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Posts Tagged ‘Fire protection’

Fire Protection for Steel Buildings IX- Concrete Filled HSS (continuation)

Thursday, December 16th, 2010

Filling hollow structural steel columns (HSS) with concrete boosts the sections’ load-bearing capability and fire resistance. This post talks about the different types of concrete filling for HSS of steel buildings and how they perform during fires.

Plain Concrete

The fire resistance of steel sections filled with plain concrete can only be from one to two hours. When exposed to increasing temperature, the concrete cracks and its compressive strength is greatly reduced, resulting to failure of the concrete core. Fire resistance can be increased to more than an hour by decreasing the load levels. However, steel columns filled with plain concrete is extremely sensitive to loads that act away from the longitudinal axis.

Steel-Fiber-Reinforced concrete

To obtain fire resistance ratings of up to three hours without reducing the load, horizontal structural steel columns are filled with steel-fiber reinforced concrete instead of plain concrete. The presence of steel fibers increases the compressive strength of the concrete and reduces the possibility of the concrete cracking and failing, when exposed to extremely high temperature.

The improved fire performance can be attributed to the superior mechanical and thermal properties of steel-fiber reinforced concrete at high temperatures. Besides, the steel fibers provide a containment effect to the concrete core.

Other numerous advantages of steel-fiber reinforced concrete aside from increased fire resistance, include:
· Improved deformation behavior that results in gradual instead of sudden failure of the concrete
· 10% to 20% increase in load-bearing capacity
· Reduced buckling
· Suitable for a wide range of steel column dimensions

Bar-Reinforced Concrete

In reality, bar-reinforced concrete provides many of the benefits steel-fiber reinforced concrete. However, aside from the fact that they are more expensive, placing the reinforcing bars is laborious and difficult, especially in confined spaces inside steel buildings.

Source: http://www.nrc-cnrc.gc.ca/obj/irc/doc/ctu-n6_eng.pdf

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Fire Protection for Steel Buildings IX – Concrete Filled HSS

Wednesday, December 15th, 2010

Steel hollow structural section (HSS) columns, widely used in the construction of steel-framed buildings for industrial purposes, are very effective in resisting compression loads. However, to increase their fire resistance and also their load-bearing capacity, round, rectangular, and square hollow structural sections can be filled with concrete. This is another good option for fire protection of steel buildings.

Designing steel buildings with concrete-filled steel columns eliminates the need for external fire protection. As a result, architects and engineers can now expose steel in their designs, without increasing the vulnerability of steel buildings to fire. An added bonus is an increase in usable floor space and a reduction in fire protection costs.

At normal room temperature, both the concrete and the steel column carry the load. However, when exposed to heat especially in the early stages of a fire, the steel column carries most of the load since this section expands more rapidly than the concrete core.

As the heat gets more intense, the steel section gradually loses its strength, the column rapidly contracts, and the concrete filling starts to carry more and more of the load. Ultimately, the strength of the concrete decreases, and when it can no longer carry the load, either collapses or fails in compression. The amount of time it takes before the column fails, determines its fire-resistance rating.

There are three different types of concrete filling: plain concrete, steel-fiber-reinforced concrete, and bar-reinforced concrete. The steel hollow structural section can be filled off-site or erected and filled on-site. This fire protection method is typically used in steel buildings designed with exposed steel, because the steel can be easily painted.

(to be continue)

Sources:

http://www.nrc-cnrc.gc.ca/obj/irc/doc/ctu-n6_eng.pdf

http://www.modernsteel.com/Uploads/Issues/September_2009/092009_steelwise.pdf

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Fire Protection for Steel Buildings VIII – Gypsum Boards

Tuesday, December 14th, 2010

Using gypsum board-based products is another popular option for protecting the structural steel in steel buildings during a fire. Gypsum panel products are used for ceilings, exterior sheathing, liner material for elevator shafts and stairwells, and most importantly, as fire-resistant partitions and membranes.

Gypsum is a very soft naturally occurring, crystalline mineral composed of calcium sulfate dihydrate. The powdered material undergoes the process of “calcinations” to remove moisture and much of the chemically combined water. The calcined gypsum, commonly known as “plaster of Paris” or “stucco”, is the main ingredient in gypsum plaster.

Technically speaking, gypsum board, often referred to as drywall, wallboard, or plaster board, is the generic name for a type of sheet products comprising of a noncombustible gypsum core, with a paper surfacing on the face, back, and long edges.

For fire protection of steel buildings, gypsum board is the material of choice due to its fire-resistant and low flame spread properties. When exposed to fire, the chemically combined water contained in its noncombustible core is released as steam, forming a thermal barrier that effectively impedes heat transfer.

Even after all the chemically combined water has already been released, the gypsum board, with the calcined gypsum in its dense core, continues to provide a physical barrier to heat and flame.

To achieve the desired fire ratings in the field, several considerations in the design and construction of gypsum assemblies should be addressed:
· Fire resistance testing is conducted only assemblies made up of specific materials built in a specified manner. Thus, gypsum board enclosures should be designed and specified as an assembly.
· The assembly in the field requires extra attention and must be representative of the one tested, down to the last detail, including size of studs and number of fasteners.

Sources:
1. http://www.wbdg.org/design/092000.php
2. http://www.modernsteel.com/Uploads/Issues/September_2009/092009_steelwise.pdf

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Fire Protection for Steel Buildings VII – SFRM

Monday, December 13th, 2010

One important goal in the design of steel buildings, is minimizing the potential for a fire to occur and protecting lives and property if one does. Often prescribed in the applicable building code, fire resistance requirements for steel buildings, are based on building occupancies, height, area, and other building characteristics.

The design of steel buildings that are prescribed to be fire resistant, should follow this very simple inequality: Fire Resistance should be greater than or equal Fire Severity. Achieving this condition means that the metal structure must resist collapse in the event of a fire of a specified severity.

Fire resistance materials and systems aim to prevent or delay the temperature rise in structural steel. Maintaining adequate strength for the steel members for the code-required duration, gives sufficient time for safe evacuation and fire-fighting operations.

We have discussed some fire resistant materials and systems such as thin-film intumescents and epoxy-based intumescents. Another most commonly used passive fire protection material for steel buildings, is SFRM or spray-applied fire-resistant material.

SFRMs initially come as dry pre-mixed combination of gypsum or Portland cement and inorganic binders and lightweight mineral or synthetic aggregates. This dry mixture is then, mixed with water on-site to form a slurry, which is pumped and sprayed on the steel substrate.

Since SFRMs have proprietary formulations, it is important to strictly follow the manufacturer’s recommendations for mixing, application, and thickness to achieve the desired fire rating.

SFRMs are popular because of the many advantages they offer including speed and efficiency of application and cost-effectiveness. The structural steel need only to be shop cleaned of dirt, oil, grease, and loose mill prior to application, which is relatively easy and fast.

However, since the application is a wet process, precautionary measures, such as protecting on-site areas from overspray, are typically required.

Source: http://www.modernsteel.com/Uploads/Issues/September_2009/092009_steelwise.pdf

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Fire Protection for Steel Buildings VI – Epoxy Based Intumescents

Friday, December 10th, 2010

Yesterday’s post talked about thin-film intumescents, a kind of material that is sprayed on exposed structural steel components of steel buildings. This post discusses the merits of epoxy-based intumescents/mastics. By the way, intumescents are materials that produce a char, which is a poor conductor of heat, which prevents the fire from spreading.

Epoxy-based intumescents/mastics are heavy-duty fire protection materials developed primarily for structures where severe fire exposures can be encountered, such as those used for offshore and petrochemical industries. When exposed to heat, these materials develop a robust char layer that is capable of withstanding highly erosive fire exposure surroundings.

In reality, epoxy-based intumescents/mastics are also being used in other applications of steel buildings including protection of structural steel in commercial buildings and clean rooms.

Epoxy-based intumescents/mastics can be applied either through spraying or brushing. Application thicknesses, ranging from 0.2 inches to over 1 inch thick, depend on the size of the structural members and the fire-resistive ratings, which can be up to 4 hours.

The resulting char layers can be 50 to 100 times thicker than the original application, significantly higher than thin-film intumescents. Due to this considerable amount of char layer thickness, support must be provided in areas vulnerable to damage, such as flange tips. Some of the materials used are metal wires, fiberglass, or carbon scrim mesh.

All in all, epoxy-based intumescents are extremely durable, provide excellent adhesion qualities and can be aesthetically appealing. In addition, these fire protection materials performed excellently in various tests using different blast loadings. As a result of these qualities, they cost more, making them predominantly used in special applications where severe fire exposures may be encountered.

Applying epoxy-based intumescents does not require a surface primer to ensure adequate adhesion of the material with the steel surface. Just make sure the exposed structural steel is clean and free of oil, dirt, and grease.

Source: http://www.haifire.com/magazine/AESS.pdf

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Fire Protection for Steel Buildings V – Thin Film Intumescents

Thursday, December 9th, 2010

Recent interest of architects in structural expression maybe the reason why some steel buildings have been designed with exposed steel structural members. The use of exposed structural steel used to be confined to monumental structures including airport terminals, train stations, gymnasiums, arenas, and industrial applications such as warehouses and factories.

Today, architecturally exposed structural steel (AESS) can be found even in small retail stores and office lobbies. Typical fire protection materials used for structural steel include sprayed fire-resistive materials (SRFM), thin-film intumescents, epoxy-based intumescents/mastics, mineral fiber boards, gypsum boards, and intumescent mat wrap materials.

These materials can provide the required fire resistance rating when properly applied to the structural steel with the minimum thickness and density (when using SRFM) as stipulated by the International Building Code.

In choosing which material to use for steel buildings, consider the following:
· Aesthetics
· Fire safety
· Durability
· Maintenance
· Exposures
· Cost

Thin-film intumescents may be the ideal choice for meeting AESS requirements due to their diversity and cost-effectiveness when compared to epoxy-based intumescents. They are applied by brushing or spray-painting directly to the structural steel, at relatively thin thickness – approximately 0.03 to 0.40 inches.

For lower thickness ranges, thin-film intumescents can be applied in a single spray pass. For greater thickness, multiple spray applications are required and longer drying time must be allowed.

Thin film intumescents undergo a chemical change and form an insulating char layer that is 15 to 30 times thicker than the initial application thickness, when exposed to heat. Currently, these materials commonly provide 1 to 2 hours of fire resistance.

To provide for a more finished appearance, a paint finish coat may be applied over the dried film intumescent. Arguably, thin-film intumescent coatings, though more expensive than SFRM, are the most aesthetically pleasing fire-resistive materials for architecturally exposed structural steel for steel buildings.

Source: http://www.haifire.com/magazine/AESS.pdf

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Fire Protection for Steel Buildings IV

Wednesday, December 8th, 2010

Steel buildings use primary and secondary steel members that are non-combustible, allowing them to be classified by the International Building Code (IBC) as Type IIB construction. More often than not, metal roof panels used in steel buildings may not be required to be protected for fire resistance.

Due to factors including building layout, end use, and proximity to other buildings, local codes may sometimes require fire-resistive walls in certain areas but not fire-resistive roofs. Now, how do the codes classify the head-of-wall joints between fire-resistant rated walls and a non-fire-resistance rated roof deck in steel buildings?

This situation often occurs when the roof assembly is more than 20 feet above the floor of the steel building – the point where the building code often allows non-fire-resistive roofs.

The 2009 IBC has no standard for testing joints between a rated assembly and a non-rated assembly prompting the Metal Building Manufacturers Association (MBMA) and its industry partner, the American Iron and Steel Institute (AISI), designed and sponsored standard head-of-wall fire tests.

The tests conducted at Underwriters Laboratories (UL) in July 2007, evaluated three possible layouts for head-of-wall joints in steel buildings when there is a fire-rated wall assembly and a non-fire-rated roof assembly – where the roof purlin is (1) located inside the wall, (2) perpendicular to the wall, and (3) parallel to the wall.

Only the first two configurations were tested since the third layout would also fall within the existing results. The two joint assemblies passed both the standard fire exposure test and the hose stream test. The testing showed that the intersection of the fire-resistance-rated wall and the non-fire-resistance-rated roof maintained the code-required continuity of the wall to the roof rather than a fire-resistive joint requirement.

In particular, the tests showed that is not necessary to cut and reattach the roof fiberglass insulation on either side of the wall, but it should just be draped and allowed to run continuously uncut over the wall. In short, the insulation facing did not transmit the fire over the top of the rated wall to the other side.

Sources: http://bsj.iccsafe.org/june/features/how_test.html

http://www.metalmag.com/technology/fire-protection.aspx,

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Fire Protection for Steel Buildings III

Tuesday, December 7th, 2010

The International Building Code classifies buildings based on their end use or intended occupancy such as (A) Assembly, (B) Business, (E) Educational, (S) Storage, and (U) Utility and Miscellaneous. Based on this classification, the fire code requirements and the allowable height and area of the building can be determined.

The 2009 IBC further defines the code requirements for specific situations for each building type including exterior separation from other structures or the property line. For example, a building that has an exterior separation distance of less than 10 feet must have exterior fire-resistance-rated walls. This is true for any building type and occupancy, including Type IIB structures, such as steel buildings.

Type II buildings are constructed of non-combustible materials. Type IIA has fire rated building components including structural frame, walls, floors, and roofs. Type IIB construction means that the building elements are not required to be fire resistance rated but still must be non-combustible.

Therefore, as the distance between adjacent buildings increases, lesser and lesser fire-resistance ratings are required. However, this varies by construction type. Steel buildings, which are classified as non-combustible construction, require less protection than combustible construction, such as wood.

Steel buildings that are used as educational facilities, are limited to 14,500 square feet per floor, for up to 2 floors. If constructed with one-hour fire-resistive elements, steel buildings can be reclassified as Type IIA and the allowable floor area can be increased up to 26,500 square feet with an additional story level. If active fire protection is installed, such as automatic sprinklers, further increases are allowed.

The MBMA, in its bid to demonstrate how steel buildings and the specific components meet the IBC requirements for fire protection, has sponsored fire tests at Underwriters Laboratories and Factory Mutual. The components tested include structural columns, interior and exterior walls, roofs and ceilings, sprinklers, and joints between walls and ceilings.

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Fire Protection of Steel Buildings

Friday, December 3rd, 2010

Prefabricated steel buildings are a popular choice because of their versatility, cost-effectiveness, and sustainability. As a result, steel buildings make up approximately 40% of the low-rise, non-residential building market with a variety of applications including complex production facilities, warehouses and distribution centers, retail stores, shopping centers, hotels and motels, car dealerships, office complexes, airplane hangars, gymnasiums and arenas, schools, libraries, churches, medical facilities, and government buildings.

The use of steel, a fire resistant and non-combustible material, in metal buildings has resulted in the overall classification by the International Building Code, as Type IIB construction. This means that steel buildings do not require passive fire protection.

Passive fire protection (PFP) is one of three components of a building’s structural fire protection and fire safety. PFP attempts to contain fires or slow down the spread of fires, through the use of fire-resistant walls, floors, and doors.

Actually, fire protection requirements, whether passive or active, depends, among other factors, on a combination of construction type and end use. However, the end use, type of storage commodity, as well as, proximity to other buildings on or near the property where the steel building will be situated.

One of the organizations that have undertaken extensive fire testing in order to demonstrate the fire-resistant capabilities of steel buildings, is the Metal Building Manufacturers Association.

The Metal Building Manufacturers Association was founded to promote the design and construction of metal building systems in the low-rise, non-residential building marketplace, architects, building code officials, and other construction professionals. As a consequence, it sponsors various research programs and produces technical publications to assist in the proper design and specification of steel buildings.

(to be continued)

Sources: http://www.metalmag.com/technology/fire-protection.aspx, MBMA website, Wikipedia

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