BC - Chapter 9 Steel Construction
Gypsum can be used to protect both columns and beams, providing fire resistance ratings from one to four hours.
Gypsum board is used in multiple layers to attain higher fire resistance ratings.
Intumescent coatings are applied as paint. They have an applied thickness 0.03 to 0.4 inches (0.75 to 10 mm) which is less than the thickness of the spray-applied materials.
Mastic coatings function in a manner similar to intumescent coatings except they are based on more complex organic materials and their reaction to heat is more complex
The applied fireproofing can vary from 7/8 to 1-7/8 inches to produce a fire-resistive rating of one to four hours
Sprayed-on insulating material is difficult to regulate after application.
Connection types and their relative strength in fire conditions will influence the overall strength of steel structures.
3 connection types increase the mass at the point of connection to make it the last item to fail within a structural system.
The investigation of a structural collapse under any circumstance must include an evaluation of any columns and their means of support.
A beam that is simply supported by a column may become dislodged and fall off the supporting column if the column shifts or buckles.
Ceiling materials are never rated independently.
A ceiling is always rated as part of a floor and ceiling assembly.
Columns that are erected so they cannot rotate at their ends have fewer tendencies to buckle than columns that are free rotate at their ends.
A column with rigid connections will withstand buckling and twisting/rotating better than a simply supported column.
The possibility of buckling in an individual column is a function of its length, its cross-section, and the method by which the column is supported at its top and bottom
A property of a given column, known as its slenderness ratio, is used in combination with the condition of the column end, to determine the load that can be safely supported without buckling.
The loss of steel strength because of increased temperature, is often not a sudden occurrence; rather, the steel loses its strength gradually as its temperature increases.
A sharp increase in temperature may cause steel components to fail suddenly.
Steel is a good conductor of heat because it is a heavy material having a density of around 490 pounds per cubic feet (7 850 kg/m³).
A significant amount of heat is required to raise the temperature of heavy steel structural members.
Steel arches can be designed as either girder arches or trussed arches.
A trussed arch is built using truss shapes. A pin connections allow for slight movement between the two halves due to settling or temperature change.
Alloy
Alloy is a substance or mixture composed of two or more metals (or a metal and nonmetallic elements) fused together and dissolved into each other to enhance properties or usefulness of the base metal.
An important factor of steel use is the support system used with a beam.
An unrestrained steel m 20 feet (6 m) long can expand 1.4 inches significantly pushing at lateral constraints.
The fire-resistance rating may be eliminated for of construction located more than 20 feet (6 m) above the floor below for some occupancies.
Another example is the allowed reduction of the required fire resistance when an automatic sprinkler system is provided that is not otherwise required by the provisions of the code.
Steel trusses that are constructed with slender components, and simply supported on a bearing wall or beam, can easily fail and collapse under fire conditions.
As in the case of steel beams, failure will not occur unless the trusses are exposed to fire conditions.
If round bars are used for the diagonal members, the open-web joists is known as a bar joist
Bar joists are frequently used in closely spaced configurations for the support of floors or roof decks.
Beam and girder steel frames can be classified as rigid, simple, or semi-rigid.
Beams and columns in steel-frame buildings are connected by bolting or welding.
The rolling process consists of repeatedly passing of steel heated to 2,200°F (1 200°C) between large rollers until the intended shape is achieved.
Cold rolled steel (CRS) is used for members that have a thin cross-section, such as floor and roof decking and walI studs.
The fire-rated membrane floor and ceiling assemblies are listed by the testing laboratories as a total assembly.
Deviation from the laboratory specifications will affect the fire rating of the assembly
When the yield point stress. is reached (approximately 36,000 psi [250000 kPa]), the steel undergoes a pronounced deformation.
Finally, the steel breaks the ultimate stress.
The design of the connections in steel-frame buildings is important for two reasons :
First, the connection of a beam to a column transfers the loads between members. Second, connections determine the rigidity of the basic structure.
Gypsum board consists of a core of calcined gypsum, starch, water, and other additives that are sandwiched between two paper faces
Gypsum can be used as an insulating material either in the form of flat boards or as plaster.
For an SFRM to be effective, proper installation procedures must be followed, in accordance with the manufacturer's listing and recommendations.
Hangers and supports must be installed before application of the SFRM because hangers and supports added after the fact may require that fireproofing material is removed, which limits the effectiveness of the remaining material.
More durable mineral fiber products with densities greater than 20 lb/ft³ (320 kg/m³) can endure conditions of limited weather exposure and higher humidity such as might exist in parking facilities.
High-density SFRMs that use magnesium oxychloride have densities ranging from to 80 lb/ft³. (640 to 1300 kg/m³).
Open web joists are mass produced and are available with depths of up to 6 feet (2 m) and span up to 144 feet.
However, they are more frequently found with depths less than 2 feet (600 mm) and spans of 40 feet.
A round steel column that is simply resting on a concrete footing with low resistance to rotation at its ends and subsequent buckling
In contrast, a wide flange column with rigid connections at each. end has a large resistance to rotation and buckling.
The slenderness ratio compares the unbraced length of a column to the shape and area of its cross-section
In general, columns used for structural support in buildings should not have a slenderness ratio greater than 120. The higher the numerical value of the slenderness ratio, the more likely it is that buckling will occur.
Cementitious materials have densities ranging from 15 to 50 lb/ft³
Ingredients can include: -Portland cement -Gypsum -Perlite -Vermiculite
Membrane ceilings are popular partly because building utilities such as electrical wiring, automatic sprinkler piping, and ventilation ducts can be concealed above the ceiling.
It may be necessary to provide additional insulation on the back of lighting fixtures and to equip ventilation ducts with fire dampers so fire does not penetrate through the opening.
Two commonly encountered applications of the basic steel truss are the joist girder and the open web joist.
Joist girders are heavy steel trusses used to take the place of steel beams as part of the primary structural frame.
Some insulating materials are applied by spraying.
Lightweight concrete can also be used.
Low-density mineral fiber materials are relatively soft and can be easily dislodged from the steel.
Low-density fiber materials are not suitable for exterior use.
The rigid connections used in the beam and girder type of frame have a greater mass of steel at the point of connection than do simple connections.
Rigid connections are frequently found intact after a fire even after other parts of a frame have failed.
Unprotected steel structural members with less mass require less heat to reach the failure temperature.
Some fires do create temperatures in excess of 1,200°F (650°C), which is hot enough to weaken steel to its yieldpoint
During the alloying process, elements are combined to produce an end result that can be used for a specific purpose
Steel - An alloy of iron and carbon; proportions and additional elements affect the characteristics of the finished material.
The basic method by which steel is used in the design of buildings is the construction of a structural framework that supports the floors, roof, and exterior walls.
Steel also can be used in heavy or lightweight trusses to support roofs and floors -Cold-rolled steel studs are being used to construct exterior walls.
Steel arches are used to support roofs on buildings where large unobstructed floors are needed, such as gymnasiums and convention halls.
Steel arches can be constructed to span distances in excess of 300 feet.
Simple Frame The joints in a simple frame are designed primarily to support a vertical force. A degree of angular change between beams and columns can occur if some form diagonal bracing is not provided
Steel beams and trusses are frequently supported by a masonry wall.
Steel beams and trusses have largely replaced wood beams and trusses in commercial structures.
Steel framing is found in buildings of all heights and is used.in both fire-resistive and non-fire resistive buildings.
Steel rigid-frame buildings with inclined (or gabled) roofs are widely use for the construction of one-story industrial buildings, farm buildings, and a variety of other applications.
Steel rigid frames usually are used for spans from 40 to 200 feet (12 to 60m) and are fabricated by welding or bolting together steel shapes and plates.
Steel trusses provide a structural member that can carry loads across spans greater more economically than beams can.
Steel trusses are frequently used in three-dimensional space frames, in which case they are known as delta trusses because the cross-section resembles the Greek letter Delta.
Yielding and thermal expansion take place simultaneously; however, the tendency for steel to yield and, therefore, to bend or buckle is the more significant concern in most fire situations
Steel with lower stresses must be heated to a high yield point to be reached.
Steel alloys with higher yield points have less ductility
Steels for special application including bridge strands have tensile strengths as high as 300,000 psi (2,100,000kPa) but have very little ductility.
A steel column encased in 3 inches (75 mm) of concrete with a siliceous aggregate would have a fire resistance of 4-hours
Structural designers refer to use lighter-weight materials for the protection of steel.
Steel can be used in slender components such as rods and cables. Wires used in bridge cables can have steel strengths as high as 300,000 psi. (2,100,000kPa)
Such slender supports easily buckle under compression forces and therefore are limited to support of tension force
Steel expands as it is heated.
The amount of expansion, and other factors such as any restraints on the beam, are important to know because steel components are unable to push their lateral constraints will weaken, bow or buckle, and. fail, which may create several points of failure.
The heating of steel or other materials does not occur uniformly during fire incidents.
The bottom of a beam is often heated more by the fire than the top or the ends
Intumescent materials undergo a chemical reaction when exposed to the heat of a fire.
The coating material will expand to 15x - 30x its original volume, the expanded coating then acts as an insulating material to protect the steel.
A common floor design in steel-frame buildings uses lightweight concrete with a minimum thickness of 2 inches (50 mm) supported by corrugated steel decking.
The corrugated steel is, in turn, supported by open-web steel joists
Spray-applied fire-resistive materials (SFRM) are efficient and inexpensive. The most commonly used SFRMs are mineral fiber or expanded aggregate coatings such as vermiculite and perlite.
The degree of fire resistance provided will depend not only on the material but also on the thickness of the application.
The lower carbon content of steel, compared to cast iron, results in a material that is ductile rather than brittle.
The ductility of steel also allows it to be shaped by rolling as opposed to molding like concrete or cutting, like wood and stone.
One common structural steel is ASTM A36 which includes manganese, carbon, and silicon.
The ductility of this steel can be illustrated by comparing the stress exerted on the steel against the resulting deformation known as the strain
individual steel members that are rigidly welded or bolted into a large structural system are better able to resist failure than if they are simply supported.
The end restraint provided by rigid connections exerts a resistance to the deformation of individual members.
Because steel is a strong but dense material, it is not efficient to use it in the form of solid slabs or panels as is done with other materials such as wood or concrete.
The exterior envelope of a steel-frame building can consist of concrete, masonry, or glasss.
The primary two metals alloyed in steel are iron and carbon.
The higher carbon content of cast iron makes the material hard but brittle.
Types of steel columns can range from simple single piece cylindrical pipes to complex tower assemblies
The most common column cross-sections are: -Hollow cylinder -Rectangular tube -Wide flange shape similar to the cross section of an I-beam
A commonly used method of protecting a steel floor of an assembly is the membrane ceiling.
The most common method is to use mineral tiles in a steel framework suspended by wires. Acoustic tile used in dropped ceilings provides a measure of fire resistance to the steel framework above.
Protected steel is one of the two common materials used in fire-resistive construction.
The other material is reinforced concrete.
Steel connections, both in the case of rigid connections used with beam and girder frames and heavy trusses, frequently use a steel web known as a gusset plate
The primary purpose of a gusset plate is to strengthen the connection, and increase the steel. mass at the connection, decreasing the possibility of failure.
The deterioration of steel strength at elevated temperatures is the characteristic most significant to the fire service.
The speed of failure depends on several factors, including: -Type of steel -Mass of the steel members
Light-gauge steel joists are also sometimes used support flooring
The steel joists are produced with. depths of 6 to 12 inches (150 to 300 mm) and can be spaced 16 to 48 inches apart (400. to 1200 mm) depending on the span and the load to be supported.
One inherent disadvantage of steel is
The tendency to rust when exposed to air and moisture.
The ceiling material acts as a thermal barrier to protect the steel that supports the floor or ceiling above.
The use of a membrane ceiling can provide floor and ceiling assembly or a ceiling and roof assembly with a fire rating of one to three hours depending on the specific details of the installation.
No matter its mass, steel is ultimately not inherently fire-resistive.
The usual way to protect steel from the heat of a fire is to add an insulating material.
Gypsum includes approximately 20%percent entrapped water.
The water turns to steam up on exposure to fire, in doing so, absorbs the heat. This process is known as calcination. Once the moisture has been driven off, the remaining gypsum will act as an insulating material.
The light-gauge steel sheeting used in floor systems and in roofs has a large surface area compared to its mass (large surface area to mass ratio).
Therefore, unprotected, light-gauge steel sheeting may fail structurally although it will not melt.
Steel columns can be disproportionately thin compared to their length because of the high compressive strength of steel.
Thin steel columns are more likely to buckle than thicker columns made with other materials, even though the compressive strength of steel is higher.
Gable roof rigid-frame structures must be braced diagonally for structural rigidity
This brace is built with diagonal cross-members in the plane of the roof and in the vertical plane of the walls between the rigid frame sections.
In gabled rigid-frame structures, the knee joint between the roof and the wall will be the strongest part the frame and the last part to fail
This connection has greater mass than other similar places. The knee joint helps transfer the roof load to the vertical members.
Regular gypsum board has no special additives to enhance its fire resistance, although it will provide some degree of fire protection.
Type 'X' gypsum board contains additives to increase it's fire resistance and is usually required where a specified fire resistance is desired.
Semi-Rigid Frame The connections in a semi-rigid frame are not completely rigid but possess enough rigidity to provide some diagonal support to the structure.
When rigid connections are not used, lateral stability for a frame must be provided through the use of diagonal bracing or shear panels. Ideally a shear wall should be continuous from the foundation of a building to the highest story at which it is needed.
Rigid Frame The connections between the beams and the columns in a rigid frame system are designed to resist the bending forces resulting from the supported loads and lateral force.
Within this connection type, sufficient rigidity exists between the beam and the column so that no change occurs in the angle between the beam and the column as the loads are applied.
