, heated concrete ceilings collapse on top of firefighters; in steel skeleton buildings, heated concrete floors buckle upward.
• Both of these structural failures are caused by spalling, the rapid expansion of heated moisture inside the concrete.
• Small amounts of moisture, normally trapped inside concrete, expand when heated by fire and create an internal pressure within the concrete. This pressure can cause heavy sections of concrete to crack away from a ceiling and collapse down onto fire operations or on top of a firefighter.
• This type of collapse occurs in a building without a suspended ceiling, where the concrete ceiling above the fire is directly exposed to flames below.
• In steel skeleton construction, the under side of each floor is not concrete; each floor consists of light-gauge corrugated steel sheet which supports several inches of concrete floor above it.
• Heat from a fire reaching the under-side of the corrugated steel is conducted through the concrete floor directly above.
• The moisture in the concrete above the steel is heated, and the internal pressure develops in the concrete above.
• Consequently, the expanding concrete buckles upward suddenly 6 to 12 inches.
Type II Non-Combustible Type II noncombustible construction is a type in which structural elements do not qualify for Type I construction and are of an approved noncombustible or limited combustible material with sufficient fire resistive rating to withstand the effects of fire and prevent its spread from story to story. More often than not, Type II buildings are steel, Modern warehouses, small arenas, and newer churches and schools are built as noncombustible. Because the steel is not required to have significant fire-resistive coatings, Type II buildings are susceptible to steel deformation and resulting collapse. Fire spread in Type II buildings is influenced by the contents. While the structure itself will not burn, rapid collapse is possible from the content BTU release stressing the steel.
Suburban strip malls with concrete block load bearing walls and steel roof structures can be classified as Type II. Fires can spread from store to store through wall openings and shared ceiling and roof support spaces. The roof structure is often of lightweight steel that fails rapidly.
More often than not, the fire-resistive device used to protect the roof structure is a dropped-in ceiling. Missing ceiling tiles, damaged drywall, and utility penetrations can render the steel unprotected. These buildings may have combustible attachments such as facades and signs as well as significant content fire loading.
Collapse Hazards• There are three basic types of non-combustible buildings: The metal-frame structure covered by metal exterior walls, the metal-frame structure enclosed by concrete block, non-bearing exterior walls; the concrete block bearing walls supporting a metal roof structure.
• On all three types, the steel roof support system may be either one of the following: a system of solid steel girders and beams, lightweight open web bar joist, or a combination of both.
• The collapse danger to a firefighter from a non-combustible building is roof cave-in from the unprotected steel open-web bar joist.
• The main disadvantage of the open-web bar joists is its susceptibility to damage by a fire in the combustible contents inside the building.
• Tests have shown that unprotected lightweight open-web bar joist can fail when exposed to fire for five to ten minutes.
• This possibility makes it extremely dangerous for a firefighter to operate on a roof supported by steel open-web bar joists which are being heated by flames.
• The open-web bar joist is the main structural hazard of non-combustible construction.
Type III Ordinary (Exterior protected)
This type of construction is commonly referred to as ordinary construction or brick and joist construction and is a highly predominate and common construction type.
Ordinary construction consists of masonry exterior load-bearing walls that are of non combustible construction. Interior framing, floors, and roofs are made of wood or other combustible materials, whose bulk is less than that needed to qualify as heavy-timber construction.
If the floor and roof construction and their supports have a one-hour fire resistance rating and all openings through the floors (stairwells) are enclosed with partitions having a one-hour fire resistance rating, then the construction is classified as “protected ordinary construction Examples are older apartment buildings, typical late 19th and early 20th century construction, also known as “Main Street U.S.A”.
• This type construction would include brick and wood joist buildings not of the size required for heavy timber.
• These structures are more prone to burn through than collapse even under normal floor loads.
• Floor and roof sheathing tend to burn through before structural failure occurs.
• In this type of structure, firefighters tend to be under the collapse rather to be on the collapsing surface.
• Another danger may be unusually high dead loads such as roof mounted HVAC units or high concentration of stock found such as found in supply houses.
• Early collapse is possible in these localized areas with little or no warning.
• Incident command should be notified of these types of loads during the firefighting.
• Personnel should be kept out of the area until stability can be confirmed.
• Other common terms applied to Type III or ordinary construction are "Main Street USA," and a "Taxpayer." The term "Taxpayer“ evolved from "Main Street USA,“ where a building owner would operate a retail store on the first floor and live in an apartment on the second floor. It was said that the property taxes for the building were paid for by the store's income, and the shop owner lived above the store tax free.
• Buildings of ordinary construction generally are built no taller than six to eight stories due to the thickness of the supporting walls required to support the floor loads above the ground.
• The tallest building of ordinary construction is located in Chicago, Illinois, and is 15 stories high.
• Most ordinary-construction buildings on are only two or four stories high.
• Exterior walls of Type III ordinary construction generally are constructed of brick.
• A true brick wall is two or more courses of brick thick. If a solid brick wall is being built, it is customary to lay five to six layers of brick lengthwise (stretcher rows) in the wall and then lay one at a 90-degree angle (header row) to the courses of brick below.
• This serves to tie the wall together as a solid unit. The bricks laid lengthwise are called "stretchers" and the rows laid at a 90-degree angle, where only the end of the brick is seen, are called "headers." Some walls have been constructed with the headers laid in a pattern in every course or every other course.
• A true brick wall has two or more wythe, while a fascia or decorative wall will have only one wythe of brick. In reading a building, the fact that there are both headers and stretchers could be one indication that it is a solid brick wall.
• Another material commonly used for exterior wall construction is concrete block. The block is the load-bearing wall and carries the loads above.
• Some older buildings used a clay terra cotta block in place of today's modern concrete block as the load-bearing wall. Terra cotta is a material similar to brick, which is fired in a kiln to harden the material.
• The terra cotta block wall was brittle and subject to being broken when struck with heavy objects. The exterior wall was faced with one layer of brick similar to the technique used for block walls.
• One danger of veneered walls placed adjacent to block walls is that it is possible, through various building connections, that gases from a fire, such as carbon monoxide, could accumulate in the small cavity between the two walls.
• Wall thickness of a Type III building typically will range from 6 to 30 inches thick, depending upon the load it must carry. Remember that every ounce of weight from the roof load through the first floor of the building must be carried and transmitted to the foundation of the building.
• Bearing walls carry the load of the structure, which is transferred to the wall from the floor joists and roof rafters.
• Failure of a load-bearing wall can result in catastrophic structural collapse. It is critical that all emergency scene personnel be able to identify the load-bearing walls and understand the dangers associated with wall failure.
• Bearing walls are typically the longest walls of the structure.
• This may be the front (street side) or, most often, the sides of the building.
• The longest walls typically are used to support the load, since this requires the shortest span for the floor joists and roof rafters inside the building.
• Walls may have a heavy load that may push in a lateral manner instead of directly downward on the wall, and tend to tip the wall outward. This often happens when loads are applied directly against the walls, or when large open areas lack adequate materials to tie the two walls together.
• To counteract the push against the walls a masonry pier can be built into the block wall, a buttress (brace larger at the bottom than at the top) can be installed, or tie cables and rods can be installed, which is the most common method.
• The concern for the steel tie rods and cables is that they are subject to relaxation or elongation when heated, thus allowing the walls to move.
• The rods and cables are connected with turnbuckles that can be adjusted to compensate for the load applied above
• Type III buildings typically will have two load-bearing walls (long walls) and two curtain walls (short walls). The load of the building is placed on the load-bearing walls, which must be transferred to the ground.
• Buildings of ordinary construction often are renovated, with new void spaces created. These include horizontal voids created by dropped ceilings and vertical voids through new utility chases.
• The cockloft area is particularly vulnerable to rapid fire extension due to the heavy fire load in a confined and open space.
• It is extremely important for fire officers, Incident Commanders (IC's), and Safety Officers to read a building's construction carefully before developing their strategic goals or implementing their tactical objectives.
• The age and renovation cycles of these structures make them prone to structural collapse and rapid fire spread and instability.
Collapse Hazards
• The structural hazard of an ordinary constructed building is the parapet wall, the portion of the masonry wall that extends above the roof line.
• The collapse danger of the parapet wall is one of the reasons why the area directly in front of a fire building is so dangerous, and why firefighters are urged either to move inside the doorway or away from the front of the building altogether.
• There are several design features relating to ordinary construction which warrant close observation. These include efflorescence, parging, and spreaders. Efflorescence results when large amounts of soluble salts are used in mortar and excessive water penetrates the masonry.
• Efflorescence appears as a white powdery substance on the wall and indicates weakened mortar.
• Parging is the plastering over of a masonry wall with concrete. It is frequently a cosmetic fix for an unattractive, deteriorated wall.
• A wall out of alignment is always a sign of danger. Spreaders are intended to spread the load among one or more structural members and are frequently used to support a wall in trouble.
• They are often indicated on the outside of a wall by a circle, star, channel, or other device; arranged in a pattern they usually serve a decorative purpose. When placed at random, these spreaders provide additional strength to the wall.
Type IV Heavy Timber
Heavy Timber (Type IV)Construction is characterized by masonry walls, heavy-timber columns and beams, and heavy plank floors. Although not immune to fire, the large mass of the wooden members slows the rate of combustion. Heavy timber construction can be used where the smallest dimension of the members exceeds 5.5 in. (14 cm). When timbers are this large, they are charred but not consumed in a fire and are generally considered akin to a fire-resistant type of construction. Buildings are those of heavy timber construction. These buildings have at least 12” x 12” wood beams with non combustible walls commonly brick or block construction.
• Heavy-Timber or NFPA Type IV buildings will have four bearing exterior walls made from noncombustible materials. Exterior walls typically are thicker at the lower levels, which are meant to carry the accumulated loads from the floors above. Walls (both exterior and interior) will have sockets built into them to accept floor joists or beams from the floor support system.
• Interior load-bearing walls will be of noncombustible materials similar to the exterior walls and play a critical role both in fire separation and in supporting the heavy loads found in this type of building.
• Heavy timber construction is known to be extremely stable under fire conditions due to the size of the load bearing members with large columns and roof support trusses.
• In fire conditions that grow to a point of potential collapse created by fire conditions inside the building, the fire would be so intense that firefighters would be driven out by the heat.
• Care must be taken in any structure with previous fires in the structure. The cumulative damage may cause a large collapse.
• Collapse on an upper floor may cause lower floors to also collapse to the excess loading in these buildings
• Incident commanders must be aware of the collapse zone which is equal to one and one half times the height of the wall. As with all collapse zones personnel and apparatus need to be kept out of the collapse zone.
• Floors may have unprotected openings for stairwells, freight elevators, or conveyor devices, which will allow fire extension to the floors above.
• The positive aspect of these buildings is that they are built for strength, with heavy-timber supports for the floor and roof structure, providing a strong and stable building in the early stages of a fire.
• Heavy-timber buildings also have been called "mill construction," since this was the typical original occupancy for this type of construction. The thick, noncombustible exterior and interior walls were made from multiple courses of brick, which provided a strong base to carry heavy loads from machinery and manufactured goods.
• The floor and roof supports were constructed from large, solid or, in later years laminated timbers covered with thick planks. This building type was very popular as the Industrial Revolution developed, and often these heavy-timber buildings were constructed next to sources of water power.
• Today many of the heavy timber or mill buildings have been converted to other purposes, such as retail sales stores, multiple-family dwelling units, or office spaces.
• Most of these buildings were protected by sprinkler systems. One advantage of this method of construction was that very little of the structure was enclosed to create voids.
• Exterior wall construction had to be noncombustible, commonly brick, block, or stone. Walls at the lower levels were typically thicker than those at the top level. This stepped wall system allowed for greater load carrying capacity at the lower levels, and the loads were accumulated from the upper floors and transmitted to the building's foundation.
• Typically, the walls were 24 to 36 inches thick at ground level. Wall sockets would be placed in both the interior and exterior walls to accommodate the floor support timbers. Common walls between buildings and floor supports seldom were used in heavy-timber construction.
• Generally each building had a separation to reduce the potential of fire spreading from one building to another.
• Unlike ordinary construction, which typically had two bearing walls and two curtain walls, the heavy-timber building would have four bearing walls, and no curtain walls. Lintels or arches to transfer loads over openings were made from materials of substantial strength.
• Occasionally, additional bracing or support cables were added to the exterior walls to provide additional strength to keep the walls from pushing outward.
Collapse Hazards
• Falling masonry walls which crash to the ground and spray bouncing chunks of bricks and mortar along the street or pavement are the structural hazards of heavy timber buildings.
• This type of construction does not collapse during the early stages of a fire when interior firefighting is taking place. However, after several hours, its floors will collapse and the free-standing walls will fall into the street and on to the roofs of lower buildings nearby.
• Consequently, withdrawing to protect exposures is the strategy used at a fire involving heavy timber construction when the initial attack fails.
Type V Wood Frame
Buildings include wood frame buildings with standard dimensional lumber like 2”x 4” up to and including 2” x 12” lumber.
• This type of construction will tends to burn through sheathing prior to collapse, but would not apply to wood frame construction using light weight truss and wood floor I beam construction.
• Buildings with lightweight wood truss floor and/or roof assemblies (collapse in 5-10 minutes).
• Expect any house and most multi-family residential structures built after about 1970 to have some form of lightweight construction. If not sure, open the ceiling just inside the door as you enter and in a multi-story house open it again when you reach the top floor
Collapse Hazards
• The structural hazard of a wood frame building is the combustible bearing wall constructed of 2x4 inch wood studs.
• A wood frame building is a bearing wall structure. The two side walls are usually bearing (that is, supporting a load other than their own weight); the front and rear walls are usually non-bearing.
• The structural supports of the side bearing walls are only 2x4 inches in size and roof joists also 2x10 inches.
• Firefighters should know that wood-frame buildings use smaller structural members to support larger structural members, and the weak link in this design is smaller structural supports, the 2x4 inch bearing walls.
• Failure of a bearing wall will trigger simultaneous failure of the floors and roof.
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